Functional Endpoints

Spring WebFlux includes WebFlux.fn, a lightweight functional programming model in which functions are used to route and handle requests and contracts are designed for immutability. It is an alternative to the annotation-based programming model but otherwise runs on the same Reactive Core foundation.

Overview

See equivalent in the Servlet stack

In WebFlux.fn, an HTTP request is handled with a HandlerFunction: a function that takes ServerRequest and returns a delayed ServerResponse (i.e. Mono<ServerResponse>). Both the request and the response object have immutable contracts that offer JDK 8-friendly access to the HTTP request and response. HandlerFunction is the equivalent of the body of a @RequestMapping method in the annotation-based programming model.

Incoming requests are routed to a handler function with a RouterFunction: a function that takes ServerRequest and returns a delayed HandlerFunction (i.e. Mono<HandlerFunction>). When the router function matches, a handler function is returned; otherwise an empty Mono. RouterFunction is the equivalent of a @RequestMapping annotation, but with the major difference that router functions provide not just data, but also behavior.

RouterFunctions.route() provides a router builder that facilitates the creation of routers, as the following example shows:

Java
Kotlin

import static org.springframework.http.MediaType.APPLICATION_JSON;
import static org.springframework.web.reactive.function.server.RequestPredicates.*;
import static org.springframework.web.reactive.function.server.RouterFunctions.route;

PersonRepository repository = ...
PersonHandler handler = new PersonHandler(repository);

RouterFunction<ServerResponse> route = route() (1)
	.GET("/person/{id}", accept(APPLICATION_JSON), handler::getPerson)
	.GET("/person", accept(APPLICATION_JSON), handler::listPeople)
	.POST("/person", handler::createPerson)
	.build();


public class PersonHandler {

	// ...

	public Mono<ServerResponse> listPeople(ServerRequest request) {
		// ...
	}

	public Mono<ServerResponse> createPerson(ServerRequest request) {
		// ...
	}

	public Mono<ServerResponse> getPerson(ServerRequest request) {
		// ...
	}
}

1	Create router using `route()`.

val repository: PersonRepository = ...
val handler = PersonHandler(repository)

val route = coRouter { (1)
	accept(APPLICATION_JSON).nest {
		GET("/person/{id}", handler::getPerson)
		GET("/person", handler::listPeople)
	}
	POST("/person", handler::createPerson)
}


class PersonHandler(private val repository: PersonRepository) {

	// ...

	suspend fun listPeople(request: ServerRequest): ServerResponse {
		// ...
	}

	suspend fun createPerson(request: ServerRequest): ServerResponse {
		// ...
	}

	suspend fun getPerson(request: ServerRequest): ServerResponse {
		// ...
	}
}

1	Create router using Coroutines router DSL; a Reactive alternative is also available via `router { }`.

One way to run a RouterFunction is to turn it into an HttpHandler and install it through one of the built-in server adapters:

RouterFunctions.toHttpHandler(RouterFunction)
RouterFunctions.toHttpHandler(RouterFunction, HandlerStrategies)

Most applications can run through the WebFlux Java configuration, see Running a Server.

HandlerFunction

See equivalent in the Servlet stack

ServerRequest and ServerResponse are immutable interfaces that offer JDK 8-friendly access to the HTTP request and response. Both request and response provide Reactive Streams back pressure against the body streams. The request body is represented with a Reactor Flux or Mono. The response body is represented with any Reactive Streams Publisher, including Flux and Mono. For more on that, see Reactive Libraries.

ServerRequest

ServerRequest provides access to the HTTP method, URI, headers, and query parameters, while access to the body is provided through the body methods.

The following example extracts the request body to a Mono<String>:

Java
Kotlin

Mono<String> string = request.bodyToMono(String.class);

val string = request.awaitBody<String>()

The following example extracts the body to a Flux<Person> (or a Flow<Person> in Kotlin), where Person objects are decoded from some serialized form, such as JSON or XML:

Java
Kotlin

Flux<Person> people = request.bodyToFlux(Person.class);

val people = request.bodyToFlow<Person>()

The preceding examples are shortcuts that use the more general ServerRequest.body(BodyExtractor), which accepts the BodyExtractor functional strategy interface. The utility class BodyExtractors provides access to a number of instances. For example, the preceding examples can also be written as follows:

Java
Kotlin

Mono<String> string = request.body(BodyExtractors.toMono(String.class));
Flux<Person> people = request.body(BodyExtractors.toFlux(Person.class));

	val string = request.body(BodyExtractors.toMono(String::class.java)).awaitSingle()
	val people = request.body(BodyExtractors.toFlux(Person::class.java)).asFlow()

The following example shows how to access form data:

Java
Kotlin

Mono<MultiValueMap<String, String>> map = request.formData();

val map = request.awaitFormData()

The following example shows how to access multipart data as a map:

Java
Kotlin

Mono<MultiValueMap<String, Part>> map = request.multipartData();

val map = request.awaitMultipartData()

The following example shows how to access multipart data, one at a time, in streaming fashion:

Java
Kotlin

Flux<PartEvent> allPartEvents = request.bodyToFlux(PartEvent.class);
allPartsEvents.windowUntil(PartEvent::isLast)
      .concatMap(p -> p.switchOnFirst((signal, partEvents) -> {
          if (signal.hasValue()) {
              PartEvent event = signal.get();
              if (event instanceof FormPartEvent formEvent) {
                  String value = formEvent.value();
                  // handle form field
              }
              else if (event instanceof FilePartEvent fileEvent) {
                  String filename = fileEvent.filename();
                  Flux<DataBuffer> contents = partEvents.map(PartEvent::content);
                  // handle file upload
              }
              else {
                  return Mono.error(new RuntimeException("Unexpected event: " + event));
              }
          }
          else {
              return partEvents; // either complete or error signal
          }
      }));

val parts = request.bodyToFlux<PartEvent>()
allPartsEvents.windowUntil(PartEvent::isLast)
    .concatMap {
        it.switchOnFirst { signal, partEvents ->
            if (signal.hasValue()) {
                val event = signal.get()
                if (event is FormPartEvent) {
                    val value: String = event.value();
                    // handle form field
                } else if (event is FilePartEvent) {
                    val filename: String = event.filename();
                    val contents: Flux<DataBuffer> = partEvents.map(PartEvent::content);
                    // handle file upload
                } else {
                    return Mono.error(RuntimeException("Unexpected event: " + event));
                }
            } else {
                return partEvents; // either complete or error signal
            }
        }
    }
}

Note that the body contents of the PartEvent objects must be completely consumed, relayed, or released to avoid memory leaks.

ServerResponse

ServerResponse provides access to the HTTP response and, since it is immutable, you can use a build method to create it. You can use the builder to set the response status, to add response headers, or to provide a body. The following example creates a 200 (OK) response with JSON content:

Java
Kotlin

Mono<Person> person = ...
ServerResponse.ok().contentType(MediaType.APPLICATION_JSON).body(person, Person.class);

val person: Person = ...
ServerResponse.ok().contentType(MediaType.APPLICATION_JSON).bodyValue(person)

The following example shows how to build a 201 (CREATED) response with a Location header and no body:

Java
Kotlin

URI location = ...
ServerResponse.created(location).build();

val location: URI = ...
ServerResponse.created(location).build()

Depending on the codec used, it is possible to pass hint parameters to customize how the body is serialized or deserialized. For example, to specify a Jackson JSON view:

Java
Kotlin

ServerResponse.ok().hint(Jackson2CodecSupport.JSON_VIEW_HINT, MyJacksonView.class).body(...);

ServerResponse.ok().hint(Jackson2CodecSupport.JSON_VIEW_HINT, MyJacksonView::class.java).body(...)

Handler Classes

We can write a handler function as a lambda, as the following example shows:

Java
Kotlin

HandlerFunction<ServerResponse> helloWorld =
  request -> ServerResponse.ok().bodyValue("Hello World");

val helloWorld = HandlerFunction<ServerResponse> { ServerResponse.ok().bodyValue("Hello World") }

That is convenient, but in an application we need multiple functions, and multiple inline lambda’s can get messy. Therefore, it is useful to group related handler functions together into a handler class, which has a similar role as @Controller in an annotation-based application. For example, the following class exposes a reactive Person repository:

Java
Kotlin

import static org.springframework.http.MediaType.APPLICATION_JSON;
import static org.springframework.web.reactive.function.server.ServerResponse.ok;

public class PersonHandler {

	private final PersonRepository repository;

	public PersonHandler(PersonRepository repository) {
		this.repository = repository;
	}

	public Mono<ServerResponse> listPeople(ServerRequest request) { (1)
		Flux<Person> people = repository.allPeople();
		return ok().contentType(APPLICATION_JSON).body(people, Person.class);
	}

	public Mono<ServerResponse> createPerson(ServerRequest request) { (2)
		Mono<Person> person = request.bodyToMono(Person.class);
		return ok().build(repository.savePerson(person));
	}

	public Mono<ServerResponse> getPerson(ServerRequest request) { (3)
		int personId = Integer.valueOf(request.pathVariable("id"));
		return repository.getPerson(personId)
			.flatMap(person -> ok().contentType(APPLICATION_JSON).bodyValue(person))
			.switchIfEmpty(ServerResponse.notFound().build());
	}
}

1	`listPeople` is a handler function that returns all `Person` objects found in the repository as JSON.
2	`createPerson` is a handler function that stores a new `Person` contained in the request body. Note that `PersonRepository.savePerson(Person)` returns `Mono<Void>`: an empty `Mono` that emits a completion signal when the person has been read from the request and stored. So we use the `build(Publisher<Void>)` method to send a response when that completion signal is received (that is, when the `Person` has been saved).
3	`getPerson` is a handler function that returns a single person, identified by the `id` path variable. We retrieve that `Person` from the repository and create a JSON response, if it is found. If it is not found, we use `switchIfEmpty(Mono<T>)` to return a 404 Not Found response.

class PersonHandler(private val repository: PersonRepository) {

	suspend fun listPeople(request: ServerRequest): ServerResponse { (1)
		val people: Flow<Person> = repository.allPeople()
		return ok().contentType(APPLICATION_JSON).bodyAndAwait(people);
	}

	suspend fun createPerson(request: ServerRequest): ServerResponse { (2)
		val person = request.awaitBody<Person>()
		repository.savePerson(person)
		return ok().buildAndAwait()
	}

	suspend fun getPerson(request: ServerRequest): ServerResponse { (3)
		val personId = request.pathVariable("id").toInt()
		return repository.getPerson(personId)?.let { ok().contentType(APPLICATION_JSON).bodyValueAndAwait(it) }
				?: ServerResponse.notFound().buildAndAwait()

	}
}

1	`listPeople` is a handler function that returns all `Person` objects found in the repository as JSON.
2	`createPerson` is a handler function that stores a new `Person` contained in the request body. Note that `PersonRepository.savePerson(Person)` is a suspending function with no return type.
3	`getPerson` is a handler function that returns a single person, identified by the `id` path variable. We retrieve that `Person` from the repository and create a JSON response, if it is found. If it is not found, we return a 404 Not Found response.

Validation

A functional endpoint can use Spring’s validation facilities to apply validation to the request body. For example, given a custom Spring Validator implementation for a Person:

Java
Kotlin

public class PersonHandler {

	private final Validator validator = new PersonValidator(); (1)

	// ...

	public Mono<ServerResponse> createPerson(ServerRequest request) {
		Mono<Person> person = request.bodyToMono(Person.class).doOnNext(this::validate); (2)
		return ok().build(repository.savePerson(person));
	}

	private void validate(Person person) {
		Errors errors = new BeanPropertyBindingResult(person, "person");
		validator.validate(person, errors);
		if (errors.hasErrors()) {
			throw new ServerWebInputException(errors.toString()); (3)
		}
	}
}

1	Create `Validator` instance.
2	Apply validation.
3	Raise exception for a 400 response.

class PersonHandler(private val repository: PersonRepository) {

	private val validator = PersonValidator() (1)

	// ...

	suspend fun createPerson(request: ServerRequest): ServerResponse {
		val person = request.awaitBody<Person>()
		validate(person) (2)
		repository.savePerson(person)
		return ok().buildAndAwait()
	}

	private fun validate(person: Person) {
		val errors: Errors = BeanPropertyBindingResult(person, "person");
		validator.validate(person, errors);
		if (errors.hasErrors()) {
			throw ServerWebInputException(errors.toString()) (3)
		}
	}
}

1	Create `Validator` instance.
2	Apply validation.
3	Raise exception for a 400 response.

Handlers can also use the standard bean validation API (JSR-303) by creating and injecting a global Validator instance based on LocalValidatorFactoryBean. See Spring Validation.

`RouterFunction`

See equivalent in the Servlet stack

Router functions are used to route the requests to the corresponding HandlerFunction. Typically, you do not write router functions yourself, but rather use a method on the RouterFunctions utility class to create one. RouterFunctions.route() (no parameters) provides you with a fluent builder for creating a router function, whereas RouterFunctions.route(RequestPredicate, HandlerFunction) offers a direct way to create a router.

Generally, it is recommended to use the route() builder, as it provides convenient short-cuts for typical mapping scenarios without requiring hard-to-discover static imports. For instance, the router function builder offers the method GET(String, HandlerFunction) to create a mapping for GET requests; and POST(String, HandlerFunction) for POSTs.

Besides HTTP method-based mapping, the route builder offers a way to introduce additional predicates when mapping to requests. For each HTTP method there is an overloaded variant that takes a RequestPredicate as a parameter, though which additional constraints can be expressed.

Predicates

You can write your own RequestPredicate, but the RequestPredicates utility class offers commonly used implementations, based on the request path, HTTP method, content-type, and so on. The following example uses a request predicate to create a constraint based on the Accept header:

Java
Kotlin

RouterFunction<ServerResponse> route = RouterFunctions.route()
	.GET("/hello-world", accept(MediaType.TEXT_PLAIN),
		request -> ServerResponse.ok().bodyValue("Hello World")).build();

val route = coRouter {
	GET("/hello-world", accept(TEXT_PLAIN)) {
		ServerResponse.ok().bodyValueAndAwait("Hello World")
	}
}

You can compose multiple request predicates together by using:

RequestPredicate.and(RequestPredicate) — both must match.
RequestPredicate.or(RequestPredicate) — either can match.

Many of the predicates from RequestPredicates are composed. For example, RequestPredicates.GET(String) is composed from RequestPredicates.method(HttpMethod) and RequestPredicates.path(String). The example shown above also uses two request predicates, as the builder uses RequestPredicates.GET internally, and composes that with the accept predicate.

Routes

Router functions are evaluated in order: if the first route does not match, the second is evaluated, and so on. Therefore, it makes sense to declare more specific routes before general ones. This is also important when registering router functions as Spring beans, as will be described later. Note that this behavior is different from the annotation-based programming model, where the "most specific" controller method is picked automatically.

When using the router function builder, all defined routes are composed into one RouterFunction that is returned from build(). There are also other ways to compose multiple router functions together:

add(RouterFunction) on the RouterFunctions.route() builder
RouterFunction.and(RouterFunction)
RouterFunction.andRoute(RequestPredicate, HandlerFunction) — shortcut for RouterFunction.and() with nested RouterFunctions.route().

The following example shows the composition of four routes:

Java
Kotlin

import static org.springframework.http.MediaType.APPLICATION_JSON;
import static org.springframework.web.reactive.function.server.RequestPredicates.*;

PersonRepository repository = ...
PersonHandler handler = new PersonHandler(repository);

RouterFunction<ServerResponse> otherRoute = ...

RouterFunction<ServerResponse> route = route()
	.GET("/person/{id}", accept(APPLICATION_JSON), handler::getPerson) (1)
	.GET("/person", accept(APPLICATION_JSON), handler::listPeople) (2)
	.POST("/person", handler::createPerson) (3)
	.add(otherRoute) (4)
	.build();

1	`GET /person/{id}` with an `Accept` header that matches JSON is routed to `PersonHandler.getPerson`
2	`GET /person` with an `Accept` header that matches JSON is routed to `PersonHandler.listPeople`
3	`POST /person` with no additional predicates is mapped to `PersonHandler.createPerson`, and
4	`otherRoute` is a router function that is created elsewhere, and added to the route built.

import org.springframework.http.MediaType.APPLICATION_JSON

val repository: PersonRepository = ...
val handler = PersonHandler(repository);

val otherRoute: RouterFunction<ServerResponse> = coRouter {  }

val route = coRouter {
	GET("/person/{id}", accept(APPLICATION_JSON), handler::getPerson) (1)
	GET("/person", accept(APPLICATION_JSON), handler::listPeople) (2)
	POST("/person", handler::createPerson) (3)
}.and(otherRoute) (4)

1	`GET /person/{id}` with an `Accept` header that matches JSON is routed to `PersonHandler.getPerson`
2	`GET /person` with an `Accept` header that matches JSON is routed to `PersonHandler.listPeople`
3	`POST /person` with no additional predicates is mapped to `PersonHandler.createPerson`, and
4	`otherRoute` is a router function that is created elsewhere, and added to the route built.

Nested Routes

It is common for a group of router functions to have a shared predicate, for instance a shared path. In the example above, the shared predicate would be a path predicate that matches /person, used by three of the routes. When using annotations, you would remove this duplication by using a type-level @RequestMapping annotation that maps to /person. In WebFlux.fn, path predicates can be shared through the path method on the router function builder. For instance, the last few lines of the example above can be improved in the following way by using nested routes:

Java
Kotlin

RouterFunction<ServerResponse> route = route()
	.path("/person", builder -> builder (1)
		.GET("/{id}", accept(APPLICATION_JSON), handler::getPerson)
		.GET(accept(APPLICATION_JSON), handler::listPeople)
		.POST(handler::createPerson))
	.build();

1	Note that second parameter of `path` is a consumer that takes the router builder.

val route = coRouter { (1)
	"/person".nest {
		GET("/{id}", accept(APPLICATION_JSON), handler::getPerson)
		GET(accept(APPLICATION_JSON), handler::listPeople)
		POST(handler::createPerson)
	}
}

1	Create router using Coroutines router DSL; a Reactive alternative is also available via `router { }`.

Though path-based nesting is the most common, you can nest on any kind of predicate by using the nest method on the builder. The above still contains some duplication in the form of the shared Accept-header predicate. We can further improve by using the nest method together with accept:

Java
Kotlin

RouterFunction<ServerResponse> route = route()
	.path("/person", b1 -> b1
		.nest(accept(APPLICATION_JSON), b2 -> b2
			.GET("/{id}", handler::getPerson)
			.GET(handler::listPeople))
		.POST(handler::createPerson))
	.build();

val route = coRouter {
	"/person".nest {
		accept(APPLICATION_JSON).nest {
			GET("/{id}", handler::getPerson)
			GET(handler::listPeople)
			POST(handler::createPerson)
		}
	}
}

Running a Server

See equivalent in the Servlet stack

How do you run a router function in an HTTP server? A simple option is to convert a router function to an HttpHandler by using one of the following:

RouterFunctions.toHttpHandler(RouterFunction)
RouterFunctions.toHttpHandler(RouterFunction, HandlerStrategies)

You can then use the returned HttpHandler with a number of server adapters by following HttpHandler for server-specific instructions.

A more typical option, also used by Spring Boot, is to run with a DispatcherHandler-based setup through the WebFlux Config, which uses Spring configuration to declare the components required to process requests. The WebFlux Java configuration declares the following infrastructure components to support functional endpoints:

RouterFunctionMapping: Detects one or more RouterFunction<?> beans in the Spring configuration, orders them, combines them through RouterFunction.andOther, and routes requests to the resulting composed RouterFunction.
HandlerFunctionAdapter: Simple adapter that lets DispatcherHandler invoke a HandlerFunction that was mapped to a request.
ServerResponseResultHandler: Handles the result from the invocation of a HandlerFunction by invoking the writeTo method of the ServerResponse.

The preceding components let functional endpoints fit within the DispatcherHandler request processing lifecycle and also (potentially) run side by side with annotated controllers, if any are declared. It is also how functional endpoints are enabled by the Spring Boot WebFlux starter.

The following example shows a WebFlux Java configuration (see DispatcherHandler for how to run it):

Java
Kotlin

@Configuration
@EnableWebFlux
public class WebConfig implements WebFluxConfigurer {

	@Bean
	public RouterFunction<?> routerFunctionA() {
		// ...
	}

	@Bean
	public RouterFunction<?> routerFunctionB() {
		// ...
	}

	// ...

	@Override
	public void configureHttpMessageCodecs(ServerCodecConfigurer configurer) {
		// configure message conversion...
	}

	@Override
	public void addCorsMappings(CorsRegistry registry) {
		// configure CORS...
	}

	@Override
	public void configureViewResolvers(ViewResolverRegistry registry) {
		// configure view resolution for HTML rendering...
	}
}

@Configuration
@EnableWebFlux
class WebConfig : WebFluxConfigurer {

	@Bean
	fun routerFunctionA(): RouterFunction<*> {
		// ...
	}

	@Bean
	fun routerFunctionB(): RouterFunction<*> {
		// ...
	}

	// ...

	override fun configureHttpMessageCodecs(configurer: ServerCodecConfigurer) {
		// configure message conversion...
	}

	override fun addCorsMappings(registry: CorsRegistry) {
		// configure CORS...
	}

	override fun configureViewResolvers(registry: ViewResolverRegistry) {
		// configure view resolution for HTML rendering...
	}
}

Filtering Handler Functions

See equivalent in the Servlet stack

You can filter handler functions by using the before, after, or filter methods on the routing function builder. With annotations, you can achieve similar functionality by using @ControllerAdvice, a ServletFilter, or both. The filter will apply to all routes that are built by the builder. This means that filters defined in nested routes do not apply to "top-level" routes. For instance, consider the following example:

Java
Kotlin

RouterFunction<ServerResponse> route = route()
	.path("/person", b1 -> b1
		.nest(accept(APPLICATION_JSON), b2 -> b2
			.GET("/{id}", handler::getPerson)
			.GET(handler::listPeople)
			.before(request -> ServerRequest.from(request) (1)
				.header("X-RequestHeader", "Value")
				.build()))
		.POST(handler::createPerson))
	.after((request, response) -> logResponse(response)) (2)
	.build();

1	The `before` filter that adds a custom request header is only applied to the two GET routes.
2	The `after` filter that logs the response is applied to all routes, including the nested ones.

val route = router {
	"/person".nest {
		GET("/{id}", handler::getPerson)
		GET("", handler::listPeople)
		before { (1)
			ServerRequest.from(it)
					.header("X-RequestHeader", "Value").build()
		}
		POST(handler::createPerson)
		after { _, response -> (2)
			logResponse(response)
		}
	}
}

1	The `before` filter that adds a custom request header is only applied to the two GET routes.
2	The `after` filter that logs the response is applied to all routes, including the nested ones.

The filter method on the router builder takes a HandlerFilterFunction: a function that takes a ServerRequest and HandlerFunction and returns a ServerResponse. The handler function parameter represents the next element in the chain. This is typically the handler that is routed to, but it can also be another filter if multiple are applied.

Now we can add a simple security filter to our route, assuming that we have a SecurityManager that can determine whether a particular path is allowed. The following example shows how to do so:

Java
Kotlin

SecurityManager securityManager = ...

RouterFunction<ServerResponse> route = route()
	.path("/person", b1 -> b1
		.nest(accept(APPLICATION_JSON), b2 -> b2
			.GET("/{id}", handler::getPerson)
			.GET(handler::listPeople))
		.POST(handler::createPerson))
	.filter((request, next) -> {
		if (securityManager.allowAccessTo(request.path())) {
			return next.handle(request);
		}
		else {
			return ServerResponse.status(UNAUTHORIZED).build();
		}
	})
	.build();

val securityManager: SecurityManager = ...

val route = router {
		("/person" and accept(APPLICATION_JSON)).nest {
			GET("/{id}", handler::getPerson)
			GET("", handler::listPeople)
			POST(handler::createPerson)
			filter { request, next ->
				if (securityManager.allowAccessTo(request.path())) {
					next(request)
				}
				else {
					status(UNAUTHORIZED).build();
				}
			}
		}
	}

The preceding example demonstrates that invoking the next.handle(ServerRequest) is optional. We only let the handler function be run when access is allowed.

Besides using the filter method on the router function builder, it is possible to apply a filter to an existing router function via RouterFunction.filter(HandlerFilterFunction).

CORS support for functional endpoints is provided through a dedicated CorsWebFilter.