[[rest-client-access]]
= REST Clients

The Spring Framework provides the following choices for making calls to REST endpoints:

* <<rest-webclient>> - non-blocking, reactive client w fluent API.
* <<rest-resttemplate>> - synchronous client with template method API.
* <<rest-http-interface>> - annotated interface with generated, dynamic proxy implementation.


[[rest-webclient]]
== `WebClient`

`WebClient` is a non-blocking, reactive client to perform HTTP requests. It was
introduced in 5.0 and offers an alternative to the `RestTemplate`, with support for
synchronous, asynchronous, and streaming scenarios.

`WebClient` supports the following:

* Non-blocking I/O.
* Reactive Streams back pressure.
* High concurrency with fewer hardware resources.
* Functional-style, fluent API that takes advantage of Java 8 lambdas.
* Synchronous and asynchronous interactions.
* Streaming up to or streaming down from a server.

See <<web-reactive.adoc#webflux-client, WebClient>> for more details.




[[rest-resttemplate]]
== `RestTemplate`

The `RestTemplate` provides a higher level API over HTTP client libraries. It makes it
easy to invoke REST endpoints in a single line. It exposes the following groups of
overloaded methods:

NOTE: `RestTemplate` is in maintenance mode, with only requests for minor
changes and bugs to be accepted. Please, consider using the
<<web-reactive.adoc#webflux-client, WebClient>> instead.

[[rest-overview-of-resttemplate-methods-tbl]]
.RestTemplate methods
[cols="1,3"]
|===
| Method group | Description

| `getForObject`
| Retrieves a representation via GET.

| `getForEntity`
| Retrieves a `ResponseEntity` (that is, status, headers, and body) by using GET.

| `headForHeaders`
| Retrieves all headers for a resource by using HEAD.

| `postForLocation`
| Creates a new resource by using POST and returns the `Location` header from the response.

| `postForObject`
| Creates a new resource by using POST and returns the representation from the response.

| `postForEntity`
| Creates a new resource by using POST and returns the representation from the response.

| `put`
| Creates or updates a resource by using PUT.

| `patchForObject`
| Updates a resource by using PATCH and returns the representation from the response.
Note that the JDK `HttpURLConnection` does not support `PATCH`, but Apache
HttpComponents and others do.

| `delete`
| Deletes the resources at the specified URI by using DELETE.

| `optionsForAllow`
| Retrieves allowed HTTP methods for a resource by using ALLOW.

| `exchange`
| More generalized (and less opinionated) version of the preceding methods that provides extra
flexibility when needed. It accepts a `RequestEntity` (including HTTP method, URL, headers,
and body as input) and returns a `ResponseEntity`.

These methods allow the use of `ParameterizedTypeReference` instead of `Class` to specify
a response type with generics.

| `execute`
| The most generalized way to perform a request, with full control over request
preparation and response extraction through callback interfaces.

|===

[[rest-resttemplate-create]]
=== Initialization

The default constructor uses `java.net.HttpURLConnection` to perform requests. You can
switch to a different HTTP library with an implementation of `ClientHttpRequestFactory`.
There is built-in support for the following:

* Apache HttpComponents
* Netty
* OkHttp

For example, to switch to Apache HttpComponents, you can use the following:

[source,java,indent=0,subs="verbatim,quotes"]
----
	RestTemplate template = new RestTemplate(new HttpComponentsClientHttpRequestFactory());
----

Each `ClientHttpRequestFactory` exposes configuration options specific to the underlying
HTTP client library -- for example, for credentials, connection pooling, and other details.

TIP: Note that the `java.net` implementation for HTTP requests can raise an exception when
accessing the status of a response that represents an error (such as 401). If this is an
issue, switch to another HTTP client library.

NOTE: `RestTemplate` can be instrumented for observability, in order to produce metrics and traces.
See the <<integration.adoc#integration.observability.http-client.resttemplate,RestTemplate Observability support>> section.

[[rest-resttemplate-uri]]
==== URIs

Many of the `RestTemplate` methods accept a URI template and URI template variables,
either as a `String` variable argument, or as `Map<String,String>`.

The following example uses a `String` variable argument:

[source,java,indent=0,subs="verbatim,quotes"]
----
	String result = restTemplate.getForObject(
			"https://example.com/hotels/{hotel}/bookings/{booking}", String.class, "42", "21");
----

The following example uses a `Map<String, String>`:

[source,java,indent=0,subs="verbatim,quotes"]
----
	Map<String, String> vars = Collections.singletonMap("hotel", "42");

	String result = restTemplate.getForObject(
			"https://example.com/hotels/{hotel}/rooms/{hotel}", String.class, vars);
----

Keep in mind URI templates are automatically encoded, as the following example shows:

[source,java,indent=0,subs="verbatim,quotes"]
----
	restTemplate.getForObject("https://example.com/hotel list", String.class);

	// Results in request to "https://example.com/hotel%20list"
----

You can use the `uriTemplateHandler` property of `RestTemplate` to customize how URIs
are encoded. Alternatively, you can prepare a `java.net.URI` and pass it into one of
the `RestTemplate` methods that accepts a `URI`.

For more details on working with and encoding URIs, see <<web.adoc#mvc-uri-building, URI Links>>.

[[rest-template-headers]]
==== Headers

You can use the `exchange()` methods to specify request headers, as the following example shows:

[source,java,indent=0,subs="verbatim,quotes"]
----
	String uriTemplate = "https://example.com/hotels/{hotel}";
	URI uri = UriComponentsBuilder.fromUriString(uriTemplate).build(42);

	RequestEntity<Void> requestEntity = RequestEntity.get(uri)
			.header("MyRequestHeader", "MyValue")
			.build();

	ResponseEntity<String> response = template.exchange(requestEntity, String.class);

	String responseHeader = response.getHeaders().getFirst("MyResponseHeader");
	String body = response.getBody();
----

You can obtain response headers through many `RestTemplate` method variants that return
`ResponseEntity`.

[[rest-template-body]]
=== Body

Objects passed into and returned from `RestTemplate` methods are converted to and from raw
content with the help of an `HttpMessageConverter`.

On a POST, an input object is serialized to the request body, as the following example shows:

----
URI location = template.postForLocation("https://example.com/people", person);
----

You need not explicitly set the Content-Type header of the request. In most cases,
you can find a compatible message converter based on the source `Object` type, and the chosen
message converter sets the content type accordingly. If necessary, you can use the
`exchange` methods to explicitly provide the `Content-Type` request header, and that, in
turn, influences what message converter is selected.

On a GET, the body of the response is deserialized to an output `Object`, as the following example shows:

----
Person person = restTemplate.getForObject("https://example.com/people/{id}", Person.class, 42);
----

The `Accept` header of the request does not need to be explicitly set. In most cases,
a compatible message converter can be found based on the expected response type, which
then helps to populate the `Accept` header. If necessary, you can use the `exchange`
methods to provide the `Accept` header explicitly.

By default, `RestTemplate` registers all built-in
<<rest-message-conversion, message converters>>, depending on classpath checks that help
to determine what optional conversion libraries are present. You can also set the message
converters to use explicitly.

[[rest-message-conversion]]
==== Message Conversion
[.small]#<<web-reactive.adoc#webflux-codecs, See equivalent in the Reactive stack>>#

The `spring-web` module contains the `HttpMessageConverter` contract for reading and
writing the body of HTTP requests and responses through `InputStream` and `OutputStream`.
`HttpMessageConverter` instances are used on the client side (for example, in the `RestTemplate`) and
on the server side (for example, in Spring MVC REST controllers).

Concrete implementations for the main media (MIME) types are provided in the framework
and are, by default, registered with the `RestTemplate` on the client side and with
`RequestMappingHandlerAdapter` on the server side (see
<<web.adoc#mvc-config-message-converters, Configuring Message Converters>>).

The implementations of `HttpMessageConverter` are described in the following sections.
For all converters, a default media type is used, but you can override it by setting the
`supportedMediaTypes` bean property. The following table describes each implementation:

[[rest-message-converters-tbl]]
.HttpMessageConverter Implementations
[cols="1,3"]
|===
| MessageConverter | Description

| `StringHttpMessageConverter`
| An `HttpMessageConverter` implementation that can read and write `String` instances from the HTTP
request and response. By default, this converter supports all text media types
(`text/{asterisk}`) and writes with a `Content-Type` of `text/plain`.

| `FormHttpMessageConverter`
| An `HttpMessageConverter` implementation that can read and write form data from the HTTP
request and response. By default, this converter reads and writes the
`application/x-www-form-urlencoded` media type. Form data is read from and written into a
`MultiValueMap<String, String>`. The converter can also write (but not read) multipart
data read from a `MultiValueMap<String, Object>`. By default, `multipart/form-data` is
supported. As of Spring Framework 5.2, additional multipart subtypes can be supported for
writing form data. Consult the javadoc for `FormHttpMessageConverter` for further details.

| `ByteArrayHttpMessageConverter`
| An `HttpMessageConverter` implementation that can read and write byte arrays from the
HTTP request and response. By default, this converter supports all media types (`{asterisk}/{asterisk}`)
and writes with a `Content-Type` of `application/octet-stream`. You can override this
by setting the `supportedMediaTypes` property and overriding `getContentType(byte[])`.

| `MarshallingHttpMessageConverter`
| An `HttpMessageConverter` implementation that can read and write XML by using Spring's
`Marshaller` and `Unmarshaller` abstractions from the `org.springframework.oxm` package.
This converter requires a `Marshaller` and `Unmarshaller` before it can be used. You can inject these
through constructor or bean properties. By default, this converter supports
`text/xml` and `application/xml`.

| `MappingJackson2HttpMessageConverter`
| An `HttpMessageConverter` implementation that can read and write JSON by using Jackson's
`ObjectMapper`. You can customize JSON mapping as needed through the use of Jackson's
provided annotations. When you need further control (for cases where custom JSON
serializers/deserializers need to be provided for specific types), you can inject a custom `ObjectMapper`
through the `ObjectMapper` property. By default, this
converter supports `application/json`.

| `MappingJackson2XmlHttpMessageConverter`
| An `HttpMessageConverter` implementation that can read and write XML by using
https://github.com/FasterXML/jackson-dataformat-xml[Jackson XML] extension's
`XmlMapper`. You can customize XML mapping as needed through the use of JAXB
or Jackson's provided annotations. When you need further control (for cases where custom XML
serializers/deserializers need to be provided for specific types), you can inject a custom `XmlMapper`
through the `ObjectMapper` property. By default, this
converter supports `application/xml`.

| `SourceHttpMessageConverter`
| An `HttpMessageConverter` implementation that can read and write
`javax.xml.transform.Source` from the HTTP request and response. Only `DOMSource`,
`SAXSource`, and `StreamSource` are supported. By default, this converter supports
`text/xml` and `application/xml`.

| `BufferedImageHttpMessageConverter`
| An `HttpMessageConverter` implementation that can read and write
`java.awt.image.BufferedImage` from the HTTP request and response. This converter reads
and writes the media type supported by the Java I/O API.

|===

[[rest-template-jsonview]]
=== Jackson JSON Views

You can specify a https://www.baeldung.com/jackson-json-view-annotation[Jackson JSON View]
to serialize only a subset of the object properties, as the following example shows:

[source,java,indent=0,subs="verbatim,quotes"]
----
	MappingJacksonValue value = new MappingJacksonValue(new User("eric", "7!jd#h23"));
	value.setSerializationView(User.WithoutPasswordView.class);

	RequestEntity<MappingJacksonValue> requestEntity =
		RequestEntity.post(new URI("https://example.com/user")).body(value);

	ResponseEntity<String> response = template.exchange(requestEntity, String.class);
----

[[rest-template-multipart]]
=== Multipart

To send multipart data, you need to provide a `MultiValueMap<String, Object>` whose values
may be an `Object` for part content, a `Resource` for a file part, or an `HttpEntity` for
part content with headers. For example:

[source,java,indent=0,subs="verbatim,quotes"]
----
	MultiValueMap<String, Object> parts = new LinkedMultiValueMap<>();

	parts.add("fieldPart", "fieldValue");
	parts.add("filePart", new FileSystemResource("...logo.png"));
	parts.add("jsonPart", new Person("Jason"));

	HttpHeaders headers = new HttpHeaders();
	headers.setContentType(MediaType.APPLICATION_XML);
	parts.add("xmlPart", new HttpEntity<>(myBean, headers));
----

In most cases, you do not have to specify the `Content-Type` for each part. The content
type is determined automatically based on the `HttpMessageConverter` chosen to serialize
it or, in the case of a `Resource` based on the file extension. If necessary, you can
explicitly provide the `MediaType` with an `HttpEntity` wrapper.

Once the `MultiValueMap` is ready, you can pass it to the `RestTemplate`, as show below:

[source,java,indent=0,subs="verbatim,quotes"]
----
	MultiValueMap<String, Object> parts = ...;
	template.postForObject("https://example.com/upload", parts, Void.class);
----

If the `MultiValueMap` contains at least one non-`String` value, the `Content-Type` is set
to `multipart/form-data` by the `FormHttpMessageConverter`. If the `MultiValueMap` has
`String` values the `Content-Type` is defaulted to `application/x-www-form-urlencoded`.
If necessary the `Content-Type` may also be set explicitly.


[[rest-http-interface]]
== HTTP Interface

The Spring Framework lets you define an HTTP service as a Java interface with annotated
methods for HTTP exchanges. You can then generate a proxy that implements this interface
and performs the exchanges. This helps to simplify HTTP remote access which often
involves a facade that wraps the details of using the underlying HTTP client.

One, declare an interface with `@HttpExchange` methods:

[source,java,indent=0,subs="verbatim,quotes"]
----
	interface RepositoryService {

		@GetExchange("/repos/{owner}/{repo}")
		Repository getRepository(@PathVariable String owner, @PathVariable String repo);

		// more HTTP exchange methods...

	}
----

Two, create a proxy that will perform the declared HTTP exchanges:

[source,java,indent=0,subs="verbatim,quotes"]
----
	WebClient client = WebClient.builder().baseUrl("https://api.github.com/").build();
	HttpServiceProxyFactory factory = HttpServiceProxyFactory.builder(WebClientAdapter.forClient(client)).build();

	RepositoryService service = factory.createClient(RepositoryService.class);
----

`@HttpExchange` is supported at the type level where it applies to all methods:

[source,java,indent=0,subs="verbatim,quotes"]
----
	@HttpExchange(url = "/repos/{owner}/{repo}", accept = "application/vnd.github.v3+json")
	interface RepositoryService {

		@GetExchange
		Repository getRepository(@PathVariable String owner, @PathVariable String repo);

		@PatchExchange(contentType = MediaType.APPLICATION_FORM_URLENCODED_VALUE)
		void updateRepository(@PathVariable String owner, @PathVariable String repo,
				@RequestParam String name, @RequestParam String description, @RequestParam String homepage);

	}
----


[[rest-http-interface-method-parameters]]
=== Method Parameters

Annotated, HTTP exchange methods support flexible method signatures with the following
method parameters:

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

| `URI`
| Dynamically set the URL for the request, overriding the annotation's `url` attribute.

| `HttpMethod`
| Dynamically set the HTTP method for the request, overriding the annotation's `method` attribute

| `@RequestHeader`
| Add a request header or multiple headers. The argument may be a `Map<String, ?>` or
  `MultiValueMap<String, ?>` with multiple headers, a `Collection<?>` of values, or an
  individual value. Type conversion is supported for non-String values.

| `@PathVariable`
| Add a variable for expand a placeholder in the request URL. The argument may be a
  `Map<String, ?>` with multiple variables, or an individual value. Type conversion
  is supported for non-String values.

| `@RequestBody`
| Provide the body of the request either as an Object to be serialized, or a
  Reactive Streams `Publisher` such as `Mono`, `Flux`, or any other async type supported
  through the configured `ReactiveAdapterRegistry`.

| `@RequestParam`
| Add a request parameter or multiple parameters. The argument may be a `Map<String, ?>`
  or `MultiValueMap<String, ?>` with multiple parameters, a `Collection<?>` of values, or
  an individual value. Type conversion is supported for non-String values.

  When `"content-type"` is set to `"application/x-www-form-urlencoded"`, request
  parameters are encoded in the request body. Otherwise, they are added as URL query
  parameters.

| `@RequestPart`
| Add a request part, which may be a String (form field), `Resource` (file part),
  Object (entity to be encoded, e.g. as JSON), `HttpEntity` (part content and headers),
  a Spring `Part`, or Reactive Streams `Publisher` of any of the above.

| `@CookieValue`
| Add a cookie or multiple cookies. The argument may be a `Map<String, ?>` or
  `MultiValueMap<String, ?>` with multiple cookies, a `Collection<?>` of values, or an
  individual value. Type conversion is supported for non-String values.

|===


[[rest-http-interface-return-values]]
=== Return Values

Annotated, HTTP exchange methods support the following return values:

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

| `void`, `Mono<Void>`
| Perform the given request, and release the response content, if any.

| `HttpHeaders`, `Mono<HttpHeaders>`
| Perform the given request, release the response content, if any, and return the
  response headers.

| `<T>`, `Mono<T>`
| Perform the given request and decode the response content to the declared return type.

| `<T>`, `Flux<T>`
| Perform the given request and decode the response content to a stream of the declared
  element type.

| `ResponseEntity<Void>`, `Mono<ResponseEntity<Void>>`
| Perform the given request, and release the response content, if any, and return a
  `ResponseEntity` with the status and headers.

| `ResponseEntity<T>`, `Mono<ResponseEntity<T>>`
| Perform the given request, decode the response content to the declared return type, and
  return a `ResponseEntity` with the status, headers, and the decoded body.

| `Mono<ResponseEntity<Flux<T>>`
| Perform the given request, decode the response content to a stream of the declared
  element type, and return a `ResponseEntity` with the status, headers, and the decoded
  response body stream.

|===

TIP: You can also use any other async or reactive types registered in the
`ReactiveAdapterRegistry`.


[[rest-http-interface-exceptions]]
=== Exception Handling

By default, `WebClient` raises `WebClientResponseException` for 4xx and 5xx HTTP status
codes. To customize this, you can register a response status handler that applies to all
responses performed through the client:

[source,java,indent=0,subs="verbatim,quotes"]
----
	WebClient webClient = WebClient.builder()
			.defaultStatusHandler(HttpStatusCode::isError, resp -> ...)
			.build();

	WebClientAdapter clientAdapter = WebClientAdapter.forClient(webClient);
	HttpServiceProxyFactory factory = HttpServiceProxyFactory
			.builder(clientAdapter).build();
----

For more details and options, such as suppressing error status codes, see the Javadoc of
`defaultStatusHandler` in `WebClient.Builder`.
