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Functional Endpoints

Spring Web MVC includes WebMvc.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 DispatcherServlet.

Overview

See equivalent in the Reactive stack

In WebMvc.fn, an HTTP request is handled with a HandlerFunction: a function that takes ServerRequest and returns a 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 an optional HandlerFunction (i.e. Optional<HandlerFunction>). When the router function matches, a handler function is returned; otherwise an empty Optional. 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.servlet.function.RequestPredicates.*;
import static org.springframework.web.servlet.function.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 ServerResponse listPeople(ServerRequest request) {
		// ...
	}

	public ServerResponse createPerson(ServerRequest request) {
		// ...
	}

	public ServerResponse getPerson(ServerRequest request) {
		// ...
	}
}
1 Create router using route(). 
import org.springframework.web.servlet.function.router

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

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


class PersonHandler(private val repository: PersonRepository) {

	// ...

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

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

	fun getPerson(request: ServerRequest): ServerResponse {
		// ...
	}
}
1 Create router using the router DSL.

If you register the RouterFunction as a bean, for instance by exposing it in a @Configuration class, it will be auto-detected by the servlet, as explained in Running a Server.

HandlerFunction

See equivalent in the Reactive stack

ServerRequest and ServerResponse are immutable interfaces that offer JDK 8-friendly access to the HTTP request and response, including headers, body, method, and status code.

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 String:

  • Java

  • Kotlin

String string = request.body(String.class);
val string = request.body<String>()

The following example extracts the body to a List<Person>, where Person objects are decoded from a serialized form, such as JSON or XML:

  • Java

  • Kotlin

List<Person> people = request.body(new ParameterizedTypeReference<List<Person>>() {});
val people = request.body<Person>()

The following example shows how to access parameters:

  • Java

  • Kotlin

MultiValueMap<String, String> params = request.params();
val map = request.params()

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

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

You can also use an asynchronous result as the body, in the form of a CompletableFuture, Publisher, or any other type supported by the ReactiveAdapterRegistry. For instance:

  • Java

  • Kotlin

Mono<Person> person = webClient.get().retrieve().bodyToMono(Person.class);
ServerResponse.ok().contentType(MediaType.APPLICATION_JSON).body(person);
val person = webClient.get().retrieve().awaitBody<Person>()
ServerResponse.ok().contentType(MediaType.APPLICATION_JSON).body(person)

If not just the body, but also the status or headers are based on an asynchronous type, you can use the static async method on ServerResponse, which accepts CompletableFuture<ServerResponse>, Publisher<ServerResponse>, or any other asynchronous type supported by the ReactiveAdapterRegistry. For instance:

  • Java

Mono<ServerResponse> asyncResponse = webClient.get().retrieve().bodyToMono(Person.class)
  .map(p -> ServerResponse.ok().header("Name", p.name()).body(p));
ServerResponse.async(asyncResponse);

Server-Sent Events can be provided via the static sse method on ServerResponse. The builder provided by that method allows you to send Strings, or other objects as JSON. For example:

  • Java

  • Kotlin

public RouterFunction<ServerResponse> sse() {
	return route(GET("/sse"), request -> ServerResponse.sse(sseBuilder -> {
				// Save the sseBuilder object somewhere..
			}));
}

// In some other thread, sending a String
sseBuilder.send("Hello world");

// Or an object, which will be transformed into JSON
Person person = ...
sseBuilder.send(person);

// Customize the event by using the other methods
sseBuilder.id("42")
		.event("sse event")
		.data(person);

// and done at some point
sseBuilder.complete();
fun sse(): RouterFunction<ServerResponse> = router {
	GET("/sse") { request -> ServerResponse.sse { sseBuilder ->
		// Save the sseBuilder object somewhere..
	}
}

// In some other thread, sending a String
sseBuilder.send("Hello world")

// Or an object, which will be transformed into JSON
val person = ...
sseBuilder.send(person)

// Customize the event by using the other methods
sseBuilder.id("42")
		.event("sse event")
		.data(person)

// and done at some point
sseBuilder.complete()

Handler Classes

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

  • Java

  • Kotlin

HandlerFunction<ServerResponse> helloWorld =
  request -> ServerResponse.ok().body("Hello World");
val helloWorld: (ServerRequest) -> ServerResponse =
  { ServerResponse.ok().body("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 ServerResponse listPeople(ServerRequest request) { (1)
		List<Person> people = repository.allPeople();
		return ok().contentType(APPLICATION_JSON).body(people);
	}

	public ServerResponse createPerson(ServerRequest request) throws Exception { (2)
		Person person = request.body(Person.class);
		repository.savePerson(person);
		return ok().build();
	}

	public ServerResponse getPerson(ServerRequest request) { (3)
		int personId = Integer.parseInt(request.pathVariable("id"));
		Person person = repository.getPerson(personId);
		if (person != null) {
			return ok().contentType(APPLICATION_JSON).body(person);
		}
		else {
			return 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.
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. 
class PersonHandler(private val repository: PersonRepository) {

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

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

	fun getPerson(request: ServerRequest): ServerResponse { (3)
		val personId = request.pathVariable("id").toInt()
		return repository.getPerson(personId)?.let { ok().contentType(APPLICATION_JSON).body(it) }
				?: 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.
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 ServerResponse createPerson(ServerRequest request) {
		Person person = request.body(Person.class);
		validate(person); (2)
		repository.savePerson(person);
		return ok().build();
	}

	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)

	// ...

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

	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 Reactive 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, through 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().body("Hello World")).build();
import org.springframework.web.servlet.function.router

val route = router {
	GET("/hello-world", accept(TEXT_PLAIN)) {
		ServerResponse.ok().body("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.servlet.function.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
import org.springframework.web.servlet.function.router

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

val otherRoute = router {  }

val route = router {
	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 WebMvc.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. 
import org.springframework.web.servlet.function.router

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

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();
import org.springframework.web.servlet.function.router

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

Running a Server

See equivalent in the Reactive stack

You typically run router functions in a DispatcherHandler-based setup through the MVC Config, which uses Spring configuration to declare the components required to process requests. The MVC 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.

The preceding components let functional endpoints fit within the DispatcherServlet 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 Web starter.

The following example shows a WebFlux Java configuration:

  • Java

  • Kotlin

@Configuration
@EnableMvc
public class WebConfig implements WebMvcConfigurer {

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

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

	// ...

	@Override
	public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
		// configure message conversion...
	}

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

	@Override
	public void configureViewResolvers(ViewResolverRegistry registry) {
		// configure view resolution for HTML rendering...
	}
}
@Configuration
@EnableMvc
class WebConfig : WebMvcConfigurer {

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

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

	// ...

	override fun configureMessageConverters(converters: List<HttpMessageConverter<*>>) {
		// 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 Reactive 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. 
import org.springframework.web.servlet.function.router

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();
import org.springframework.web.servlet.function.router

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 CorsFilter.