[[cors]]
= CORS Support

== Introduction

For security reasons, browsers prohibit AJAX calls to resources residing outside the
current origin. For example, as you're checking your bank account in one tab, you
could have the evil.com website open in another tab. The scripts from evil.com should not
be able to make AJAX requests to your bank API (e.g., withdrawing money from your account!)
using your credentials.

http://en.wikipedia.org/wiki/Cross-origin_resource_sharing[Cross-origin resource sharing]
(CORS) is a http://www.w3.org/TR/cors/[W3C specification] implemented by
http://caniuse.com/#feat=cors[most browsers] that allows you to specify in a flexible
way what kind of cross domain requests are authorized, instead of using some less secured
and less powerful hacks like IFRAME or JSONP.

As of Spring Framework 4.2, CORS is supported out of the box. CORS requests
(https://github.com/spring-projects/spring-framework/blob/master/spring-webmvc/src/main/java/org/springframework/web/servlet/FrameworkServlet.java#L906[including preflight ones with an `OPTIONS` method])
are automatically dispatched to the various registered ++HandlerMapping++s. They handle
CORS preflight requests and intercept CORS simple and actual requests thanks to a
http://docs.spring.io/spring/docs/4.2.x/javadoc-api/org/springframework/web/cors/CorsProcessor.html[CorsProcessor]
implementation (https://github.com/spring-projects/spring-framework/blob/master/spring-web/src/main/java/org/springframework/web/cors/DefaultCorsProcessor.java[DefaultCorsProcessor]
by default) in order to add the relevant CORS response headers (like `Access-Control-Allow-Origin`)
based on the CORS configuration you have provided.

[NOTE]
====
Since CORS requests are automatically dispatched, you *do not need* to change the
`DispatcherServlet` `dispatchOptionsRequest` init parameter value; using its default value
(`false`) is the recommended approach.
====

== Controller method CORS configuration

You can add an
http://docs.spring.io/spring/docs/4.2.x/javadoc-api/org/springframework/web/bind/annotation/CrossOrigin.html[`@CrossOrigin`]
annotation to your `@RequestMapping` annotated handler method in order to enable CORS on
it. By default `@CrossOrigin` allows all origins and the HTTP methods specified in the
`@RequestMapping` annotation:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
@RestController
@RequestMapping("/account")
public class AccountController {

	@CrossOrigin
	@RequestMapping("/{id}")
	public Account retrieve(@PathVariable Long id) {
		// ...
	}

	@RequestMapping(method = RequestMethod.DELETE, path = "/{id}")
	public void remove(@PathVariable Long id) {
		// ...
	}
}
----

It is also possible to enable CORS for the whole controller:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
@CrossOrigin(origins = "http://domain2.com", maxAge = 3600)
@RestController
@RequestMapping("/account")
public class AccountController {

	@RequestMapping("/{id}")
	public Account retrieve(@PathVariable Long id) {
		// ...
	}

	@RequestMapping(method = RequestMethod.DELETE, path = "/{id}")
	public void remove(@PathVariable Long id) {
		// ...
	}
}
----

In the above example CORS support is enabled for both the `retrieve()` and the `remove()`
handler methods, and you can also see how you can customize the CORS configuration using
`@CrossOrigin` attributes.

You can even use both controller-level and method-level CORS configurations; Spring will
then combine attributes from both annotations to create merged CORS configuration.

[source,java,indent=0]
[subs="verbatim,quotes"]
----
@CrossOrigin(maxAge = 3600)
@RestController
@RequestMapping("/account")
public class AccountController {

	@CrossOrigin("http://domain2.com")
	@RequestMapping("/{id}")
	public Account retrieve(@PathVariable Long id) {
		// ...
	}

	@RequestMapping(method = RequestMethod.DELETE, path = "/{id}")
	public void remove(@PathVariable Long id) {
		// ...
	}
}
----

== Global CORS configuration

In addition to fine-grained, annotation-based configuration you'll probably want to
define some global CORS configuration as well. This is similar to using filters but can
be declared within Spring MVC and combined with fine-grained `@CrossOrigin` configuration.
By default all origins and `GET`, `HEAD`, and `POST` methods are allowed.

=== JavaConfig

Enabling CORS for the whole application is as simple as:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
@Configuration
@EnableWebMvc
public class WebConfig extends WebMvcConfigurerAdapter {

	@Override
	public void addCorsMappings(CorsRegistry registry) {
		registry.addMapping("/**");
	}
}
----

You can easily change any properties, as well as only apply this CORS configuration to a
specific path pattern:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
@Configuration
@EnableWebMvc
public class WebConfig extends WebMvcConfigurerAdapter {

	@Override
	public void addCorsMappings(CorsRegistry registry) {
		registry.addMapping("/api/**")
			.allowedOrigins("http://domain2.com")
			.allowedMethods("PUT", "DELETE")
			.allowedHeaders("header1", "header2", "header3")
			.exposedHeaders("header1", "header2")
			.allowCredentials(false).maxAge(3600);
	}
}
----

=== XML namespace

The following minimal XML configuration enables CORS for the `/**` path pattern with
the same default properties as with the aforementioned JavaConfig examples:

[source,xml,indent=0]
[subs="verbatim"]
----
<mvc:cors>
	<mvc:mapping path="/**" />
</mvc:cors>
----

It is also possible to declare several CORS mappings with customized properties:

[source,xml,indent=0]
[subs="verbatim"]
----
<mvc:cors>

	<mvc:mapping path="/api/**"
		allowed-origins="http://domain1.com, http://domain2.com"
		allowed-methods="GET, PUT"
		allowed-headers="header1, header2, header3"
		exposed-headers="header1, header2" allow-credentials="false"
		max-age="123" />

	<mvc:mapping path="/resources/**"
		allowed-origins="http://domain1.com" />

</mvc:cors>
----

== Advanced Customization

http://docs.spring.io/spring/docs/4.2.x/javadoc-api/org/springframework/web/cors/CorsConfiguration.html[CorsConfiguration]
allows you to specify how the CORS requests should be processed: allowed origins, headers, methods, etc.
It can be provided in various ways:

 * http://docs.spring.io/spring/docs/4.2.x/javadoc-api/org/springframework/web/servlet/handler/AbstractHandlerMapping.html#setCorsConfiguration-java.util.Map-[`AbstractHandlerMapping#setCorsConfiguration()`]
   allows to specify a `Map` with several http://docs.spring.io/spring/docs/4.2.x/javadoc-api/org/springframework/web/cors/CorsConfiguration.html[CorsConfiguration]
   instances mapped to path patterns like `/api/**`.
 * Subclasses can provide their own `CorsConfiguration` by overriding the
   `AbstractHandlerMapping#getCorsConfiguration(Object, HttpServletRequest)` method.
 * Handlers can implement the http://docs.spring.io/spring/docs/4.2.x/javadoc-api/org/springframework/web/cors/CorsConfigurationSource.html[`CorsConfigurationSource`]
   interface (like https://github.com/spring-projects/spring-framework/blob/master/spring-webmvc/src/main/java/org/springframework/web/servlet/resource/ResourceHttpRequestHandler.java[`ResourceHttpRequestHandler`]
   now does) in order to provide a http://docs.spring.io/spring/docs/4.2.x/javadoc-api/org/springframework/web/cors/CorsConfiguration.html[CorsConfiguration]
   instance for each request.