[[webflux-client]]
= WebClient

Spring WebFlux includes a reactive, non-blocking `WebClient` for HTTP requests. The client
has a functional, fluent API with reactive types for declarative composition, see
<<web-reactive.adoc#webflux-reactive-libraries>>. WebFlux client and server rely on the
same non-blocking <<web-reactive.adoc#webflux-codecs, codecs>> to encode and decode request
and response content.

Internally `WebClient` delegates to an HTTP client library. By default, it uses
https://github.com/reactor/reactor-netty[Reactor Netty], there is built-in support for
the Jetty https://github.com/jetty-project/jetty-reactive-httpclient[reactive HttpClient],
and others can be plugged in through a `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)`

The above methods use the Reactor Netty `HttpClient` with default settings and expect
`io.projectreactor.netty:reactor-netty` to be on the classpath.

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

* `uriBuilderFactory`: Customized `UriBuilderFactory` to use as a base URL.
* `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.

The following example configures <<web-reactive.adoc#webflux-codecs, HTTP codecs>>:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	ExchangeStrategies strategies = ExchangeStrategies.builder()
			.codecs(configurer -> {
				// ...
			})
			.build();

	WebClient client = WebClient.builder()
			.exchangeStrategies(strategies)
			.build();
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	val strategies = ExchangeStrategies.builder()
			.codecs {
				// ...
			}
			.build()

	val client = WebClient.builder()
			.exchangeStrategies(strategies)
			.build()
----

Once built, a `WebClient` instance is immutable. However, you can clone it and build a
modified copy without affecting the original instance, as the following example shows:

[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-reactor]]
=== Reactor Netty

To customize Reactor Netty settings, simple 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()
			.tcpConfiguration(client ->
					client.option(ChannelOption.CONNECT_TIMEOUT_MILLIS, 10000));
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	import io.netty.channel.ChannelOption

	val httpClient = HttpClient.create()
			.tcpConfiguration { it.option(ChannelOption.CONNECT_TIMEOUT_MILLIS, 10000)}
----

To configure a read and/or write timeout values:

[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()
			.tcpConfiguration(client ->
					client.doOnConnected(conn -> conn
							.addHandlerLast(new ReadTimeoutHandler(10))
							.addHandlerLast(new WriteTimeoutHandler(10))));
----
[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().tcpConfiguration {
		it.doOnConnected { conn -> conn
				.addHandlerLast(ReadTimeoutHandler(10))
				.addHandlerLast(WriteTimeoutHandler(10))
		}
	}
----



[[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(...);
	ClientHttpConnector connector = new JettyClientHttpConnector(httpClient);

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

	val webClient = WebClient.builder().clientConnector(connector).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-retrieve]]
== `retrieve()`

The `retrieve()` method is the easiest way to get a response body and decode it.
The following example shows how to do so:

[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>()
----

You can also get a stream of objects decoded from the response, as the following example shows:

[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, responses with 4xx or 5xx status codes result in an
`WebClientResponseException` or one of its HTTP status specific sub-classes, such as
`WebClientResponseException.BadRequest`, `WebClientResponseException.NotFound`, and others.
You can also use the `onStatus` method to customize the resulting exception,
as the following example shows:

[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>()
----

When `onStatus` is used, if the response is expected to have content, then the `onStatus`
callback should consume it. If not, the content will be automatically drained to ensure
resources are released.




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

The `exchange()` method provides more control than the `retrieve` method. The following example is equivalent
to `retrieve()` but also provides access to the `ClientResponse`:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	Mono<Person> result = client.get()
			.uri("/persons/{id}", id).accept(MediaType.APPLICATION_JSON)
			.exchange()
			.flatMap(response -> 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)
			.awaitExchange()
			.awaitBody<Person>()
----

At this level, you can also create a full `ResponseEntity`:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	Mono<ResponseEntity<Person>> result = client.get()
			.uri("/persons/{id}", id).accept(MediaType.APPLICATION_JSON)
			.exchange()
			.flatMap(response -> response.toEntity(Person.class));
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	val result = client.get()
			.uri("/persons/{id}", id).accept(MediaType.APPLICATION_JSON)
			.awaitExchange()
			.toEntity<Person>()
----

Note that (unlike `retrieve()`), with `exchange()`, there are no automatic error signals for
4xx and 5xx responses. You have to check the status code and decide how to proceed.

[CAUTION]
====
When using `exchange()`, you have to make sure that the body is always consumed or released,
even when an exception occurs (see <<core.adoc#databuffers-using,Using DataBuffer>>).
Typically, you do this by invoking either `bodyTo*` or `toEntity*` on `ClientResponse`
to convert the body into an object of the desired type, but
you can also invoke `releaseBody()` to discard the body contents without consuming it or
`toBodilessEntity()` to get just the status and headers (while discarding the body).

Finally, there is `bodyToMono(Void.class)`, which should only be used if no response content is
expected.
If the response does have content, the connection is closed and is not placed back in the pool,
because it is not left in a reusable state.
====



[[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]]
== Client 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()
----

Filters apply globally to every request. To change a filter's behavior for a specific
request, you can add request attributes to the `ClientRequest` that can then be accessed
by all filters in the chain, as the following example shows:

[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>()
----

You can also replicate an existing `WebClient`, insert new filters, or remove already
registered filters. The following example, 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-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.
