[[mvc]]
:chapter: mvc
= Spring Web MVC
:doc-spring-security: {docs-site}/spring-security/reference

Spring Web MVC is the original web framework built on the Servlet API and has been included
in the Spring Framework from the very beginning. The formal name, "`Spring Web MVC,`"
comes from the name of its source module
({spring-framework-main-code}/spring-webmvc[`spring-webmvc`]),
but it is more commonly known as "`Spring MVC`".

Parallel to Spring Web MVC, Spring Framework 5.0 introduced a reactive-stack web framework
whose name, "`Spring WebFlux,`" is also based on its source module
({spring-framework-main-code}/spring-webflux[`spring-webflux`]).
This section covers Spring Web MVC. The <<web-reactive.adoc#spring-web-reactive, next section>>
covers Spring WebFlux.

For baseline information and compatibility with Servlet container and Jakarta EE version
ranges, see the Spring Framework
https://github.com/spring-projects/spring-framework/wiki/Spring-Framework-Versions[Wiki].




[[mvc-servlet]]
== DispatcherServlet
[.small]#<<web-reactive.adoc#webflux-dispatcher-handler, WebFlux>>#

Spring MVC, as many other web frameworks, is designed around the front controller
pattern where a central `Servlet`, the `DispatcherServlet`, provides a shared algorithm
for request processing, while actual work is performed by configurable delegate components.
This model is flexible and supports diverse workflows.

The `DispatcherServlet`, as any `Servlet`, needs to be declared and mapped according
to the Servlet specification by using Java configuration or in `web.xml`.
In turn, the `DispatcherServlet` uses Spring configuration to discover
the delegate components it needs for request mapping, view resolution, exception
handling, <<mvc-servlet-special-bean-types, and more>>.

The following example of the Java configuration registers and initializes
the `DispatcherServlet`, which is auto-detected by the Servlet container
(see <<mvc-container-config>>):

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	public class MyWebApplicationInitializer implements WebApplicationInitializer {

		@Override
		public void onStartup(ServletContext servletContext) {

			// Load Spring web application configuration
			AnnotationConfigWebApplicationContext context = new AnnotationConfigWebApplicationContext();
			context.register(AppConfig.class);

			// Create and register the DispatcherServlet
			DispatcherServlet servlet = new DispatcherServlet(context);
			ServletRegistration.Dynamic registration = servletContext.addServlet("app", servlet);
			registration.setLoadOnStartup(1);
			registration.addMapping("/app/*");
		}
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	class MyWebApplicationInitializer : WebApplicationInitializer {

		override fun onStartup(servletContext: ServletContext) {

			// Load Spring web application configuration
			val context = AnnotationConfigWebApplicationContext()
			context.register(AppConfig::class.java)

			// Create and register the DispatcherServlet
			val servlet = DispatcherServlet(context)
			val registration = servletContext.addServlet("app", servlet)
			registration.setLoadOnStartup(1)
			registration.addMapping("/app/*")
		}
	}
----

NOTE: In addition to using the ServletContext API directly, you can also extend
`AbstractAnnotationConfigDispatcherServletInitializer` and override specific methods
(see the example under <<mvc-servlet-context-hierarchy>>).

NOTE: For programmatic use cases, a `GenericWebApplicationContext` can be used as an
alternative to `AnnotationConfigWebApplicationContext`. See the
{api-spring-framework}/web/context/support/GenericWebApplicationContext.html[`GenericWebApplicationContext`]
javadoc for details.

The following example of `web.xml` configuration registers and initializes the `DispatcherServlet`:

[source,xml,indent=0,subs="verbatim,quotes"]
----
<web-app>

	<listener>
		<listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
	</listener>

	<context-param>
		<param-name>contextConfigLocation</param-name>
		<param-value>/WEB-INF/app-context.xml</param-value>
	</context-param>

	<servlet>
		<servlet-name>app</servlet-name>
		<servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
		<init-param>
			<param-name>contextConfigLocation</param-name>
			<param-value></param-value>
		</init-param>
		<load-on-startup>1</load-on-startup>
	</servlet>

	<servlet-mapping>
		<servlet-name>app</servlet-name>
		<url-pattern>/app/*</url-pattern>
	</servlet-mapping>

</web-app>
----

NOTE: Spring Boot follows a different initialization sequence. Rather than hooking into
the lifecycle of the Servlet container, Spring Boot uses Spring configuration to
bootstrap itself and the embedded Servlet container. `Filter` and `Servlet` declarations
are detected in Spring configuration and registered with the Servlet container.
For more details, see the
https://docs.spring.io/spring-boot/docs/current/reference/htmlsingle/#boot-features-embedded-container[Spring Boot documentation].



[[mvc-servlet-context-hierarchy]]
=== Context Hierarchy

`DispatcherServlet` expects a `WebApplicationContext` (an extension of a plain
`ApplicationContext`) for its own configuration. `WebApplicationContext` has a link to the
`ServletContext` and the `Servlet` with which it is associated. It is also bound to the `ServletContext`
such that applications can use static methods on `RequestContextUtils` to look up the
`WebApplicationContext` if they need access to it.

For many applications, having a single `WebApplicationContext` is simple and suffices.
It is also possible to have a context hierarchy where one root `WebApplicationContext`
is shared across multiple `DispatcherServlet` (or other `Servlet`) instances, each with
its own child `WebApplicationContext` configuration.
See <<core.adoc#context-introduction,Additional Capabilities of the `ApplicationContext`>>
for more on the context hierarchy feature.

The root `WebApplicationContext` typically contains infrastructure beans, such as data repositories and
business services that need to be shared across multiple `Servlet` instances. Those beans
are effectively inherited and can be overridden (that is, re-declared) in the Servlet-specific
child `WebApplicationContext`, which typically contains beans local to the given `Servlet`.
The following image shows this relationship:

image::images/mvc-context-hierarchy.png[]

The following example configures a `WebApplicationContext` hierarchy:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	public class MyWebAppInitializer extends AbstractAnnotationConfigDispatcherServletInitializer {

		@Override
		protected Class<?>[] getRootConfigClasses() {
			return new Class<?>[] { RootConfig.class };
		}

		@Override
		protected Class<?>[] getServletConfigClasses() {
			return new Class<?>[] { App1Config.class };
		}

		@Override
		protected String[] getServletMappings() {
			return new String[] { "/app1/*" };
		}
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	class MyWebAppInitializer : AbstractAnnotationConfigDispatcherServletInitializer() {

		override fun getRootConfigClasses(): Array<Class<*>> {
			return arrayOf(RootConfig::class.java)
		}

		override fun getServletConfigClasses(): Array<Class<*>> {
			return arrayOf(App1Config::class.java)
		}

		override fun getServletMappings(): Array<String> {
			return arrayOf("/app1/*")
		}
	}
----

TIP: If an application context hierarchy is not required, applications can return all
configuration through `getRootConfigClasses()` and `null` from `getServletConfigClasses()`.

The following example shows the `web.xml` equivalent:

[source,xml,indent=0,subs="verbatim,quotes"]
----
<web-app>

	<listener>
		<listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
	</listener>

	<context-param>
		<param-name>contextConfigLocation</param-name>
		<param-value>/WEB-INF/root-context.xml</param-value>
	</context-param>

	<servlet>
		<servlet-name>app1</servlet-name>
		<servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
		<init-param>
			<param-name>contextConfigLocation</param-name>
			<param-value>/WEB-INF/app1-context.xml</param-value>
		</init-param>
		<load-on-startup>1</load-on-startup>
	</servlet>

	<servlet-mapping>
		<servlet-name>app1</servlet-name>
		<url-pattern>/app1/*</url-pattern>
	</servlet-mapping>

</web-app>
----

TIP: If an application context hierarchy is not required, applications may configure a
"`root`" context only and leave the `contextConfigLocation` Servlet parameter empty.



[[mvc-servlet-special-bean-types]]
=== Special Bean Types
[.small]#<<web-reactive.adoc#webflux-special-bean-types, WebFlux>>#

The `DispatcherServlet` delegates to special beans to process requests and render the
appropriate responses. By "`special beans`" we mean Spring-managed `Object` instances that
implement framework contracts. Those usually come with built-in contracts, but
you can customize their properties and extend or replace them.

The following table lists the special beans detected by the `DispatcherServlet`:

[[mvc-webappctx-special-beans-tbl]]
[cols="1,2", options="header"]
|===
| Bean type| Explanation

| `HandlerMapping`
| Map a request to a handler along with a list of
  <<mvc-handlermapping-interceptor, interceptors>> for pre- and post-processing.
  The mapping is based on some criteria, the details of which vary by `HandlerMapping`
  implementation.

  The two main `HandlerMapping` implementations are `RequestMappingHandlerMapping`
  (which supports `@RequestMapping` annotated methods) and `SimpleUrlHandlerMapping`
  (which maintains explicit registrations of URI path patterns to handlers).

| `HandlerAdapter`
| Help the `DispatcherServlet` to invoke a handler mapped to a request, regardless of
  how the handler is actually invoked. For example, invoking an annotated controller
  requires resolving annotations. The main purpose of a `HandlerAdapter` is
  to shield the `DispatcherServlet` from such details.

| <<mvc-exceptionhandlers, `HandlerExceptionResolver`>>
| Strategy to resolve exceptions, possibly mapping them to handlers, to HTML error
  views, or other targets. See <<mvc-exceptionhandlers>>.

| <<mvc-viewresolver, `ViewResolver`>>
| Resolve logical `String`-based view names returned from a handler to an actual `View`
  with which to render to the response. See <<mvc-viewresolver>> and <<mvc-view>>.

| <<mvc-localeresolver, `LocaleResolver`>>, <<mvc-timezone, LocaleContextResolver>>
| Resolve the `Locale` a client is using and possibly their time zone, in order to be able
  to offer internationalized views. See <<mvc-localeresolver>>.

| <<mvc-themeresolver, `ThemeResolver`>>
| Resolve themes your web application can use -- for example, to offer personalized layouts.
  See <<mvc-themeresolver>>.

| <<mvc-multipart, `MultipartResolver`>>
| Abstraction for parsing a multi-part request (for example, browser form file upload) with
  the help of some multipart parsing library. See <<mvc-multipart>>.

| <<mvc-flash-attributes, `FlashMapManager`>>
| Store and retrieve the "`input`" and the "`output`" `FlashMap` that can be used to pass
  attributes from one request to another, usually across a redirect.
  See <<mvc-flash-attributes>>.
|===



[[mvc-servlet-config]]
=== Web MVC Config
[.small]#<<web-reactive.adoc#webflux-framework-config, WebFlux>>#

Applications can declare the infrastructure beans listed in <<mvc-servlet-special-bean-types>>
that are required to process requests. The `DispatcherServlet` checks the
`WebApplicationContext` for each special bean. If there are no matching bean types,
it falls back on the default types listed in
{spring-framework-main-code}/spring-webmvc/src/main/resources/org/springframework/web/servlet/DispatcherServlet.properties[`DispatcherServlet.properties`].

In most cases, the <<mvc-config>> is the best starting point. It declares the required
beans in either Java or XML and provides a higher-level configuration callback API to
customize it.

NOTE: Spring Boot relies on the MVC Java configuration to configure Spring MVC and
provides many extra convenient options.



[[mvc-container-config]]
=== Servlet Config

In a Servlet environment, you have the option of configuring the Servlet container
programmatically as an alternative or in combination with a `web.xml` file.
The following example registers a `DispatcherServlet`:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	import org.springframework.web.WebApplicationInitializer;

	public class MyWebApplicationInitializer implements WebApplicationInitializer {

		@Override
		public void onStartup(ServletContext container) {
			XmlWebApplicationContext appContext = new XmlWebApplicationContext();
			appContext.setConfigLocation("/WEB-INF/spring/dispatcher-config.xml");

			ServletRegistration.Dynamic registration = container.addServlet("dispatcher", new DispatcherServlet(appContext));
			registration.setLoadOnStartup(1);
			registration.addMapping("/");
		}
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	import org.springframework.web.WebApplicationInitializer

	class MyWebApplicationInitializer : WebApplicationInitializer {

		override fun onStartup(container: ServletContext) {
			val appContext = XmlWebApplicationContext()
			appContext.setConfigLocation("/WEB-INF/spring/dispatcher-config.xml")

			val registration = container.addServlet("dispatcher", DispatcherServlet(appContext))
			registration.setLoadOnStartup(1)
			registration.addMapping("/")
		}
	}
----


`WebApplicationInitializer` is an interface provided by Spring MVC that ensures your
implementation is detected and automatically used to initialize any Servlet 3 container.
An abstract base class implementation of `WebApplicationInitializer` named
`AbstractDispatcherServletInitializer` makes it even easier to register the
`DispatcherServlet` by overriding methods to specify the servlet mapping and the
location of the `DispatcherServlet` configuration.

This is recommended for applications that use Java-based Spring configuration, as the
following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	public class MyWebAppInitializer extends AbstractAnnotationConfigDispatcherServletInitializer {

		@Override
		protected Class<?>[] getRootConfigClasses() {
			return null;
		}

		@Override
		protected Class<?>[] getServletConfigClasses() {
			return new Class<?>[] { MyWebConfig.class };
		}

		@Override
		protected String[] getServletMappings() {
			return new String[] { "/" };
		}
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	class MyWebAppInitializer : AbstractAnnotationConfigDispatcherServletInitializer() {

		override fun getRootConfigClasses(): Array<Class<*>>? {
			return null
		}

		override fun getServletConfigClasses(): Array<Class<*>>? {
			return arrayOf(MyWebConfig::class.java)
		}

		override fun getServletMappings(): Array<String> {
			return arrayOf("/")
		}
	}
----

If you use XML-based Spring configuration, you should extend directly from
`AbstractDispatcherServletInitializer`, as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	public class MyWebAppInitializer extends AbstractDispatcherServletInitializer {

		@Override
		protected WebApplicationContext createRootApplicationContext() {
			return null;
		}

		@Override
		protected WebApplicationContext createServletApplicationContext() {
			XmlWebApplicationContext cxt = new XmlWebApplicationContext();
			cxt.setConfigLocation("/WEB-INF/spring/dispatcher-config.xml");
			return cxt;
		}

		@Override
		protected String[] getServletMappings() {
			return new String[] { "/" };
		}
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	class MyWebAppInitializer : AbstractDispatcherServletInitializer() {

		override fun createRootApplicationContext(): WebApplicationContext? {
			return null
		}

		override fun createServletApplicationContext(): WebApplicationContext {
			return XmlWebApplicationContext().apply {
				setConfigLocation("/WEB-INF/spring/dispatcher-config.xml")
			}
		}

		override fun getServletMappings(): Array<String> {
			return arrayOf("/")
		}
	}
----

`AbstractDispatcherServletInitializer` also provides a convenient way to add `Filter`
instances and have them be automatically mapped to the `DispatcherServlet`, as the
following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	public class MyWebAppInitializer extends AbstractDispatcherServletInitializer {

		// ...

		@Override
		protected Filter[] getServletFilters() {
			return new Filter[] {
				new HiddenHttpMethodFilter(), new CharacterEncodingFilter() };
		}
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	class MyWebAppInitializer : AbstractDispatcherServletInitializer() {

		// ...

		override fun getServletFilters(): Array<Filter> {
			return arrayOf(HiddenHttpMethodFilter(), CharacterEncodingFilter())
		}
	}
----

Each filter is added with a default name based on its concrete type and automatically
mapped to the `DispatcherServlet`.

The `isAsyncSupported` protected method of `AbstractDispatcherServletInitializer`
provides a single place to enable async support on the `DispatcherServlet` and all
filters mapped to it. By default, this flag is set to `true`.

Finally, if you need to further customize the `DispatcherServlet` itself, you can
override the `createDispatcherServlet` method.



[[mvc-servlet-sequence]]
=== Processing
[.small]#<<web-reactive.adoc#webflux-dispatcher-handler-sequence, WebFlux>>#

The `DispatcherServlet` processes requests as follows:

* The `WebApplicationContext` is searched for and bound in the request as an attribute
  that the controller and other elements in the process can use. It is bound by default
  under the `DispatcherServlet.WEB_APPLICATION_CONTEXT_ATTRIBUTE` key.
* The locale resolver is bound to the request to let elements in the process
  resolve the locale to use when processing the request (rendering the view, preparing
  data, and so on). If you do not need locale resolving, you do not need the locale resolver.
* The theme resolver is bound to the request to let elements such as views determine
  which theme to use. If you do not use themes, you can ignore it.
* If you specify a multipart file resolver, the request is inspected for multiparts. If
  multiparts are found, the request is wrapped in a `MultipartHttpServletRequest` for
  further processing by other elements in the process. See <<mvc-multipart>> for further
  information about multipart handling.
* An appropriate handler is searched for. If a handler is found, the execution chain
  associated with the handler (preprocessors, postprocessors, and controllers) is
  run to prepare a model for rendering. Alternatively, for annotated
  controllers, the response can be rendered (within the `HandlerAdapter`) instead of
  returning a view.
* If a model is returned, the view is rendered. If no model is returned (maybe due to
  a preprocessor or postprocessor intercepting the request, perhaps for security
  reasons), no view is rendered, because the request could already have been fulfilled.

The `HandlerExceptionResolver` beans declared in the `WebApplicationContext` are used to
resolve exceptions thrown during request processing. Those exception resolvers allow
customizing the logic to address exceptions. See <<mvc-exceptionhandlers>> for more details.

For HTTP caching support, handlers can use the `checkNotModified` methods of `WebRequest`,
along with further options for annotated controllers as described in
<<mvc-caching-etag-lastmodified,HTTP Caching for Controllers>>.

You can customize individual `DispatcherServlet` instances by adding Servlet
initialization parameters (`init-param` elements) to the Servlet declaration in the
`web.xml` file. The following table lists the supported parameters:

[[mvc-disp-servlet-init-params-tbl]]
.DispatcherServlet initialization parameters
|===
| Parameter| Explanation

| `contextClass`
| Class that implements `ConfigurableWebApplicationContext`, to be instantiated and
  locally configured by this Servlet. By default, `XmlWebApplicationContext` is used.

| `contextConfigLocation`
| String that is passed to the context instance (specified by `contextClass`) to
  indicate where contexts can be found. The string consists potentially of multiple
  strings (using a comma as a delimiter) to support multiple contexts. In the case of
  multiple context locations with beans that are defined twice, the latest location
  takes precedence.

| `namespace`
| Namespace of the `WebApplicationContext`. Defaults to `[servlet-name]-servlet`.

| `throwExceptionIfNoHandlerFound`
| Whether to throw a `NoHandlerFoundException` when no handler was found for a request.
  The exception can then be caught with a `HandlerExceptionResolver` (for example, by using an
  `@ExceptionHandler` controller method) and handled as any others.

  By default, this is set to `false`, in which case the `DispatcherServlet` sets the
  response status to 404 (NOT_FOUND) without raising an exception.

  Note that, if <<mvc-default-servlet-handler,default servlet handling>>  is
  also configured, unresolved requests are always forwarded to the default servlet
  and a 404 is never raised.
|===



[[mvc-handlermapping-path]]
=== Path Matching

The Servlet API exposes the full request path as `requestURI` and further sub-divides it
into `contextPath`, `servletPath`, and `pathInfo` whose values vary depending on how a
Servlet is mapped. From these inputs, Spring MVC needs to determine the lookup path to
use for mapping handlers, which should exclude the `contextPath` and any `servletMapping`
prefix, if applicable.

The `servletPath` and `pathInfo` are decoded and that makes them impossible to compare
directly to the full `requestURI` in order to derive the lookupPath and that makes it
necessary to decode the `requestURI`. However this introduces its own issues because the
path may contain encoded reserved characters such as `"/"` or `";"` that can in turn
alter the structure of the path after they are decoded which can also lead to security
issues. In addition, Servlet containers may normalize the `servletPath` to varying
degrees which makes it further impossible to perform `startsWith` comparisons against
the `requestURI`.

This is why it is best to avoid reliance on the `servletPath` which comes with the
prefix-based `servletPath` mapping type. If the `DispatcherServlet` is mapped as the
default Servlet with `"/"` or otherwise without a prefix with `"/*"` and the Servlet
container is 4.0+ then Spring MVC is able to detect the Servlet mapping type and avoid
use of the `servletPath` and `pathInfo` altogether. On a 3.1 Servlet container,
assuming the same Servlet mapping types, the equivalent can be achieved by providing
a `UrlPathHelper` with `alwaysUseFullPath=true` via <<mvc-config-path-matching>> in
the MVC config.

Fortunately the default Servlet mapping `"/"` is a good choice. However, there is still
an issue in that the `requestURI` needs to be decoded to make it possible to compare to
controller mappings. This is again undesirable because of the potential to decode
reserved characters that alter the path structure. If such characters are not expected,
then you can reject them (like the Spring Security HTTP firewall), or you can configure
`UrlPathHelper` with `urlDecode=false` but controller mappings will need to match to the
encoded path which may not always work well. Furthermore, sometimes the
`DispatcherServlet` needs to share the URL space with another Servlet and may need to
be mapped by prefix.

The above issues are addressed when using `PathPatternParser` and parsed patterns, as
an alternative to String path matching with `AntPathMatcher`. The `PathPatternParser` has
been available for use in Spring MVC from version 5.3, and is enabled by default from
version 6.0. Unlike `AntPathMatcher` which needs either the lookup path decoded or the
controller mapping encoded, a parsed `PathPattern` matches to a parsed representation
of the path called `RequestPath`, one path segment at a time. This allows decoding and
sanitizing path segment values individually without the risk of altering the structure
of the path. Parsed `PathPattern` also supports the use of `servletPath` prefix mapping
as long as a Servlet path mapping is used and the prefix is kept simple, i.e. it has no
encoded characters. For pattern syntax details and comparison, see
<<mvc-ann-requestmapping-pattern-comparison>>.




[[mvc-handlermapping-interceptor]]
=== Interception

All `HandlerMapping` implementations support handler interceptors that are useful when
you want to apply specific functionality to certain requests -- for example, checking for
a principal. Interceptors must implement `HandlerInterceptor` from the
`org.springframework.web.servlet` package with three methods that should provide enough
flexibility to do all kinds of pre-processing and post-processing:

* `preHandle(..)`: Before the actual handler is run
* `postHandle(..)`: After the handler is run
* `afterCompletion(..)`: After the complete request has finished

The `preHandle(..)` method returns a boolean value. You can use this method to break or
continue the processing of the execution chain. When this method returns `true`, the
handler execution chain continues. When it returns false, the `DispatcherServlet`
assumes the interceptor itself has taken care of requests (and, for example, rendered an
appropriate view) and does not continue executing the other interceptors and the actual
handler in the execution chain.

See <<mvc-config-interceptors>> in the section on MVC configuration for examples of how to
configure interceptors. You can also register them directly by using setters on individual
`HandlerMapping` implementations.

`postHandle` method is less useful with `@ResponseBody` and `ResponseEntity` methods for
which the response is written and committed within the `HandlerAdapter` and before
`postHandle`. That means it is too late to make any changes to the response, such as adding
an extra header. For such scenarios, you can implement `ResponseBodyAdvice` and either
declare it as an <<mvc-ann-controller-advice>> bean or configure it directly on
`RequestMappingHandlerAdapter`.




[[mvc-exceptionhandlers]]
=== Exceptions
[.small]#<<web-reactive.adoc#webflux-dispatcher-exceptions, WebFlux>>#

If an exception occurs during request mapping or is thrown from a request handler (such as
a `@Controller`), the `DispatcherServlet` delegates to a chain of `HandlerExceptionResolver`
beans to resolve the exception and provide alternative handling, which is typically an
error response.

The following table lists the available `HandlerExceptionResolver` implementations:

[cols="1,2", options="header"]
.HandlerExceptionResolver implementations
|===
| `HandlerExceptionResolver` | Description

| `SimpleMappingExceptionResolver`
| A mapping between exception class names and error view names. Useful for rendering
  error pages in a browser application.

| {api-spring-framework}/web/servlet/mvc/support/DefaultHandlerExceptionResolver.html[`DefaultHandlerExceptionResolver`]
| Resolves exceptions raised by Spring MVC and maps them to HTTP status codes.
  See also alternative `ResponseEntityExceptionHandler` and <<mvc-ann-rest-exceptions>>.

| `ResponseStatusExceptionResolver`
| Resolves exceptions with the `@ResponseStatus` annotation and maps them to HTTP status
  codes based on the value in the annotation.

| `ExceptionHandlerExceptionResolver`
| Resolves exceptions by invoking an `@ExceptionHandler` method in a `@Controller` or a
  `@ControllerAdvice` class. See <<mvc-ann-exceptionhandler, @ExceptionHandler methods>>.
|===


[[mvc-exceptionhandlers-handling]]
==== Chain of Resolvers

You can form an exception resolver chain by declaring multiple `HandlerExceptionResolver`
beans in your Spring configuration and setting their `order` properties as needed.
The higher the order property, the later the exception resolver is positioned.

The contract of `HandlerExceptionResolver` specifies that it can return:

* a `ModelAndView` that points to an error view.
* An empty `ModelAndView` if the exception was handled within the resolver.
* `null` if the exception remains unresolved, for subsequent resolvers to try, and, if the
exception remains at the end, it is allowed to bubble up to the Servlet container.

The <<mvc-config>> automatically declares built-in resolvers for default Spring MVC
exceptions, for `@ResponseStatus` annotated exceptions, and for support of
`@ExceptionHandler` methods. You can customize that list or replace it.


[[mvc-ann-customer-servlet-container-error-page]]
==== Container Error Page

If an exception remains unresolved by any `HandlerExceptionResolver` and is, therefore,
left to propagate or if the response status is set to an error status (that is, 4xx, 5xx),
Servlet containers can render a default error page in HTML. To customize the default
error page of the container, you can declare an error page mapping in `web.xml`.
The following example shows how to do so:

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<error-page>
		<location>/error</location>
	</error-page>
----

Given the preceding example, when an exception bubbles up or the response has an error status, the
Servlet container makes an ERROR dispatch within the container to the configured URL
(for example, `/error`). This is then processed by the `DispatcherServlet`, possibly mapping it
to a `@Controller`, which could be implemented to return an error view name with a model
or to render a JSON response, as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@RestController
	public class ErrorController {

		@RequestMapping(path = "/error")
		public Map<String, Object> handle(HttpServletRequest request) {
			Map<String, Object> map = new HashMap<String, Object>();
			map.put("status", request.getAttribute("jakarta.servlet.error.status_code"));
			map.put("reason", request.getAttribute("jakarta.servlet.error.message"));
			return map;
		}
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@RestController
	class ErrorController {

		@RequestMapping(path = ["/error"])
		fun handle(request: HttpServletRequest): Map<String, Any> {
			val map = HashMap<String, Any>()
			map["status"] = request.getAttribute("jakarta.servlet.error.status_code")
			map["reason"] = request.getAttribute("jakarta.servlet.error.message")
			return map
		}
	}
----

TIP: The Servlet API does not provide a way to create error page mappings in Java. You can,
however, use both a `WebApplicationInitializer` and a minimal `web.xml`.



[[mvc-viewresolver]]
=== View Resolution
[.small]#<<web-reactive.adoc#webflux-viewresolution, WebFlux>>#

Spring MVC defines the `ViewResolver` and `View` interfaces that let you render
models in a browser without tying you to a specific view technology. `ViewResolver`
provides a mapping between view names and actual views. `View` addresses the preparation
of data before handing over to a specific view technology.

The following table provides more details on the `ViewResolver` hierarchy:

[[mvc-view-resolvers-tbl]]
.ViewResolver implementations
|===
| ViewResolver| Description

| `AbstractCachingViewResolver`
| Subclasses of `AbstractCachingViewResolver` cache view instances that they resolve.
  Caching improves performance of certain view technologies. You can turn off the
  cache by setting the `cache` property to `false`. Furthermore, if you must refresh
  a certain view at runtime (for example, when a FreeMarker template is modified),
  you can use the `removeFromCache(String viewName, Locale loc)` method.

| `UrlBasedViewResolver`
| Simple implementation of the `ViewResolver` interface that effects the direct
  resolution of logical view names to URLs without an explicit mapping definition.
  This is appropriate if your logical names match the names of your view resources
  in a straightforward manner, without the need for arbitrary mappings.

| `InternalResourceViewResolver`
| Convenient subclass of `UrlBasedViewResolver` that supports `InternalResourceView` (in
  effect, Servlets and JSPs) and subclasses such as `JstlView` and `TilesView`. You can
  specify the view class for all views generated by this resolver by using `setViewClass(..)`.
  See the {api-spring-framework}/web/reactive/result/view/UrlBasedViewResolver.html[`UrlBasedViewResolver`]
  javadoc for details.

| `FreeMarkerViewResolver`
| Convenient subclass of `UrlBasedViewResolver` that supports `FreeMarkerView` and
  custom subclasses of them.

| `ContentNegotiatingViewResolver`
| Implementation of the `ViewResolver` interface that resolves a view based on the
  request file name or `Accept` header. See <<mvc-multiple-representations>>.

| `BeanNameViewResolver`
| Implementation of the `ViewResolver` interface that interprets a view name as a
  bean name in the current application context. This is a very flexible variant which
  allows for mixing and matching different view types based on distinct view names.
  Each such `View` can be defined as a bean e.g. in XML or in configuration classes.
|===


[[mvc-viewresolver-handling]]
==== Handling
[.small]#<<web-reactive.adoc#webflux-viewresolution-handling, WebFlux>>#

You can chain view resolvers by declaring more than one resolver bean and, if necessary, by
setting the `order` property to specify ordering. Remember, the higher the order property,
the later the view resolver is positioned in the chain.

The contract of a `ViewResolver` specifies that it can return null to indicate that the
view could not be found. However, in the case of JSPs and `InternalResourceViewResolver`,
the only way to figure out if a JSP exists is to perform a dispatch through
`RequestDispatcher`. Therefore, you must always configure an `InternalResourceViewResolver`
to be last in the overall order of view resolvers.

Configuring view resolution is as simple as adding `ViewResolver` beans to your Spring
configuration. The <<mvc-config>> provides a dedicated configuration API for
<<mvc-config-view-resolvers>> and for adding logic-less
<<mvc-config-view-controller, View Controllers>> which are useful for HTML template
rendering without controller logic.


[[mvc-redirecting-redirect-prefix]]
==== Redirecting
[.small]#<<web-reactive.adoc#webflux-redirecting-redirect-prefix, WebFlux>>#

The special `redirect:` prefix in a view name lets you perform a redirect. The
`UrlBasedViewResolver` (and its subclasses) recognize this as an instruction that a
redirect is needed. The rest of the view name is the redirect URL.

The net effect is the same as if the controller had returned a `RedirectView`, but now
the controller itself can operate in terms of logical view names. A logical view
name (such as `redirect:/myapp/some/resource`) redirects relative to the current
Servlet context, while a name such as `redirect:https://myhost.com/some/arbitrary/path`
redirects to an absolute URL.

Note that, if a controller method is annotated with the `@ResponseStatus`, the annotation
value takes precedence over the response status set by `RedirectView`.


[[mvc-redirecting-forward-prefix]]
==== Forwarding

You can also use a special `forward:` prefix for view names that are
ultimately resolved by `UrlBasedViewResolver` and subclasses. This creates an
`InternalResourceView`, which does a `RequestDispatcher.forward()`.
Therefore, this prefix is not useful with `InternalResourceViewResolver` and
`InternalResourceView` (for JSPs), but it can be helpful if you use another view
technology but still want to force a forward of a resource to be handled by the
Servlet/JSP engine. Note that you may also chain multiple view resolvers, instead.


[[mvc-multiple-representations]]
==== Content Negotiation
[.small]#<<web-reactive.adoc#webflux-multiple-representations, WebFlux>>#

{api-spring-framework}/web/servlet/view/ContentNegotiatingViewResolver.html[`ContentNegotiatingViewResolver`]
does not resolve views itself but rather delegates
to other view resolvers and selects the view that resembles the representation requested
by the client. The representation can be determined from the `Accept` header or from a
query parameter (for example, `"/path?format=pdf"`).

The `ContentNegotiatingViewResolver` selects an appropriate `View` to handle the request
by comparing the request media types with the media type (also known as
`Content-Type`) supported by the `View` associated with each of its `ViewResolvers`. The
first `View` in the list that has a compatible `Content-Type` returns the representation
to the client. If a compatible view cannot be supplied by the `ViewResolver` chain,
the list of views specified through the `DefaultViews` property is consulted. This
latter option is appropriate for singleton `Views` that can render an appropriate
representation of the current resource regardless of the logical view name. The `Accept`
header can include wildcards (for example `text/{asterisk}`), in which case a `View` whose
`Content-Type` is `text/xml` is a compatible match.

See <<mvc-config-view-resolvers>> under <<mvc-config>> for configuration details.



[[mvc-localeresolver]]
=== Locale

Most parts of Spring's architecture support internationalization, as the Spring web
MVC framework does. `DispatcherServlet` lets you automatically resolve messages
by using the client's locale. This is done with `LocaleResolver` objects.

When a request comes in, the `DispatcherServlet` looks for a locale resolver and, if it
finds one, it tries to use it to set the locale. By using the `RequestContext.getLocale()`
method, you can always retrieve the locale that was resolved by the locale resolver.

In addition to automatic locale resolution, you can also attach an interceptor to the
handler mapping (see <<mvc-handlermapping-interceptor>> for more information on handler
mapping interceptors) to change the locale under specific circumstances (for example,
based on a parameter in the request).

Locale resolvers and interceptors are defined in the
`org.springframework.web.servlet.i18n` package and are configured in your application
context in the normal way. The following selection of locale resolvers is included in
Spring.

* <<mvc-timezone>>
* <<mvc-localeresolver-acceptheader>>
* <<mvc-localeresolver-cookie>>
* <<mvc-localeresolver-session>>
* <<mvc-localeresolver-interceptor>>


[[mvc-timezone]]
==== Time Zone

In addition to obtaining the client's locale, it is often useful to know its time zone.
The `LocaleContextResolver` interface offers an extension to `LocaleResolver` that lets
resolvers provide a richer `LocaleContext`, which may include time zone information.

When available, the user's `TimeZone` can be obtained by using the
`RequestContext.getTimeZone()` method. Time zone information is automatically used
by any Date/Time `Converter` and `Formatter` objects that are registered with Spring's
`ConversionService`.


[[mvc-localeresolver-acceptheader]]
==== Header Resolver

This locale resolver inspects the `accept-language` header in the request that was sent
by the client (for example, a web browser). Usually, this header field contains the locale of
the client's operating system. Note that this resolver does not support time zone
information.


[[mvc-localeresolver-cookie]]
==== Cookie Resolver

This locale resolver inspects a `Cookie` that might exist on the client to see if a
`Locale` or `TimeZone` is specified. If so, it uses the specified details. By using the
properties of this locale resolver, you can specify the name of the cookie as well as the
maximum age. The following example defines a `CookieLocaleResolver`:

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<bean id="localeResolver" class="org.springframework.web.servlet.i18n.CookieLocaleResolver">

		<property name="cookieName" value="clientlanguage"/>

		<!-- in seconds. If set to -1, the cookie is not persisted (deleted when browser shuts down) -->
		<property name="cookieMaxAge" value="100000"/>

	</bean>
----

The following table describes the properties `CookieLocaleResolver`:

[[mvc-cookie-locale-resolver-props-tbl]]
.CookieLocaleResolver properties
[cols="1,1,4"]
|===
| Property | Default | Description

| `cookieName`
| classname + LOCALE
| The name of the cookie

| `cookieMaxAge`
| Servlet container default
| The maximum time a cookie persists on the client. If `-1` is specified, the
  cookie will not be persisted. It is available only until the client shuts down
  the browser.

| `cookiePath`
| /
| Limits the visibility of the cookie to a certain part of your site. When `cookiePath` is
  specified, the cookie is visible only to that path and the paths below it.
|===


[[mvc-localeresolver-session]]
==== Session Resolver

The `SessionLocaleResolver` lets you retrieve `Locale` and `TimeZone` from the
session that might be associated with the user's request. In contrast to
`CookieLocaleResolver`, this strategy stores locally chosen locale settings in the
Servlet container's `HttpSession`. As a consequence, those settings are temporary
for each session and are, therefore, lost when each session ends.

Note that there is no direct relationship with external session management mechanisms,
such as the Spring Session project. This `SessionLocaleResolver` evaluates and
modifies the corresponding `HttpSession` attributes against the current `HttpServletRequest`.


[[mvc-localeresolver-interceptor]]
==== Locale Interceptor

You can enable changing of locales by adding the `LocaleChangeInterceptor` to one of the
`HandlerMapping` definitions. It detects a parameter in the request and changes the locale
accordingly, calling the `setLocale` method on the `LocaleResolver` in the dispatcher's
application context. The next example shows that calls to all `{asterisk}.view` resources
that contain a parameter named `siteLanguage` now changes the locale. So, for example,
a request for the URL, `https://www.sf.net/home.view?siteLanguage=nl`, changes the site
language to Dutch. The following example shows how to intercept the locale:

[source,xml,indent=0,subs="verbatim"]
----
	<bean id="localeChangeInterceptor"
			class="org.springframework.web.servlet.i18n.LocaleChangeInterceptor">
		<property name="paramName" value="siteLanguage"/>
	</bean>

	<bean id="localeResolver"
			class="org.springframework.web.servlet.i18n.CookieLocaleResolver"/>

	<bean id="urlMapping"
			class="org.springframework.web.servlet.handler.SimpleUrlHandlerMapping">
		<property name="interceptors">
			<list>
				<ref bean="localeChangeInterceptor"/>
			</list>
		</property>
		<property name="mappings">
			<value>/**/*.view=someController</value>
		</property>
	</bean>
----



[[mvc-themeresolver]]
=== Themes

You can apply Spring Web MVC framework themes to set the overall look-and-feel of your
application, thereby enhancing user experience. A theme is a collection of static
resources, typically style sheets and images, that affect the visual style of the
application.

WARNING: as of 6.0 support for themes has been deprecated theme in favor of using CSS,
and without any special support on the server side.


[[mvc-themeresolver-defining]]
==== Defining a theme

To use themes in your web application, you must set up an implementation of the
`org.springframework.ui.context.ThemeSource` interface. The `WebApplicationContext`
interface extends `ThemeSource` but delegates its responsibilities to a dedicated
implementation. By default, the delegate is an
`org.springframework.ui.context.support.ResourceBundleThemeSource` implementation that
loads properties files from the root of the classpath. To use a custom `ThemeSource`
implementation or to configure the base name prefix of the `ResourceBundleThemeSource`,
you can register a bean in the application context with the reserved name, `themeSource`.
The web application context automatically detects a bean with that name and uses it.

When you use the `ResourceBundleThemeSource`, a theme is defined in a simple properties
file. The properties file lists the resources that make up the theme, as the following example shows:

[literal,subs="verbatim,quotes"]
----
styleSheet=/themes/cool/style.css
background=/themes/cool/img/coolBg.jpg
----

The keys of the properties are the names that refer to the themed elements from view
code. For a JSP, you typically do this using the `spring:theme` custom tag, which is
very similar to the `spring:message` tag. The following JSP fragment uses the theme
defined in the previous example to customize the look and feel:

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<%@ taglib prefix="spring" uri="http://www.springframework.org/tags"%>
	<html>
		<head>
			<link rel="stylesheet" href="<spring:theme code='styleSheet'/>" type="text/css"/>
		</head>
		<body style="background=<spring:theme code='background'/>">
			...
		</body>
	</html>
----

By default, the `ResourceBundleThemeSource` uses an empty base name prefix. As a result,
the properties files are loaded from the root of the classpath. Thus, you would put the
`cool.properties` theme definition in a directory at the root of the classpath (for
example, in `/WEB-INF/classes`). The `ResourceBundleThemeSource` uses the standard Java
resource bundle loading mechanism, allowing for full internationalization of themes. For
example, we could have a `/WEB-INF/classes/cool_nl.properties` that references a special
background image with Dutch text on it.


[[mvc-themeresolver-resolving]]
==== Resolving Themes

After you define themes, as described in the <<mvc-themeresolver-defining, preceding section>>,
you decide which theme to use. The `DispatcherServlet` looks for a bean named `themeResolver`
to find out which `ThemeResolver` implementation to use. A theme resolver works in much the same
way as a `LocaleResolver`. It detects the theme to use for a particular request and can also
alter the request's theme. The following table describes the theme resolvers provided by Spring:

[[mvc-theme-resolver-impls-tbl]]
.ThemeResolver implementations
[cols="1,4"]
|===
| Class | Description

| `FixedThemeResolver`
| Selects a fixed theme, set by using the `defaultThemeName` property.

| `SessionThemeResolver`
| The theme is maintained in the user's HTTP session. It needs to be set only once for
  each session but is not persisted between sessions.

| `CookieThemeResolver`
| The selected theme is stored in a cookie on the client.
|===

Spring also provides a `ThemeChangeInterceptor` that lets theme changes on every
request with a simple request parameter.



[[mvc-multipart]]
=== Multipart Resolver
[.small]#<<web-reactive.adoc#webflux-multipart, WebFlux>>#

`MultipartResolver` from the `org.springframework.web.multipart` package is a strategy
for parsing multipart requests including file uploads. There is one implementation
based on https://commons.apache.org/proper/commons-fileupload[Commons FileUpload] and
another based on Servlet multipart request parsing.

To enable multipart handling, you need to declare a `MultipartResolver` bean in your
`DispatcherServlet` Spring configuration with a name of `multipartResolver`.
The `DispatcherServlet` detects it and applies it to the incoming request. When a POST
with a content type of `multipart/form-data` is received, the resolver parses the
content wraps the current `HttpServletRequest` as a `MultipartHttpServletRequest` to
provide access to resolved files in addition to exposing parts as request parameters.


[[mvc-multipart-resolver-commons]]
==== Apache Commons `FileUpload`

To use Apache Commons `FileUpload`, you can configure a bean of type
`CommonsMultipartResolver` with a name of `multipartResolver`. You also need to have
the `commons-fileupload` jar as a dependency on your classpath.

This resolver variant delegates to a local library within the application, providing
maximum portability across Servlet containers. As an alternative, consider standard
Servlet multipart resolution through the container's own parser as discussed below.

[NOTE]
====
Commons FileUpload traditionally applies to POST requests only but accepts any
`multipart/` content type. See the
{api-spring-framework}/web/multipart/commons/CommonsMultipartResolver.html[`CommonsMultipartResolver`]
javadoc for details and configuration options.
====


[[mvc-multipart-resolver-standard]]
==== Servlet Multipart Parsing

Servlet multipart parsing needs to be enabled through Servlet container configuration.
To do so:

* In Java, set a `MultipartConfigElement` on the Servlet registration.
* In `web.xml`, add a `"<multipart-config>"` section to the servlet declaration.

The following example shows how to set a `MultipartConfigElement` on the Servlet registration:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	public class AppInitializer extends AbstractAnnotationConfigDispatcherServletInitializer {

		// ...

		@Override
		protected void customizeRegistration(ServletRegistration.Dynamic registration) {

			// Optionally also set maxFileSize, maxRequestSize, fileSizeThreshold
			registration.setMultipartConfig(new MultipartConfigElement("/tmp"));
		}

	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	class AppInitializer : AbstractAnnotationConfigDispatcherServletInitializer() {

		// ...

		override fun customizeRegistration(registration: ServletRegistration.Dynamic) {

			// Optionally also set maxFileSize, maxRequestSize, fileSizeThreshold
			registration.setMultipartConfig(MultipartConfigElement("/tmp"))
		}

	}
----

Once the Servlet multipart configuration is in place, you can add a bean of type
`StandardServletMultipartResolver` with a name of `multipartResolver`.

[NOTE]
====
This resolver variant uses your Servlet container's multipart parser as-is,
potentially exposing the application to container implementation differences.
By default, it will try to parse any `multipart/` content type with any HTTP
method but this may not be supported across all Servlet containers. See the
{api-spring-framework}/web/multipart/support/StandardServletMultipartResolver.html[`StandardServletMultipartResolver`]
javadoc for details and configuration options.
====



[[mvc-logging]]
=== Logging
[.small]#<<web-reactive.adoc#webflux-logging, WebFlux>>#

DEBUG-level logging in Spring MVC is designed to be compact, minimal, and
human-friendly. It focuses on high-value bits of information that are useful over and
over again versus others that are useful only when debugging a specific issue.

TRACE-level logging generally follows the same principles as DEBUG (and, for example, also
should not be a fire hose) but can be used for debugging any issue. In addition, some log
messages may show a different level of detail at TRACE versus DEBUG.

Good logging comes from the experience of using the logs. If you spot anything that does
not meet the stated goals, please let us know.


[[mvc-logging-sensitive-data]]
==== Sensitive Data
[.small]#<<web-reactive.adoc#webflux-logging-sensitive-data, WebFlux>>#

DEBUG and TRACE logging may log sensitive information. This is why request parameters and
headers are masked by default and their logging in full must be enabled explicitly
through the `enableLoggingRequestDetails` property on `DispatcherServlet`.

The following example shows how to do so by using Java configuration:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
public class MyInitializer
		extends AbstractAnnotationConfigDispatcherServletInitializer {

	@Override
	protected Class<?>[] getRootConfigClasses() {
		return ... ;
	}

	@Override
	protected Class<?>[] getServletConfigClasses() {
		return ... ;
	}

	@Override
	protected String[] getServletMappings() {
		return ... ;
	}

	@Override
	protected void customizeRegistration(ServletRegistration.Dynamic registration) {
		registration.setInitParameter("enableLoggingRequestDetails", "true");
	}

}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	class MyInitializer : AbstractAnnotationConfigDispatcherServletInitializer() {

		override fun getRootConfigClasses(): Array<Class<*>>? {
			return ...
		}

		override fun getServletConfigClasses(): Array<Class<*>>? {
			return ...
		}

		override fun getServletMappings(): Array<String> {
			return ...
		}

		override fun customizeRegistration(registration: ServletRegistration.Dynamic) {
			registration.setInitParameter("enableLoggingRequestDetails", "true")
		}
	}
----




[[filters]]
== Filters
[.small]#<<web-reactive.adoc#webflux-filters, WebFlux>>#

The `spring-web` module provides some useful filters:

* <<filters-http-put>>
* <<filters-forwarded-headers>>
* <<filters-shallow-etag>>
* <<filters-cors>>



[[filters-http-put]]
=== Form Data

Browsers can submit form data only through HTTP GET or HTTP POST but non-browser clients can also
use HTTP PUT, PATCH, and DELETE. The Servlet API requires `ServletRequest.getParameter{asterisk}()`
methods to support form field access only for HTTP POST.

The `spring-web` module provides `FormContentFilter` to intercept HTTP PUT, PATCH, and DELETE
requests with a content type of `application/x-www-form-urlencoded`, read the form data from
the body of the request, and wrap the `ServletRequest` to make the form data
available through the `ServletRequest.getParameter{asterisk}()` family of methods.



[[filters-forwarded-headers]]
=== Forwarded Headers
[.small]#<<web-reactive.adoc#webflux-forwarded-headers, WebFlux>>#

As a request goes through proxies (such as load balancers) the host, port, and
scheme may change, and that makes it a challenge to create links that point to the correct
host, port, and scheme from a client perspective.

https://tools.ietf.org/html/rfc7239[RFC 7239] defines the `Forwarded` HTTP header
that proxies can use to provide information about the original request. There are other
non-standard headers, too, including `X-Forwarded-Host`, `X-Forwarded-Port`,
`X-Forwarded-Proto`, `X-Forwarded-Ssl`, and `X-Forwarded-Prefix`.

`ForwardedHeaderFilter` is a Servlet filter that modifies the request in order to
a) change the host, port, and scheme based on `Forwarded` headers, and b) to remove those
headers to eliminate further impact. The filter relies on wrapping the request, and
therefore it must be ordered ahead of other filters, such as `RequestContextFilter`, that
should work with the modified and not the original request.

There are security considerations for forwarded headers since an application cannot know
if the headers were added by a proxy, as intended, or by a malicious client. This is why
a proxy at the boundary of trust should be configured to remove untrusted `Forwarded`
headers that come from the outside. You can also configure the `ForwardedHeaderFilter`
with `removeOnly=true`, in which case it removes but does not use the headers.

In order to support <<mvc-ann-async,asynchronous requests>> and error dispatches this
filter should be mapped with `DispatcherType.ASYNC` and also `DispatcherType.ERROR`.
If using Spring Framework's `AbstractAnnotationConfigDispatcherServletInitializer`
(see <<mvc-container-config>>) all filters are automatically registered for all dispatch
types. However if registering the filter via `web.xml` or in Spring Boot via a
`FilterRegistrationBean` be sure to include `DispatcherType.ASYNC` and
`DispatcherType.ERROR` in addition to `DispatcherType.REQUEST`.



[[filters-shallow-etag]]
=== Shallow ETag

The `ShallowEtagHeaderFilter` filter creates a "`shallow`" ETag by caching the content
written to the response and computing an MD5 hash from it. The next time a client sends,
it does the same, but it also compares the computed value against the `If-None-Match`
request header and, if the two are equal, returns a 304 (NOT_MODIFIED).

This strategy saves network bandwidth but not CPU, as the full response must be computed
for each request. Other strategies at the controller level, described earlier, can avoid
the computation. See <<mvc-caching>>.

This filter has a `writeWeakETag` parameter that configures the filter to write weak ETags
similar to the following: `W/"02a2d595e6ed9a0b24f027f2b63b134d6"` (as defined in
https://tools.ietf.org/html/rfc7232#section-2.3[RFC 7232 Section 2.3]).

In order to support <<mvc-ann-async,asynchronous requests>> this filter must be mapped
with `DispatcherType.ASYNC` so that the filter can delay and successfully generate an
ETag to the end of the last async dispatch. If using Spring Framework's
`AbstractAnnotationConfigDispatcherServletInitializer` (see <<mvc-container-config>>)
all filters are automatically registered for all dispatch types. However if registering
the filter via `web.xml` or in Spring Boot via a `FilterRegistrationBean` be sure to include
`DispatcherType.ASYNC`.



[[filters-cors]]
=== CORS
[.small]#<<web-reactive.adoc#webflux-filters-cors, WebFlux>>#

Spring MVC provides fine-grained support for CORS configuration through annotations on
controllers. However, when used with Spring Security, we advise relying on the built-in
`CorsFilter` that must be ordered ahead of Spring Security's chain of filters.

See the sections on <<mvc-cors>> and the <<mvc-cors-filter>> for more details.




[[mvc-controller]]
== Annotated Controllers
[.small]#<<web-reactive.adoc#webflux-controller, WebFlux>>#

Spring MVC provides an annotation-based programming model where `@Controller` and
`@RestController` components use annotations to express request mappings, request input,
exception handling, and more. Annotated controllers have flexible method signatures and
do not have to extend base classes nor implement specific interfaces.
The following example shows a controller defined by annotations:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Controller
	public class HelloController {

		@GetMapping("/hello")
		public String handle(Model model) {
			model.addAttribute("message", "Hello World!");
			return "index";
		}
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	import org.springframework.ui.set

	@Controller
	class HelloController {

		@GetMapping("/hello")
		fun handle(model: Model): String {
			model["message"] = "Hello World!"
			return "index"
		}
	}
----

In the preceding example, the method accepts a `Model` and returns a view name as a `String`,
but many other options exist and are explained later in this chapter.

TIP: Guides and tutorials on https://spring.io/guides[spring.io] use the annotation-based
programming model described in this section.



[[mvc-ann-controller]]
=== Declaration
[.small]#<<web-reactive.adoc#webflux-ann-controller, WebFlux>>#

You can define controller beans by using a standard Spring bean definition in the
Servlet's `WebApplicationContext`. The `@Controller` stereotype allows for auto-detection,
aligned with Spring general support for detecting `@Component` classes in the classpath
and auto-registering bean definitions for them. It also acts as a stereotype for the
annotated class, indicating its role as a web component.

To enable auto-detection of such `@Controller` beans, you can add component scanning to
your Java configuration, as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Configuration
	@ComponentScan("org.example.web")
	public class WebConfig {

		// ...
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@Configuration
	@ComponentScan("org.example.web")
	class WebConfig {

		// ...
	}
----

The following example shows the XML configuration equivalent of the preceding example:

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<?xml version="1.0" encoding="UTF-8"?>
	<beans xmlns="http://www.springframework.org/schema/beans"
		xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
		xmlns:p="http://www.springframework.org/schema/p"
		xmlns:context="http://www.springframework.org/schema/context"
		xsi:schemaLocation="
			http://www.springframework.org/schema/beans
			https://www.springframework.org/schema/beans/spring-beans.xsd
			http://www.springframework.org/schema/context
			https://www.springframework.org/schema/context/spring-context.xsd">

		<context:component-scan base-package="org.example.web"/>

		<!-- ... -->

	</beans>
----

`@RestController` is a <<core.adoc#beans-meta-annotations, composed annotation>> that is
itself meta-annotated with `@Controller` and `@ResponseBody` to indicate a controller whose
every method inherits the type-level `@ResponseBody` annotation and, therefore, writes
directly to the response body versus view resolution and rendering with an HTML template.


[[mvc-ann-requestmapping-proxying]]
==== AOP Proxies
[.small]#<<web-reactive.adoc#webflux-ann-requestmapping-proxying, WebFlux>>#

In some cases, you may need to decorate a controller with an AOP proxy at runtime.
One example is if you choose to have `@Transactional` annotations directly on the
controller. When this is the case, for controllers specifically, we recommend
using class-based proxying. This is automatically the case with such annotations
directly on the controller.

If the controller implements an interface, and needs AOP proxying, you may need to
explicitly configure class-based proxying. For example, with `@EnableTransactionManagement`
you can change to `@EnableTransactionManagement(proxyTargetClass = true)`, and with
`<tx:annotation-driven/>` you can change to `<tx:annotation-driven proxy-target-class="true"/>`.

NOTE: Keep in mind that as of 6.0, with interface proxying, Spring MVC no longer detects
controllers based solely on a type-level `@RequestMapping` annotation on the interface.
Please, enable class based proxying, or otherwise the interface must also have an
`@Controller` annotation.



[[mvc-ann-requestmapping]]
=== Request Mapping
[.small]#<<web-reactive.adoc#webflux-ann-requestmapping, WebFlux>>#

You can use the `@RequestMapping` annotation to map requests to controllers methods. It has
various attributes to match by URL, HTTP method, request parameters, headers, and media
types. You can use it at the class level to express shared mappings or at the method level
to narrow down to a specific endpoint mapping.

There are also HTTP method specific shortcut variants of `@RequestMapping`:

* `@GetMapping`
* `@PostMapping`
* `@PutMapping`
* `@DeleteMapping`
* `@PatchMapping`

The shortcuts are <<mvc-ann-requestmapping-composed>> that are provided because,
arguably, most controller methods should be mapped to a specific HTTP method versus
using `@RequestMapping`, which, by default, matches to all HTTP methods.
A `@RequestMapping` is still needed at the class level to express shared mappings.

The following example has type and method level mappings:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@RestController
	@RequestMapping("/persons")
	class PersonController {

		@GetMapping("/{id}")
		public Person getPerson(@PathVariable Long id) {
			// ...
		}

		@PostMapping
		@ResponseStatus(HttpStatus.CREATED)
		public void add(@RequestBody Person person) {
			// ...
		}
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@RestController
	@RequestMapping("/persons")
	class PersonController {

		@GetMapping("/{id}")
		fun getPerson(@PathVariable id: Long): Person {
			// ...
		}

		@PostMapping
		@ResponseStatus(HttpStatus.CREATED)
		fun add(@RequestBody person: Person) {
			// ...
		}
	}
----



[[mvc-ann-requestmapping-uri-templates]]
==== URI patterns
[.small]#<<web-reactive.adoc#webflux-ann-requestmapping-uri-templates, WebFlux>>#

`@RequestMapping` methods can be mapped using URL patterns. There are two alternatives:

* `PathPattern` -- a pre-parsed pattern matched against the URL path also pre-parsed as
`PathContainer`. Designed for web use, this solution deals effectively with encoding and
path parameters, and matches efficiently.
* `AntPathMatcher` -- match String patterns against a String path. This is the original
solution also used in Spring configuration to select resources on the classpath, on the
filesystem, and other locations. It is less efficient and the String path input is a
challenge for dealing effectively with encoding and other issues with URLs.

`PathPattern` is the recommended solution for web applications and it is the only choice in
Spring WebFlux. It was enabled for use in Spring MVC from version 5.3 and is enabled by
default from version 6.0. See <<mvc-config-path-matching, MVC config>> for
customizations of path matching options.

`PathPattern` supports the same pattern syntax as `AntPathMatcher`. In addition, it also
supports the capturing pattern, e.g. `+{*spring}+`, for matching 0 or more path segments
at the end of a path. `PathPattern` also restricts the use of `+**+` for matching multiple
path segments such that it's only allowed at the end of a pattern. This eliminates many
cases of ambiguity when choosing the best matching pattern for a given request.
For full pattern syntax please refer to
{api-spring-framework}/web/util/pattern/PathPattern.html[PathPattern] and
{api-spring-framework}/util/AntPathMatcher.html[AntPathMatcher].

Some example patterns:

* `+"/resources/ima?e.png"+` - match one character in a path segment
* `+"/resources/*.png"+` - match zero or more characters in a path segment
* `+"/resources/**"+` - match multiple path segments
* `+"/projects/{project}/versions"+` - match a path segment and capture it as a variable
* `+"/projects/{project:[a-z]+}/versions"+` - match and capture a variable with a regex

Captured URI variables can be accessed with `@PathVariable`. For example:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@GetMapping("/owners/{ownerId}/pets/{petId}")
	public Pet findPet(@PathVariable Long ownerId, @PathVariable Long petId) {
		// ...
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@GetMapping("/owners/{ownerId}/pets/{petId}")
	fun findPet(@PathVariable ownerId: Long, @PathVariable petId: Long): Pet {
		// ...
	}
----


You can declare URI variables at the class and method levels, as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Controller
	@RequestMapping("/owners/{ownerId}")
	public class OwnerController {

		@GetMapping("/pets/{petId}")
		public Pet findPet(@PathVariable Long ownerId, @PathVariable Long petId) {
			// ...
		}
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@Controller
	@RequestMapping("/owners/{ownerId}")
	class OwnerController {

		@GetMapping("/pets/{petId}")
		fun findPet(@PathVariable ownerId: Long, @PathVariable petId: Long): Pet {
			// ...
		}
	}
----

URI variables are automatically converted to the appropriate type, or `TypeMismatchException`
is raised. Simple types (`int`, `long`, `Date`, and so on) are supported by default and you can
register support for any other data type.
See <<mvc-ann-typeconversion>> and <<mvc-ann-initbinder>>.

You can explicitly name URI variables (for example, `@PathVariable("customId")`), but you can
leave that detail out if the names are the same and your code is compiled with debugging
information or with the `-parameters` compiler flag on Java 8.

The syntax `{varName:regex}` declares a URI variable with a regular expression that has
syntax of `{varName:regex}`. For example, given URL `"/spring-web-3.0.5.jar"`, the following method
extracts the name, version, and file extension:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@GetMapping("/{name:[a-z-]+}-{version:\\d\\.\\d\\.\\d}{ext:\\.[a-z]+}")
	public void handle(@PathVariable String name, @PathVariable String version, @PathVariable String ext) {
		// ...
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@GetMapping("/{name:[a-z-]+}-{version:\\d\\.\\d\\.\\d}{ext:\\.[a-z]+}")
	fun handle(@PathVariable name: String, @PathVariable version: String, @PathVariable ext: String) {
		// ...
	}
----

URI path patterns can also have embedded `${...}` placeholders that are resolved on startup
by using `PropertySourcesPlaceholderConfigurer` against local, system, environment, and
other property sources. You can use this, for example, to parameterize a base URL based on
some external configuration.



[[mvc-ann-requestmapping-pattern-comparison]]
==== Pattern Comparison
[.small]#<<web-reactive.adoc#webflux-ann-requestmapping-pattern-comparison, WebFlux>>#

When multiple patterns match a URL, the best match must be selected. This is done with
one of the following depending on whether use of parsed `PathPattern` is enabled for use or not:

* {api-spring-framework}/web/util/pattern/PathPattern.html#SPECIFICITY_COMPARATOR[`PathPattern.SPECIFICITY_COMPARATOR`]
* {api-spring-framework}/util/AntPathMatcher.html#getPatternComparator-java.lang.String-[`AntPathMatcher.getPatternComparator(String path)`]

Both help to sort patterns with more specific ones on top. A pattern is less specific if
it has a lower count of URI variables (counted as 1), single wildcards (counted as 1),
and double wildcards (counted as 2). Given an equal score, the longer pattern is chosen.
Given the same score and length, the pattern with more URI variables than wildcards is
chosen.

The default mapping pattern (`/{asterisk}{asterisk}`) is excluded from scoring and always
sorted last. Also, prefix patterns (such as `/public/{asterisk}{asterisk}`) are considered less
specific than other pattern that do not have double wildcards.

For the full details, follow the above links to the pattern Comparators.


[[mvc-ann-requestmapping-suffix-pattern-match]]
==== Suffix Match

Starting in 5.3, by default Spring MVC no longer performs `.{asterisk}` suffix pattern
matching where a controller mapped to `/person` is also implicitly mapped to
`/person.{asterisk}`. As a consequence path extensions are no longer used to interpret
the requested content type for the response -- for example, `/person.pdf`, `/person.xml`,
and so on.

Using file extensions in this way was necessary when browsers used to send `Accept` headers
that were hard to interpret consistently. At present, that is no longer a necessity and
using the `Accept` header should be the preferred choice.

Over time, the use of file name extensions has proven problematic in a variety of ways.
It can cause ambiguity when overlain with the use of URI variables, path parameters, and
URI encoding. Reasoning about URL-based authorization
and security (see next section for more details) also becomes more difficult.

To completely disable the use of path extensions in versions prior to 5.3, set the following:

* `useSuffixPatternMatching(false)`, see <<mvc-config-path-matching, PathMatchConfigurer>>
* `favorPathExtension(false)`, see <<mvc-config-content-negotiation, ContentNegotiationConfigurer>>

Having a way to request content types other than through the `"Accept"` header can still
be useful, e.g. when typing a URL in a browser. A safe alternative to path extensions is
to use the query parameter strategy. If you must use file extensions, consider restricting
them to a list of explicitly registered extensions through the `mediaTypes` property of
<<mvc-config-content-negotiation,ContentNegotiationConfigurer>>.


[[mvc-ann-requestmapping-rfd]]
==== Suffix Match and RFD

A reflected file download (RFD) attack is similar to XSS in that it relies on request input
(for example, a query parameter and a URI variable) being reflected in the response. However, instead of
inserting JavaScript into HTML, an RFD attack relies on the browser switching to perform a
download and treating the response as an executable script when double-clicked later.

In Spring MVC, `@ResponseBody` and `ResponseEntity` methods are at risk, because
they can render different content types, which clients can request through URL path extensions.
Disabling suffix pattern matching and using path extensions for content negotiation
lower the risk but are not sufficient to prevent RFD attacks.

To prevent RFD attacks, prior to rendering the response body, Spring MVC adds a
`Content-Disposition:inline;filename=f.txt` header to suggest a fixed and safe download
file. This is done only if the URL path contains a file extension that is neither
allowed as safe nor explicitly registered for content negotiation. However, it can
potentially have side effects when URLs are typed directly into a browser.

Many common path extensions are allowed as safe by default. Applications with custom
`HttpMessageConverter` implementations can explicitly register file extensions for content
negotiation to avoid having a `Content-Disposition` header added for those extensions.
See <<mvc-config-content-negotiation>>.

See https://pivotal.io/security/cve-2015-5211[CVE-2015-5211] for additional
recommendations related to RFD.


[[mvc-ann-requestmapping-consumes]]
==== Consumable Media Types
[.small]#<<web-reactive.adoc#webflux-ann-requestmapping-consumes, WebFlux>>#

You can narrow the request mapping based on the `Content-Type` of the request,
as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@PostMapping(path = "/pets", consumes = "application/json") // <1>
	public void addPet(@RequestBody Pet pet) {
		// ...
	}
----
<1> Using a `consumes` attribute to narrow the mapping by the content type.

[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@PostMapping("/pets", consumes = ["application/json"]) // <1>
	fun addPet(@RequestBody pet: Pet) {
		// ...
	}
----
<1> Using a `consumes` attribute to narrow the mapping by the content type.

The `consumes` attribute also supports negation expressions -- for example, `!text/plain` means any
content type other than `text/plain`.

You can declare a shared `consumes` attribute at the class level. Unlike most other
request-mapping attributes, however, when used at the class level, a method-level `consumes` attribute
overrides rather than extends the class-level declaration.

TIP: `MediaType` provides constants for commonly used media types, such as
`APPLICATION_JSON_VALUE` and `APPLICATION_XML_VALUE`.


[[mvc-ann-requestmapping-produces]]
==== Producible Media Types
[.small]#<<web-reactive.adoc#webflux-ann-requestmapping-produces, WebFlux>>#

You can narrow the request mapping based on the `Accept` request header and the list of
content types that a controller method produces, as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@GetMapping(path = "/pets/{petId}", produces = "application/json") // <1>
	@ResponseBody
	public Pet getPet(@PathVariable String petId) {
		// ...
	}
----
<1> Using a `produces` attribute to narrow the mapping by the content type.

[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@GetMapping("/pets/{petId}", produces = ["application/json"]) // <1>
	@ResponseBody
	fun getPet(@PathVariable petId: String): Pet {
		// ...
	}
----
<1> Using a `produces` attribute to narrow the mapping by the content type.

The media type can specify a character set. Negated expressions are supported -- for example,
`!text/plain` means any content type other than "text/plain".

You can declare a shared `produces` attribute at the class level. Unlike most other
request-mapping attributes, however, when used at the class level, a method-level `produces` attribute
overrides rather than extends the class-level declaration.

TIP: `MediaType` provides constants for commonly used media types, such as
`APPLICATION_JSON_VALUE` and `APPLICATION_XML_VALUE`.


[[mvc-ann-requestmapping-params-and-headers]]
==== Parameters, headers
[.small]#<<web-reactive.adoc#webflux-ann-requestmapping-params-and-headers, WebFlux>>#

You can narrow request mappings based on request parameter conditions. You can test for the
presence of a request parameter (`myParam`), for the absence of one (`!myParam`), or for a
specific value (`myParam=myValue`). The following example shows how to test for a specific value:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@GetMapping(path = "/pets/{petId}", params = "myParam=myValue") // <1>
	public void findPet(@PathVariable String petId) {
		// ...
	}
----
<1> Testing whether `myParam` equals `myValue`.

[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@GetMapping("/pets/{petId}", params = ["myParam=myValue"]) // <1>
	fun findPet(@PathVariable petId: String) {
		// ...
	}
----
<1> Testing whether `myParam` equals `myValue`.

You can also use the same with request header conditions, as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@GetMapping(path = "/pets", headers = "myHeader=myValue") // <1>
	public void findPet(@PathVariable String petId) {
		// ...
	}
----
<1> Testing whether `myHeader` equals `myValue`.

[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@GetMapping("/pets", headers = ["myHeader=myValue"]) // <1>
	fun findPet(@PathVariable petId: String) {
		// ...
	}
----

TIP: You can match `Content-Type` and `Accept` with the headers condition, but it is better to use
<<mvc-ann-requestmapping-consumes, consumes>> and <<mvc-ann-requestmapping-produces, produces>>
instead.


[[mvc-ann-requestmapping-head-options]]
==== HTTP HEAD, OPTIONS
[.small]#<<web-reactive.adoc#webflux-ann-requestmapping-head-options, WebFlux>>#

`@GetMapping` (and `@RequestMapping(method=HttpMethod.GET)`) support HTTP HEAD
transparently for request mapping. Controller methods do not need to change.
A response wrapper, applied in `jakarta.servlet.http.HttpServlet`, ensures a `Content-Length`
header is set to the number of bytes written (without actually writing to the response).

`@GetMapping` (and `@RequestMapping(method=HttpMethod.GET)`) are implicitly mapped to
and support HTTP HEAD. An HTTP HEAD request is processed as if it were HTTP GET except
that, instead of writing the body, the number of bytes are counted and the `Content-Length`
header is set.

By default, HTTP OPTIONS is handled by setting the `Allow` response header to the list of HTTP
methods listed in all `@RequestMapping` methods that have matching URL patterns.

For a `@RequestMapping` without HTTP method declarations, the `Allow` header is set to
`GET,HEAD,POST,PUT,PATCH,DELETE,OPTIONS`. Controller methods should always declare the
supported HTTP methods (for example, by using the HTTP method specific variants:
`@GetMapping`, `@PostMapping`, and others).

You can explicitly map the `@RequestMapping` method to HTTP HEAD and HTTP OPTIONS, but that
is not necessary in the common case.


[[mvc-ann-requestmapping-composed]]
==== Custom Annotations
[.small]#<<web-reactive.adoc#mvc-ann-requestmapping-head-options, WebFlux>>#

Spring MVC supports the use of <<core.adoc#beans-meta-annotations, composed annotations>>
for request mapping. Those are annotations that are themselves meta-annotated with
`@RequestMapping` and composed to redeclare a subset (or all) of the `@RequestMapping`
attributes with a narrower, more specific purpose.

`@GetMapping`, `@PostMapping`, `@PutMapping`, `@DeleteMapping`, and `@PatchMapping` are
examples of composed annotations. They are provided because, arguably, most
controller methods should be mapped to a specific HTTP method versus using `@RequestMapping`,
which, by default, matches to all HTTP methods. If you need an example of composed
annotations, look at how those are declared.

Spring MVC also supports custom request-mapping attributes with custom request-matching
logic. This is a more advanced option that requires subclassing
`RequestMappingHandlerMapping` and overriding the `getCustomMethodCondition` method, where
you can check the custom attribute and return your own `RequestCondition`.


[[mvc-ann-requestmapping-registration]]
==== Explicit Registrations
[.small]#<<web-reactive.adoc#webflux-ann-requestmapping-registration, WebFlux>>#

You can programmatically register handler methods, which you can use for dynamic
registrations or for advanced cases, such as different instances of the same handler
under different URLs. The following example registers a handler method:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Configuration
	public class MyConfig {

		@Autowired
		public void setHandlerMapping(RequestMappingHandlerMapping mapping, UserHandler handler) // <1>
				throws NoSuchMethodException {

			RequestMappingInfo info = RequestMappingInfo
					.paths("/user/{id}").methods(RequestMethod.GET).build(); // <2>

			Method method = UserHandler.class.getMethod("getUser", Long.class); // <3>

			mapping.registerMapping(info, handler, method); // <4>
		}
	}
----
<1> Inject the target handler and the handler mapping for controllers.
<2> Prepare the request mapping meta data.
<3> Get the handler method.
<4> Add the registration.

[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@Configuration
	class MyConfig {

		@Autowired
		fun setHandlerMapping(mapping: RequestMappingHandlerMapping, handler: UserHandler) { // <1>
			val info = RequestMappingInfo.paths("/user/{id}").methods(RequestMethod.GET).build() // <2>
			val method = UserHandler::class.java.getMethod("getUser", Long::class.java) // <3>
			mapping.registerMapping(info, handler, method) // <4>
		}
	}
----
<1> Inject the target handler and the handler mapping for controllers.
<2> Prepare the request mapping meta data.
<3> Get the handler method.
<4> Add the registration.



[[mvc-ann-methods]]
=== Handler Methods
[.small]#<<web-reactive.adoc#webflux-ann-methods, WebFlux>>#

`@RequestMapping` handler methods have a flexible signature and can choose from a range of
supported controller method arguments and return values.


[[mvc-ann-arguments]]
==== Method Arguments
[.small]#<<web-reactive.adoc#webflux-ann-arguments, WebFlux>>#

The next table describes the supported controller method arguments. Reactive types are not supported
for any arguments.

JDK 8's `java.util.Optional` is supported as a method argument in combination with
annotations that have a `required` attribute (for example, `@RequestParam`, `@RequestHeader`,
and others) and is equivalent to `required=false`.

[cols="1,2", options="header"]
|===
| Controller method argument | Description

| `WebRequest`, `NativeWebRequest`
| Generic access to request parameters and request and session attributes, without direct
  use of the Servlet API.

| `jakarta.servlet.ServletRequest`, `jakarta.servlet.ServletResponse`
| Choose any specific request or response type -- for example, `ServletRequest`, `HttpServletRequest`,
  or Spring's `MultipartRequest`, `MultipartHttpServletRequest`.

| `jakarta.servlet.http.HttpSession`
| Enforces the presence of a session. As a consequence, such an argument is never `null`.
  Note that session access is not thread-safe. Consider setting the
  `RequestMappingHandlerAdapter` instance's `synchronizeOnSession` flag to `true` if multiple
  requests are allowed to concurrently access a session.

| `jakarta.servlet.http.PushBuilder`
| Servlet 4.0 push builder API for programmatic HTTP/2 resource pushes.
  Note that, per the Servlet specification, the injected `PushBuilder` instance can be null if the client
  does not support that HTTP/2 feature.

| `java.security.Principal`
| Currently authenticated user -- possibly a specific `Principal` implementation class if known.

  Note that this argument is not resolved eagerly, if it is annotated in order to allow a custom resolver to resolve it
  before falling back on default resolution via `HttpServletRequest#getUserPrincipal`.
  For example, the Spring Security `Authentication` implements `Principal` and would be injected as such via
  `HttpServletRequest#getUserPrincipal`, unless it is also annotated with `@AuthenticationPrincipal` in which case it
  is resolved by a custom Spring Security resolver through `Authentication#getPrincipal`.

| `HttpMethod`
| The HTTP method of the request.

| `java.util.Locale`
| The current request locale, determined by the most specific `LocaleResolver` available (in
  effect, the configured `LocaleResolver` or `LocaleContextResolver`).

| `java.util.TimeZone` + `java.time.ZoneId`
| The time zone associated with the current request, as determined by a `LocaleContextResolver`.

| `java.io.InputStream`, `java.io.Reader`
| For access to the raw request body as exposed by the Servlet API.

| `java.io.OutputStream`, `java.io.Writer`
| For access to the raw response body as exposed by the Servlet API.

| `@PathVariable`
| For access to URI template variables. See <<mvc-ann-requestmapping-uri-templates>>.

| `@MatrixVariable`
| For access to name-value pairs in URI path segments. See <<mvc-ann-matrix-variables>>.

| `@RequestParam`
| For access to the Servlet request parameters, including multipart files. Parameter values
  are converted to the declared method argument type. See <<mvc-ann-requestparam>> as well
  as <<mvc-multipart-forms>>.

  Note that use of `@RequestParam` is optional for simple parameter values.
  See "`Any other argument`", at the end of this table.

| `@RequestHeader`
| For access to request headers. Header values are converted to the declared method argument
  type. See <<mvc-ann-requestheader>>.

| `@CookieValue`
| For access to cookies. Cookies values are converted to the declared method argument
  type. See <<mvc-ann-cookievalue>>.

| `@RequestBody`
| For access to the HTTP request body. Body content is converted to the declared method
  argument type by using `HttpMessageConverter` implementations. See <<mvc-ann-requestbody>>.

| `HttpEntity<B>`
| For access to request headers and body. The body is converted with an `HttpMessageConverter`.
  See <<mvc-ann-httpentity>>.

| `@RequestPart`
| For access to a part in a `multipart/form-data` request, converting the part's body
  with an `HttpMessageConverter`. See <<mvc-multipart-forms>>.

| `java.util.Map`, `org.springframework.ui.Model`, `org.springframework.ui.ModelMap`
| For access to the model that is used in HTML controllers and exposed to templates as
  part of view rendering.

| `RedirectAttributes`
| Specify attributes to use in case of a redirect (that is, to be appended to the query
  string) and flash attributes to be stored temporarily until the request after redirect.
  See <<mvc-redirecting-passing-data>> and <<mvc-flash-attributes>>.

| `@ModelAttribute`
| For access to an existing attribute in the model (instantiated if not present) with
  data binding and validation applied. See <<mvc-ann-modelattrib-method-args>> as well as
  <<mvc-ann-modelattrib-methods>> and <<mvc-ann-initbinder>>.

  Note that use of `@ModelAttribute` is optional (for example, to set its attributes).
  See "`Any other argument`" at the end of this table.

| `Errors`, `BindingResult`
| For access to errors from validation and data binding for a command object
  (that is, a `@ModelAttribute` argument) or errors from the validation of a `@RequestBody` or
  `@RequestPart` arguments. You must declare an `Errors`, or `BindingResult` argument
  immediately after the validated method argument.

| `SessionStatus` + class-level `@SessionAttributes`
| For marking form processing complete, which triggers cleanup of session attributes
  declared through a class-level `@SessionAttributes` annotation. See
  <<mvc-ann-sessionattributes>> for more details.

| `UriComponentsBuilder`
| For preparing a URL relative to the current request's host, port, scheme, context path, and
  the literal part of the servlet mapping. See <<mvc-uri-building>>.

| `@SessionAttribute`
| For access to any session attribute, in contrast to model attributes stored in the session
  as a result of a class-level `@SessionAttributes` declaration. See
  <<mvc-ann-sessionattribute>> for more details.

| `@RequestAttribute`
| For access to request attributes. See <<mvc-ann-requestattrib>> for more details.

| Any other argument
| If a method argument is not matched to any of the earlier values in this table and it is
  a simple type (as determined by
	{api-spring-framework}/beans/BeanUtils.html#isSimpleProperty-java.lang.Class-[BeanUtils#isSimpleProperty]),
  it is resolved as a `@RequestParam`. Otherwise, it is resolved as a `@ModelAttribute`.
|===


[[mvc-ann-return-types]]
==== Return Values
[.small]#<<web-reactive.adoc#webflux-ann-return-types, WebFlux>>#

The next table describes the supported controller method return values. Reactive types are
supported for all return values.

[cols="1,2", options="header"]
|===
| Controller method return value | Description

| `@ResponseBody`
| The return value is converted through `HttpMessageConverter` implementations and written to the
  response. See <<mvc-ann-responsebody>>.

| `HttpEntity<B>`, `ResponseEntity<B>`
| The return value that specifies the full response (including HTTP headers and body) is to be converted
  through `HttpMessageConverter` implementations and written to the response.
  See <<mvc-ann-responseentity>>.

| `HttpHeaders`
| For returning a response with headers and no body.

| `ErrorResponse`
| To render an RFC 7807 error response with details in the body,
  see <<mvc-ann-rest-exceptions>>

| `ProblemDetail`
| To render an RFC 7807 error response with details in the body,
  see <<mvc-ann-rest-exceptions>>

| `String`
| A view name to be resolved with `ViewResolver` implementations and used together with the implicit
  model -- determined through command objects and `@ModelAttribute` methods. The handler
  method can also programmatically enrich the model by declaring a `Model` argument
  (see <<mvc-ann-requestmapping-registration>>).

| `View`
| A `View` instance to use for rendering together with the implicit model -- determined
  through command objects and `@ModelAttribute` methods. The handler method can also
  programmatically enrich the model by declaring a `Model` argument
  (see <<mvc-ann-requestmapping-registration>>).

| `java.util.Map`, `org.springframework.ui.Model`
| Attributes to be added to the implicit model, with the view name implicitly determined
  through a `RequestToViewNameTranslator`.

| `@ModelAttribute`
| An attribute to be added to the model, with the view name implicitly determined through
  a `RequestToViewNameTranslator`.

  Note that `@ModelAttribute` is optional. See "Any other return value" at the end of
  this table.

| `ModelAndView` object
| The view and model attributes to use and, optionally, a response status.

| `void`
| A method with a `void` return type (or `null` return value) is considered to have fully
  handled the response if it also has a `ServletResponse`, an `OutputStream` argument, or
  an `@ResponseStatus` annotation. The same is also true if the controller has made a positive
  `ETag` or `lastModified` timestamp check (see <<mvc-caching-etag-lastmodified>> for details).

  If none of the above is true, a `void` return type can also indicate "`no response body`" for
  REST controllers or a default view name selection for HTML controllers.

| `DeferredResult<V>`
| Produce any of the preceding return values asynchronously from any thread -- for example, as a
  result of some event or callback. See <<mvc-ann-async>> and <<mvc-ann-async-deferredresult>>.

| `Callable<V>`
| Produce any of the above return values asynchronously in a Spring MVC-managed thread.
  See <<mvc-ann-async>> and <<mvc-ann-async-callable>>.

| `ListenableFuture<V>`,
  `java.util.concurrent.CompletionStage<V>`,
  `java.util.concurrent.CompletableFuture<V>`
| Alternative to `DeferredResult`, as a convenience (for example, when an underlying service
  returns one of those).

| `ResponseBodyEmitter`, `SseEmitter`
| Emit a stream of objects asynchronously to be written to the response with
  `HttpMessageConverter` implementations. Also supported as the body of a `ResponseEntity`.
  See <<mvc-ann-async>> and <<mvc-ann-async-http-streaming>>.

| `StreamingResponseBody`
| Write to the response `OutputStream` asynchronously. Also supported as the body of a
  `ResponseEntity`. See <<mvc-ann-async>> and <<mvc-ann-async-http-streaming>>.

| Reactor and other reactive types registered via `ReactiveAdapterRegistry`
| A single value type, e.g. `Mono`, is comparable to returning `DeferredResult`.
  A multi-value type, e.g. `Flux`, may be treated as a stream depending on the requested
  media type, e.g. "text/event-stream", "application/json+stream", or otherwise is
  collected to a List and rendered as a single value. See <<mvc-ann-async>> and
  <<mvc-ann-async-reactive-types>>.

| Other return values
| If a return value remains unresolved in any other way, it is treated as a model
  attribute, unless it is a simple type as determined by
  {api-spring-framework}/beans/BeanUtils.html#isSimpleProperty-java.lang.Class-[BeanUtils#isSimpleProperty],
  in which case it remains unresolved.
|===


[[mvc-ann-typeconversion]]
==== Type Conversion
[.small]#<<web-reactive.adoc#webflux-ann-typeconversion, WebFlux>>#

Some annotated controller method arguments that represent `String`-based request input (such as
`@RequestParam`, `@RequestHeader`, `@PathVariable`, `@MatrixVariable`, and `@CookieValue`)
can require type conversion if the argument is declared as something other than `String`.

For such cases, type conversion is automatically applied based on the configured converters.
By default, simple types (`int`, `long`, `Date`, and others) are supported. You can customize
type conversion through a `WebDataBinder` (see <<mvc-ann-initbinder>>) or by registering
`Formatters` with the `FormattingConversionService`.
See <<core.adoc#format, Spring Field Formatting>>.

A practical issue in type conversion is the treatment of an empty String source value.
Such a value is treated as missing if it becomes `null` as a result of type conversion.
This can be the case for `Long`, `UUID`, and other target types. If you want to allow `null`
to be injected, either use the `required` flag on the argument annotation, or declare the
argument as `@Nullable`.

[NOTE]
====
As of 5.3, non-null arguments will be enforced even after type conversion. If your handler
method intends to accept a null value as well, either declare your argument as `@Nullable`
or mark it as `required=false` in the corresponding `@RequestParam`, etc. annotation. This is
a best practice and the recommended solution for regressions encountered in a 5.3 upgrade.

Alternatively, you may specifically handle e.g. the resulting `MissingPathVariableException`
in the case of a required `@PathVariable`. A null value after conversion will be treated like
an empty original value, so the corresponding `Missing...Exception` variants will be thrown.
====


[[mvc-ann-matrix-variables]]
==== Matrix Variables
[.small]#<<web-reactive.adoc#webflux-ann-matrix-variables, WebFlux>>#

https://tools.ietf.org/html/rfc3986#section-3.3[RFC 3986] discusses name-value pairs in
path segments. In Spring MVC, we refer to those as "`matrix variables`" based on an
https://www.w3.org/DesignIssues/MatrixURIs.html["`old post`"] by Tim Berners-Lee, but they
can be also be referred to as URI path parameters.

Matrix variables can appear in any path segment, with each variable separated by a semicolon and
multiple values separated by comma (for example, `/cars;color=red,green;year=2012`). Multiple
values can also be specified through repeated variable names (for example,
`color=red;color=green;color=blue`).

If a URL is expected to contain matrix variables, the request mapping for a controller
method must use a URI variable to mask that variable content and ensure the request can
be matched successfully independent of matrix variable order and presence.
The following example uses a matrix variable:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	// GET /pets/42;q=11;r=22

	@GetMapping("/pets/{petId}")
	public void findPet(@PathVariable String petId, @MatrixVariable int q) {

		// petId == 42
		// q == 11
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	// GET /pets/42;q=11;r=22

	@GetMapping("/pets/{petId}")
	fun findPet(@PathVariable petId: String, @MatrixVariable q: Int) {

		// petId == 42
		// q == 11
	}
----

Given that all path segments may contain matrix variables, you may sometimes need to
disambiguate which path variable the matrix variable is expected to be in.
The following example shows how to do so:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	// GET /owners/42;q=11/pets/21;q=22

	@GetMapping("/owners/{ownerId}/pets/{petId}")
	public void findPet(
			@MatrixVariable(name="q", pathVar="ownerId") int q1,
			@MatrixVariable(name="q", pathVar="petId") int q2) {

		// q1 == 11
		// q2 == 22
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	// GET /owners/42;q=11/pets/21;q=22

	@GetMapping("/owners/{ownerId}/pets/{petId}")
	fun findPet(
			@MatrixVariable(name = "q", pathVar = "ownerId") q1: Int,
			@MatrixVariable(name = "q", pathVar = "petId") q2: Int) {

		// q1 == 11
		// q2 == 22
	}
----

A matrix variable may be defined as optional and a default value specified, as the
following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	// GET /pets/42

	@GetMapping("/pets/{petId}")
	public void findPet(@MatrixVariable(required=false, defaultValue="1") int q) {

		// q == 1
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	// GET /pets/42

	@GetMapping("/pets/{petId}")
	fun findPet(@MatrixVariable(required = false, defaultValue = "1") q: Int) {

		// q == 1
	}
----

To get all matrix variables, you can use a `MultiValueMap`, as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	// GET /owners/42;q=11;r=12/pets/21;q=22;s=23

	@GetMapping("/owners/{ownerId}/pets/{petId}")
	public void findPet(
			@MatrixVariable MultiValueMap<String, String> matrixVars,
			@MatrixVariable(pathVar="petId") MultiValueMap<String, String> petMatrixVars) {

		// matrixVars: ["q" : [11,22], "r" : 12, "s" : 23]
		// petMatrixVars: ["q" : 22, "s" : 23]
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	// GET /owners/42;q=11;r=12/pets/21;q=22;s=23

	@GetMapping("/owners/{ownerId}/pets/{petId}")
	fun findPet(
			@MatrixVariable matrixVars: MultiValueMap<String, String>,
			@MatrixVariable(pathVar="petId") petMatrixVars: MultiValueMap<String, String>) {

		// matrixVars: ["q" : [11,22], "r" : 12, "s" : 23]
		// petMatrixVars: ["q" : 22, "s" : 23]
	}
----

Note that you need to enable the use of matrix variables. In the MVC Java configuration,
you need to set a `UrlPathHelper` with `removeSemicolonContent=false` through
<<mvc-config-path-matching>>. In the MVC XML namespace, you can set
`<mvc:annotation-driven enable-matrix-variables="true"/>`.


[[mvc-ann-requestparam]]
==== `@RequestParam`
[.small]#<<web-reactive.adoc#webflux-ann-requestparam, WebFlux>>#

You can use the `@RequestParam` annotation to bind Servlet request parameters (that is,
query parameters or form data) to a method argument in a controller.

The following example shows how to do so:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Controller
	@RequestMapping("/pets")
	public class EditPetForm {

		// ...

		@GetMapping
		public String setupForm(@RequestParam("petId") int petId, Model model) { <1>
			Pet pet = this.clinic.loadPet(petId);
			model.addAttribute("pet", pet);
			return "petForm";
		}

		// ...

	}
----
<1> Using `@RequestParam` to bind `petId`.

[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	import org.springframework.ui.set

	@Controller
	@RequestMapping("/pets")
	class EditPetForm {

		// ...

		@GetMapping
		fun setupForm(@RequestParam("petId") petId: Int, model: Model): String { // <1>
			val pet = this.clinic.loadPet(petId);
			model["pet"] = pet
			return "petForm"
		}

		// ...

	}
----
<1> Using `@RequestParam` to bind `petId`.

By default, method parameters that use this annotation are required, but you can specify that
a method parameter is optional by setting the `@RequestParam` annotation's `required` flag to
`false` or by declaring the argument with an `java.util.Optional` wrapper.

Type conversion is automatically applied if the target method parameter type is not
`String`. See <<mvc-ann-typeconversion>>.

Declaring the argument type as an array or list allows for resolving multiple parameter
values for the same parameter name.

When an `@RequestParam` annotation is declared as a `Map<String, String>` or
`MultiValueMap<String, String>`, without a parameter name specified in the annotation,
then the map is populated with the request parameter values for each given parameter name.

Note that use of `@RequestParam` is optional (for example, to set its attributes).
By default, any argument that is a simple value type (as determined by
{api-spring-framework}/beans/BeanUtils.html#isSimpleProperty-java.lang.Class-[BeanUtils#isSimpleProperty])
and is not resolved by any other argument resolver, is treated as if it were annotated
with `@RequestParam`.


[[mvc-ann-requestheader]]
==== `@RequestHeader`
[.small]#<<web-reactive.adoc#webflux-ann-requestheader, WebFlux>>#

You can use the `@RequestHeader` annotation to bind a request header to a method argument in a
controller.

Consider the following request, with headers:

[literal]
[subs="verbatim,quotes"]
----
Host                    localhost:8080
Accept                  text/html,application/xhtml+xml,application/xml;q=0.9
Accept-Language         fr,en-gb;q=0.7,en;q=0.3
Accept-Encoding         gzip,deflate
Accept-Charset          ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive              300
----

The following example gets the value of the `Accept-Encoding` and `Keep-Alive` headers:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@GetMapping("/demo")
	public void handle(
			@RequestHeader("Accept-Encoding") String encoding, // <1>
			@RequestHeader("Keep-Alive") long keepAlive) { // <2>
		//...
	}
----
<1> Get the value of the `Accept-Encoding` header.
<2> Get the value of the `Keep-Alive` header.

[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@GetMapping("/demo")
	fun handle(
			@RequestHeader("Accept-Encoding") encoding: String, // <1>
			@RequestHeader("Keep-Alive") keepAlive: Long) { // <2>
		//...
	}
----
<1> Get the value of the `Accept-Encoding` header.
<2> Get the value of the `Keep-Alive` header.

If the target method parameter type is not
`String`, type conversion is automatically applied. See <<mvc-ann-typeconversion>>.

When an `@RequestHeader` annotation is used on a `Map<String, String>`,
`MultiValueMap<String, String>`, or `HttpHeaders` argument, the map is populated
with all header values.

TIP: Built-in support is available for converting a comma-separated string into an
array or collection of strings or other types known to the type conversion system. For
example, a method parameter annotated with `@RequestHeader("Accept")` can be of type
`String` but also `String[]` or `List<String>`.


[[mvc-ann-cookievalue]]
==== `@CookieValue`
[.small]#<<web-reactive.adoc#webflux-ann-cookievalue, WebFlux>>#

You can use the `@CookieValue` annotation to bind the value of an HTTP cookie to a method argument
in a controller.

Consider a request with the following cookie:

[literal,subs="verbatim,quotes"]
----
JSESSIONID=415A4AC178C59DACE0B2C9CA727CDD84
----

The following example shows how to get the cookie value:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@GetMapping("/demo")
	public void handle(@CookieValue("JSESSIONID") String cookie) { <1>
		//...
	}
----
<1> Get the value of the `JSESSIONID` cookie.

[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@GetMapping("/demo")
	fun handle(@CookieValue("JSESSIONID") cookie: String) { // <1>
		//...
	}
----
<1> Get the value of the `JSESSIONID` cookie.

If the target method parameter type is not `String`, type conversion is applied automatically.
See <<mvc-ann-typeconversion>>.


[[mvc-ann-modelattrib-method-args]]
==== `@ModelAttribute`
[.small]#<<web-reactive.adoc#webflux-ann-modelattrib-method-args, WebFlux>>#

You can use the `@ModelAttribute` annotation on a method argument to access an attribute from
the model or have it be instantiated if not present. The model attribute is also overlain with
values from HTTP Servlet request parameters whose names match to field names. This is referred
to as data binding, and it saves you from having to deal with parsing and converting individual
query parameters and form fields. The following example shows how to do so:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@PostMapping("/owners/{ownerId}/pets/{petId}/edit")
	public String processSubmit(@ModelAttribute Pet pet) {
		// method logic...
	}
----

[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
@PostMapping("/owners/{ownerId}/pets/{petId}/edit")
fun processSubmit(@ModelAttribute pet: Pet): String {
	// method logic...
}
----

The `Pet` instance above is sourced in one of the following ways:

* Retrieved from the model where it may have been added by a
  <<mvc-ann-modelattrib-methods,@ModelAttribute method>>.
* Retrieved from the HTTP session if the model attribute was listed in
  the class-level <<mvc-ann-sessionattributes>> annotation.
* Obtained through a `Converter` where the model attribute name matches the name of a
  request value such as a path variable or a request parameter (see next example).
* Instantiated using its default constructor.
* Instantiated through a "`primary constructor`" with arguments that match to Servlet
  request parameters. Argument names are determined through JavaBeans
  `@ConstructorProperties` or through runtime-retained parameter names in the bytecode.

One alternative to using a <<mvc-ann-modelattrib-methods,@ModelAttribute method>> to
supply it or relying on the framework to create the model attribute, is to have a
`Converter<String, T>` to provide the instance. This is applied when the model attribute
name matches to the name of a request value such as a path variable or a request
parameter, and there is a `Converter` from `String` to the model attribute type.
In the following example, the model attribute name is `account` which matches the URI
path variable `account`, and there is a registered `Converter<String, Account>` which
could load the `Account` from a data store:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@PutMapping("/accounts/{account}")
	public String save(@ModelAttribute("account") Account account) {
		// ...
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@PutMapping("/accounts/{account}")
	fun save(@ModelAttribute("account") account: Account): String {
		// ...
	}
----

After the model attribute instance is obtained, data binding is applied. The
`WebDataBinder` class matches Servlet request parameter names (query parameters and form
fields) to field names on the target `Object`. Matching fields are populated after type
conversion is applied, where necessary. For more on data binding (and validation), see
<<core.adoc#validation, Validation>>. For more on customizing data binding, see
<<mvc-ann-initbinder>>.

Data binding can result in errors. By default, a `BindException` is raised. However, to check
for such errors in the controller method, you can add a `BindingResult` argument immediately next
to the `@ModelAttribute`, as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@PostMapping("/owners/{ownerId}/pets/{petId}/edit")
	public String processSubmit(@ModelAttribute("pet") Pet pet, BindingResult result) { // <1>
		if (result.hasErrors()) {
			return "petForm";
		}
		// ...
	}
----
<1> Adding a `BindingResult` next to the `@ModelAttribute`.

[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@PostMapping("/owners/{ownerId}/pets/{petId}/edit")
	fun processSubmit(@ModelAttribute("pet") pet: Pet, result: BindingResult): String { // <1>
		if (result.hasErrors()) {
			return "petForm"
		}
		// ...
	}
----
<1> Adding a `BindingResult` next to the `@ModelAttribute`.

In some cases, you may want access to a model attribute without data binding. For such
cases, you can inject the `Model` into the controller and access it directly or,
alternatively, set `@ModelAttribute(binding=false)`, as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@ModelAttribute
	public AccountForm setUpForm() {
		return new AccountForm();
	}

	@ModelAttribute
	public Account findAccount(@PathVariable String accountId) {
		return accountRepository.findOne(accountId);
	}

	@PostMapping("update")
	public String update(@Valid AccountForm form, BindingResult result,
			@ModelAttribute(binding=false) Account account) { // <1>
		// ...
	}
----
<1> Setting `@ModelAttribute(binding=false)`.

[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@ModelAttribute
	fun setUpForm(): AccountForm {
		return AccountForm()
	}

	@ModelAttribute
	fun findAccount(@PathVariable accountId: String): Account {
		return accountRepository.findOne(accountId)
	}

	@PostMapping("update")
	fun update(@Valid form: AccountForm, result: BindingResult,
			   @ModelAttribute(binding = false) account: Account): String { // <1>
		// ...
	}
----
<1> Setting `@ModelAttribute(binding=false)`.

You can automatically apply validation after data binding by adding the
`jakarta.validation.Valid` annotation or Spring's `@Validated` annotation
(<<core.adoc#validation-beanvalidation, Bean Validation>> and
<<core.adoc#validation, Spring validation>>). The following example shows how to do so:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
		@PostMapping("/owners/{ownerId}/pets/{petId}/edit")
		public String processSubmit(@Valid @ModelAttribute("pet") Pet pet, BindingResult result) { // <1>
			if (result.hasErrors()) {
				return "petForm";
			}
			// ...
		}
----
<1> Validate the `Pet` instance.

[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@PostMapping("/owners/{ownerId}/pets/{petId}/edit")
	fun processSubmit(@Valid @ModelAttribute("pet") pet: Pet, result: BindingResult): String { // <1>
		if (result.hasErrors()) {
			return "petForm"
		}
		// ...
	}
----

Note that using `@ModelAttribute` is optional (for example, to set its attributes).
By default, any argument that is not a simple value type (as determined by
{api-spring-framework}/beans/BeanUtils.html#isSimpleProperty-java.lang.Class-[BeanUtils#isSimpleProperty])
and is not resolved by any other argument resolver is treated as if it were annotated
with `@ModelAttribute`.


[[mvc-ann-sessionattributes]]
==== `@SessionAttributes`
[.small]#<<web-reactive.adoc#webflux-ann-sessionattributes, WebFlux>>#

`@SessionAttributes` is used to store model attributes in the HTTP Servlet session between
requests. It is a type-level annotation that declares the session attributes used by a
specific controller. This typically lists the names of model attributes or types of
model attributes that should be transparently stored in the session for subsequent
requests to access.

The following example uses the `@SessionAttributes` annotation:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Controller
	@SessionAttributes("pet") // <1>
	public class EditPetForm {
		// ...
	}
----
<1> Using the `@SessionAttributes` annotation.

[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@Controller
	@SessionAttributes("pet") // <1>
	class EditPetForm {
		// ...
	}
----
<1> Using the `@SessionAttributes` annotation.

On the first request, when a model attribute with the name, `pet`, is added to the model,
it is automatically promoted to and saved in the HTTP Servlet session. It remains there
until another controller method uses a `SessionStatus` method argument to clear the
storage, as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Controller
	@SessionAttributes("pet") // <1>
	public class EditPetForm {

		// ...

		@PostMapping("/pets/{id}")
		public String handle(Pet pet, BindingResult errors, SessionStatus status) {
			if (errors.hasErrors) {
				// ...
			}
			status.setComplete(); // <2>
			// ...
		}
	}
----
<1> Storing the `Pet` value in the Servlet session.
<2> Clearing the `Pet` value from the Servlet session.

[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
@Controller
@SessionAttributes("pet") // <1>
class EditPetForm {

	// ...

	@PostMapping("/pets/{id}")
	fun handle(pet: Pet, errors: BindingResult, status: SessionStatus): String {
		if (errors.hasErrors()) {
			// ...
		}
		status.setComplete() // <2>
		// ...
	}
}
----
<1> Storing the `Pet` value in the Servlet session.
<2> Clearing the `Pet` value from the Servlet session.


[[mvc-ann-sessionattribute]]
==== `@SessionAttribute`
[.small]#<<web-reactive.adoc#webflux-ann-sessionattribute, WebFlux>>#

If you need access to pre-existing session attributes that are managed globally
(that is, outside the controller -- for example, by a filter) and may or may not be present,
you can use the `@SessionAttribute` annotation on a method parameter,
as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@RequestMapping("/")
	public String handle(@SessionAttribute User user) { <1>
		// ...
	}
----
<1> Using a `@SessionAttribute` annotation.

[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@RequestMapping("/")
	fun handle(@SessionAttribute user: User): String { // <1>
		// ...
	}
----

For use cases that require adding or removing session attributes, consider injecting
`org.springframework.web.context.request.WebRequest` or
`jakarta.servlet.http.HttpSession` into the controller method.

For temporary storage of model attributes in the session as part of a controller
workflow, consider using `@SessionAttributes` as described in
<<mvc-ann-sessionattributes>>.


[[mvc-ann-requestattrib]]
==== `@RequestAttribute`
[.small]#<<web-reactive.adoc#webflux-ann-requestattrib, WebFlux>>#

Similar to `@SessionAttribute`, you can use the `@RequestAttribute` annotations to
access pre-existing request attributes created earlier (for example, by a Servlet `Filter`
or `HandlerInterceptor`):

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@GetMapping("/")
	public String handle(@RequestAttribute Client client) { // <1>
		// ...
	}
----
<1> Using the `@RequestAttribute` annotation.

[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@GetMapping("/")
	fun handle(@RequestAttribute client: Client): String { // <1>
		// ...
	}
----
<1> Using the `@RequestAttribute` annotation.


[[mvc-redirecting-passing-data]]
==== Redirect Attributes

By default, all model attributes are considered to be exposed as URI template variables in
the redirect URL. Of the remaining attributes, those that are primitive types or
collections or arrays of primitive types are automatically appended as query parameters.

Appending primitive type attributes as query parameters can be the desired result if a
model instance was prepared specifically for the redirect. However, in annotated
controllers, the model can contain additional attributes added for rendering purposes (for example,
drop-down field values). To avoid the possibility of having such attributes appear in the
URL, a `@RequestMapping` method can declare an argument of type `RedirectAttributes` and
use it to specify the exact attributes to make available to `RedirectView`. If the method
does redirect, the content of `RedirectAttributes` is used.  Otherwise, the content of the
model is used.

The `RequestMappingHandlerAdapter` provides a flag called
`ignoreDefaultModelOnRedirect`, which you can use to indicate that the content of the default
`Model` should never be used if a controller method redirects. Instead, the controller
method should declare an attribute of type `RedirectAttributes` or, if it does not do so,
no attributes should be passed on to `RedirectView`. Both the MVC namespace and the MVC
Java configuration keep this flag set to `false`, to maintain backwards compatibility.
However, for new applications, we recommend setting it to `true`.

Note that URI template variables from the present request are automatically made
available when expanding a redirect URL, and you don't need to explicitly add them
through `Model` or `RedirectAttributes`. The following example shows how to define a redirect:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@PostMapping("/files/{path}")
	public String upload(...) {
		// ...
		return "redirect:files/{path}";
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@PostMapping("/files/{path}")
	fun upload(...): String {
		// ...
		return "redirect:files/{path}"
	}
----

Another way of passing data to the redirect target is by using flash attributes. Unlike
other redirect attributes, flash attributes are saved in the HTTP session (and, hence, do
not appear in the URL). See <<mvc-flash-attributes>> for more information.


[[mvc-flash-attributes]]
==== Flash Attributes

Flash attributes provide a way for one request to store attributes that are intended for use in
another. This is most commonly needed when redirecting -- for example, the
Post-Redirect-Get pattern. Flash attributes are saved temporarily before the
redirect (typically in the session) to be made available to the request after the
redirect and are removed immediately.

Spring MVC has two main abstractions in support of flash attributes. `FlashMap` is used
to hold flash attributes, while `FlashMapManager` is used to store, retrieve, and manage
`FlashMap` instances.

Flash attribute support is always "`on`" and does not need to be enabled explicitly.
However, if not used, it never causes HTTP session creation. On each request, there is an
"`input`" `FlashMap` with attributes passed from a previous request (if any) and an
"`output`" `FlashMap` with attributes to save for a subsequent request. Both `FlashMap`
instances are accessible from anywhere in Spring MVC through static methods in
`RequestContextUtils`.

Annotated controllers typically do not need to work with `FlashMap` directly. Instead, a
`@RequestMapping` method can accept an argument of type `RedirectAttributes` and use it
to add flash attributes for a redirect scenario. Flash attributes added through
`RedirectAttributes` are automatically propagated to the "`output`" FlashMap. Similarly,
after the redirect, attributes from the "`input`" `FlashMap` are automatically added to the
`Model` of the controller that serves the target URL.

.Matching requests to flash attributes
****
The concept of flash attributes exists in many other web frameworks and has proven to sometimes
be exposed to concurrency issues. This is because, by definition, flash attributes
are to be stored until the next request. However the very "`next`" request may not be the
intended recipient but another asynchronous request (for example, polling or resource requests),
in which case the flash attributes are removed too early.

To reduce the possibility of such issues, `RedirectView` automatically "`stamps`"
`FlashMap` instances with the path and query parameters of the target redirect URL. In
turn, the default `FlashMapManager` matches that information to incoming requests when
it looks up the "`input`" `FlashMap`.

This does not entirely eliminate the possibility of a concurrency issue but
reduces it greatly with information that is already available in the redirect URL.
Therefore, we recommend that you use flash attributes mainly for redirect scenarios.
****


[[mvc-multipart-forms]]
==== Multipart
[.small]#<<web-reactive.adoc#webflux-multipart-forms, WebFlux>>#

After a `MultipartResolver` has been <<mvc-multipart,enabled>>, the content of POST
requests with `multipart/form-data` is parsed and accessible as regular request
parameters. The following example accesses one regular form field and one uploaded
file:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Controller
	public class FileUploadController {

		@PostMapping("/form")
		public String handleFormUpload(@RequestParam("name") String name,
				@RequestParam("file") MultipartFile file) {

			if (!file.isEmpty()) {
				byte[] bytes = file.getBytes();
				// store the bytes somewhere
				return "redirect:uploadSuccess";
			}
			return "redirect:uploadFailure";
		}
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@Controller
	class FileUploadController {

		@PostMapping("/form")
		fun handleFormUpload(@RequestParam("name") name: String,
							@RequestParam("file") file: MultipartFile): String {

			if (!file.isEmpty) {
				val bytes = file.bytes
				// store the bytes somewhere
				return "redirect:uploadSuccess"
			}
			return "redirect:uploadFailure"
		}
	}
----

Declaring the argument type as a `List<MultipartFile>` allows for resolving multiple
files for the same parameter name.

When the `@RequestParam` annotation is declared as a `Map<String, MultipartFile>` or
`MultiValueMap<String, MultipartFile>`, without a parameter name specified in the annotation,
then the map is populated with the multipart files for each given parameter name.

NOTE: With Servlet multipart parsing, you may also declare `jakarta.servlet.http.Part`
instead of Spring's `MultipartFile`, as a method argument or collection value type.

You can also use multipart content as part of data binding to a
<<mvc-ann-modelattrib-method-args, command object>>. For example, the form field
and file from the preceding example could be fields on a form object,
as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	class MyForm {

		private String name;

		private MultipartFile file;

		// ...
	}

	@Controller
	public class FileUploadController {

		@PostMapping("/form")
		public String handleFormUpload(MyForm form, BindingResult errors) {
			if (!form.getFile().isEmpty()) {
				byte[] bytes = form.getFile().getBytes();
				// store the bytes somewhere
				return "redirect:uploadSuccess";
			}
			return "redirect:uploadFailure";
		}
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	class MyForm(val name: String, val file: MultipartFile, ...)

	@Controller
	class FileUploadController {

		@PostMapping("/form")
		fun handleFormUpload(form: MyForm, errors: BindingResult): String {
			if (!form.file.isEmpty) {
				val bytes = form.file.bytes
				// store the bytes somewhere
				return "redirect:uploadSuccess"
			}
			return "redirect:uploadFailure"
		}
	}
----


Multipart requests can also be submitted from non-browser clients in a RESTful service
scenario. The following example shows a file with JSON:

[literal,subs="verbatim,quotes"]
----
POST /someUrl
Content-Type: multipart/mixed

--edt7Tfrdusa7r3lNQc79vXuhIIMlatb7PQg7Vp
Content-Disposition: form-data; name="meta-data"
Content-Type: application/json; charset=UTF-8
Content-Transfer-Encoding: 8bit

{
	"name": "value"
}
--edt7Tfrdusa7r3lNQc79vXuhIIMlatb7PQg7Vp
Content-Disposition: form-data; name="file-data"; filename="file.properties"
Content-Type: text/xml
Content-Transfer-Encoding: 8bit
... File Data ...
----

You can access the "meta-data" part with `@RequestParam` as a `String` but you'll
probably want it deserialized from JSON (similar to `@RequestBody`). Use the
`@RequestPart` annotation to access a multipart after converting it with an
<<integration.adoc#rest-message-conversion, HttpMessageConverter>>:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@PostMapping("/")
	public String handle(@RequestPart("meta-data") MetaData metadata,
			@RequestPart("file-data") MultipartFile file) {
		// ...
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@PostMapping("/")
	fun handle(@RequestPart("meta-data") metadata: MetaData,
			@RequestPart("file-data") file: MultipartFile): String {
		// ...
	}
----

You can use `@RequestPart` in combination with `jakarta.validation.Valid` or use Spring's
`@Validated` annotation, both of which cause Standard Bean Validation to be applied.
By default, validation errors cause a `MethodArgumentNotValidException`, which is turned
into a 400 (BAD_REQUEST) response. Alternatively, you can handle validation errors locally
within the controller through an `Errors` or `BindingResult` argument,
as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@PostMapping("/")
	public String handle(@Valid @RequestPart("meta-data") MetaData metadata,
			BindingResult result) {
		// ...
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@PostMapping("/")
	fun handle(@Valid @RequestPart("meta-data") metadata: MetaData,
			result: BindingResult): String {
		// ...
	}
----



[[mvc-ann-requestbody]]
==== `@RequestBody`
[.small]#<<web-reactive.adoc#webflux-ann-requestbody, WebFlux>>#

You can use the `@RequestBody` annotation to have the request body read and deserialized into an
`Object` through an <<integration.adoc#rest-message-conversion, `HttpMessageConverter`>>.
The following example uses a `@RequestBody` argument:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@PostMapping("/accounts")
	public void handle(@RequestBody Account account) {
		// ...
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@PostMapping("/accounts")
	fun handle(@RequestBody account: Account) {
		// ...
	}
----


You can use the <<mvc-config-message-converters>> option of the <<mvc-config>> to
configure or customize message conversion.

You can use `@RequestBody` in combination with `jakarta.validation.Valid` or Spring's
`@Validated` annotation, both of which cause Standard Bean Validation to be applied.
By default, validation errors cause a `MethodArgumentNotValidException`, which is turned
into a 400 (BAD_REQUEST) response. Alternatively, you can handle validation errors locally
within the controller through an `Errors` or `BindingResult` argument,
as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@PostMapping("/accounts")
	public void handle(@Valid @RequestBody Account account, BindingResult result) {
		// ...
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@PostMapping("/accounts")
	fun handle(@Valid @RequestBody account: Account, result: BindingResult) {
		// ...
	}
----


[[mvc-ann-httpentity]]
==== HttpEntity
[.small]#<<web-reactive.adoc#webflux-ann-httpentity, WebFlux>>#

`HttpEntity` is more or less identical to using <<mvc-ann-requestbody>> but is based on a
container object that exposes request headers and body. The following listing shows an example:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@PostMapping("/accounts")
	public void handle(HttpEntity<Account> entity) {
		// ...
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@PostMapping("/accounts")
	fun handle(entity: HttpEntity<Account>) {
		// ...
	}
----



[[mvc-ann-responsebody]]
==== `@ResponseBody`
[.small]#<<web-reactive.adoc#webflux-ann-responsebody, WebFlux>>#

You can use the `@ResponseBody` annotation on a method to have the return serialized
to the response body through an
<<integration.adoc#rest-message-conversion, HttpMessageConverter>>.
The following listing shows an example:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@GetMapping("/accounts/{id}")
	@ResponseBody
	public Account handle() {
		// ...
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@GetMapping("/accounts/{id}")
	@ResponseBody
	fun handle(): Account {
		// ...
	}
----

`@ResponseBody` is also supported at the class level, in which case it is inherited by
all controller methods. This is the effect of `@RestController`, which is nothing more
than a meta-annotation marked with `@Controller` and `@ResponseBody`.

You can use `@ResponseBody` with reactive types.
See <<mvc-ann-async>> and <<mvc-ann-async-reactive-types>> for more details.

You can use the <<mvc-config-message-converters>> option of the <<mvc-config>> to
configure or customize message conversion.

You can combine `@ResponseBody` methods with JSON serialization views.
See <<mvc-ann-jackson>> for details.


[[mvc-ann-responseentity]]
==== ResponseEntity
[.small]#<<web-reactive.adoc#webflux-ann-responseentity, WebFlux>>#

`ResponseEntity` is like <<mvc-ann-responsebody>> but with status and headers. For example:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@GetMapping("/something")
	public ResponseEntity<String> handle() {
		String body = ... ;
		String etag = ... ;
		return ResponseEntity.ok().eTag(etag).body(body);
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@GetMapping("/something")
	fun handle(): ResponseEntity<String> {
		val body = ...
		val etag = ...
		return ResponseEntity.ok().eTag(etag).build(body)
	}
----

Spring MVC supports using a single value <<mvc-ann-async-reactive-types, reactive type>>
to produce the `ResponseEntity` asynchronously, and/or single and multi-value reactive
types for the body. This allows the following types of async responses:

* `ResponseEntity<Mono<T>>` or `ResponseEntity<Flux<T>>` make the response status and
  headers known immediately while the body is provided asynchronously at a later point.
  Use `Mono` if the body consists of 0..1 values or `Flux` if it can produce multiple values.
* `Mono<ResponseEntity<T>>` provides all three -- response status, headers, and body,
  asynchronously at a later point. This allows the response status and headers to vary
  depending on the outcome of asynchronous request handling.


[[mvc-ann-jackson]]
==== Jackson JSON

Spring offers support for the Jackson JSON library.

[[mvc-ann-jsonview]]
===== JSON Views
[.small]#<<web-reactive.adoc#webflux-ann-jsonview, WebFlux>>#

Spring MVC provides built-in support for
https://www.baeldung.com/jackson-json-view-annotation[Jackson's Serialization Views],
which allow rendering only a subset of all fields in an `Object`. To use it with
`@ResponseBody` or `ResponseEntity` controller methods, you can use Jackson's
`@JsonView` annotation to activate a serialization view class, as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@RestController
	public class UserController {

		@GetMapping("/user")
		@JsonView(User.WithoutPasswordView.class)
		public User getUser() {
			return new User("eric", "7!jd#h23");
		}
	}

	public class User {

		public interface WithoutPasswordView {};
		public interface WithPasswordView extends WithoutPasswordView {};

		private String username;
		private String password;

		public User() {
		}

		public User(String username, String password) {
			this.username = username;
			this.password = password;
		}

		@JsonView(WithoutPasswordView.class)
		public String getUsername() {
			return this.username;
		}

		@JsonView(WithPasswordView.class)
		public String getPassword() {
			return this.password;
		}
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@RestController
	class UserController {

		@GetMapping("/user")
		@JsonView(User.WithoutPasswordView::class)
		fun getUser() = User("eric", "7!jd#h23")
	}

	class User(
			@JsonView(WithoutPasswordView::class) val username: String,
			@JsonView(WithPasswordView::class) val password: String) {

		interface WithoutPasswordView
		interface WithPasswordView : WithoutPasswordView
	}
----

NOTE: `@JsonView` allows an array of view classes, but you can specify only one per
controller method. If you need to activate multiple views, you can use a composite interface.

If you want to do the above programmatically, instead of declaring an `@JsonView` annotation,
wrap the return value with `MappingJacksonValue` and use it to supply the serialization view:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@RestController
	public class UserController {

		@GetMapping("/user")
		public MappingJacksonValue getUser() {
			User user = new User("eric", "7!jd#h23");
			MappingJacksonValue value = new MappingJacksonValue(user);
			value.setSerializationView(User.WithoutPasswordView.class);
			return value;
		}
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@RestController
	class UserController {

		@GetMapping("/user")
		fun getUser(): MappingJacksonValue {
			val value = MappingJacksonValue(User("eric", "7!jd#h23"))
			value.serializationView = User.WithoutPasswordView::class.java
			return value
		}
	}
----

For controllers that rely on view resolution, you can add the serialization view class
to the model, as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Controller
	public class UserController extends AbstractController {

		@GetMapping("/user")
		public String getUser(Model model) {
			model.addAttribute("user", new User("eric", "7!jd#h23"));
			model.addAttribute(JsonView.class.getName(), User.WithoutPasswordView.class);
			return "userView";
		}
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@Controller
	class UserController : AbstractController() {

		@GetMapping("/user")
		fun getUser(model: Model): String {
			model["user"] = User("eric", "7!jd#h23")
			model[JsonView::class.qualifiedName] = User.WithoutPasswordView::class.java
			return "userView"
		}
	}
----



[[mvc-ann-modelattrib-methods]]
=== Model
[.small]#<<web-reactive.adoc#webflux-ann-modelattrib-methods, WebFlux>>#

You can use the `@ModelAttribute` annotation:

* On a <<mvc-ann-modelattrib-method-args,method argument>> in `@RequestMapping` methods
to create or access an `Object` from the model and to bind it to the request through a
`WebDataBinder`.
* As a method-level annotation in `@Controller` or `@ControllerAdvice` classes that help
to initialize the model prior to any `@RequestMapping` method invocation.
* On a `@RequestMapping` method to mark its return value is a model attribute.

This section discusses `@ModelAttribute` methods -- the second item in the preceding list.
A controller can have any number of `@ModelAttribute` methods. All such methods are
invoked before `@RequestMapping` methods in the same controller. A `@ModelAttribute`
method can also be shared across controllers through `@ControllerAdvice`. See the section on
<<mvc-ann-controller-advice>> for more details.

`@ModelAttribute` methods have flexible method signatures. They support many of the same
arguments as `@RequestMapping` methods, except for `@ModelAttribute` itself or anything
related to the request body.

The following example shows a `@ModelAttribute` method:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@ModelAttribute
	public void populateModel(@RequestParam String number, Model model) {
		model.addAttribute(accountRepository.findAccount(number));
		// add more ...
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@ModelAttribute
	fun populateModel(@RequestParam number: String, model: Model) {
		model.addAttribute(accountRepository.findAccount(number))
		// add more ...
	}
----

The following example adds only one attribute:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@ModelAttribute
	public Account addAccount(@RequestParam String number) {
		return accountRepository.findAccount(number);
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@ModelAttribute
	fun addAccount(@RequestParam number: String): Account {
		return accountRepository.findAccount(number)
	}
----


NOTE: When a name is not explicitly specified, a default name is chosen based on the `Object`
type, as explained in the javadoc for {api-spring-framework}/core/Conventions.html[`Conventions`].
You can always assign an explicit name by using the overloaded `addAttribute` method or
through the `name` attribute on `@ModelAttribute` (for a return value).

You can also use `@ModelAttribute` as a method-level annotation on `@RequestMapping` methods,
in which case the return value of the `@RequestMapping` method is interpreted as a model
attribute. This is typically not required, as it is the default behavior in HTML controllers,
unless the return value is a `String` that would otherwise be interpreted as a view name.
`@ModelAttribute` can also customize the model attribute name, as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@GetMapping("/accounts/{id}")
	@ModelAttribute("myAccount")
	public Account handle() {
		// ...
		return account;
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@GetMapping("/accounts/{id}")
	@ModelAttribute("myAccount")
	fun handle(): Account {
		// ...
		return account
	}
----



[[mvc-ann-initbinder]]
=== `DataBinder`
[.small]#<<web-reactive.adoc#webflux-ann-initbinder, WebFlux>>#

`@Controller` or `@ControllerAdvice` classes can have `@InitBinder` methods that
initialize instances of `WebDataBinder`, and those, in turn, can:

* Bind request parameters (that is, form or query data) to a model object.
* Convert String-based request values (such as request parameters, path variables,
headers, cookies, and others) to the target type of controller method arguments.
* Format model object values as `String` values when rendering HTML forms.

`@InitBinder` methods can register controller-specific `java.beans.PropertyEditor` or
Spring `Converter` and `Formatter` components. In addition, you can use the
<<mvc-config-conversion,MVC config>> to register `Converter` and `Formatter`
types in a globally shared `FormattingConversionService`.

`@InitBinder` methods support many of the same arguments that `@RequestMapping` methods
do, except for `@ModelAttribute` (command object) arguments. Typically, they are declared
with a `WebDataBinder` argument (for registrations) and a `void` return value.
The following listing shows an example:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Controller
	public class FormController {

		@InitBinder // <1>
		public void initBinder(WebDataBinder binder) {
			SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd");
			dateFormat.setLenient(false);
			binder.registerCustomEditor(Date.class, new CustomDateEditor(dateFormat, false));
		}

		// ...
	}
----
<1> Defining an `@InitBinder` method.

[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@Controller
	class FormController {

		@InitBinder // <1>
		fun initBinder(binder: WebDataBinder) {
			val dateFormat = SimpleDateFormat("yyyy-MM-dd")
			dateFormat.isLenient = false
			binder.registerCustomEditor(Date::class.java, CustomDateEditor(dateFormat, false))
		}

		// ...
	}
----
<1> Defining an `@InitBinder` method.

Alternatively, when you use a `Formatter`-based setup through a shared
`FormattingConversionService`, you can re-use the same approach and register
controller-specific `Formatter` implementations, as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Controller
	public class FormController {

		@InitBinder // <1>
		protected void initBinder(WebDataBinder binder) {
			binder.addCustomFormatter(new DateFormatter("yyyy-MM-dd"));
		}

		// ...
	}
----
<1> Defining an `@InitBinder` method on a custom formatter.

[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@Controller
	class FormController {

		@InitBinder // <1>
		protected fun initBinder(binder: WebDataBinder) {
			binder.addCustomFormatter(DateFormatter("yyyy-MM-dd"))
		}

		// ...
	}
----
<1> Defining an `@InitBinder` method on a custom formatter.

[[mvc-ann-initbinder-model-design]]
==== Model Design
[.small]#<<web-reactive.adoc#webflux-ann-initbinder-model-design, WebFlux>>#

include::web-data-binding-model-design.adoc[]


[[mvc-ann-exceptionhandler]]
=== Exceptions
[.small]#<<web-reactive.adoc#webflux-ann-controller-exceptions, WebFlux>>#

`@Controller` and <<mvc-ann-controller-advice, @ControllerAdvice>> classes can have
`@ExceptionHandler` methods to handle exceptions from controller methods, as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Controller
	public class SimpleController {

		// ...

		@ExceptionHandler
		public ResponseEntity<String> handle(IOException ex) {
			// ...
		}
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@Controller
	class SimpleController {

		// ...

		@ExceptionHandler
		fun handle(ex: IOException): ResponseEntity<String> {
			// ...
		}
	}
----

The exception may match against a top-level exception being propagated (e.g. a direct
`IOException` being thrown) or against a nested cause within a wrapper exception (e.g.
an `IOException` wrapped inside an `IllegalStateException`). As of 5.3, this can match
at arbitrary cause levels, whereas previously only an immediate cause was considered.

For matching exception types, preferably declare the target exception as a method argument,
as the preceding example shows. When multiple exception methods match, a root exception match is
generally preferred to a cause exception match. More specifically, the `ExceptionDepthComparator`
is used to sort exceptions based on their depth from the thrown exception type.

Alternatively, the annotation declaration may narrow the exception types to match,
as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@ExceptionHandler({FileSystemException.class, RemoteException.class})
	public ResponseEntity<String> handle(IOException ex) {
		// ...
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@ExceptionHandler(FileSystemException::class, RemoteException::class)
	fun handle(ex: IOException): ResponseEntity<String> {
		// ...
	}
----

You can even use a list of specific exception types with a very generic argument signature,
as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@ExceptionHandler({FileSystemException.class, RemoteException.class})
	public ResponseEntity<String> handle(Exception ex) {
		// ...
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@ExceptionHandler(FileSystemException::class, RemoteException::class)
	fun handle(ex: Exception): ResponseEntity<String> {
		// ...
	}
----

[NOTE]
====
The distinction between root and cause exception matching can be surprising.

In the `IOException` variant shown earlier, the method is typically called with
the actual `FileSystemException` or `RemoteException` instance as the argument,
since both of them extend from `IOException`. However, if any such matching
exception is propagated within a wrapper exception which is itself an `IOException`,
the passed-in exception instance is that wrapper exception.

The behavior is even simpler in the `handle(Exception)` variant. This is
always invoked with the wrapper exception in a wrapping scenario, with the
actually matching exception to be found through `ex.getCause()` in that case.
The passed-in exception is the actual `FileSystemException` or
`RemoteException` instance only when these are thrown as top-level exceptions.
====

We generally recommend that you be as specific as possible in the argument signature,
reducing the potential for mismatches between root and cause exception types.
Consider breaking a multi-matching method into individual `@ExceptionHandler`
methods, each matching a single specific exception type through its signature.

In a multi-`@ControllerAdvice` arrangement, we recommend declaring your primary root exception
mappings on a `@ControllerAdvice` prioritized with a corresponding order. While a root
exception match is preferred to a cause, this is defined among the methods of a given
controller or `@ControllerAdvice` class. This means a cause match on a higher-priority
`@ControllerAdvice` bean is preferred to any match (for example, root) on a lower-priority
`@ControllerAdvice` bean.

Last but not least, an `@ExceptionHandler` method implementation can choose to back
out of dealing with a given exception instance by rethrowing it in its original form.
This is useful in scenarios where you are interested only in root-level matches or in
matches within a specific context that cannot be statically determined. A rethrown
exception is propagated through the remaining resolution chain, as though
the given `@ExceptionHandler` method would not have matched in the first place.

Support for `@ExceptionHandler` methods in Spring MVC is built on the `DispatcherServlet`
level, <<mvc-exceptionhandlers, HandlerExceptionResolver>> mechanism.


[[mvc-ann-exceptionhandler-args]]
==== Method Arguments
[.small]#<<webflux.adoc#webflux-ann-exceptionhandler-args, WebFlux>>#

`@ExceptionHandler` methods support the following arguments:

[cols="1,2", options="header"]
|===
| Method argument | Description

| Exception type
| For access to the raised exception.

| `HandlerMethod`
| For access to the controller method that raised the exception.

| `WebRequest`, `NativeWebRequest`
| Generic access to request parameters and request and session attributes without direct
  use of the Servlet API.

| `jakarta.servlet.ServletRequest`, `jakarta.servlet.ServletResponse`
| Choose any specific request or response type (for example, `ServletRequest` or
  `HttpServletRequest` or Spring's `MultipartRequest` or `MultipartHttpServletRequest`).

| `jakarta.servlet.http.HttpSession`
| Enforces the presence of a session. As a consequence, such an argument is never `null`. +
  Note that session access is not thread-safe. Consider setting the
  `RequestMappingHandlerAdapter` instance's `synchronizeOnSession` flag to `true` if multiple
  requests are allowed to access a session concurrently.

| `java.security.Principal`
| Currently authenticated user -- possibly a specific `Principal` implementation class if known.

| `HttpMethod`
| The HTTP method of the request.

| `java.util.Locale`
| The current request locale, determined by the most specific `LocaleResolver` available -- in
  effect, the configured `LocaleResolver` or `LocaleContextResolver`.

| `java.util.TimeZone`, `java.time.ZoneId`
| The time zone associated with the current request, as determined by a `LocaleContextResolver`.

| `java.io.OutputStream`, `java.io.Writer`
| For access to the raw response body, as exposed by the Servlet API.

| `java.util.Map`, `org.springframework.ui.Model`, `org.springframework.ui.ModelMap`
| For access to the model for an error response. Always empty.

| `RedirectAttributes`
| Specify attributes to use in case of a redirect -- (that is to be appended to the query
  string) and flash attributes to be stored temporarily until the request after the redirect.
  See <<mvc-redirecting-passing-data>> and <<mvc-flash-attributes>>.

| `@SessionAttribute`
| For access to any session attribute, in contrast to model attributes stored in the
  session as a result of a class-level `@SessionAttributes` declaration.
  See <<mvc-ann-sessionattribute>> for more details.

| `@RequestAttribute`
| For access to request attributes. See <<mvc-ann-requestattrib>> for more details.

|===


[[mvc-ann-exceptionhandler-return-values]]
==== Return Values
[.small]#<<webflux.adoc#webflux-ann-exceptionhandler-return-values, WebFlux>>#

`@ExceptionHandler` methods support the following return values:

[cols="1,2", options="header"]
|===
| Return value | Description

| `@ResponseBody`
| The return value is converted through `HttpMessageConverter` instances and written to the
  response. See <<mvc-ann-responsebody>>.

| `HttpEntity<B>`, `ResponseEntity<B>`
| The return value specifies that the full response (including the HTTP headers and the body)
  be converted through `HttpMessageConverter` instances and written to the response.
  See <<mvc-ann-responseentity>>.

| `ErrorResponse`
| To render an RFC 7807 error response with details in the body,
see <<mvc-ann-rest-exceptions>>

| `ProblemDetail`
| To render an RFC 7807 error response with details in the body,
see <<mvc-ann-rest-exceptions>>

| `String`
| A view name to be resolved with `ViewResolver` implementations and used together with the
  implicit model -- determined through command objects and `@ModelAttribute` methods.
  The handler method can also programmatically enrich the model by declaring a `Model`
  argument (described earlier).

| `View`
| A `View` instance to use for rendering together with the implicit model -- determined
  through command objects and `@ModelAttribute` methods. The handler method may also
  programmatically enrich the model by declaring a `Model` argument (descried earlier).

| `java.util.Map`, `org.springframework.ui.Model`
| Attributes to be added to the implicit model with the view name implicitly determined
  through a `RequestToViewNameTranslator`.

| `@ModelAttribute`
| An attribute to be added to the model with the view name implicitly determined through
  a `RequestToViewNameTranslator`.

  Note that `@ModelAttribute` is optional. See "`Any other return value`" at the end of
  this table.

| `ModelAndView` object
| The view and model attributes to use and, optionally, a response status.

| `void`
| A method with a `void` return type (or `null` return value) is considered to have fully
  handled the response if it also has a `ServletResponse` an `OutputStream` argument, or
  a `@ResponseStatus` annotation. The same is also true if the controller has made a positive
  `ETag` or `lastModified` timestamp check (see <<mvc-caching-etag-lastmodified>> for details).

  If none of the above is true, a `void` return type can also indicate "`no response body`" for
  REST controllers or default view name selection for HTML controllers.

| Any other return value
| If a return value is not matched to any of the above and is not a simple type (as determined by
  {api-spring-framework}/beans/BeanUtils.html#isSimpleProperty-java.lang.Class-[BeanUtils#isSimpleProperty]),
  by default, it is treated as a model attribute to be added to the model. If it is a simple type,
  it remains unresolved.
|===



[[mvc-ann-controller-advice]]
=== Controller Advice
[.small]#<<web-reactive.adoc#webflux-ann-controller-advice, WebFlux>>#

`@ExceptionHandler`, `@InitBinder`, and `@ModelAttribute` methods apply only to the
`@Controller` class, or class hierarchy, in which they are declared. If, instead, they
are declared in an `@ControllerAdvice` or `@RestControllerAdvice` class, then they apply
to any controller. Moreover, as of 5.3, `@ExceptionHandler` methods in `@ControllerAdvice`
can be used to handle exceptions from any `@Controller` or any other handler.

`@ControllerAdvice` is meta-annotated with `@Component` and therefore can be registered as
a Spring bean through <<core.adoc#beans-java-instantiating-container-scan,
component scanning>>. `@RestControllerAdvice` is meta-annotated with `@ControllerAdvice`
and `@ResponseBody`, and that means `@ExceptionHandler` methods will have their return
value rendered via response body message conversion, rather than via HTML views.

On startup, `RequestMappingHandlerMapping` and `ExceptionHandlerExceptionResolver` detect
controller advice beans and apply them at runtime. Global `@ExceptionHandler` methods,
from an `@ControllerAdvice`, are applied _after_ local ones, from the `@Controller`.
By contrast, global `@ModelAttribute` and `@InitBinder` methods are applied _before_ local ones.

The `@ControllerAdvice` annotation has attributes that let you narrow the set of controllers
and handlers that they apply to. For example:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	// Target all Controllers annotated with @RestController
	@ControllerAdvice(annotations = RestController.class)
	public class ExampleAdvice1 {}

	// Target all Controllers within specific packages
	@ControllerAdvice("org.example.controllers")
	public class ExampleAdvice2 {}

	// Target all Controllers assignable to specific classes
	@ControllerAdvice(assignableTypes = {ControllerInterface.class, AbstractController.class})
	public class ExampleAdvice3 {}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	// Target all Controllers annotated with @RestController
	@ControllerAdvice(annotations = [RestController::class])
	class ExampleAdvice1

	// Target all Controllers within specific packages
	@ControllerAdvice("org.example.controllers")
	class ExampleAdvice2

	// Target all Controllers assignable to specific classes
	@ControllerAdvice(assignableTypes = [ControllerInterface::class, AbstractController::class])
	class ExampleAdvice3
----

The selectors in the preceding example are evaluated at runtime and may negatively impact
performance if used extensively. See the
{api-spring-framework}/web/bind/annotation/ControllerAdvice.html[`@ControllerAdvice`]
javadoc for more details.

include::webmvc-functional.adoc[leveloffset=+1]



[[mvc-uri-building]]
== URI Links
[.small]#<<web-reactive.adoc#webflux-uri-building, WebFlux>>#

This section describes various options available in the Spring Framework to work with URI's.

include::web-uris.adoc[leveloffset=+2]



[[mvc-servleturicomponentsbuilder]]
=== Relative Servlet Requests

You can use `ServletUriComponentsBuilder` to create URIs relative to the current request,
as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	HttpServletRequest request = ...

	// Re-uses scheme, host, port, path, and query string...

	URI uri = ServletUriComponentsBuilder.fromRequest(request)
			.replaceQueryParam("accountId", "{id}")
			.build("123");
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	val request: HttpServletRequest = ...

	// Re-uses scheme, host, port, path, and query string...

	val uri = ServletUriComponentsBuilder.fromRequest(request)
			.replaceQueryParam("accountId", "{id}")
			.build("123")
----

You can create URIs relative to the context path, as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	HttpServletRequest request = ...

	// Re-uses scheme, host, port, and context path...

	URI uri = ServletUriComponentsBuilder.fromContextPath(request)
			.path("/accounts")
			.build()
			.toUri();
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	val request: HttpServletRequest = ...

	// Re-uses scheme, host, port, and context path...

	val uri = ServletUriComponentsBuilder.fromContextPath(request)
			.path("/accounts")
			.build()
			.toUri()
----

You can create URIs relative to a Servlet (for example, `/main/{asterisk}`),
as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	HttpServletRequest request = ...

	// Re-uses scheme, host, port, context path, and Servlet mapping prefix...

	URI uri = ServletUriComponentsBuilder.fromServletMapping(request)
			.path("/accounts")
			.build()
			.toUri();
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	val request: HttpServletRequest = ...

	// Re-uses scheme, host, port, context path, and Servlet mapping prefix...

	val uri = ServletUriComponentsBuilder.fromServletMapping(request)
			.path("/accounts")
			.build()
			.toUri()
----

NOTE: As of 5.1, `ServletUriComponentsBuilder` ignores information from the `Forwarded` and
`X-Forwarded-*` headers, which specify the client-originated address. Consider using the
<<filters-forwarded-headers, `ForwardedHeaderFilter`>> to extract and use or to discard
such headers.



[[mvc-links-to-controllers]]
=== Links to Controllers

Spring MVC provides a mechanism to prepare links to controller methods. For example,
the following MVC controller allows for link creation:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Controller
	@RequestMapping("/hotels/{hotel}")
	public class BookingController {

		@GetMapping("/bookings/{booking}")
		public ModelAndView getBooking(@PathVariable Long booking) {
			// ...
		}
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@Controller
	@RequestMapping("/hotels/{hotel}")
	class BookingController {

		@GetMapping("/bookings/{booking}")
		fun getBooking(@PathVariable booking: Long): ModelAndView {
			// ...
		}
	}
----

You can prepare a link by referring to the method by name, as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	UriComponents uriComponents = MvcUriComponentsBuilder
		.fromMethodName(BookingController.class, "getBooking", 21).buildAndExpand(42);

	URI uri = uriComponents.encode().toUri();
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	val uriComponents = MvcUriComponentsBuilder
		.fromMethodName(BookingController::class.java, "getBooking", 21).buildAndExpand(42)

	val uri = uriComponents.encode().toUri()
----

In the preceding example, we provide actual method argument values (in this case, the long value: `21`)
to be used as a path variable and inserted into the URL. Furthermore, we provide the
value, `42`, to fill in any remaining URI variables, such as the `hotel` variable inherited
from the type-level request mapping. If the method had more arguments, we could supply null for
arguments not needed for the URL. In general, only `@PathVariable` and `@RequestParam` arguments
are relevant for constructing the URL.

There are additional ways to use `MvcUriComponentsBuilder`. For example, you can use a technique
akin to mock testing through proxies to avoid referring to the controller method by name, as the following example shows
(the example assumes static import of `MvcUriComponentsBuilder.on`):

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	UriComponents uriComponents = MvcUriComponentsBuilder
		.fromMethodCall(on(BookingController.class).getBooking(21)).buildAndExpand(42);

	URI uri = uriComponents.encode().toUri();
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	val uriComponents = MvcUriComponentsBuilder
		.fromMethodCall(on(BookingController::class.java).getBooking(21)).buildAndExpand(42)

	val uri = uriComponents.encode().toUri()
----

NOTE: Controller method signatures are limited in their design when they are supposed to be usable for
link creation with `fromMethodCall`. Aside from needing a proper parameter signature,
there is a technical limitation on the return type (namely, generating a runtime proxy
for link builder invocations), so the return type must not be `final`. In particular,
the common `String` return type for view names does not work here. You should use `ModelAndView`
or even plain `Object` (with a `String` return value) instead.

The earlier examples use static methods in `MvcUriComponentsBuilder`. Internally, they rely
on `ServletUriComponentsBuilder` to prepare a base URL from the scheme, host, port,
context path, and servlet path of the current request. This works well in most cases.
However, sometimes, it can be insufficient. For example, you may be outside the context of
a request (such as a batch process that prepares links) or perhaps you need to insert a path
prefix (such as a locale prefix that was removed from the request path and needs to be
re-inserted into links).

For such cases, you can use the static `fromXxx` overloaded methods that accept a
`UriComponentsBuilder` to use a base URL. Alternatively, you can create an instance of `MvcUriComponentsBuilder`
with a base URL and then use the instance-based `withXxx` methods. For example, the
following listing uses `withMethodCall`:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	UriComponentsBuilder base = ServletUriComponentsBuilder.fromCurrentContextPath().path("/en");
	MvcUriComponentsBuilder builder = MvcUriComponentsBuilder.relativeTo(base);
	builder.withMethodCall(on(BookingController.class).getBooking(21)).buildAndExpand(42);

	URI uri = uriComponents.encode().toUri();
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	val base = ServletUriComponentsBuilder.fromCurrentContextPath().path("/en")
	val builder = MvcUriComponentsBuilder.relativeTo(base)
	builder.withMethodCall(on(BookingController::class.java).getBooking(21)).buildAndExpand(42)

	val uri = uriComponents.encode().toUri()
----

NOTE: As of 5.1, `MvcUriComponentsBuilder` ignores information from the `Forwarded` and
`X-Forwarded-*` headers, which specify the client-originated address. Consider using the
<<filters-forwarded-headers, ForwardedHeaderFilter>> to extract and use or to discard
such headers.



[[mvc-links-to-controllers-from-views]]
=== Links in Views

In views such as Thymeleaf, FreeMarker, or JSP, you can build links to annotated controllers
by referring to the implicitly or explicitly assigned name for each request mapping.

Consider the following example:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@RequestMapping("/people/{id}/addresses")
	public class PersonAddressController {

		@RequestMapping("/{country}")
		public HttpEntity<PersonAddress> getAddress(@PathVariable String country) { ... }
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@RequestMapping("/people/{id}/addresses")
	class PersonAddressController {

		@RequestMapping("/{country}")
		fun getAddress(@PathVariable country: String): HttpEntity<PersonAddress> { ... }
	}
----

Given the preceding controller, you can prepare a link from a JSP, as follows:

[source,jsp,indent=0,subs="verbatim,quotes"]
----
<%@ taglib uri="http://www.springframework.org/tags" prefix="s" %>
...
<a href="${s:mvcUrl('PAC#getAddress').arg(0,'US').buildAndExpand('123')}">Get Address</a>
----

The preceding example relies on the `mvcUrl` function declared in the Spring tag library
(that is, META-INF/spring.tld), but it is easy to define your own function or prepare a
similar one for other templating technologies.

Here is how this works. On startup, every `@RequestMapping` is assigned a default name
through `HandlerMethodMappingNamingStrategy`, whose default implementation uses the
capital letters of the class and the method name (for example, the `getThing` method in
`ThingController` becomes "TC#getThing"). If there is a name clash, you can use
`@RequestMapping(name="..")` to assign an explicit name or implement your own
`HandlerMethodMappingNamingStrategy`.




[[mvc-ann-async]]
== Asynchronous Requests
[.small]#<<mvc-ann-async-vs-webflux, Compared to WebFlux>>#

Spring MVC has an extensive integration with Servlet asynchronous request
<<mvc-ann-async-processing,processing>>:

* <<mvc-ann-async-deferredresult, `DeferredResult`>> and <<mvc-ann-async-callable, `Callable`>>
return values in controller methods provide basic support for a single asynchronous
return value.
* Controllers can <<mvc-ann-async-http-streaming,stream>> multiple values, including
<<mvc-ann-async-sse, SSE>> and <<mvc-ann-async-output-stream, raw data>>.
* Controllers can use reactive clients and return
<<mvc-ann-async-reactive-types, reactive types>> for response handling.



[[mvc-ann-async-deferredresult]]
=== `DeferredResult`
[.small]#<<mvc-ann-async-vs-webflux, Compared to WebFlux>>#

Once the asynchronous request processing feature is <<mvc-ann-async-configuration, enabled>>
in the Servlet container, controller methods can wrap any supported controller method
return value with `DeferredResult`, as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@GetMapping("/quotes")
	@ResponseBody
	public DeferredResult<String> quotes() {
		DeferredResult<String> deferredResult = new DeferredResult<String>();
		// Save the deferredResult somewhere..
		return deferredResult;
	}

	// From some other thread...
	deferredResult.setResult(result);
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@GetMapping("/quotes")
	@ResponseBody
	fun quotes(): DeferredResult<String> {
		val deferredResult = DeferredResult<String>()
		// Save the deferredResult somewhere..
		return deferredResult
	}

	// From some other thread...
	deferredResult.setResult(result)
----

The controller can produce the return value asynchronously, from a different thread -- for
example, in response to an external event (JMS message), a scheduled task, or other event.



[[mvc-ann-async-callable]]
=== `Callable`
[.small]#<<mvc-ann-async-vs-webflux, Compared to WebFlux>>#

A controller can wrap any supported return value with `java.util.concurrent.Callable`,
as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@PostMapping
	public Callable<String> processUpload(final MultipartFile file) {

		return new Callable<String>() {
			public String call() throws Exception {
				// ...
				return "someView";
			}
		};
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@PostMapping
	fun processUpload(file: MultipartFile) = Callable<String> {
		// ...
		"someView"
	}
----

The return value can then be obtained by running the given task through the
<<mvc-ann-async-configuration-spring-mvc, configured>> `TaskExecutor`.



[[mvc-ann-async-processing]]
=== Processing
[.small]#<<mvc-ann-async-vs-webflux, Compared to WebFlux>>#

Here is a very concise overview of Servlet asynchronous request processing:

* A `ServletRequest` can be put in asynchronous mode by calling `request.startAsync()`.
  The main effect of doing so is that the Servlet (as well as any filters) can exit, but
  the response remains open to let processing complete later.
* The call to `request.startAsync()` returns `AsyncContext`, which you can use for
  further control over asynchronous processing. For example, it provides the `dispatch` method,
  which is similar to a forward from the Servlet API, except that it lets an
  application resume request processing on a Servlet container thread.
* The `ServletRequest` provides access to the current `DispatcherType`, which you can
  use to distinguish between processing the initial request, an asynchronous
  dispatch, a forward, and other dispatcher types.

`DeferredResult` processing works as follows:

* The controller returns a `DeferredResult` and saves it in some in-memory
  queue or list where it can be accessed.
* Spring MVC calls `request.startAsync()`.
* Meanwhile, the `DispatcherServlet` and all configured filters exit the request
  processing thread, but the response remains open.
* The application sets the `DeferredResult` from some thread, and Spring MVC
  dispatches the request back to the Servlet container.
* The `DispatcherServlet` is invoked again, and processing resumes with the
  asynchronously produced return value.

`Callable` processing works as follows:

* The controller returns a `Callable`.
* Spring MVC calls `request.startAsync()` and submits the `Callable` to
  a `TaskExecutor` for processing in a separate thread.
* Meanwhile, the `DispatcherServlet` and all filters exit the Servlet container thread,
  but the response remains open.
* Eventually the `Callable` produces a result, and Spring MVC dispatches the request back
  to the Servlet container to complete processing.
* The `DispatcherServlet` is invoked again, and processing resumes with the
  asynchronously produced return value from the `Callable`.

For further background and context, you can also read
https://spring.io/blog/2012/05/07/spring-mvc-3-2-preview-introducing-servlet-3-async-support[the
blog posts] that introduced asynchronous request processing support in Spring MVC 3.2.


[[mvc-ann-async-exceptions]]
==== Exception Handling

When you use a `DeferredResult`, you can choose whether to call `setResult` or
`setErrorResult` with an exception. In both cases, Spring MVC dispatches the request back
to the Servlet container to complete processing. It is then treated either as if the
controller method returned the given value or as if it produced the given exception.
The exception then goes through the regular exception handling mechanism (for example, invoking
`@ExceptionHandler` methods).

When you use `Callable`, similar processing logic occurs, the main difference being that
the result is returned from the `Callable` or an exception is raised by it.


[[mvc-ann-async-interception]]
==== Interception

`HandlerInterceptor` instances can be of type `AsyncHandlerInterceptor`, to receive the
`afterConcurrentHandlingStarted` callback on the initial request that starts asynchronous
processing (instead of `postHandle` and `afterCompletion`).

`HandlerInterceptor` implementations can also register a `CallableProcessingInterceptor`
or a `DeferredResultProcessingInterceptor`, to integrate more deeply with the
lifecycle of an asynchronous request (for example, to handle a timeout event). See
{api-spring-framework}/web/servlet/AsyncHandlerInterceptor.html[`AsyncHandlerInterceptor`]
for more details.

`DeferredResult` provides `onTimeout(Runnable)` and `onCompletion(Runnable)` callbacks.
See the {api-spring-framework}/web/context/request/async/DeferredResult.html[javadoc of `DeferredResult`]
for more details. `Callable` can be substituted for `WebAsyncTask` that exposes additional
methods for timeout and completion callbacks.


[[mvc-ann-async-vs-webflux]]
==== Compared to WebFlux

The Servlet API was originally built for making a single pass through the Filter-Servlet
chain. Asynchronous request processing lets applications exit the Filter-Servlet chain
but leave the response open for further processing. The Spring MVC asynchronous support
is built around that mechanism. When a controller returns a `DeferredResult`, the
Filter-Servlet chain is exited, and the Servlet container thread is released. Later, when
the `DeferredResult` is set, an `ASYNC` dispatch (to the same URL) is made, during which the
controller is mapped again but, rather than invoking it, the `DeferredResult` value is used
(as if the controller returned it) to resume processing.

By contrast, Spring WebFlux is neither built on the Servlet API, nor does it need such an
asynchronous request processing feature, because it is asynchronous by design. Asynchronous
handling is built into all framework contracts and is intrinsically supported through all
stages of request processing.

From a programming model perspective, both Spring MVC and Spring WebFlux support
asynchronous and <<mvc-ann-async-reactive-types>> as return values in controller methods.
Spring MVC even supports streaming, including reactive back pressure. However, individual
writes to the response remain blocking (and are performed on a separate thread), unlike WebFlux,
which relies on non-blocking I/O and does not need an extra thread for each write.

Another fundamental difference is that Spring MVC does not support asynchronous or reactive
types in controller method arguments (for example, `@RequestBody`, `@RequestPart`, and others),
nor does it have any explicit support for asynchronous and reactive types as model attributes.
Spring WebFlux does support all that.



[[mvc-ann-async-http-streaming]]
=== HTTP Streaming
[.small]#<<web-reactive.adoc#webflux-codecs-streaming, WebFlux>>#

You can use `DeferredResult` and `Callable` for a single asynchronous return value.
What if you want to produce multiple asynchronous values and have those written to the
response? This section describes how to do so.


[[mvc-ann-async-objects]]
==== Objects

You can use the `ResponseBodyEmitter` return value to produce a stream of objects, where
each object is serialized with an
<<integration.adoc#rest-message-conversion, `HttpMessageConverter`>> and written to the
response, as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@GetMapping("/events")
	public ResponseBodyEmitter handle() {
		ResponseBodyEmitter emitter = new ResponseBodyEmitter();
		// Save the emitter somewhere..
		return emitter;
	}

	// In some other thread
	emitter.send("Hello once");

	// and again later on
	emitter.send("Hello again");

	// and done at some point
	emitter.complete();
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@GetMapping("/events")
	fun handle() = ResponseBodyEmitter().apply {
		// Save the emitter somewhere..
	}

	// In some other thread
	emitter.send("Hello once")

	// and again later on
	emitter.send("Hello again")

	// and done at some point
	emitter.complete()
----

You can also use `ResponseBodyEmitter` as the body in a `ResponseEntity`, letting you
customize the status and headers of the response.

When an `emitter` throws an `IOException` (for example, if the remote client went away), applications
are not responsible for cleaning up the connection and should not invoke `emitter.complete`
or `emitter.completeWithError`. Instead, the servlet container automatically initiates an
`AsyncListener` error notification, in which Spring MVC makes a `completeWithError` call.
This call, in turn, performs one final `ASYNC` dispatch to the application, during which Spring MVC
invokes the configured exception resolvers and completes the request.


[[mvc-ann-async-sse]]
==== SSE

`SseEmitter` (a subclass of `ResponseBodyEmitter`) provides support for
https://www.w3.org/TR/eventsource/[Server-Sent Events], where events sent from the server
are formatted according to the W3C SSE specification. To produce an SSE
stream from a controller, return `SseEmitter`, as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@GetMapping(path="/events", produces=MediaType.TEXT_EVENT_STREAM_VALUE)
	public SseEmitter handle() {
		SseEmitter emitter = new SseEmitter();
		// Save the emitter somewhere..
		return emitter;
	}

	// In some other thread
	emitter.send("Hello once");

	// and again later on
	emitter.send("Hello again");

	// and done at some point
	emitter.complete();
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@GetMapping("/events", produces = [MediaType.TEXT_EVENT_STREAM_VALUE])
	fun handle() = SseEmitter().apply {
		// Save the emitter somewhere..
	}

	// In some other thread
	emitter.send("Hello once")

	// and again later on
	emitter.send("Hello again")

	// and done at some point
	emitter.complete()
----

While SSE is the main option for streaming into browsers, note that Internet Explorer
does not support Server-Sent Events. Consider using Spring's
<<web.adoc#websocket, WebSocket messaging>> with
<<web.adoc#websocket-fallback, SockJS fallback>> transports (including SSE) that target
a wide range of browsers.

See also <<mvc-ann-async-objects, previous section>> for notes on exception handling.


[[mvc-ann-async-output-stream]]
==== Raw Data

Sometimes, it is useful to bypass message conversion and stream directly to the response
`OutputStream` (for example, for a file download). You can use the `StreamingResponseBody`
return value type to do so, as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@GetMapping("/download")
	public StreamingResponseBody handle() {
		return new StreamingResponseBody() {
			@Override
			public void writeTo(OutputStream outputStream) throws IOException {
				// write...
			}
		};
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@GetMapping("/download")
	fun handle() = StreamingResponseBody {
		// write...
	}
----

You can use `StreamingResponseBody` as the body in a `ResponseEntity` to
customize the status and headers of the response.



[[mvc-ann-async-reactive-types]]
=== Reactive Types
[.small]#<<web-reactive.adoc#webflux-codecs-streaming, WebFlux>>#

Spring MVC supports use of reactive client libraries in a controller (also read
<<web-reactive.adoc#webflux-reactive-libraries, Reactive Libraries>> in the WebFlux section).
This includes the `WebClient` from `spring-webflux` and others, such as Spring Data
reactive data repositories. In such scenarios, it is convenient to be able to return
reactive types from the controller method.

Reactive return values are handled as follows:

* A single-value promise is adapted to, similar to using `DeferredResult`. Examples
include `Mono` (Reactor) or `Single` (RxJava).
* A multi-value stream with a streaming media type (such as `application/x-ndjson`
or `text/event-stream`) is adapted to, similar to using `ResponseBodyEmitter` or
`SseEmitter`. Examples include `Flux` (Reactor) or `Observable` (RxJava).
Applications can also return `Flux<ServerSentEvent>` or `Observable<ServerSentEvent>`.
* A multi-value stream with any other media type (such as `application/json`) is adapted
to, similar to using `DeferredResult<List<?>>`.

TIP: Spring MVC supports Reactor and RxJava through the
{api-spring-framework}/core/ReactiveAdapterRegistry.html[`ReactiveAdapterRegistry`] from
`spring-core`, which lets it adapt from multiple reactive libraries.

For streaming to the response, reactive back pressure is supported, but writes to the
response are still blocking and are run on a separate thread through the
<<mvc-ann-async-configuration-spring-mvc, configured>> `TaskExecutor`, to avoid
blocking the upstream source (such as a `Flux` returned from `WebClient`).
By default, `SimpleAsyncTaskExecutor` is used for the blocking writes, but that is not
suitable under load. If you plan to stream with a reactive type, you should use the
<<mvc-ann-async-configuration-spring-mvc, MVC configuration>> to configure a task executor.



[[mvc-ann-async-context-propagation]]
=== Context Propagation

It is common to propagate context via `java.lang.ThreadLocal`. This works transparently
for handling on the same thread, but requires additional work for asynchronous handling
across multiple threads. The Micrometer
https://github.com/micrometer-metrics/context-propagation#context-propagation-library[Context Propagation]
library simplifies context propagation across threads, and across context mechanisms such
as `ThreadLocal` values,
Reactor https://projectreactor.io/docs/core/release/reference/#context[context],
GraphQL Java https://www.graphql-java.com/documentation/concerns/#context-objects[context],
and others.

If Micrometer Context Propagation is present on the classpath, when a controller method
returns a <<mvc-ann-async-reactive-types,reactive type>> such as `Flux` or `Mono`, all
`ThreadLocal` values, for which there is a registered `io.micrometer.ThreadLocalAccessor`,
are written to the Reactor `Context` as key-value pairs, using the key assigned by the
`ThreadLocalAccessor`.

For other asynchronous handling scenarios, you can use the Context Propagation library
directly. For example:

[source,java,indent=0,subs="verbatim,quotes"]
.Java
----
	// Capture ThreadLocal values from the main thread ...
	ContextSnapshot snapshot = ContextSnapshot.captureAll();

	// On a different thread: restore ThreadLocal values
	try (ContextSnapshot.Scope scoped = snapshot.setThreadLocals()) {
		// ...
	}
----

For more details, see the
https://micrometer.io/docs/contextPropagation[documentation] of the Micrometer Context
Propagation library.



[[mvc-ann-async-disconnects]]
=== Disconnects
[.small]#<<web-reactive.adoc#webflux-codecs-streaming, WebFlux>>#

The Servlet API does not provide any notification when a remote client goes away.
Therefore, while streaming to the response, whether through <<mvc-ann-async-sse, SseEmitter>>
or <<mvc-ann-async-reactive-types, reactive types>>, it is important to send data periodically,
since the write fails if the client has disconnected. The send could take the form of an
empty (comment-only) SSE event or any other data that the other side would have to interpret
as a heartbeat and ignore.

Alternatively, consider using web messaging solutions (such as
<<websocket-stomp, STOMP over WebSocket>> or WebSocket with <<websocket-fallback, SockJS>>)
that have a built-in heartbeat mechanism.



[[mvc-ann-async-configuration]]
=== Configuration
[.small]#<<mvc-ann-async-vs-webflux, Compared to WebFlux>>#

The asynchronous request processing feature must be enabled at the Servlet container level.
The MVC configuration also exposes several options for asynchronous requests.


[[mvc-ann-async-configuration-servlet3]]
==== Servlet Container

Filter and Servlet declarations have an `asyncSupported` flag that needs to be set to `true`
to enable asynchronous request processing. In addition, Filter mappings should be
declared to handle the `ASYNC` `jakarta.servlet.DispatchType`.

In Java configuration, when you use `AbstractAnnotationConfigDispatcherServletInitializer`
to initialize the Servlet container, this is done automatically.

In `web.xml` configuration, you can add `<async-supported>true</async-supported>` to the
`DispatcherServlet` and to `Filter` declarations and add
`<dispatcher>ASYNC</dispatcher>` to filter mappings.


[[mvc-ann-async-configuration-spring-mvc]]
==== Spring MVC

The MVC configuration exposes the following options related to asynchronous request processing:

* Java configuration: Use the `configureAsyncSupport` callback on `WebMvcConfigurer`.
* XML namespace: Use the `<async-support>` element under `<mvc:annotation-driven>`.

You can configure the following:

* Default timeout value for async requests, which if not set, depends
on the underlying Servlet container.
* `AsyncTaskExecutor` to use for blocking writes when streaming with
<<mvc-ann-async-reactive-types>> and for executing `Callable` instances returned from
controller methods. We highly recommended configuring this property if you
stream with reactive types or have controller methods that return `Callable`, since
by default, it is a `SimpleAsyncTaskExecutor`.
* `DeferredResultProcessingInterceptor` implementations and `CallableProcessingInterceptor` implementations.

Note that you can also set the default timeout value on a `DeferredResult`,
a `ResponseBodyEmitter`, and an `SseEmitter`. For a `Callable`, you can use
`WebAsyncTask` to provide a timeout value.


include::webmvc-cors.adoc[leveloffset=+1]


[[mvc-ann-rest-exceptions]]
== Error Responses
[.small]#<<web-reactive.adoc#webflux-ann-rest-exceptions, WebFlux>>#

A common requirement for REST services is to include details in the body of error
responses. The Spring Framework supports the "Problem Details for HTTP APIs"
specification, https://www.rfc-editor.org/rfc/rfc7807.html[RFC 7807].

The following are the main abstractions for this support:

- `ProblemDetail` -- representation for an RFC 7807 problem detail; a simple container
for both standard fields defined in the spec, and for non-standard ones.
- `ErrorResponse` -- contract to expose HTTP error response details including HTTP
status, response headers, and a body in the format of RFC 7807; this allows exceptions to
encapsulate and expose the details of how they map to an HTTP response. All Spring MVC
exceptions implement this.
- `ErrorResponseException` -- basic `ErrorResponse` implementation that others
can use as a convenient base class.
- `ResponseEntityExceptionHandler` -- convenient base class for an
<<mvc-ann-controller-advice,@ControllerAdvice>> that handles all Spring MVC exceptions,
and any `ErrorResponseException`, and renders an error response with a body.



[[mvc-ann-rest-exceptions-render]]
=== Render
[.small]#<<web-reactive.adoc#webflux-ann-rest-exceptions-render, WebFlux>>#

You can return `ProblemDetail` or `ErrorResponse` from any `@ExceptionHandler` or from
any `@RequestMapping` method to render an RFC 7807 response. This is processed as follows:

- The `status` property of `ProblemDetail` determines the HTTP status.
- The `instance` property of `ProblemDetail` is set from the current URL path, if not
already set.
- For content negotiation, the Jackson `HttpMessageConverter` prefers
"application/problem+json" over "application/json" when rendering a `ProblemDetail`,
and also falls back on it if no compatible media type is found.

To enable RFC 7807 responses for Spring WebFlux exceptions and for any
`ErrorResponseException`, extend `ResponseEntityExceptionHandler` and declare it as an
<<mvc-ann-controller-advice,@ControllerAdvice>> in Spring configuration. The handler
has an `@ExceptionHandler` method that handles any `ErrorResponse` exception, which
includes all built-in web exceptions. You can add more exception handling methods, and
use a protected method to map any exception to a `ProblemDetail`.



[[mvc-ann-rest-exceptions-non-standard]]
=== Non-Standard Fields
[.small]#<<web-reactive.adoc#webflux-ann-rest-exceptions-non-standard, WebFlux>>#

You can extend an RFC 7807 response with non-standard fields in one of two ways.

One, insert into the "properties" `Map` of `ProblemDetail`. When using the Jackson
library, the Spring Framework registers `ProblemDetailJacksonMixin` that ensures this
"properties" `Map` is unwrapped and rendered as top level JSON properties in the
response, and likewise any unknown property during deserialization is inserted into
this `Map`.

You can also extend `ProblemDetail` to add dedicated non-standard properties.
The copy constructor in `ProblemDetail` allows a subclass to make it easy to be created
from an existing `ProblemDetail`. This could be done centrally, e.g. from an
`@ControllerAdvice` such as `ResponseEntityExceptionHandler` that re-creates the
`ProblemDetail` of an exception into a subclass with the additional non-standard fields.



[[mvc-ann-rest-exceptions-i18n]]
=== Internationalization
[.small]#<<web-reactive.adoc#webflux-ann-rest-exceptions-i18n, WebFlux>>#

It is a common requirement to internationalize error response details, and good practice
to customize the problem details for Spring MVC exceptions. This is supported as follows:

- Each `ErrorResponse` exposes a message code and arguments to resolve the "detail" field
through a <<core.adoc#context-functionality-messagesource,MessageSource>>.
The actual message code value is parameterized with placeholders, e.g.
`"HTTP method {0} not supported"` to be expanded from the arguments.
- Each `ErrorResponse` also exposes a message code to resolve the "title" field.
- `ResponseEntityExceptionHandler` uses the message code and arguments to resolve the
"detail" and the "title" fields.

By default, the message code for the "detail" field is "problemDetail." + the fully
qualified exception class name. Some exceptions may expose additional message codes in
which case a suffix is added to the default message code. The table below lists message
arguments and codes for Spring MVC exceptions:

[[mvc-ann-rest-exceptions-codes]]
[cols="1,1,2", options="header"]
|===
| Exception | Message Code | Message Code Arguments

| `AsyncRequestTimeoutException`
| (default)
|

| `ConversionNotSupportedException`
| (default)
| `{0}` property name, `{1}` property value

| `HttpMediaTypeNotAcceptableException`
| (default)
| `{0}` list of supported media types

| `HttpMediaTypeNotAcceptableException`
| (default) + ".parseError"
|

| `HttpMediaTypeNotSupportedException`
| (default)
| `{0}` the media type that is not supported, `{1}` list of supported media types

| `HttpMediaTypeNotSupportedException`
| (default) + ".parseError"
|

| `HttpMessageNotReadableException`
| (default)
|

| `HttpMessageNotWritableException`
| (default)
|

| `HttpRequestMethodNotSupportedException`
| (default)
| `{0}` the current HTTP method, `{1}` the list of supported HTTP methods

| `MethodArgumentNotValidException`
| (default)
| `{0}` the list of global errors, `{1}` the list of field errors.
  Message codes and arguments for each error within the `BindingResult` are also resolved
  via `MessageSource`.

| `MissingRequestHeaderException`
| (default)
| `{0}` the header name

| `MissingServletRequestParameterException`
| (default)
| `{0}` the request parameter name

| `MissingMatrixVariableException`
| (default)
| `{0}` the matrix variable name

| `MissingPathVariableException`
| (default)
| `{0}` the path variable name

| `MissingRequestCookieException`
| (default)
| `{0}` the cookie name

| `MissingServletRequestPartException`
| (default)
| `{0}` the part name

| `NoHandlerFoundException`
| (default)
|

| `TypeMismatchException`
| (default)
| `{0}` property name, `{1}` property value

| `UnsatisfiedServletRequestParameterException`
| (default)
| `{0}` the list of parameter conditions

|===

By default, the message code for the "title" field is "problemDetail.title." + the fully
qualified exception class name.



[[mvc-ann-rest-exceptions-client]]
=== Client Handling
[.small]#<<web-reactive.adoc#webflux-ann-rest-exceptions-client, WebFlux>>#

A client application can catch `WebClientResponseException`, when using the `WebClient`,
or `RestClientResponseException` when using the `RestTemplate`, and use their
`getResponseBodyAs` methods to decode the error response body to any target type such as
`ProblemDetail`, or a subclass of `ProblemDetail`.



[[mvc-web-security]]
== Web Security
[.small]#<<web-reactive.adoc#webflux-web-security, WebFlux>>#

The https://spring.io/projects/spring-security[Spring Security] project provides support
for protecting web applications from malicious exploits. See the Spring Security
reference documentation, including:

* {doc-spring-security}/servlet/integrations/mvc.html[Spring MVC Security]
* {doc-spring-security}/servlet/test/mockmvc/setup.html[Spring MVC Test Support]
* {doc-spring-security}/features/exploits/csrf.html#csrf-protection[CSRF protection]
* {doc-spring-security}/features/exploits/headers.html[Security Response Headers]

https://hdiv.org/[HDIV] is another web security framework that integrates with Spring MVC.




[[mvc-caching]]
== HTTP Caching
[.small]#<<web-reactive.adoc#webflux-caching, WebFlux>>#

HTTP caching can significantly improve the performance of a web application. HTTP caching
revolves around the `Cache-Control` response header and, subsequently, conditional request
headers (such as `Last-Modified` and `ETag`). `Cache-Control` advises private (for example, browser)
and public (for example, proxy) caches on how to cache and re-use responses. An `ETag` header is used
to make a conditional request that may result in a 304 (NOT_MODIFIED) without a body,
if the content has not changed. `ETag` can be seen as a more sophisticated successor to
the `Last-Modified` header.

This section describes the HTTP caching-related options that are available in Spring Web MVC.



[[mvc-caching-cachecontrol]]
=== `CacheControl`
[.small]#<<web-reactive.adoc#webflux-caching-cachecontrol, WebFlux>>#

{api-spring-framework}/http/CacheControl.html[`CacheControl`] provides support for
configuring settings related to the `Cache-Control` header and is accepted as an argument
in a number of places:

* {api-spring-framework}/web/servlet/mvc/WebContentInterceptor.html[`WebContentInterceptor`]
* {api-spring-framework}/web/servlet/support/WebContentGenerator.html[`WebContentGenerator`]
* <<mvc-caching-etag-lastmodified>>
* <<mvc-caching-static-resources>>

While https://tools.ietf.org/html/rfc7234#section-5.2.2[RFC 7234] describes all possible
directives for the `Cache-Control` response header, the `CacheControl` type takes a
use case-oriented approach that focuses on the common scenarios:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	// Cache for an hour - "Cache-Control: max-age=3600"
	CacheControl ccCacheOneHour = CacheControl.maxAge(1, TimeUnit.HOURS);

	// Prevent caching - "Cache-Control: no-store"
	CacheControl ccNoStore = CacheControl.noStore();

	// Cache for ten days in public and private caches,
	// public caches should not transform the response
	// "Cache-Control: max-age=864000, public, no-transform"
	CacheControl ccCustom = CacheControl.maxAge(10, TimeUnit.DAYS).noTransform().cachePublic();
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	// Cache for an hour - "Cache-Control: max-age=3600"
	val ccCacheOneHour = CacheControl.maxAge(1, TimeUnit.HOURS)

	// Prevent caching - "Cache-Control: no-store"
	val ccNoStore = CacheControl.noStore()

	// Cache for ten days in public and private caches,
	// public caches should not transform the response
	// "Cache-Control: max-age=864000, public, no-transform"
	val ccCustom = CacheControl.maxAge(10, TimeUnit.DAYS).noTransform().cachePublic()
----

`WebContentGenerator` also accepts a simpler `cachePeriod` property (defined in seconds) that
works as follows:

* A `-1` value does not generate a `Cache-Control` response header.
* A `0` value prevents caching by using the `'Cache-Control: no-store'` directive.
* An `n > 0` value caches the given response for `n` seconds by using the
`'Cache-Control: max-age=n'` directive.



[[mvc-caching-etag-lastmodified]]
=== Controllers
[.small]#<<web-reactive.adoc#webflux-caching-etag-lastmodified, WebFlux>>#

Controllers can add explicit support for HTTP caching. We recommended doing so, since the
`lastModified` or `ETag` value for a resource needs to be calculated before it can be compared
against conditional request headers. A controller can add an `ETag` header and `Cache-Control`
settings to a `ResponseEntity`, as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@GetMapping("/book/{id}")
	public ResponseEntity<Book> showBook(@PathVariable Long id) {

		Book book = findBook(id);
		String version = book.getVersion();

		return ResponseEntity
				.ok()
				.cacheControl(CacheControl.maxAge(30, TimeUnit.DAYS))
				.eTag(version) // lastModified is also available
				.body(book);
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@GetMapping("/book/{id}")
	fun showBook(@PathVariable id: Long): ResponseEntity<Book> {

		val book = findBook(id);
		val version = book.getVersion()

		return ResponseEntity
				.ok()
				.cacheControl(CacheControl.maxAge(30, TimeUnit.DAYS))
				.eTag(version) // lastModified is also available
				.body(book)
	}
----

The preceding example sends a 304 (NOT_MODIFIED) response with an empty body if the comparison
to the conditional request headers indicates that the content has not changed. Otherwise, the
`ETag` and `Cache-Control` headers are added to the response.

You can also make the check against conditional request headers in the controller,
as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@RequestMapping
	public String myHandleMethod(WebRequest request, Model model) {

		long eTag = ... // <1>

		if (request.checkNotModified(eTag)) {
			return null; // <2>
		}

		model.addAttribute(...); // <3>
		return "myViewName";
	}
----
<1> Application-specific calculation.
<2> The response has been set to 304 (NOT_MODIFIED) -- no further processing.
<3> Continue with the request processing.

[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@RequestMapping
	fun myHandleMethod(request: WebRequest, model: Model): String? {

		val eTag: Long = ... // <1>

		if (request.checkNotModified(eTag)) {
			return null // <2>
		}

		model[...] = ... // <3>
		return "myViewName"
	}
----
<1> Application-specific calculation.
<2> The response has been set to 304 (NOT_MODIFIED) -- no further processing.
<3> Continue with the request processing.


There are three variants for checking conditional requests against `eTag` values, `lastModified`
values, or both. For conditional `GET` and `HEAD` requests, you can set the response to
304 (NOT_MODIFIED). For conditional `POST`, `PUT`, and `DELETE`, you can instead set the response
to 412 (PRECONDITION_FAILED), to prevent concurrent modification.



[[mvc-caching-static-resources]]
=== Static Resources
[.small]#<<web-reactive.adoc#webflux-caching-static-resources, WebFlux>>#

You should serve static resources with a `Cache-Control` and conditional response headers
for optimal performance. See the section on configuring <<mvc-config-static-resources>>.



[[mvc-httpcaching-shallowetag]]
=== `ETag` Filter

You can use the `ShallowEtagHeaderFilter` to add "`shallow`" `eTag` values that are computed from the
response content and, thus, save bandwidth but not CPU time. See <<filters-shallow-etag>>.

include::webmvc-view.adoc[leveloffset=+1]




[[mvc-config]]
== MVC Config
[.small]#<<web-reactive.adoc#webflux-config, WebFlux>>#

The MVC Java configuration and the MVC XML namespace provide default configuration
suitable for most applications and a configuration API to customize it.

For more advanced customizations, which are not available in the configuration API,
see <<mvc-config-advanced-java>> and <<mvc-config-advanced-xml>>.

You do not need to understand the underlying beans created by the MVC Java configuration
and the MVC namespace. If you want to learn more, see <<mvc-servlet-special-bean-types>>
and <<mvc-servlet-config>>.



[[mvc-config-enable]]
=== Enable MVC Configuration
[.small]#<<web-reactive.adoc#webflux-config-enable, WebFlux>>#

In Java configuration, you can use the `@EnableWebMvc` annotation to enable MVC
configuration, as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Configuration
	@EnableWebMvc
	public class WebConfig {
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@Configuration
	@EnableWebMvc
	class WebConfig
----

In XML configuration, you can use the `<mvc:annotation-driven>` element to enable MVC
configuration, as the following example shows:

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<?xml version="1.0" encoding="UTF-8"?>
	<beans xmlns="http://www.springframework.org/schema/beans"
		xmlns:mvc="http://www.springframework.org/schema/mvc"
		xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
		xsi:schemaLocation="
			http://www.springframework.org/schema/beans
			https://www.springframework.org/schema/beans/spring-beans.xsd
			http://www.springframework.org/schema/mvc
			https://www.springframework.org/schema/mvc/spring-mvc.xsd">

		<mvc:annotation-driven/>

	</beans>
----

The preceding example registers a number of Spring MVC
<<mvc-servlet-special-bean-types, infrastructure beans>> and adapts to dependencies
available on the classpath (for example, payload converters for JSON, XML, and others).



[[mvc-config-customize]]
=== MVC Config API
[.small]#<<web-reactive.adoc#webflux-config-customize, WebFlux>>#

In Java configuration, you can implement the `WebMvcConfigurer` interface, as the
following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Configuration
	@EnableWebMvc
	public class WebConfig implements WebMvcConfigurer {

		// Implement configuration methods...
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@Configuration
	@EnableWebMvc
	class WebConfig : WebMvcConfigurer {

		// Implement configuration methods...
	}
----


In XML, you can check attributes and sub-elements of `<mvc:annotation-driven/>`. You can
view the https://schema.spring.io/mvc/spring-mvc.xsd[Spring MVC XML schema] or use
the code completion feature of your IDE to discover what attributes and
sub-elements are available.



[[mvc-config-conversion]]
=== Type Conversion
[.small]#<<web-reactive.adoc#webflux-config-conversion, WebFlux>>#

By default, formatters for various number and date types are installed, along with support
for customization via `@NumberFormat` and `@DateTimeFormat` on fields.

To register custom formatters and converters in Java config, use the following:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Configuration
	@EnableWebMvc
	public class WebConfig implements WebMvcConfigurer {

		@Override
		public void addFormatters(FormatterRegistry registry) {
			// ...
		}
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@Configuration
	@EnableWebMvc
	class WebConfig : WebMvcConfigurer {

		override fun addFormatters(registry: FormatterRegistry) {
			// ...
		}
	}
----

To do the same in XML config, use the following:

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<?xml version="1.0" encoding="UTF-8"?>
	<beans xmlns="http://www.springframework.org/schema/beans"
		xmlns:mvc="http://www.springframework.org/schema/mvc"
		xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
		xsi:schemaLocation="
			http://www.springframework.org/schema/beans
			https://www.springframework.org/schema/beans/spring-beans.xsd
			http://www.springframework.org/schema/mvc
			https://www.springframework.org/schema/mvc/spring-mvc.xsd">

		<mvc:annotation-driven conversion-service="conversionService"/>

		<bean id="conversionService"
				class="org.springframework.format.support.FormattingConversionServiceFactoryBean">
			<property name="converters">
				<set>
					<bean class="org.example.MyConverter"/>
				</set>
			</property>
			<property name="formatters">
				<set>
					<bean class="org.example.MyFormatter"/>
					<bean class="org.example.MyAnnotationFormatterFactory"/>
				</set>
			</property>
			<property name="formatterRegistrars">
				<set>
					<bean class="org.example.MyFormatterRegistrar"/>
				</set>
			</property>
		</bean>

	</beans>
----

By default Spring MVC considers the request Locale when parsing and formatting date
values. This works for forms where dates are represented as Strings with "input" form
fields. For "date" and "time" form fields, however, browsers use a fixed format defined
in the HTML spec. For such cases date and time formatting can be customized as follows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Configuration
	@EnableWebMvc
	public class WebConfig implements WebMvcConfigurer {

		@Override
		public void addFormatters(FormatterRegistry registry) {
			DateTimeFormatterRegistrar registrar = new DateTimeFormatterRegistrar();
			registrar.setUseIsoFormat(true);
			registrar.registerFormatters(registry);
      	}
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@Configuration
	@EnableWebMvc
	class WebConfig : WebMvcConfigurer {

		override fun addFormatters(registry: FormatterRegistry) {
			val registrar = DateTimeFormatterRegistrar()
			registrar.setUseIsoFormat(true)
			registrar.registerFormatters(registry)
		}
	}
----

NOTE: See <<core.adoc#format-FormatterRegistrar-SPI, the `FormatterRegistrar` SPI>>
and the `FormattingConversionServiceFactoryBean` for more information on when to use
FormatterRegistrar implementations.



[[mvc-config-validation]]
=== Validation
[.small]#<<web-reactive.adoc#webflux-config-validation, WebFlux>>#

By default, if <<core.adoc#validation-beanvalidation-overview, Bean Validation>> is present
on the classpath (for example, Hibernate Validator), the `LocalValidatorFactoryBean` is
registered as a global <<core.adoc#validator, Validator>> for use with `@Valid` and
`Validated` on controller method arguments.

In Java configuration, you can customize the global `Validator` instance, as the
following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Configuration
	@EnableWebMvc
	public class WebConfig implements WebMvcConfigurer {

		@Override
		public Validator getValidator() {
			// ...
		}
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@Configuration
	@EnableWebMvc
	class WebConfig : WebMvcConfigurer {

		override fun getValidator(): Validator {
			// ...
		}
	}
----

The following example shows how to achieve the same configuration in XML:

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<?xml version="1.0" encoding="UTF-8"?>
	<beans xmlns="http://www.springframework.org/schema/beans"
		xmlns:mvc="http://www.springframework.org/schema/mvc"
		xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
		xsi:schemaLocation="
			http://www.springframework.org/schema/beans
			https://www.springframework.org/schema/beans/spring-beans.xsd
			http://www.springframework.org/schema/mvc
			https://www.springframework.org/schema/mvc/spring-mvc.xsd">

		<mvc:annotation-driven validator="globalValidator"/>

	</beans>
----

Note that you can also register `Validator` implementations locally, as the following
example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Controller
	public class MyController {

		@InitBinder
		protected void initBinder(WebDataBinder binder) {
			binder.addValidators(new FooValidator());
		}
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@Controller
	class MyController {

		@InitBinder
		protected fun initBinder(binder: WebDataBinder) {
			binder.addValidators(FooValidator())
		}
	}
----

TIP: If you need to have a `LocalValidatorFactoryBean` injected somewhere, create a bean and
mark it with `@Primary` in order to avoid conflict with the one declared in the MVC configuration.



[[mvc-config-interceptors]]
=== Interceptors

In Java configuration, you can register interceptors to apply to incoming requests, as
the following example shows:

[source,java,indent=0,subs="verbatim",role="primary"]
.Java
----
	@Configuration
	@EnableWebMvc
	public class WebConfig implements WebMvcConfigurer {

		@Override
		public void addInterceptors(InterceptorRegistry registry) {
			registry.addInterceptor(new LocaleChangeInterceptor());
			registry.addInterceptor(new ThemeChangeInterceptor()).addPathPatterns("/**").excludePathPatterns("/admin/**");
		}
	}
----
[source,kotlin,indent=0,subs="verbatim",role="secondary"]
.Kotlin
----
	@Configuration
	@EnableWebMvc
	class WebConfig : WebMvcConfigurer {

		override fun addInterceptors(registry: InterceptorRegistry) {
			registry.addInterceptor(LocaleChangeInterceptor())
			registry.addInterceptor(ThemeChangeInterceptor()).addPathPatterns("/**").excludePathPatterns("/admin/**")
		}
	}
----

The following example shows how to achieve the same configuration in XML:

[source,xml,indent=0,subs="verbatim"]
----
	<mvc:interceptors>
		<bean class="org.springframework.web.servlet.i18n.LocaleChangeInterceptor"/>
		<mvc:interceptor>
			<mvc:mapping path="/**"/>
			<mvc:exclude-mapping path="/admin/**"/>
			<bean class="org.springframework.web.servlet.theme.ThemeChangeInterceptor"/>
		</mvc:interceptor>
	</mvc:interceptors>
----

NOTE: Mapped interceptors are not ideally suited as a security layer due to the potential
for a mismatch with annotated controller path matching, which can also match trailing
slashes and path extensions transparently, along with other path matching options. Many
of these options have been deprecated but the potential for a mismatch remains.
Generally, we recommend using Spring Security which includes a dedicated
https://docs.spring.io/spring-security/reference/servlet/integrations/mvc.html#mvc-requestmatcher[MvcRequestMatcher]
to align with Spring MVC path matching and also has a security firewall that blocks many
unwanted characters in URL paths.




[[mvc-config-content-negotiation]]
=== Content Types
[.small]#<<web-reactive.adoc#webflux-config-content-negotiation, WebFlux>>#

You can configure how Spring MVC determines the requested media types from the request
(for example, `Accept` header, URL path extension, query parameter, and others).

By default, only the `Accept` header is checked.

If you must use URL-based content type resolution, consider using the query parameter
strategy over path extensions. See
<<mvc-ann-requestmapping-suffix-pattern-match>> and <<mvc-ann-requestmapping-rfd>> for
more details.

In Java configuration, you can customize requested content type resolution, as the
following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Configuration
	@EnableWebMvc
	public class WebConfig implements WebMvcConfigurer {

		@Override
		public void configureContentNegotiation(ContentNegotiationConfigurer configurer) {
			configurer.mediaType("json", MediaType.APPLICATION_JSON);
			configurer.mediaType("xml", MediaType.APPLICATION_XML);
		}
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@Configuration
	@EnableWebMvc
	class WebConfig : WebMvcConfigurer {

		override fun configureContentNegotiation(configurer: ContentNegotiationConfigurer) {
			configurer.mediaType("json", MediaType.APPLICATION_JSON)
			configurer.mediaType("xml", MediaType.APPLICATION_XML)
		}
	}
----


The following example shows how to achieve the same configuration in XML:

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<mvc:annotation-driven content-negotiation-manager="contentNegotiationManager"/>

	<bean id="contentNegotiationManager" class="org.springframework.web.accept.ContentNegotiationManagerFactoryBean">
		<property name="mediaTypes">
			<value>
				json=application/json
				xml=application/xml
			</value>
		</property>
	</bean>
----



[[mvc-config-message-converters]]
=== Message Converters
[.small]#<<web-reactive.adoc#webflux-config-message-codecs, WebFlux>>#

You can customize `HttpMessageConverter` in Java configuration by overriding
{api-spring-framework}/web/servlet/config/annotation/WebMvcConfigurer.html#configureMessageConverters-java.util.List-[`configureMessageConverters()`]
(to replace the default converters created by Spring MVC) or by overriding
{api-spring-framework}/web/servlet/config/annotation/WebMvcConfigurer.html#extendMessageConverters-java.util.List-[`extendMessageConverters()`]
(to customize the default converters or add additional converters to the default ones).

The following example adds XML and Jackson JSON converters with a customized
`ObjectMapper` instead of the default ones:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Configuration
	@EnableWebMvc
	public class WebConfiguration implements WebMvcConfigurer {

		@Override
		public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
			Jackson2ObjectMapperBuilder builder = new Jackson2ObjectMapperBuilder()
					.indentOutput(true)
					.dateFormat(new SimpleDateFormat("yyyy-MM-dd"))
					.modulesToInstall(new ParameterNamesModule());
			converters.add(new MappingJackson2HttpMessageConverter(builder.build()));
			converters.add(new MappingJackson2XmlHttpMessageConverter(builder.createXmlMapper(true).build()));
		}
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@Configuration
	@EnableWebMvc
	class WebConfiguration : WebMvcConfigurer {

		override fun configureMessageConverters(converters: MutableList<HttpMessageConverter<*>>) {
			val builder = Jackson2ObjectMapperBuilder()
					.indentOutput(true)
					.dateFormat(SimpleDateFormat("yyyy-MM-dd"))
					.modulesToInstall(ParameterNamesModule())
			converters.add(MappingJackson2HttpMessageConverter(builder.build()))
			converters.add(MappingJackson2XmlHttpMessageConverter(builder.createXmlMapper(true).build()))
----

In the preceding example,
{api-spring-framework}/http/converter/json/Jackson2ObjectMapperBuilder.html[`Jackson2ObjectMapperBuilder`]
is used to create a common configuration for both `MappingJackson2HttpMessageConverter` and
`MappingJackson2XmlHttpMessageConverter` with indentation enabled, a customized date format,
and the registration of
https://github.com/FasterXML/jackson-module-parameter-names[`jackson-module-parameter-names`],
Which adds support for accessing parameter names (a feature added in Java 8).

This builder customizes Jackson's default properties as follows:

* https://fasterxml.github.io/jackson-databind/javadoc/2.6/com/fasterxml/jackson/databind/DeserializationFeature.html#FAIL_ON_UNKNOWN_PROPERTIES[`DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES`] is disabled.
* https://fasterxml.github.io/jackson-databind/javadoc/2.6/com/fasterxml/jackson/databind/MapperFeature.html#DEFAULT_VIEW_INCLUSION[`MapperFeature.DEFAULT_VIEW_INCLUSION`] is disabled.

It also automatically registers the following well-known modules if they are detected on the classpath:

* https://github.com/FasterXML/jackson-datatype-joda[jackson-datatype-joda]: Support for Joda-Time types.
* https://github.com/FasterXML/jackson-datatype-jsr310[jackson-datatype-jsr310]: Support for Java 8 Date and Time API types.
* https://github.com/FasterXML/jackson-datatype-jdk8[jackson-datatype-jdk8]: Support for other Java 8 types, such as `Optional`.
* https://github.com/FasterXML/jackson-module-kotlin[`jackson-module-kotlin`]: Support for Kotlin classes and data classes.

NOTE: Enabling indentation with Jackson XML support requires
https://search.maven.org/#search%7Cgav%7C1%7Cg%3A%22org.codehaus.woodstox%22%20AND%20a%3A%22woodstox-core-asl%22[`woodstox-core-asl`]
dependency in addition to https://search.maven.org/#search%7Cga%7C1%7Ca%3A%22jackson-dataformat-xml%22[`jackson-dataformat-xml`] one.

Other interesting Jackson modules are available:

* https://github.com/zalando/jackson-datatype-money[jackson-datatype-money]: Support for `javax.money` types (unofficial module).
* https://github.com/FasterXML/jackson-datatype-hibernate[jackson-datatype-hibernate]: Support for Hibernate-specific types and properties (including lazy-loading aspects).

The following example shows how to achieve the same configuration in XML:

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<mvc:annotation-driven>
		<mvc:message-converters>
			<bean class="org.springframework.http.converter.json.MappingJackson2HttpMessageConverter">
				<property name="objectMapper" ref="objectMapper"/>
			</bean>
			<bean class="org.springframework.http.converter.xml.MappingJackson2XmlHttpMessageConverter">
				<property name="objectMapper" ref="xmlMapper"/>
			</bean>
		</mvc:message-converters>
	</mvc:annotation-driven>

	<bean id="objectMapper" class="org.springframework.http.converter.json.Jackson2ObjectMapperFactoryBean"
		  p:indentOutput="true"
		  p:simpleDateFormat="yyyy-MM-dd"
		  p:modulesToInstall="com.fasterxml.jackson.module.paramnames.ParameterNamesModule"/>

	<bean id="xmlMapper" parent="objectMapper" p:createXmlMapper="true"/>
----



[[mvc-config-view-controller]]
=== View Controllers

This is a shortcut for defining a `ParameterizableViewController` that immediately
forwards to a view when invoked. You can use it in static cases when there is no Java controller
logic to run before the view generates the response.

The following example of Java configuration forwards a request for `/` to a view called `home`:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Configuration
	@EnableWebMvc
	public class WebConfig implements WebMvcConfigurer {

		@Override
		public void addViewControllers(ViewControllerRegistry registry) {
			registry.addViewController("/").setViewName("home");
		}
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@Configuration
	@EnableWebMvc
	class WebConfig : WebMvcConfigurer {

		override fun addViewControllers(registry: ViewControllerRegistry) {
			registry.addViewController("/").setViewName("home")
		}
	}
----

The following example achieves the same thing as the preceding example, but with XML, by
using the `<mvc:view-controller>` element:

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<mvc:view-controller path="/" view-name="home"/>
----

If an `@RequestMapping` method is mapped to a URL for any HTTP method then a view
controller cannot be used to handle the same URL. This is because a match by URL to an
annotated controller is considered a strong enough indication of endpoint ownership so
that a 405 (METHOD_NOT_ALLOWED), a 415 (UNSUPPORTED_MEDIA_TYPE), or similar response can
be sent to the client to help with debugging. For this reason it is recommended to avoid
splitting URL handling across an annotated controller and a view controller.



[[mvc-config-view-resolvers]]
=== View Resolvers
[.small]#<<web-reactive.adoc#webflux-config-view-resolvers, WebFlux>>#

The MVC configuration simplifies the registration of view resolvers.

The following Java configuration example configures content negotiation view
resolution by using JSP and Jackson as a default `View` for JSON rendering:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Configuration
	@EnableWebMvc
	public class WebConfig implements WebMvcConfigurer {

		@Override
		public void configureViewResolvers(ViewResolverRegistry registry) {
			registry.enableContentNegotiation(new MappingJackson2JsonView());
			registry.jsp();
		}
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@Configuration
	@EnableWebMvc
	class WebConfig : WebMvcConfigurer {

		override fun configureViewResolvers(registry: ViewResolverRegistry) {
			registry.enableContentNegotiation(MappingJackson2JsonView())
			registry.jsp()
		}
	}
----


The following example shows how to achieve the same configuration in XML:

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<mvc:view-resolvers>
		<mvc:content-negotiation>
			<mvc:default-views>
				<bean class="org.springframework.web.servlet.view.json.MappingJackson2JsonView"/>
			</mvc:default-views>
		</mvc:content-negotiation>
		<mvc:jsp/>
	</mvc:view-resolvers>
----

Note, however, that FreeMarker, Tiles, Groovy Markup, and script templates also require
configuration of the underlying view technology.

The MVC namespace provides dedicated elements. The following example works with FreeMarker:

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<mvc:view-resolvers>
		<mvc:content-negotiation>
			<mvc:default-views>
				<bean class="org.springframework.web.servlet.view.json.MappingJackson2JsonView"/>
			</mvc:default-views>
		</mvc:content-negotiation>
		<mvc:freemarker cache="false"/>
	</mvc:view-resolvers>

	<mvc:freemarker-configurer>
		<mvc:template-loader-path location="/freemarker"/>
	</mvc:freemarker-configurer>
----

In Java configuration, you can add the respective `Configurer` bean,
as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Configuration
	@EnableWebMvc
	public class WebConfig implements WebMvcConfigurer {

		@Override
		public void configureViewResolvers(ViewResolverRegistry registry) {
			registry.enableContentNegotiation(new MappingJackson2JsonView());
			registry.freeMarker().cache(false);
		}

		@Bean
		public FreeMarkerConfigurer freeMarkerConfigurer() {
			FreeMarkerConfigurer configurer = new FreeMarkerConfigurer();
			configurer.setTemplateLoaderPath("/freemarker");
			return configurer;
		}
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@Configuration
	@EnableWebMvc
	class WebConfig : WebMvcConfigurer {

		override fun configureViewResolvers(registry: ViewResolverRegistry) {
			registry.enableContentNegotiation(MappingJackson2JsonView())
			registry.freeMarker().cache(false)
		}

		@Bean
		fun freeMarkerConfigurer() = FreeMarkerConfigurer().apply {
			setTemplateLoaderPath("/freemarker")
		}
	}
----



[[mvc-config-static-resources]]
=== Static Resources
[.small]#<<web-reactive.adoc#webflux-config-static-resources, WebFlux>>#

This option provides a convenient way to serve static resources from a list of
{api-spring-framework}/core/io/Resource.html[`Resource`]-based locations.

In the next example, given a request that starts with `/resources`, the relative path is
used to find and serve static resources relative to `/public` under the web application
root or on the classpath under `/static`. The resources are served with a one-year future
expiration to ensure maximum use of the browser cache and a reduction in HTTP requests
made by the browser. The `Last-Modified` information is deduced from `Resource#lastModified`
so that HTTP conditional requests are supported with `"Last-Modified"` headers.

The following listing shows how to do so with Java configuration:

[source,java,indent=0,subs="verbatim",role="primary"]
.Java
----
	@Configuration
	@EnableWebMvc
	public class WebConfig implements WebMvcConfigurer {

		@Override
		public void addResourceHandlers(ResourceHandlerRegistry registry) {
			registry.addResourceHandler("/resources/**")
					.addResourceLocations("/public", "classpath:/static/")
					.setCacheControl(CacheControl.maxAge(Duration.ofDays(365)));
		}
	}
----
[source,kotlin,indent=0,subs="verbatim",role="secondary"]
.Kotlin
----
	@Configuration
	@EnableWebMvc
	class WebConfig : WebMvcConfigurer {

		override fun addResourceHandlers(registry: ResourceHandlerRegistry) {
			registry.addResourceHandler("/resources/**")
					.addResourceLocations("/public", "classpath:/static/")
					.setCacheControl(CacheControl.maxAge(Duration.ofDays(365)))
		}
	}
----

The following example shows how to achieve the same configuration in XML:

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<mvc:resources mapping="/resources/**"
		location="/public, classpath:/static/"
		cache-period="31556926" />
----

See also
<<mvc-caching-static-resources, HTTP caching support for static resources>>.

The resource handler also supports a chain of
{api-spring-framework}/web/servlet/resource/ResourceResolver.html[`ResourceResolver`] implementations and
{api-spring-framework}/web/servlet/resource/ResourceTransformer.html[`ResourceTransformer`] implementations,
which you can use to create a toolchain for working with optimized resources.

You can use the `VersionResourceResolver` for versioned resource URLs based on an MD5 hash
computed from the content, a fixed application version, or other. A
`ContentVersionStrategy` (MD5 hash) is a good choice -- with some notable exceptions, such as
JavaScript resources used with a module loader.

The following example shows how to use `VersionResourceResolver` in Java configuration:

[source,java,indent=0,subs="verbatim",role="primary"]
.Java
----
	@Configuration
	@EnableWebMvc
	public class WebConfig implements WebMvcConfigurer {

		@Override
		public void addResourceHandlers(ResourceHandlerRegistry registry) {
			registry.addResourceHandler("/resources/**")
					.addResourceLocations("/public/")
					.resourceChain(true)
					.addResolver(new VersionResourceResolver().addContentVersionStrategy("/**"));
		}
	}
----
[source,kotlin,indent=0,subs="verbatim",role="secondary"]
.Kotlin
----
	@Configuration
	@EnableWebMvc
	class WebConfig : WebMvcConfigurer {

		override fun addResourceHandlers(registry: ResourceHandlerRegistry) {
			registry.addResourceHandler("/resources/**")
					.addResourceLocations("/public/")
					.resourceChain(true)
					.addResolver(VersionResourceResolver().addContentVersionStrategy("/**"))
		}
	}
----

The following example shows how to achieve the same configuration in XML:

[source,xml,indent=0,subs="verbatim"]
----
<mvc:resources mapping="/resources/**" location="/public/">
	<mvc:resource-chain resource-cache="true">
		<mvc:resolvers>
			<mvc:version-resolver>
				<mvc:content-version-strategy patterns="/**"/>
			</mvc:version-resolver>
		</mvc:resolvers>
	</mvc:resource-chain>
</mvc:resources>
----

You can then use `ResourceUrlProvider` to rewrite URLs and apply the full chain of resolvers and
transformers -- for example, to insert versions. The MVC configuration provides a `ResourceUrlProvider`
bean so that it can be injected into others. You can also make the rewrite transparent with the
`ResourceUrlEncodingFilter` for Thymeleaf, JSPs, FreeMarker, and others with URL tags that
rely on `HttpServletResponse#encodeURL`.

Note that, when using both `EncodedResourceResolver` (for example, for serving gzipped or
brotli-encoded resources) and `VersionResourceResolver`, you must register them in this order.
That ensures content-based versions are always computed reliably, based on the unencoded file.

For https://www.webjars.org/documentation[WebJars], versioned URLs like
`/webjars/jquery/1.2.0/jquery.min.js` are the recommended and most efficient way to use them.
The related resource location is configured out of the box with Spring Boot (or can be configured
manually via `ResourceHandlerRegistry`) and does not require to add the
`org.webjars:webjars-locator-core` dependency.

Version-less URLs like `/webjars/jquery/jquery.min.js` are supported through the
`WebJarsResourceResolver` which is automatically registered when the
`org.webjars:webjars-locator-core` library is present on the classpath, at the cost of a
classpath scanning that could slow down application startup. The resolver can re-write URLs to
include the version of the jar and can also match against incoming URLs without versions
-- for example, from `/webjars/jquery/jquery.min.js` to `/webjars/jquery/1.2.0/jquery.min.js`.

TIP: The Java configuration based on `ResourceHandlerRegistry` provides further options
for fine-grained control, e.g. last-modified behavior and optimized resource resolution.



[[mvc-default-servlet-handler]]
=== Default Servlet

Spring MVC allows for mapping the `DispatcherServlet` to `/` (thus overriding the mapping
of the container's default Servlet), while still allowing static resource requests to be
handled by the container's default Servlet. It configures a
`DefaultServletHttpRequestHandler` with a URL mapping of `/**` and the lowest priority
relative to other URL mappings.

This handler forwards all requests to the default Servlet. Therefore, it must
remain last in the order of all other URL `HandlerMappings`. That is the
case if you use `<mvc:annotation-driven>`. Alternatively, if you set up your
own customized `HandlerMapping` instance, be sure to set its `order` property to a value
lower than that of the `DefaultServletHttpRequestHandler`, which is `Integer.MAX_VALUE`.

The following example shows how to enable the feature by using the default setup:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Configuration
	@EnableWebMvc
	public class WebConfig implements WebMvcConfigurer {

		@Override
		public void configureDefaultServletHandling(DefaultServletHandlerConfigurer configurer) {
			configurer.enable();
		}
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@Configuration
	@EnableWebMvc
	class WebConfig : WebMvcConfigurer {

		override fun configureDefaultServletHandling(configurer: DefaultServletHandlerConfigurer) {
			configurer.enable()
		}
	}
----

The following example shows how to achieve the same configuration in XML:

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<mvc:default-servlet-handler/>
----

The caveat to overriding the `/` Servlet mapping is that the `RequestDispatcher` for the
default Servlet must be retrieved by name rather than by path. The
`DefaultServletHttpRequestHandler` tries to auto-detect the default Servlet for
the container at startup time, using a list of known names for most of the major Servlet
containers (including Tomcat, Jetty, GlassFish, JBoss, Resin, WebLogic, and WebSphere).
If the default Servlet has been custom-configured with a different name, or if a
different Servlet container is being used where the default Servlet name is unknown,
then you must explicitly provide the default Servlet's name, as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Configuration
	@EnableWebMvc
	public class WebConfig implements WebMvcConfigurer {

		@Override
		public void configureDefaultServletHandling(DefaultServletHandlerConfigurer configurer) {
			configurer.enable("myCustomDefaultServlet");
		}
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@Configuration
	@EnableWebMvc
	class WebConfig : WebMvcConfigurer {

		override fun configureDefaultServletHandling(configurer: DefaultServletHandlerConfigurer) {
			configurer.enable("myCustomDefaultServlet")
		}
	}
----


The following example shows how to achieve the same configuration in XML:

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<mvc:default-servlet-handler default-servlet-name="myCustomDefaultServlet"/>
----



[[mvc-config-path-matching]]
=== Path Matching
[.small]#<<web-reactive.adoc#webflux-config-path-matching, WebFlux>>#

You can customize options related to path matching and treatment of the URL.
For details on the individual options, see the
{api-spring-framework}/web/servlet/config/annotation/PathMatchConfigurer.html[`PathMatchConfigurer`] javadoc.

The following example shows how to customize path matching in Java configuration:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Configuration
	@EnableWebMvc
	public class WebConfig implements WebMvcConfigurer {

		@Override
		public void configurePathMatch(PathMatchConfigurer configurer) {
			configurer.addPathPrefix("/api", HandlerTypePredicate.forAnnotation(RestController.class));
		}

		private PathPatternParser patternParser() {
			// ...
		}
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@Configuration
	@EnableWebMvc
	class WebConfig : WebMvcConfigurer {

		override fun configurePathMatch(configurer: PathMatchConfigurer) {
			configurer.addPathPrefix("/api", HandlerTypePredicate.forAnnotation(RestController::class.java))
		}

		fun patternParser(): PathPatternParser {
			//...
		}
	}
----

The following example shows how to customize path matching in XML configuration:

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<mvc:annotation-driven>
		<mvc:path-matching
			path-helper="pathHelper"
			path-matcher="pathMatcher"/>
	</mvc:annotation-driven>

	<bean id="pathHelper" class="org.example.app.MyPathHelper"/>
	<bean id="pathMatcher" class="org.example.app.MyPathMatcher"/>
----



[[mvc-config-advanced-java]]
=== Advanced Java Config
[.small]#<<web-reactive.adoc#webflux-config-advanced-java, WebFlux>>#

`@EnableWebMvc` imports `DelegatingWebMvcConfiguration`, which:

* Provides default Spring configuration for Spring MVC applications
* Detects and delegates to `WebMvcConfigurer` implementations to customize that configuration.

For advanced mode, you can remove `@EnableWebMvc` and extend directly from
`DelegatingWebMvcConfiguration` instead of implementing `WebMvcConfigurer`,
as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Configuration
	public class WebConfig extends DelegatingWebMvcConfiguration {

		// ...
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@Configuration
	class WebConfig : DelegatingWebMvcConfiguration() {

		// ...
	}
----

You can keep existing methods in `WebConfig`, but you can now also override bean declarations
from the base class, and you can still have any number of other `WebMvcConfigurer` implementations on
the classpath.



[[mvc-config-advanced-xml]]
=== Advanced XML Config

The MVC namespace does not have an advanced mode. If you need to customize a property on
a bean that you cannot change otherwise, you can use the `BeanPostProcessor` lifecycle
hook of the Spring `ApplicationContext`, as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Component
	public class MyPostProcessor implements BeanPostProcessor {

		public Object postProcessBeforeInitialization(Object bean, String name) throws BeansException {
			// ...
		}
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@Component
	class MyPostProcessor : BeanPostProcessor {

		override fun postProcessBeforeInitialization(bean: Any, name: String): Any {
			// ...
		}
	}
----


Note that you need to declare `MyPostProcessor` as a bean, either explicitly in XML or
by letting it be detected through a `<component-scan/>` declaration.




[[mvc-http2]]
== HTTP/2
[.small]#<<web-reactive.adoc#webflux-http2, WebFlux>>#

Servlet 4 containers are required to support HTTP/2, and Spring Framework 5 is compatible
with Servlet API 4. From a programming model perspective, there is nothing specific that
applications need to do. However, there are considerations related to server configuration.
For more details, see the
https://github.com/spring-projects/spring-framework/wiki/HTTP-2-support[HTTP/2 wiki page].

The Servlet API does expose one construct related to HTTP/2. You can use the
`jakarta.servlet.http.PushBuilder` to proactively push resources to clients, and it
is supported as a <<mvc-ann-arguments, method argument>> to `@RequestMapping` methods.
