An introduction to Reactive Web

Unlike the Servlet stack, the Reactive Web Stack is rewritten from scratch using ReactiveStreams and Reactor API.

There are two important concepts in the Reactive Web stack to handle web request.

• WebHandler - which is the high-level APIs to assemble the resources eg. WebFilter, ExceptionHandler, etc. to handle web requests from client.
• HttpHandler - which is a low-level API to adapt web handlers to the underlying runtime environment, such as Netty, as well as Servlet 3.1+ container which did provide async capability.

You can review the Application classes in the Getting Started part.

Enable WebFlux

To activate Reactive Web, create a @Configurationclass, add an additional @EnableWebFlux to activate WebFlux support.

@Configuration
@EnableWebFlux
class WebConfig {}

Similar to the existing WebMvcConfigurer in the Servlet stack. There is a WebFluxConfigurer interface for you to customize the details of web resources, validation, CORS, etc.

You can create a configuration class to derive this interface and override the methods to customize the built-in configuration.

@Configuration
@EnableWebFlux
class WebConfig implements WebFluxConfigurer {}

NOTE: For Spring Boot applications, no extra steps are required to activate WebFlux if spring-boot-starter-webflux is included in the classpath.

RxJava 2 and RxJava 3

By default, Spring WebFlux heavily depends on Reactor, but it also supports RxJava 2 and RxJava 3 APIs via adapters.

To use RxJava APIs instead of Reactor's API, firstly add RxJava in project dependencies.

<dependency>
    <groupId>io.reactivex.rxjava3</groupId>
    <artifactId>rxjava</artifactId>
    <version>3.0.7</version>
</dependency>

Then you can use RxJava API freely in the @RestController like the APIs in the reactive Repository .

@RestController
@RequestMapping(value = "/posts")
class PostController {

    private final PostRepository posts;

    public PostController(PostRepository posts) {
        this.posts = posts;
    }

    @GetMapping(value = "")
    public Observable<Post> all() {
        return this.posts.findAll();
    }

    @GetMapping(value = "/{id}")
    public Maybe<Post> get(@PathVariable(value = "id") UUID id) {
        return this.posts.findById(id);
    }

    @PostMapping(value = "")
    public Single<Post> create(Post post) {
        return this.posts.save(post);
    }
}

For the complete codes, check spring-reactive-sample/rxjava2 and spring-reactive-sample/rxjava3.

Java 9 Flow API

As a member of the ReactiveStreams family, Spring WebFlux also supports it. You can use Flow.Publisher as the controller method return type or parameters.

NOTE: Make sure you are using Java 9 or the later versions.

@RestController
@RequestMapping(value = "/posts")
public class PostController {

    @GetMapping
    public Flow.Publisher<Post> all() {
        // see: https://stackoverflow.com/questions/46597924/spring-5-supports-java-9-flow-apis-in-its-reactive-feature
        return JdkFlowAdapter.publisherToFlowPublisher(
                Flux.just(
                        new Post(1L, "post one", "content of post one"),
                        new Post(2L, "post two", "content of post two")
                )
        );
    }

}

For the complete codes, check spring-reactive-sample/java9.

SmallRye Mutiny

SmallRye Mutiny support is added in Spring Framework 5.3.10. For developers, you can use Mutiny APIs instead of Reactor to build Spring WebFlux applications.

Add the following project dependencies.

<dependencyManagement>
    <dependencies>
        <dependency>
            <groupId>io.smallrye.reactive</groupId>
            <artifactId>mutiny-bom</artifactId>
            <version>1.0.0</version>
            <type>pom</type>
            <scope>import</scope>
        </dependency>
    </dependencies>
</dependencyManagement>

<dependencies>

    <dependency>
        <groupId>io.smallrye.reactive</groupId>
        <artifactId>mutiny</artifactId>
    </dependency>
    ...

</dependencies>

There is a Controller exmaple which is written in SmallRye Mutiny.

@RestController
@RequestMapping(value = "/posts")
class PostController {

    private final PostRepository posts;

    public PostController(PostRepository posts) {
        this.posts = posts;
    }

    @GetMapping(value = "")
    public Multi<Post> all() {
        return this.posts.findAll();
    }

    @GetMapping(value = "/{id}")
    public Uni<Post> get(@PathVariable(value = "id") UUID id) {
        return this.posts.findById(id);
    }

    @PostMapping(value = "")
    public Uni<ResponseEntity<?>> create(@RequestBody Post post) {
        return this.posts.save(post).map(p -> ResponseEntity.created(URI.create("/posts/" + p.getId())).build());
    }

}

For the complete codes, check spring-reactive-sample/smallrye-mutiny.