[[webflux-client]]
= WebClient

Spring WebFlux includes a client to perform HTTP requests with. `WebClient` has a
functional, fluent API based on Reactor, see <<web-reactive.adoc#webflux-reactive-libraries>>,
which enables declarative composition of asynchronous logic without the need to deal with
threads or concurrency. It is fully non-blocking, it supports streaming, and relies on
the same <<web-reactive.adoc#webflux-codecs, codecs>> that are also used to encode and
decode request and response content on the server side.

`WebClient` needs an HTTP client library to perform requests with. There is built-in
support for the following:

* https://github.com/reactor/reactor-netty[Reactor Netty]
* https://github.com/jetty-project/jetty-reactive-httpclient[Jetty Reactive HttpClient]
* https://hc.apache.org/index.html[Apache HttpComponents]
* Others can be plugged via `ClientHttpConnector`.




[[webflux-client-builder]]
== Configuration

The simplest way to create a `WebClient` is through one of the static factory methods:

* `WebClient.create()`
* `WebClient.create(String baseUrl)`

You can also use `WebClient.builder()` with further options:

* `uriBuilderFactory`: Customized `UriBuilderFactory` to use as a base URL.
* `defaultUriVariables`: default values to use when expanding URI templates.
* `defaultHeader`: Headers for every request.
* `defaultCookie`: Cookies for every request.
* `defaultRequest`: `Consumer` to customize every request.
* `filter`: Client filter for every request.
* `exchangeStrategies`: HTTP message reader/writer customizations.
* `clientConnector`: HTTP client library settings.

For example:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	WebClient client = WebClient.builder()
			.codecs(configurer -> ... )
			.build();
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	val webClient = WebClient.builder()
			.codecs { configurer -> ... }
			.build()
----

Once built, a `WebClient` is immutable. However, you can clone it and build a
modified copy as follows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	WebClient client1 = WebClient.builder()
			.filter(filterA).filter(filterB).build();

	WebClient client2 = client1.mutate()
			.filter(filterC).filter(filterD).build();

	// client1 has filterA, filterB

	// client2 has filterA, filterB, filterC, filterD
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	val client1 = WebClient.builder()
			.filter(filterA).filter(filterB).build()

	val client2 = client1.mutate()
			.filter(filterC).filter(filterD).build()

	// client1 has filterA, filterB

	// client2 has filterA, filterB, filterC, filterD
----

[[webflux-client-builder-maxinmemorysize]]
=== MaxInMemorySize

Codecs have <<web-reactive.adoc#webflux-codecs-limits,limits>> for buffering data in
memory to avoid application memory issues. By the default those are set to 256KB.
If that's not enough you'll get the following error:

----
org.springframework.core.io.buffer.DataBufferLimitException: Exceeded limit on max bytes to buffer
----

To change the limit for default codecs, use the following:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	WebClient webClient = WebClient.builder()
			.codecs(configurer -> configurer.defaultCodecs().maxInMemorySize(2 * 1024 * 1024))
			.build();
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	val webClient = WebClient.builder()
			.codecs { configurer -> configurer.defaultCodecs().maxInMemorySize(2 * 1024 * 1024) }
			.build()
----



[[webflux-client-builder-reactor]]
=== Reactor Netty

To customize Reactor Netty settings, provide a pre-configured `HttpClient`:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	HttpClient httpClient = HttpClient.create().secure(sslSpec -> ...);

	WebClient webClient = WebClient.builder()
			.clientConnector(new ReactorClientHttpConnector(httpClient))
			.build();
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	val httpClient = HttpClient.create().secure { ... }

	val webClient = WebClient.builder()
		.clientConnector(ReactorClientHttpConnector(httpClient))
		.build()
----


[[webflux-client-builder-reactor-resources]]
==== Resources

By default, `HttpClient` participates in the global Reactor Netty resources held in
`reactor.netty.http.HttpResources`, including event loop threads and a connection pool.
This is the recommended mode, since fixed, shared resources are preferred for event loop
concurrency. In this mode global resources remain active until the process exits.

If the server is timed with the process, there is typically no need for an explicit
shutdown. However, if the server can start or stop in-process (for example, a Spring MVC
application deployed as a WAR), you can declare a Spring-managed bean of type
`ReactorResourceFactory` with `globalResources=true` (the default) to ensure that the Reactor
Netty global resources are shut down when the Spring `ApplicationContext` is closed,
as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Bean
	public ReactorResourceFactory reactorResourceFactory() {
		return new ReactorResourceFactory();
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@Bean
	fun reactorResourceFactory() = ReactorResourceFactory()
----

You can also choose not to participate in the global Reactor Netty resources. However,
in this mode, the burden is on you to ensure that all Reactor Netty client and server
instances use shared resources, as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Bean
	public ReactorResourceFactory resourceFactory() {
		ReactorResourceFactory factory = new ReactorResourceFactory();
		factory.setUseGlobalResources(false); // <1>
		return factory;
	}

	@Bean
	public WebClient webClient() {

		Function<HttpClient, HttpClient> mapper = client -> {
			// Further customizations...
		};

		ClientHttpConnector connector =
				new ReactorClientHttpConnector(resourceFactory(), mapper); // <2>

		return WebClient.builder().clientConnector(connector).build(); // <3>
	}
----
<1> Create resources independent of global ones.
<2> Use the `ReactorClientHttpConnector` constructor with resource factory.
<3> Plug the connector into the `WebClient.Builder`.

[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@Bean
	fun resourceFactory() = ReactorResourceFactory().apply {
		isUseGlobalResources = false // <1>
	}

	@Bean
	fun webClient(): WebClient {

		val mapper: (HttpClient) -> HttpClient = {
			// Further customizations...
		}

		val connector = ReactorClientHttpConnector(resourceFactory(), mapper) // <2>

		return WebClient.builder().clientConnector(connector).build() // <3>
	}
----
<1> Create resources independent of global ones.
<2> Use the `ReactorClientHttpConnector` constructor with resource factory.
<3> Plug the connector into the `WebClient.Builder`.


[[webflux-client-builder-reactor-timeout]]
==== Timeouts

To configure a connection timeout:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	import io.netty.channel.ChannelOption;

	HttpClient httpClient = HttpClient.create()
			.option(ChannelOption.CONNECT_TIMEOUT_MILLIS, 10000);

	WebClient webClient = WebClient.builder()
			.clientConnector(new ReactorClientHttpConnector(httpClient))
			.build();
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	import io.netty.channel.ChannelOption

	val httpClient = HttpClient.create()
			.option(ChannelOption.CONNECT_TIMEOUT_MILLIS, 10000);

	val webClient = WebClient.builder()
			.clientConnector(new ReactorClientHttpConnector(httpClient))
			.build();
----

To configure a read or write timeout:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	import io.netty.handler.timeout.ReadTimeoutHandler;
	import io.netty.handler.timeout.WriteTimeoutHandler;

	HttpClient httpClient = HttpClient.create()
			.doOnConnected(conn -> conn
					.addHandlerLast(new ReadTimeoutHandler(10))
					.addHandlerLast(new WriteTimeoutHandler(10)));

	// Create WebClient...

----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	import io.netty.handler.timeout.ReadTimeoutHandler
	import io.netty.handler.timeout.WriteTimeoutHandler

	val httpClient = HttpClient.create()
			.doOnConnected { conn -> conn
					.addHandlerLast(new ReadTimeoutHandler(10))
					.addHandlerLast(new WriteTimeoutHandler(10))
			}

	// Create WebClient...
----

To configure a response timeout for all requests:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	HttpClient httpClient = HttpClient.create()
			.responseTimeout(Duration.ofSeconds(2));

	// Create WebClient...
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	val httpClient = HttpClient.create()
			.responseTimeout(Duration.ofSeconds(2));

	// Create WebClient...
----

To configure a response timeout for a specific request:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	WebClient.create().get()
			.uri("https://example.org/path")
			.httpRequest(httpRequest -> {
				HttpClientRequest reactorRequest = httpRequest.getNativeRequest();
				reactorRequest.responseTimeout(Duration.ofSeconds(2));
			})
			.retrieve()
			.bodyToMono(String.class);
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	WebClient.create().get()
			.uri("https://example.org/path")
			.httpRequest { httpRequest: ClientHttpRequest ->
				val reactorRequest = httpRequest.getNativeRequest<HttpClientRequest>()
				reactorRequest.responseTimeout(Duration.ofSeconds(2))
			}
			.retrieve()
			.bodyToMono(String::class.java)
----



[[webflux-client-builder-jetty]]
=== Jetty

The following example shows how to customize Jetty `HttpClient` settings:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	HttpClient httpClient = new HttpClient();
	httpClient.setCookieStore(...);

	WebClient webClient = WebClient.builder()
			.clientConnector(new JettyClientHttpConnector(httpClient))
			.build();
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	val httpClient = HttpClient()
	httpClient.cookieStore = ...

	val webClient = WebClient.builder()
			.clientConnector(new JettyClientHttpConnector(httpClient))
			.build();
----

By default, `HttpClient` creates its own resources (`Executor`, `ByteBufferPool`, `Scheduler`),
which remain active until the process exits or `stop()` is called.

You can share resources between multiple instances of the Jetty client (and server) and
ensure that the resources are shut down when the Spring `ApplicationContext` is closed by
declaring a Spring-managed bean of type `JettyResourceFactory`, as the following example
shows:

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

	@Bean
	public WebClient webClient() {

		HttpClient httpClient = new HttpClient();
		// Further customizations...
		
		ClientHttpConnector connector =
				new JettyClientHttpConnector(httpClient, resourceFactory()); <1>

		return WebClient.builder().clientConnector(connector).build(); <2>
	}
----
<1> Use the `JettyClientHttpConnector` constructor with resource factory.
<2> Plug the connector into the `WebClient.Builder`.

[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@Bean
	fun resourceFactory() = JettyResourceFactory()

	@Bean
	fun webClient(): WebClient {

		val httpClient = HttpClient()
		// Further customizations...
			
		val connector = JettyClientHttpConnector(httpClient, resourceFactory()) // <1>

		return WebClient.builder().clientConnector(connector).build() // <2>
	}
----
<1> Use the `JettyClientHttpConnector` constructor with resource factory.
<2> Plug the connector into the `WebClient.Builder`.



[[webflux-client-builder-http-components]]
=== HttpComponents

The following example shows how to customize Apache HttpComponents `HttpClient` settings:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	HttpAsyncClientBuilder clientBuilder = HttpAsyncClients.custom();
	clientBuilder.setDefaultRequestConfig(...);
	CloseableHttpAsyncClient client = clientBuilder.build();
	ClientHttpConnector connector = new HttpComponentsClientHttpConnector(client);

	WebClient webClient = WebClient.builder().clientConnector(connector).build();
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	val client = HttpAsyncClients.custom().apply {
		setDefaultRequestConfig(...)
	}.build()
	val connector = HttpComponentsClientHttpConnector(client)
	val webClient = WebClient.builder().clientConnector(connector).build()
----


[[webflux-client-retrieve]]
== `retrieve()`

The `retrieve()` method can be used to declare how to extract the response. For example:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	WebClient client = WebClient.create("https://example.org");

	Mono<ResponseEntity<Person>> result = client.get()
			.uri("/persons/{id}", id).accept(MediaType.APPLICATION_JSON)
			.retrieve()
			.toEntity(Person.class);
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	val client = WebClient.create("https://example.org")

	val result = client.get()
			.uri("/persons/{id}", id).accept(MediaType.APPLICATION_JSON)
			.retrieve()
			.toEntity<Person>().awaitSingle()
----

Or to get only the body:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	WebClient client = WebClient.create("https://example.org");

	Mono<Person> result = client.get()
			.uri("/persons/{id}", id).accept(MediaType.APPLICATION_JSON)
			.retrieve()
			.bodyToMono(Person.class);
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	val client = WebClient.create("https://example.org")

	val result = client.get()
			.uri("/persons/{id}", id).accept(MediaType.APPLICATION_JSON)
			.retrieve()
			.awaitBody<Person>()
----

To get a stream of decoded objects:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	Flux<Quote> result = client.get()
			.uri("/quotes").accept(MediaType.TEXT_EVENT_STREAM)
			.retrieve()
			.bodyToFlux(Quote.class);
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	val result = client.get()
			.uri("/quotes").accept(MediaType.TEXT_EVENT_STREAM)
			.retrieve()
			.bodyToFlow<Quote>()
----

By default, 4xx or 5xx responses result in an `WebClientResponseException`, including
sub-classes for specific HTTP status codes. To customize the handling of error
responses, use `onStatus` handlers as follows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	Mono<Person> result = client.get()
			.uri("/persons/{id}", id).accept(MediaType.APPLICATION_JSON)
			.retrieve()
			.onStatus(HttpStatus::is4xxClientError, response -> ...)
			.onStatus(HttpStatus::is5xxServerError, response -> ...)
			.bodyToMono(Person.class);
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	val result = client.get()
			.uri("/persons/{id}", id).accept(MediaType.APPLICATION_JSON)
			.retrieve()
			.onStatus(HttpStatus::is4xxClientError) { ... }
			.onStatus(HttpStatus::is5xxServerError) { ... }
			.awaitBody<Person>()
----




[[webflux-client-exchange]]
== Exchange

The `exchangeToMono()` and `exchangeToFlux()` methods (or `awaitExchange { }` and `exchangeToFlow { }` in Kotlin)
are useful for more advanced cases that require more control, such as to decode the response differently
depending on the response status:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	Mono<Object> entityMono = client.get()
			.uri("/persons/1")
			.accept(MediaType.APPLICATION_JSON)
			.exchangeToMono(response -> {
				if (response.statusCode().equals(HttpStatus.OK)) {
					return response.bodyToMono(Person.class);
				}
				else if (response.statusCode().is4xxClientError()) {
					// Suppress error status code
					return response.bodyToMono(ErrorContainer.class);
				}
				else {
					// Turn to error
					return response.createException().flatMap(Mono::error);
				}
			});
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
val entity = client.get()
  .uri("/persons/1")
  .accept(MediaType.APPLICATION_JSON)
  .awaitExchange {
		if (response.statusCode() == HttpStatus.OK) {
			 return response.awaitBody<Person>()
		}
		else if (response.statusCode().is4xxClientError) {
			 return response.awaitBody<ErrorContainer>()
		}
		else {
			 throw response.createExceptionAndAwait()
		}
  }
----

When using the above, after the returned `Mono` or `Flux` completes, the response body
is checked and if not consumed it is released to prevent memory and connection leaks.
Therefore the response cannot be decoded further downstream. It is up to the provided
function to declare how to decode the response if needed.




[[webflux-client-body]]
== Request Body

The request body can be encoded from any asynchronous type handled by `ReactiveAdapterRegistry`,
like `Mono` or Kotlin Coroutines `Deferred` as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	Mono<Person> personMono = ... ;

	Mono<Void> result = client.post()
			.uri("/persons/{id}", id)
			.contentType(MediaType.APPLICATION_JSON)
			.body(personMono, Person.class)
			.retrieve()
			.bodyToMono(Void.class);
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	val personDeferred: Deferred<Person> = ...

	client.post()
			.uri("/persons/{id}", id)
			.contentType(MediaType.APPLICATION_JSON)
			.body<Person>(personDeferred)
			.retrieve()
			.awaitBody<Unit>()
----

You can also have a stream of objects be encoded, as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	Flux<Person> personFlux = ... ;

	Mono<Void> result = client.post()
			.uri("/persons/{id}", id)
			.contentType(MediaType.APPLICATION_STREAM_JSON)
			.body(personFlux, Person.class)
			.retrieve()
			.bodyToMono(Void.class);
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	val people: Flow<Person> = ...

	client.post()
			.uri("/persons/{id}", id)
			.contentType(MediaType.APPLICATION_JSON)
			.body(people)
			.retrieve()
			.awaitBody<Unit>()
----

Alternatively, if you have the actual value, you can use the `bodyValue` shortcut method,
as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	Person person = ... ;

	Mono<Void> result = client.post()
			.uri("/persons/{id}", id)
			.contentType(MediaType.APPLICATION_JSON)
			.bodyValue(person)
			.retrieve()
			.bodyToMono(Void.class);
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	val person: Person = ...

	client.post()
			.uri("/persons/{id}", id)
			.contentType(MediaType.APPLICATION_JSON)
			.bodyValue(person)
			.retrieve()
			.awaitBody<Unit>()
----



[[webflux-client-body-form]]
=== Form Data

To send form data, you can provide a `MultiValueMap<String, String>` as the body. Note that the
content is automatically set to `application/x-www-form-urlencoded` by the
`FormHttpMessageWriter`. The following example shows how to use `MultiValueMap<String, String>`:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	MultiValueMap<String, String> formData = ... ;

	Mono<Void> result = client.post()
			.uri("/path", id)
			.bodyValue(formData)
			.retrieve()
			.bodyToMono(Void.class);
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	val formData: MultiValueMap<String, String> = ...

	client.post()
			.uri("/path", id)
			.bodyValue(formData)
			.retrieve()
			.awaitBody<Unit>()
----

You can also supply form data in-line by using `BodyInserters`, as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	import static org.springframework.web.reactive.function.BodyInserters.*;

	Mono<Void> result = client.post()
			.uri("/path", id)
			.body(fromFormData("k1", "v1").with("k2", "v2"))
			.retrieve()
			.bodyToMono(Void.class);
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	import org.springframework.web.reactive.function.BodyInserters.*

	client.post()
			.uri("/path", id)
			.body(fromFormData("k1", "v1").with("k2", "v2"))
			.retrieve()
			.awaitBody<Unit>()
----



[[webflux-client-body-multipart]]
=== Multipart Data

To send multipart data, you need to provide a `MultiValueMap<String, ?>` whose values are
either `Object` instances that represent part content or `HttpEntity` instances that represent the content and
headers for a part. `MultipartBodyBuilder` provides a convenient API to prepare a
multipart request. The following example shows how to create a `MultiValueMap<String, ?>`:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	MultipartBodyBuilder builder = new MultipartBodyBuilder();
	builder.part("fieldPart", "fieldValue");
	builder.part("filePart1", new FileSystemResource("...logo.png"));
	builder.part("jsonPart", new Person("Jason"));
	builder.part("myPart", part); // Part from a server request

	MultiValueMap<String, HttpEntity<?>> parts = builder.build();
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	val builder = MultipartBodyBuilder().apply {
		part("fieldPart", "fieldValue")
		part("filePart1", new FileSystemResource("...logo.png"))
		part("jsonPart", new Person("Jason"))
		part("myPart", part) // Part from a server request	
	}
	
	val parts = builder.build()
----

In most cases, you do not have to specify the `Content-Type` for each part. The content
type is determined automatically based on the `HttpMessageWriter` chosen to serialize it
or, in the case of a `Resource`, based on the file extension. If necessary, you can
explicitly provide the `MediaType` to use for each part through one of the overloaded
builder `part` methods.

Once a `MultiValueMap` is prepared, the easiest way to pass it to the `WebClient` is
through the `body` method, as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	MultipartBodyBuilder builder = ...;

	Mono<Void> result = client.post()
			.uri("/path", id)
			.body(builder.build())
			.retrieve()
			.bodyToMono(Void.class);
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	val builder: MultipartBodyBuilder = ...

	client.post()
			.uri("/path", id)
			.body(builder.build())
			.retrieve()
			.awaitBody<Unit>()
----

If the `MultiValueMap` contains at least one non-`String` value, which could also
represent regular form data (that is, `application/x-www-form-urlencoded`), you need not
set the `Content-Type` to `multipart/form-data`. This is always the case when using
`MultipartBodyBuilder`, which ensures an `HttpEntity` wrapper.

As an alternative to `MultipartBodyBuilder`, you can also provide multipart content,
inline-style, through the built-in `BodyInserters`, as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	import static org.springframework.web.reactive.function.BodyInserters.*;

	Mono<Void> result = client.post()
			.uri("/path", id)
			.body(fromMultipartData("fieldPart", "value").with("filePart", resource))
			.retrieve()
			.bodyToMono(Void.class);
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	import org.springframework.web.reactive.function.BodyInserters.*

	client.post()
			.uri("/path", id)
			.body(fromMultipartData("fieldPart", "value").with("filePart", resource))
			.retrieve()
			.awaitBody<Unit>()
----



[[webflux-client-filter]]
== Filters

You can register a client filter (`ExchangeFilterFunction`) through the `WebClient.Builder`
in order to intercept and modify requests, as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	WebClient client = WebClient.builder()
			.filter((request, next) -> {

				ClientRequest filtered = ClientRequest.from(request)
						.header("foo", "bar")
						.build();

				return next.exchange(filtered);
			})
			.build();
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	val client = WebClient.builder()
			.filter { request, next ->

				val filtered = ClientRequest.from(request)
						.header("foo", "bar")
						.build()

				next.exchange(filtered)
			}
			.build()
----

This can be used for cross-cutting concerns, such as authentication. The following example uses
a filter for basic authentication through a static factory method:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	import static org.springframework.web.reactive.function.client.ExchangeFilterFunctions.basicAuthentication;

	WebClient client = WebClient.builder()
			.filter(basicAuthentication("user", "password"))
			.build();
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	import org.springframework.web.reactive.function.client.ExchangeFilterFunctions.basicAuthentication

	val client = WebClient.builder()
			.filter(basicAuthentication("user", "password"))
			.build()
----

You can create a new `WebClient` instance by using another as a starting point. This allows
insert or removing filters without affecting the original `WebClient`. Below is an example
that inserts a basic authentication filter at index 0:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	import static org.springframework.web.reactive.function.client.ExchangeFilterFunctions.basicAuthentication;

	WebClient client = webClient.mutate()
			.filters(filterList -> {
				filterList.add(0, basicAuthentication("user", "password"));
			})
			.build();
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	val client = webClient.mutate()
			.filters { it.add(0, basicAuthentication("user", "password")) }
			.build()
----


[[webflux-client-attributes]]
== Attributes

You can add attributes to a request. This is convenient if you want to pass information
through the filter chain and influence the behavior of filters for a given request.
For example:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	WebClient client = WebClient.builder()
			.filter((request, next) -> {
				Optional<Object> usr = request.attribute("myAttribute");
				// ...
			})
			.build();

	client.get().uri("https://example.org/")
			.attribute("myAttribute", "...")
			.retrieve()
			.bodyToMono(Void.class);

		}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	val client = WebClient.builder()
			.filter { request, _ ->
				val usr = request.attributes()["myAttribute"];
				// ...
			}
			.build()

		client.get().uri("https://example.org/")
				.attribute("myAttribute", "...")
				.retrieve()
				.awaitBody<Unit>()
----

Note that you can configure a `defaultRequest` callback globally at the
`WebClient.Builder` level which lets you insert attributes into all requests,
which could be used for example in a Spring MVC application to populate
request attributes based on `ThreadLocal` data.


[[webflux-client-context]]
== Context

<<webflux-client-attributes>> provide a convenient way to pass information to the filter
chain but they only influence the current request. If you want to pass information that
propagates to additional requests that are nested, e.g. via `flatMap`, or executed after,
e.g. via `concatMap`, then you'll need to use the Reactor `Context`.

The Reactor `Context` needs to be populated at the end of a reactive chain in order to
apply to all operations. For example:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	WebClient client = WebClient.builder()
			.filter((request, next) ->
					Mono.deferContextual(contextView -> {
						String value = contextView.get("foo");
						// ...
					}))
			.build();

	client.get().uri("https://example.org/")
			.retrieve()
			.bodyToMono(String.class)
			.flatMap(body -> {
					// perform nested request (context propagates automatically)...
			})
			.contextWrite(context -> context.put("foo", ...));
----



[[webflux-client-synchronous]]
== Synchronous Use

`WebClient` can be used in synchronous style by blocking at the end for the result:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	Person person = client.get().uri("/person/{id}", i).retrieve()
		.bodyToMono(Person.class)
		.block();

	List<Person> persons = client.get().uri("/persons").retrieve()
		.bodyToFlux(Person.class)
		.collectList()
		.block();
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	val person = runBlocking {
		client.get().uri("/person/{id}", i).retrieve()
				.awaitBody<Person>()
	}
	
	val persons = runBlocking {
		client.get().uri("/persons").retrieve()
				.bodyToFlow<Person>()
				.toList()
	}
----

However if multiple calls need to be made, it's more efficient to avoid blocking on each
response individually, and instead wait for the combined result:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	Mono<Person> personMono = client.get().uri("/person/{id}", personId)
			.retrieve().bodyToMono(Person.class);

	Mono<List<Hobby>> hobbiesMono = client.get().uri("/person/{id}/hobbies", personId)
			.retrieve().bodyToFlux(Hobby.class).collectList();

	Map<String, Object> data = Mono.zip(personMono, hobbiesMono, (person, hobbies) -> {
				Map<String, String> map = new LinkedHashMap<>();
				map.put("person", person);
				map.put("hobbies", hobbies);
				return map;
			})
			.block();
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	val data = runBlocking {
			val personDeferred = async {
				client.get().uri("/person/{id}", personId)
						.retrieve().awaitBody<Person>()
			}

			val hobbiesDeferred = async {
				client.get().uri("/person/{id}/hobbies", personId)
						.retrieve().bodyToFlow<Hobby>().toList()
			}

			mapOf("person" to personDeferred.await(), "hobbies" to hobbiesDeferred.await())
		}
----

The above is merely one example. There are lots of other patterns and operators for putting
together a reactive pipeline that makes many remote calls, potentially some nested,
inter-dependent, without ever blocking until the end.

[NOTE]
====
With `Flux` or `Mono`, you should never have to block in a Spring MVC or Spring WebFlux controller.
Simply return the resulting reactive type from the controller method. The same principle apply to
Kotlin Coroutines and Spring WebFlux, just use suspending function or return `Flow` in your
controller method . 
====




[[webflux-client-testing]]
== Testing

To test code that uses the `WebClient`, you can use a mock web server, such as the
https://github.com/square/okhttp#mockwebserver[OkHttp MockWebServer]. To see an example
of its use, check out
https://github.com/spring-projects/spring-framework/blob/master/spring-webflux/src/test/java/org/springframework/web/reactive/function/client/WebClientIntegrationTests.java[`WebClientIntegrationTests`]
in the Spring Framework test suite or the
https://github.com/square/okhttp/tree/master/samples/static-server[`static-server`]
sample in the OkHttp repository.
