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

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

* xref:integration/rest-clients.adoc#rest-restclient[`RestClient`] - synchronous client with a fluent API.
* xref:integration/rest-clients.adoc#rest-webclient[`WebClient`] - non-blocking, reactive client with fluent API.
* xref:integration/rest-clients.adoc#rest-resttemplate[`RestTemplate`] - synchronous client with template method API.
* xref:integration/rest-clients.adoc#rest-http-interface[HTTP Interface] - annotated interface with generated, dynamic proxy implementation.


[[rest-restclient]]
== `RestClient`

The `RestClient` is a synchronous HTTP client that offers a modern, fluent API.
It offers an abstraction over HTTP libraries that allows for convenient conversion from a Java object to an HTTP request, and the creation of objects from an HTTP response.

=== Creating a `RestClient`

The `RestClient` is created using one of the static `create` methods.
You can also use `builder()` to get a builder with further options, such as specifying which HTTP library to use (see <<rest-request-factories>>) and which message converters to use (see <<rest-message-conversion>>), setting a default URI, default path variables, default request headers, or `uriBuilderFactory`, or registering interceptors and initializers.

Once created (or built), the `RestClient` can be used safely by multiple threads.

The following sample shows how to create a default `RestClient`, and how to build a custom one.

[tabs]
======
Java::
+
[source,java,indent=0,subs="verbatim"]
----
RestClient defaultClient = RestClient.create();

RestClient customClient = RestClient.builder()
  .requestFactory(new HttpComponentsClientHttpRequestFactory())
  .messageConverters(converters -> converters.add(new MyCustomMessageConverter()))
  .baseUrl("https://example.com")
  .defaultUriVariables(Map.of("variable", "foo"))
  .defaultHeader("My-Header", "Foo")
  .requestInterceptor(myCustomInterceptor)
  .requestInitializer(myCustomInitializer)
  .build();
----

Kotlin::
+
[source,kotlin,indent=0,subs="verbatim"]
----
val defaultClient = RestClient.create()

val customClient = RestClient.builder()
  .requestFactory(HttpComponentsClientHttpRequestFactory())
  .messageConverters { converters -> converters.add(MyCustomMessageConverter()) }
  .baseUrl("https://example.com")
  .defaultUriVariables(mapOf("variable" to "foo"))
  .defaultHeader("My-Header", "Foo")
  .requestInterceptor(myCustomInterceptor)
  .requestInitializer(myCustomInitializer)
  .build()
----
======

=== Using the `RestClient`

When making an HTTP request with the `RestClient`, the first thing to specify is which HTTP method to use.
This can be done with `method(HttpMethod)` or with the convenience methods `get()`, `head()`, `post()`, and so on.

==== Request URL

Next, the request URI can be specified with the `uri` methods.
This step is optional and can be skipped if the `RestClient` is configured with a default URI.
The URL is typically specified as a `String`, with optional URI template variables.
The following example configures a GET request to `https://example.com/orders/42`:

[tabs]
======
Java::
+
[source,java,indent=0,subs="verbatim,quotes"]
----
int id = 42;
restClient.get()
  .uri("https://example.com/orders/{id}", id)
  ....
----

Kotlin::
+
[source,kotlin,indent=0,subs="verbatim,quotes"]
----
val id = 42
restClient.get()
  .uri("https://example.com/orders/{id}", id)
  ...
----
======

A function can also be used for more controls, such as specifying xref:web/webmvc/mvc-uri-building.adoc[request parameters].

String URLs are encoded by default, but this can be changed by building a client with a custom `uriBuilderFactory`.
The URL can also be provided with a function or as a `java.net.URI`, both of which are not encoded.
For more details on working with and encoding URIs, see xref:web/webmvc/mvc-uri-building.adoc[URI Links].

==== Request headers and body

If necessary, the HTTP request can be manipulated by adding request headers with `header(String, String)`, `headers(Consumer<HttpHeaders>`, or with the convenience methods `accept(MediaType...)`, `acceptCharset(Charset...)` and so on.
For HTTP requests that can contain a body (`POST`, `PUT`, and `PATCH`), additional methods are available: `contentType(MediaType)`, and `contentLength(long)`.

The request body itself can be set by `body(Object)`, which internally uses <<rest-message-conversion>>.
Alternatively, the request body can be set using a `ParameterizedTypeReference`, allowing you to use generics.
Finally, the body can be set to a callback function that writes to an `OutputStream`.

==== Retrieving the response

Once the request has been set up, the HTTP response is accessed by invoking `retrieve()`.
The response body can be accessed by using `body(Class)` or `body(ParameterizedTypeReference)` for parameterized types like lists.
The `body` method converts the response contents into various types – for instance, bytes can be converted into a `String`, JSON can be converted into objects using Jackson, and so on (see <<rest-message-conversion>>).

The response can also be converted into a `ResponseEntity`, giving access to the response headers as well as the body.

This sample shows how `RestClient` can be used to perform a simple `GET` request.

[tabs]
======
Java::
+
[source,java,indent=0,subs="verbatim,quotes"]
----
String result = restClient.get() <1>
  .uri("https://example.com") <2>
  .retrieve() <3>
  .body(String.class); <4>

System.out.println(result); <5>
----
<1> Set up a GET request
<2> Specify the URL to connect to
<3> Retrieve the response
<4> Convert the response into a string
<5> Print the result

Kotlin::
+
[source,kotlin,indent=0,subs="verbatim,quotes"]
----
val result= restClient.get() <1>
  .uri("https://example.com") <2>
  .retrieve() <3>
  .body<String>() <4>

println(result) <5>
----
<1> Set up a GET request
<2> Specify the URL to connect to
<3> Retrieve the response
<4> Convert the response into a string
<5> Print the result
======

Access to the response status code and headers is provided through `ResponseEntity`:

[tabs]
======
Java::
+
[source,java,indent=0,subs="verbatim,quotes"]
----
ResponseEntity<String> result = restClient.get() <1>
  .uri("https://example.com") <1>
  .retrieve()
  .toEntity(String.class); <2>

System.out.println("Response status: " + result.getStatusCode()); <3>
System.out.println("Response headers: " + result.getHeaders()); <3>
System.out.println("Contents: " + result.getBody()); <3>
----
<1> Set up a GET request for the specified URL
<2> Convert the response into a `ResponseEntity`
<3> Print the result

Kotlin::
+
[source,kotlin,indent=0,subs="verbatim,quotes"]
----
val result = restClient.get() <1>
  .uri("https://example.com") <1>
  .retrieve()
  .toEntity<String>() <2>

println("Response status: " + result.statusCode) <3>
println("Response headers: " + result.headers) <3>
println("Contents: " + result.body) <3>
----
<1> Set up a GET request for the specified URL
<2> Convert the response into a `ResponseEntity`
<3> Print the result
======

`RestClient` can convert JSON to objects, using the Jackson library.
Note the usage of URI variables in this sample and that the `Accept` header is set to JSON.

[tabs]
======
Java::
+
[source,java,indent=0,subs="verbatim,quotes"]
----
int id = ...;
Pet pet = restClient.get()
  .uri("https://petclinic.example.com/pets/{id}", id) <1>
  .accept(APPLICATION_JSON) <2>
  .retrieve()
  .body(Pet.class); <3>
----
<1> Using URI variables
<2> Set the `Accept` header to `application/json`
<3> Convert the JSON response into a `Pet` domain object

Kotlin::
+
[source,kotlin,indent=0,subs="verbatim,quotes"]
----
val id = ...
val pet = restClient.get()
  .uri("https://petclinic.example.com/pets/{id}", id) <1>
  .accept(APPLICATION_JSON) <2>
  .retrieve()
  .body<Pet>() <3>
----
<1> Using URI variables
<2> Set the `Accept` header to `application/json`
<3> Convert the JSON response into a `Pet` domain object
======

In the next sample, `RestClient` is used to perform a POST request that contains JSON, which again is converted using Jackson.

[tabs]
======
Java::
+
[source,java,indent=0,subs="verbatim,quotes"]
----
Pet pet = ... <1>
ResponseEntity<Void> response = restClient.post() <2>
  .uri("https://petclinic.example.com/pets/new") <2>
  .contentType(APPLICATION_JSON) <3>
  .body(pet) <4>
  .retrieve()
  .toBodilessEntity(); <5>
----
<1> Create a `Pet` domain object
<2> Set up a POST request, and the URL to connect to
<3> Set the `Content-Type` header to `application/json`
<4> Use `pet` as the request body
<5> Convert the response into a response entity with no body.

Kotlin::
+
[source,kotlin,indent=0,subs="verbatim,quotes"]
----
val pet: Pet = ... <1>
val response = restClient.post() <2>
  .uri("https://petclinic.example.com/pets/new") <2>
  .contentType(APPLICATION_JSON) <3>
  .body(pet) <4>
  .retrieve()
  .toBodilessEntity() <5>
----
<1> Create a `Pet` domain object
<2> Set up a POST request, and the URL to connect to
<3> Set the `Content-Type` header to `application/json`
<4> Use `pet` as the request body
<5> Convert the response into a response entity with no body.
======

==== Error handling

By default, `RestClient` throws a subclass of `RestClientException` when retrieving a response with a 4xx or 5xx status code.
This behavior can be overridden using `onStatus`.

[tabs]
======
Java::
+
[source,java,indent=0,subs="verbatim,quotes"]
----
String result = restClient.get() <1>
  .uri("https://example.com/this-url-does-not-exist") <1>
  .retrieve()
  .onStatus(HttpStatusCode::is4xxClientError, (request, response) -> { <2>
      throw new MyCustomRuntimeException(response.getStatusCode(), response.getHeaders()); <3>
  })
  .body(String.class);
----
<1> Create a GET request for a URL that returns a 404 status code
<2> Set up a status handler for all 4xx status codes
<3> Throw a custom exception

Kotlin::
+
[source,kotlin,indent=0,subs="verbatim,quotes"]
----
val result = restClient.get() <1>
  .uri("https://example.com/this-url-does-not-exist") <1>
  .retrieve()
  .onStatus(HttpStatusCode::is4xxClientError) { _, response -> <2>
    throw MyCustomRuntimeException(response.getStatusCode(), response.getHeaders()) } <3>
  .body<String>()
----
<1> Create a GET request for a URL that returns a 404 status code
<2> Set up a status handler for all 4xx status codes
<3> Throw a custom exception
======

==== Exchange

For more advanced scenarios, the `RestClient` gives access to the underlying HTTP request and response through the `exchange()` method, which can be used instead of `retrieve()`.
Status handlers are not applied when use `exchange()`, because the exchange function already provides access to the full response, allowing you to perform any error handling necessary.

[tabs]
======
Java::
+
[source,java,indent=0,subs="verbatim,quotes"]
----
Pet result = restClient.get()
  .uri("https://petclinic.example.com/pets/{id}", id)
  .accept(APPLICATION_JSON)
  .exchange((request, response) -> { <1>
    if (response.getStatusCode().is4xxClientError()) { <2>
      throw new MyCustomRuntimeException(response.getStatusCode(), response.getHeaders()); <2>
    }
    else {
      Pet pet = convertResponse(response); <3>
      return pet;
    }
  });
----
<1> `exchange` provides the request and response
<2> Throw an exception when the response has a 4xx status code
<3> Convert the response into a Pet domain object

Kotlin::
+
[source,kotlin,indent=0,subs="verbatim,quotes"]
----
val result = restClient.get()
  .uri("https://petclinic.example.com/pets/{id}", id)
  .accept(MediaType.APPLICATION_JSON)
  .exchange { request, response -> <1>
    if (response.getStatusCode().is4xxClientError()) { <2>
      throw MyCustomRuntimeException(response.getStatusCode(), response.getHeaders()) <2>
    } else {
      val pet: Pet = convertResponse(response) <3>
      pet
    }
  }
----
<1> `exchange` provides the request and response
<2> Throw an exception when the response has a 4xx status code
<3> Convert the response into a Pet domain object
======


[[rest-message-conversion]]
=== HTTP Message Conversion

xref:web/webmvc/message-converters.adoc#message-converters[See the supported HTTP message converters in the dedicated section].

==== Jackson JSON Views

To serialize only a subset of the object properties, you can specify a {baeldung-blog}/jackson-json-view-annotation[Jackson JSON View], as the following example shows:

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

ResponseEntity<Void> response = restClient.post() // or RestTemplate.postForEntity
  .contentType(APPLICATION_JSON)
  .body(value)
  .retrieve()
  .toBodilessEntity();

----

==== 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"]
----
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));

// send using RestClient.post or RestTemplate.postForEntity
----

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 use it as the body of a `POST` request, using  `RestClient.post().body(parts)` (or `RestTemplate.postForObject`).

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` defaults to `application/x-www-form-urlencoded`.
If necessary the `Content-Type` may also be set explicitly.

[[rest-request-factories]]
=== Client Request Factories

To execute the HTTP request, `RestClient` uses a client HTTP library.
These libraries are adapted via the `ClientRequestFactory` interface.
Various implementations are available:

* `JdkClientHttpRequestFactory` for Java's `HttpClient`
* `HttpComponentsClientHttpRequestFactory` for use with Apache HTTP Components `HttpClient`
* `JettyClientHttpRequestFactory` for Jetty's `HttpClient`
* `ReactorNettyClientRequestFactory` for Reactor Netty's `HttpClient`
* `SimpleClientHttpRequestFactory` as a simple default


If no request factory is specified when the `RestClient` was built, it will use the Apache or Jetty `HttpClient` if they are available on the classpath.
Otherwise, if the `java.net.http` module is loaded, it will use Java's `HttpClient`.
Finally, it will resort to the simple default.

TIP: Note that the `SimpleClientHttpRequestFactory` may raise an exception when accessing the status of a response that represents an error (e.g. 401).
If this is an issue, use any of the alternative request factories.

[[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 xref:web/webflux-webclient.adoc[WebClient] for more details.




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

The `RestTemplate` provides a high-level API over HTTP client libraries in the form of a classic Spring Template class.
It exposes the following groups of overloaded methods:

NOTE: The xref:integration/rest-clients.adoc#rest-restclient[`RestClient`] offers a more modern API for synchronous HTTP access.
For asynchronous and streaming scenarios, consider the reactive xref:web/webflux-webclient.adoc[WebClient].

[[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.

|===

=== Initialization

`RestTemplate` uses the same HTTP library abstraction as `RestClient`.
By default, it uses the `SimpleClientHttpRequestFactory`, but this can be changed via the constructor.
See <<rest-request-factories>>.

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

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

Objects passed into and returned from `RestTemplate` methods are converted to and from HTTP messages with the help of an `HttpMessageConverter`, see <<rest-message-conversion>>.

=== Migrating from `RestTemplate` to `RestClient`

The following table shows `RestClient` equivalents for `RestTemplate` methods.
It can be used to migrate from the latter to the former.

.RestClient equivalents for RestTemplate methods
[cols="1,1", options="header"]
|===
| `RestTemplate` method | `RestClient` equivalent

| `getForObject(String, Class, Object...)`
| `get()
.uri(String, Object...)
.retrieve()
.body(Class)`

| `getForObject(String, Class, Map)`
| `get()
.uri(String, Map)
.retrieve()
.body(Class)`

| `getForObject(URI, Class)`
| `get()
.uri(URI)
.retrieve()
.body(Class)`


| `getForEntity(String, Class, Object...)`
| `get()
.uri(String, Object...)
.retrieve()
.toEntity(Class)`

| `getForEntity(String, Class, Map)`
| `get()
.uri(String, Map)
.retrieve()
.toEntity(Class)`

| `getForEntity(URI, Class)`
| `get()
.uri(URI)
.retrieve()
.toEntity(Class)`


| `headForHeaders(String, Object...)`
| `head()
.uri(String, Object...)
.retrieve()
.toBodilessEntity()
.getHeaders()`

| `headForHeaders(String, Map)`
| `head()
.uri(String, Map)
.retrieve()
.toBodilessEntity()
.getHeaders()`

| `headForHeaders(URI)`
| `head()
.uri(URI)
.retrieve()
.toBodilessEntity()
.getHeaders()`


| `postForLocation(String, Object, Object...)`
| `post()
.uri(String, Object...)
.body(Object).retrieve()
.toBodilessEntity()
.getLocation()`

| `postForLocation(String, Object, Map)`
| `post()
.uri(String, Map)
.body(Object)
.retrieve()
.toBodilessEntity()
.getLocation()`

| `postForLocation(URI, Object)`
| `post()
.uri(URI)
.body(Object)
.retrieve()
.toBodilessEntity()
.getLocation()`


| `postForObject(String, Object, Class, Object...)`
| `post()
.uri(String, Object...)
.body(Object)
.retrieve()
.body(Class)`

| `postForObject(String, Object, Class, Map)`
| `post()
.uri(String, Map)
.body(Object)
.retrieve()
.body(Class)`

| `postForObject(URI, Object, Class)`
| `post()
.uri(URI)
.body(Object)
.retrieve()
.body(Class)`


| `postForEntity(String, Object, Class, Object...)`
| `post()
.uri(String, Object...)
.body(Object)
.retrieve()
.toEntity(Class)`

| `postForEntity(String, Object, Class, Map)`
| `post()
.uri(String, Map)
.body(Object)
.retrieve()
.toEntity(Class)`

| `postForEntity(URI, Object, Class)`
| `post()
.uri(URI)
.body(Object)
.retrieve()
.toEntity(Class)`


| `put(String, Object, Object...)`
| `put()
.uri(String, Object...)
.body(Object)
.retrieve()
.toBodilessEntity()`

| `put(String, Object, Map)`
| `put()
.uri(String, Map)
.body(Object)
.retrieve()
.toBodilessEntity()`

| `put(URI, Object)`
| `put()
.uri(URI)
.body(Object)
.retrieve()
.toBodilessEntity()`


| `patchForObject(String, Object, Class, Object...)`
| `patch()
.uri(String, Object...)
.body(Object)
.retrieve()
.body(Class)`

| `patchForObject(String, Object, Class, Map)`
| `patch()
.uri(String, Map)
.body(Object)
.retrieve()
.body(Class)`

| `patchForObject(URI, Object, Class)`
| `patch()
.uri(URI)
.body(Object)
.retrieve()
.body(Class)`


| `delete(String, Object...)`
| `delete()
.uri(String, Object...)
.retrieve()
.toBodilessEntity()`

| `delete(String, Map)`
| `delete()
.uri(String, Map)
.retrieve()
.toBodilessEntity()`

| `delete(URI)`
| `delete()
.uri(URI)
.retrieve()
.toBodilessEntity()`


| `optionsForAllow(String, Object...)`
| `options()
.uri(String, Object...)
.retrieve()
.toBodilessEntity()
.getAllow()`

| `optionsForAllow(String, Map)`
| `options()
.uri(String, Map)
.retrieve()
.toBodilessEntity()
.getAllow()`

| `optionsForAllow(URI)`
| `options()
.uri(URI)
.retrieve()
.toBodilessEntity()
.getAllow()`


| `exchange(String, HttpMethod, HttpEntity, Class, Object...)`
| `method(HttpMethod)
.uri(String, Object...)
.headers(Consumer<HttpHeaders>)
.body(Object)
.retrieve()
.toEntity(Class)` footnote:http-entity[`HttpEntity` headers and body have to be supplied to the `RestClient` via `headers(Consumer<HttpHeaders>)` and `body(Object)`.]

| `exchange(String, HttpMethod, HttpEntity, Class, Map)`
| `method(HttpMethod)
.uri(String, Map)
.headers(Consumer<HttpHeaders>)
.body(Object)
.retrieve()
.toEntity(Class)` footnote:http-entity[]

| `exchange(URI, HttpMethod, HttpEntity, Class)`
| `method(HttpMethod)
.uri(URI)
.headers(Consumer<HttpHeaders>)
.body(Object)
.retrieve()
.toEntity(Class)` footnote:http-entity[]


| `exchange(String, HttpMethod, HttpEntity, ParameterizedTypeReference, Object...)`
| `method(HttpMethod)
.uri(String, Object...)
.headers(Consumer<HttpHeaders>)
.body(Object)
.retrieve()
.toEntity(ParameterizedTypeReference)` footnote:http-entity[]

| `exchange(String, HttpMethod, HttpEntity, ParameterizedTypeReference, Map)`
| `method(HttpMethod)
.uri(String, Map)
.headers(Consumer<HttpHeaders>)
.body(Object)
.retrieve()
.toEntity(ParameterizedTypeReference)` footnote:http-entity[]

| `exchange(URI, HttpMethod, HttpEntity, ParameterizedTypeReference)`
| `method(HttpMethod)
.uri(URI)
.headers(Consumer<HttpHeaders>)
.body(Object)
.retrieve()
.toEntity(ParameterizedTypeReference)` footnote:http-entity[]


| `exchange(RequestEntity, Class)`
| `method(HttpMethod)
.uri(URI)
.headers(Consumer<HttpHeaders>)
.body(Object)
.retrieve()
.toEntity(Class)` footnote:request-entity[`RequestEntity` method, URI, headers and body have to be supplied to the `RestClient` via `method(HttpMethod)`, `uri(URI)`, `headers(Consumer<HttpHeaders>)` and `body(Object)`.]

| `exchange(RequestEntity, ParameterizedTypeReference)`
| `method(HttpMethod)
.uri(URI)
.headers(Consumer<HttpHeaders>)
.body(Object)
.retrieve()
.toEntity(ParameterizedTypeReference)` footnote:request-entity[]


| `execute(String, HttpMethod, RequestCallback, ResponseExtractor, Object...)`
| `method(HttpMethod)
.uri(String, Object...)
.exchange(ExchangeFunction)`

| `execute(String, HttpMethod, RequestCallback, ResponseExtractor, Map)`
| `method(HttpMethod)
.uri(String, Map)
.exchange(ExchangeFunction)`

| `execute(URI, HttpMethod, RequestCallback, ResponseExtractor)`
| `method(HttpMethod)
.uri(URI)
.exchange(ExchangeFunction)`

|===


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

The Spring Framework lets you define an HTTP service as a Java interface with
`@HttpExchange` methods. You can pass such an interface to `HttpServiceProxyFactory`
to create a proxy which performs requests through an HTTP client such as `RestClient`
or `WebClient`. You can also implement the interface from an `@Controller` for server
request handling.

Start by creating the 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...

	}
----

Now you can create a proxy that performs requests when methods are called.

For `RestClient`:

[source,java,indent=0,subs="verbatim,quotes"]
----
	RestClient restClient = RestClient.builder().baseUrl("https://api.github.com/").build();
	RestClientAdapter adapter = RestClientAdapter.create(restClient);
	HttpServiceProxyFactory factory = HttpServiceProxyFactory.builderFor(adapter).build();

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

For `WebClient`:

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

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

For `RestTemplate`:

[source,java,indent=0,subs="verbatim,quotes"]
----
	RestTemplate restTemplate = new RestTemplate();
	restTemplate.setUriTemplateHandler(new DefaultUriBuilderFactory("https://api.github.com/"));
	RestTemplateAdapter adapter = RestTemplateAdapter.create(restTemplate);
	HttpServiceProxyFactory factory = HttpServiceProxyFactory.builderFor(adapter).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.

| `UriBuilderFactory`
| Provide a `UriBuilderFactory` to expand the URI template and URI variables with.
  In effect, replaces the `UriBuilderFactory` (and its base URL) of the underlying client.

| `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. This overrides
  the annotation's `headers` attribute.

| `@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.

| `@RequestAttribute`
| Provide an `Object` to add as a request attribute. Only supported by `RestClient`
  and `WebClient`.

| `@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.

| `MultipartFile`
| Add a request part from a `MultipartFile`, typically used in a Spring MVC controller
  where it represents an uploaded file.

| `@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.

|===

Method parameters cannot be `null` unless the `required` attribute (where available on a
parameter annotation) is set to `false`, or the parameter is marked optional as determined by
{spring-framework-api}/core/MethodParameter.html#isOptional()[`MethodParameter#isOptional`].



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

The supported return values depend on the underlying client.

Clients adapted to `HttpExchangeAdapter` such as `RestClient` and `RestTemplate`
support synchronous return values:

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

| `void`
| Perform the given request.

| `HttpHeaders`
| Perform the given request and return the response headers.

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

| `ResponseEntity<Void>`
| Perform the given request and return a `ResponseEntity` with the status and headers.

| `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.

|===

Clients adapted to `ReactorHttpExchangeAdapter` such as `WebClient`, support all of above
as well as reactive variants. The table below shows Reactor types, but you can also use
other reactive types that are supported through the `ReactiveAdapterRegistry`:

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

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

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

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

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

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

| `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.

|===

By default, the timeout for synchronous return values with `ReactorHttpExchangeAdapter`
depends on how the underlying HTTP client is configured. You can set a `blockTimeout`
value on the adapter level as well, but we recommend relying on timeout settings of the
underlying HTTP client, which operates at a lower level and provides more control.


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

To customize error response handling, you need to configure the underlying HTTP client.

For `RestClient`:

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

[source,java,indent=0,subs="verbatim,quotes"]
----
	RestClient restClient = RestClient.builder()
			.defaultStatusHandler(HttpStatusCode::isError, (request, response) -> ...)
			.build();

	RestClientAdapter adapter = RestClientAdapter.create(restClient);
	HttpServiceProxyFactory factory = HttpServiceProxyFactory.builderFor(adapter).build();
----

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

For `WebClient`:

By default, `WebClient` raises `WebClientResponseException` for 4xx and 5xx HTTP status codes.
To customize this, 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 adapter = WebClientAdapter.create(webClient);
	HttpServiceProxyFactory factory = HttpServiceProxyFactory.builder(adapter).build();
----

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

For `RestTemplate`:

By default, `RestTemplate` raises `RestClientException` for 4xx and 5xx HTTP status codes.
To customize this, register an error handler that applies to all responses
performed through the client:

[source,java,indent=0,subs="verbatim,quotes"]
----
    RestTemplate restTemplate = new RestTemplate();
    restTemplate.setErrorHandler(myErrorHandler);

    RestTemplateAdapter adapter = RestTemplateAdapter.create(restTemplate);
    HttpServiceProxyFactory factory = HttpServiceProxyFactory.builderFor(adapter).build();
----

For more details and options, see the Javadoc of `setErrorHandler` in `RestTemplate` and
the `ResponseErrorHandler` hierarchy.

