23. WebFlux framework

This section provides basic information on the reactive programming support for Web applications in Spring Framework 5.

23.1 Introduction

23.1.1 What is Reactive Programming?

In plain terms reactive programming is about non-blocking applications that are asynchronous and event-driven and require a small number of threads to scale vertically (i.e. within the JVM) rather than horizontally (i.e. through clustering).

A key aspect of reactive applications is the concept of backpressure which is a mechanism to ensure producers don’t overwhelm consumers. For example in a pipeline of reactive components extending from the database to the HTTP response when the HTTP connection is too slow the data repository can also slow down or stop completely until network capacity frees up.

Reactive programming also leads to a major shift from imperative to declarative async composition of logic. It is comparable to writing blocking code vs using the CompletableFuture from Java 8 to compose follow-up actions via lambda expressions.

For a longer introduction check the blog series "Notes on Reactive Programming" by Dave Syer.

23.1.2 Reactive API and Building Blocks

Spring Framework 5 embraces Reactive Streams as the contract for communicating backpressure across async components and libraries. Reactive Streams is a specification created through industry collaboration that has also been adopted in Java 9 as java.util.concurrent.Flow.

The Spring Framework uses Reactor internally for its own reactive support. Reactor is a Reactive Streams implementation that further extends the basic Reactive Streams Publisher contract with the Flux and Mono composable API types to provide declarative operations on data sequences of 0..N and 0..1.

The Spring Framework exposes Flux and Mono in many of its own reactive APIs. At the application level however, as always, Spring provides choice and fully supports the use of RxJava. For more on reactive types check the post "Understanding Reactive Types" by Sebastien Deleuze.

23.2 Spring WebFlux Module

Spring Framework 5 includes a new spring-webflux module. The module contains support for reactive HTTP and WebSocket clients as well as for reactive server web applications including REST, HTML browser, and WebSocket style interactions.

23.2.1 Server Side

On the server-side WebFlux supports 2 distinct programming models:

  • Annotation-based with @Controller and the other annotations supported also with Spring MVC
  • Functional, Java 8 lambda style routing and handling

Both programming models are executed on the same reactive foundation that adapts non-blocking HTTP runtimes to the Reactive Streams API. The diagram below shows the server-side stack including traditional, Servlet-based Spring MVC on the left from the spring-webmvc module and also the reactive stack on the right from the spring-webflux module.

webflux overview

WebFlux can run on Servlet containers with support for the Servlet 3.1 Non-Blocking IO API as well as on other async runtimes such as Netty and Undertow. Each runtime is adapted to a reactive ServerHttpRequest and ServerHttpResponse exposing the body of the request and response as Flux<DataBuffer>, rather than InputStream and OutputStream, with reactive backpressure. REST-style JSON and XML serialization and deserialization is supported on top as a Flux<Object>, and so is HTML view rendering and Server-Sent Events.

Annotation-based Programming Model

The same @Controller programming model and the same annotations used in Spring MVC are also supported in WebFlux. The main difference is that the underlying core, framework contracts — i.e. HandlerMapping, HandlerAdapter, are non-blocking and operate on the reactive ServerHttpRequest and ServerHttpResponse rather than on the HttpServletRequest and HttpServletResponse. Below is an example with a reactive controller:

public class PersonController {

	private final PersonRepository repository;

	public PersonController(PersonRepository repository) {
		this.repository = repository;

	Mono<Void> create(@RequestBody Publisher<Person> personStream) {
		return this.repository.save(personStream).then();

	Flux<Person> list() {
		return this.repository.findAll();

	Mono<Person> findById(@PathVariable String id) {
		return this.repository.findOne(id);

Functional Programming Model

Functional Programming Model

This section is to be merged into web-flux.adoc.


Incoming HTTP requests are handled by a HandlerFunction, which is essentially a function that takes a ServerRequest and returns a Mono<ServerResponse>. The annotation counterpart to a handler function would be a method with @RequestMapping.

ServerRequest and ServerResponse are immutable interfaces that offer JDK-8 friendly access to the underlying HTTP messages. Both are fully reactive by building on top of Reactor: the request expose the body as Flux or Mono; the response accepts any Reactive Streams Publisher as body.

ServerRequest gives access to various HTTP request elements: the method, URI, query parameters, and — through the separate ServerRequest.Headers interface — the headers. Access to the body is provided through the body methods. For instance, this is how to extract the request body into a Mono<String>:

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

And here is how to extract the body into a Flux, where Person is a class that can be deserialised from the contents of the body (i.e. Person is supported by Jackson if the body contains JSON, or JAXB if XML).

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

The two methods above (bodyToMono and bodyToFlux) are, in fact, convenience methods that use the generic ServerRequest.body(BodyExtractor) method. BodyExtractor is a functional strategy interface that allows you to write your own extraction logic, but common BodyExtractor instances can be found in the BodyExtractors utility class. So, the above examples can be replaced with:

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

Similarly, ServerResponse provides access to the HTTP response. Since it is immutable, you create a ServerResponse with a builder. The builder allows you to set the response status, add response headers, and provide a body. For instance, this is how to create a response with a 200 OK status, a JSON content-type, and a body:

Mono<Person> person = ...

And here is how to build a response with a 201 Created status, Location header, and empty body:

URI location = ...

Putting these together allows us to create a HandlerFunction. For instance, here is an example of a simple "Hello World" handler lambda, that returns a response with a 200 status and a body based on a String:

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

Writing handler functions as lambda’s, as we do above, is convenient, but perhaps lacks in readability and becomes less maintainable when dealing with multiple functions. Therefore, it is recommended to group related handler functions into a handler or controller class. For example, here is a class that exposes a reactive Person repository:

import static org.springframework.http.MediaType.APPLICATION_JSON;
import static org.springframework.web.reactive.function.BodyInserters.fromObject;

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 ServerResponse.ok().contentType(APPLICATION_JSON).body(people, Person.class);

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

	public Mono<ServerResponse> getPerson(ServerRequest request) { 3
		int personId = Integer.valueOf(request.pathVariable("id"));
		Mono<ServerResponse> notFound = ServerResponse.notFound().build();
		Mono<Person> personMono = this.repository.getPerson(personId);
		return personMono
				.then(person -> ServerResponse.ok().contentType(APPLICATION_JSON).body(fromObject(person)))


listPeople is a handler function that returns all Person objects found in the repository as JSON.


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, i.e. when the Person has been saved.


getPerson is a handler function that returns a single person, identified via the path variable id. We retrieve that Person via the repository, and create a JSON response if it is found. If it is not found, we use otherwiseIfEmpty(Mono<T>) to return a 404 Not Found response.


Incoming requests are routed to handler functions with a RouterFunction, which is a function that takes a ServerRequest, and returns a Mono<HandlerFunction>. If a request matches a particular route, a handler function is returned; otherwise it returns an empty Mono. The RouterFunction has a similar purpose as the @RequestMapping annotation in @Controller classes.

Typically, you do not write router functions yourself, but rather use RouterFunctions.route(RequestPredicate, HandlerFunction) to create one using a request predicate and handler function. If the predicate applies, the request is routed to the given handler function; otherwise no routing is performed, resulting in a 404 Not Found response. Though you can write your own RequestPredicate, you do not have to: the RequestPredicates utility class offers commonly used predicates, such matching based on path, HTTP method, content-type, etc. Using route, we can route to our "Hello World" handler function:

RouterFunction<ServerResponse> helloWorldRoute =
	request -> Response.ok().body(fromObject("Hello World")));

Two router functions can be composed into a new router function that routes to either handler function: if the predicate of the first route does not match, the second is evaluated. Composed router functions are evaluated in order, so it makes sense to put specific functions before generic ones. You can compose two router functions by calling RouterFunction.and(RouterFunction), or by calling RouterFunction.andRoute(RequestPredicate, HandlerFunction), which is a convenient combination of RouterFunction.and() with RouterFunctions.route().

Given the PersonHandler we showed above, we can now define a router function that routes to the respective handler functions. We use method-references to refer to the handler functions:

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> personRoute =
	route(GET("/person/{id}").and(accept(APPLICATION_JSON)), handler::getPerson)
		.andRoute(GET("/person").and(accept(APPLICATION_JSON)), handler::listPeople)
		.andRoute(POST("/person").and(contentType(APPLICATION_JSON)), handler::createPerson);

Besides router functions, you can also compose request predicates, by calling RequestPredicate.and(RequestPredicate) or RequestPredicate.or(RequestPredicate). These work as expected: for and the resulting predicate matches if both given predicates match; or matches if either predicate does. Most of the predicates found in RequestPredicates are compositions. For instance, RequestPredicates.GET(String) is a composition of RequestPredicates.method(HttpMethod) and RequestPredicates.path(String).

Running a Server

Now there is just one piece of the puzzle missing: running a router function in an HTTP server. You can convert a router function into a HttpHandler by using RouterFunctions.toHttpHandler(RouterFunction). The HttpHandler allows you to run on a wide variety of reactive runtimes: Reactor Netty, RxNetty, Servlet 3.1+, and Undertow. Here is how we run a router function in Reactor Netty, for instance:

RouterFunction<ServerResponse> route = ...
HttpHandler httpHandler = RouterFunctions.toHttpHandler(route);
ReactorHttpHandlerAdapter adapter = new ReactorHttpHandlerAdapter(httpHandler);
HttpServer server = HttpServer.create(HOST, PORT);

For Tomcat it looks like this:

RouterFunction<ServerResponse> route = ...
HttpHandler httpHandler = RouterFunctions.toHttpHandler(route);
HttpServlet servlet = new ServletHttpHandlerAdapter(httpHandler);
Tomcat server = new Tomcat();
Context rootContext = server.addContext("", System.getProperty("java.io.tmpdir"));
Tomcat.addServlet(rootContext, "servlet", servlet);
rootContext.addServletMapping("/", "servlet");

TODO: DispatcherHandler


Routes mapped by a router function can be filtered by calling RouterFunction.filter(HandlerFilterFunction), where HandlerFilterFunction is essentially 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 HandlerFunction that is routed to, but can also be another FilterFunction if multiple filters are applied. With annotations, similar functionality can be achieved using @ControllerAdvice and/or a ServletFilter. Let’s add a simple security filter to our route, assuming that we have a SecurityManager that can determine whether a particular path is allowed:

import static org.springframework.http.HttpStatus.UNAUTHORIZED;

SecurityManager securityManager = ...
RouterFunction<ServerResponse> route = ...

RouterFunction<ServerResponse> filteredRoute =
	route.filter(request, next) -> {
		if (securityManager.allowAccessTo(request.path())) {
			return next.handle(request);
		else {
			return ServerResponse.status(UNAUTHORIZED).build();

You can see in this example that invoking the next.handle(ServerRequest) is optional: we only allow the handler function to be executed when access is allowed.

23.2.2 Client Side

WebFlux includes a functional, reactive WebClient that offers a fully non-blocking and reactive alternative to the RestTemplate. It exposes network input and output as a reactive ClientHttpRequest and ClientHttpResponse where the body of the request and response is a Flux<DataBuffer> rather than an InputStream and OutputStream. In addition it supports the same reactive JSON, XML, and SSE serialization mechanism as on the server side so you can work with typed objects. Below is an example of using the WebClient which requires a ClientHttpConnector implementation to plug in a specific HTTP client such as Reactor Netty:

WebClient client = WebClient.create("http://example.com");

Mono<Account> account = client.get()
		.url("/accounts/{id}", 1L)
		.then(response -> response.bodyToMono(Account.class));

The AsyncRestTemplate also supports non-blocking interactions. The main difference is it can’t support non-blocking streaming, like for example Twitter one, because fundamentally it’s still based and relies on InputStream and OutputStream.

23.2.3 Request and Response Body Conversion

The spring-core module provides reactive Encoder and Decoder contracts that enable the serialization of a Flux of bytes to and from typed objects. The spring-web module adds JSON (Jackson) and XML (JAXB) implementations for use in web applications as well as others for SSE streaming and zero-copy file transfer.

For example the request body can be one of the following way and it will be decoded automatically in both the annotation and the functional programming models:

  • Account account — the account is deserialized without blocking before the controller is invoked.
  • Mono<Account> account — the controller can use the Mono to declare logic to be executed after the account is deserialized.
  • Single<Account> account — same as with Mono but using RxJava
  • Flux<Account> accounts — input streaming scenario.
  • Observable<Account> accounts — input streaming with RxJava.

The response body can be one of the following:

  • Mono<Account> — serialize without blocking the given Account when the Mono completes.
  • Single<Account> — same but using RxJava.
  • Flux<Account> — streaming scenario, possibly SSE depending on the requested content type.
  • Observable<Account> — same but using RxJava Observable type.
  • Flowable<Account> — same but using RxJava 2 Flowable type.
  • Flux<ServerSentEvent> — SSE streaming.
  • Mono<Void> — request handling completes when the Mono completes.
  • Account — serialize without blocking the given Account; implies a synchronous, non-blocking controller method.
  • void — specific to the annotation-based programming model, request handling completes when the method returns; implies a synchronous, non-blocking controller method.

When using stream types like Flux or Observable, the media type specified in the request/response or at mapping/routing level is used to determine how the data should be serialized and flushed. For example a REST endpoint that returns a Flux<User> will be serialized by default as following:

  • application/json: a Flux<User> is handled as an asynchronous collection and serialized as a JSON array with an explicit flush when the complete event is emitted.
  • application/stream+json: a Flux<User> will be handled as a stream of User elements serialized as individual JSON object separated by new lines and explicitly flushed after each element. The WebClient supports JSON stream decoding so this is a good use case for server to server use case.
  • text/event-stream: a Flux<User> or Flux<ServerSentEvent<User>> will be handled as a stream of User or ServerSentEvent elements serialized as individual SSE elements using by default JSON for data encoding and explicit flush after each element. This is well suited for exposing a stream to browser clients. WebClient supports reading SSE streams as well.

23.2.4 Reactive WebSocket Support

WebFlux includes reactive WebSocket client and server support. Both client and server are supported on the Java WebSocket API (JSR-356), Jetty, Undertow, Reactor Netty, and RxNetty.

On the server side, declare a WebSocketHandlerAdapter and then simply add mappings to WebSocketHandler-based endpoints:

public HandlerMapping webSocketMapping() {
	Map<String, WebSocketHandler> map = new HashMap<>();
	map.put("/foo", new FooWebSocketHandler());
	map.put("/bar", new BarWebSocketHandler());

	SimpleUrlHandlerMapping mapping = new SimpleUrlHandlerMapping();
	return mapping;

public WebSocketHandlerAdapter handlerAdapter() {
	return new WebSocketHandlerAdapter();

On the client side create a WebSocketClient for one of the supported libraries listed above:

WebSocketClient client = new ReactorNettyWebSocketClient();
client.execute("ws://localhost:8080/echo"), session -> {... }).blockMillis(5000);

23.2.5 Testing

The spring-test module includes a WebTestClient that can be used to test WebFlux server endpoints with or without a running server.

Tests without a running server are comparable to MockMvc from Spring MVC where mock request and response are used instead of connecting over the network using a socket. The WebTestClient however can also perform tests against a running server.

For more see sample tests in the framework.

23.3 Getting Started

23.3.1 Spring Boot Starter

The Spring Boot Web Reactive starter available via http://start.spring.io is the fastest way to get started. It does all that’s necessary so you to start writing @Controller classes just like with Spring MVC. Simply go to http://start.spring.io, choose version 2.0.0.BUILD-SNAPSHOT, and type reactive in the dependencies box. By default the starter runs with Tomcat but the dependencies can be changed as usual with Spring Boot to switch to a different runtime. See the starter page for more details and instruction

This starter also supports the functional web API and will detect automatically RouterFunction beans. Your Spring Boot WebFlux application should use the RouterFunction or the RequestMapping approach, it is currently not possible to mix them in the same application.

23.3.2 Manual Bootstrapping

This section outlines the steps to get up and running manually.

For dependencies start with spring-webflux and spring-context. Then add jackson-databind and io.netty:netty-buffer (temporarily see SPR-14528) for JSON support. Lastly add the dependencies for one of the supported runtimes:

  • Tomcat — org.apache.tomcat.embed:tomcat-embed-core
  • Jetty — org.eclipse.jetty:jetty-server and org.eclipse.jetty:jetty-servlet
  • Reactor Netty — io.projectreactor.ipc:reactor-netty
  • RxNetty — io.reactivex:rxnetty-common and io.reactivex:rxnetty-http
  • Undertow — io.undertow:undertow-core

For the annotation-based programming model bootstrap with:

ApplicationContext context = new AnnotationConfigApplicationContext(DelegatingWebFluxConfiguration.class);  // (1)
HttpHandler handler = DispatcherHandler.toHttpHandler(context);  // (2)

The above loads default Spring Web framework configuration (1), then creates a DispatcherHandler, the main class driving request processing (2), and adapts it to HttpHandler — the lowest level Spring abstraction for reactive HTTP request handling.

For the functional programming model bootstrap as follows:

ApplicationContext context = new AnnotationConfigApplicationContext(); // (1)
context.registerBean(FooBean.class, () -> new FooBeanImpl()); // (2)
context.registerBean(BarBean.class); // (3)

HttpHandler handler = WebHttpHandlerBuilder
		.build(); // (4)

The above creates an AnnotationConfigApplicationContext instance (1) that can take advantage of the new functional bean registration API (2) to register beans using a Java 8 Supplier or just by specifying its class (3). The HttpHandler is created using WebHttpHandlerBuilder (4).

The HttpHandler can then be installed in one of the supported runtimes:

// Tomcat and Jetty (also see notes below)
HttpServlet servlet = new ServletHttpHandlerAdapter(handler);

// Reactor Netty
ReactorHttpHandlerAdapter adapter = new ReactorHttpHandlerAdapter(handler);
HttpServer.create(host, port).newHandler(adapter).block();

// RxNetty
RxNettyHttpHandlerAdapter adapter = new RxNettyHttpHandlerAdapter(handler);
HttpServer server = HttpServer.newServer(new InetSocketAddress(host, port));

// Undertow
UndertowHttpHandlerAdapter adapter = new UndertowHttpHandlerAdapter(handler);
Undertow server = Undertow.builder().addHttpListener(port, host).setHandler(adapter).build();

For Servlet containers especially with WAR deployment you can use the AbstractAnnotationConfigDispatcherHandlerInitializer which as a WebApplicationInitializer and is auto-detected by Servlet containers. It takes care of registering the ServletHttpHandlerAdapter as shown above. You will need to implement one abstract method in order to point to your Spring configuration.

23.3.3 Examples

You will find code examples useful to build reactive Web application in the following projects: