WebSocket

WebSocket is a bi-directional multiplexed protocol, it is based on HTTP protocol. WebSocket is wildly used in client and server real-time communications, such as online gaming, and multiple clients chat applications. It is very flexible to create your own sub protocol.

Spring WebFlux adds simple basic WebSocket support.

Server Side

Firstly create a WebSocket Handler to process the incoming messages and send back to the client.

public class PostsWebSocketHandler implements WebSocketHandler {

    private final PostRepository posts;

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

    @Override
    public List<String> getSubProtocols() {
        return Arrays.asList("test");
    }

    @Override
    public Mono<Void> handle(WebSocketSession session) {
        String protocol = session.getHandshakeInfo().getSubProtocol();
        WebSocketMessage message = session.textMessage(this.posts.findAll().takeLast(0).toString());
        return doSend(session, Mono.just(message));
    }

    // TODO: workaround for suspected RxNetty WebSocket client issue
    // https://github.com/ReactiveX/RxNetty/issues/560
    private Mono<Void> doSend(WebSocketSession session, Publisher<WebSocketMessage> output) {
        return session.send(Mono.delay(Duration.ofMillis(100)).thenMany(output));
    }

}

And register this handler in a HandlerMapping bean.

@Bean
public HandlerMapping handlerMapping() {
    Map<String, WebSocketHandler> map = new HashMap<>();
    map.put("/echo", new EchoWebSocketHandler());
    map.put("/posts", new PostsWebSocketHandler(this.posts));

    SimpleUrlHandlerMapping mapping = new SimpleUrlHandlerMapping();
    mapping.setUrlMap(map);
    return mapping;
}

In a none Spring Boot application, you have to declare a WebSocketHandlerAdapter bean.

@Bean
WebSocketHandlerAdapter webSocketHandlerAdapter(){
    return new WebSocketHandlerAdapter();
}

Source codes: spring-reactive-sample/websocket.

Client Side

Let’s move to the client side to interact with this WebSocket.

Spring provides a built-in Netty based WebSocket client to communicate with the server side.

WebSocketClient client = new ReactorNettyWebSocketClient();
//        client.execute(new URI("ws://localhost:8080/echo"), (WebSocketSession session) -> {
//            session.send().log().;
//        });

int count = 100;
Flux<String> input = Flux.range(1, count).map(index -> "msg-" + index);
ReplayProcessor<Object> output = ReplayProcessor.create(count);

client.execute(new URI("ws://localhost:8080/echo"),
               session -> {
                   log.debug("Starting to send messages");
                   return session
                       .send(input.doOnNext(s -> log.debug("outbound " + s)).map(session::textMessage))
                       .thenMany(session.receive().take(count).map(WebSocketMessage::getPayloadAsText))
                       .subscribeWith(output)
                       .doOnNext(s -> log.debug("inbound " + s))
                       .then()
                       .doOnTerminate((aVoid, ex) ->
                                      log.debug("Done with " + (ex != null ? ex.getMessage() : "success")));
               })
    .block(Duration.ofMillis(5000));

}

Source codes: spring-reactive-sample/client.

A more close to the real world example application, go to hantsy/angular-spring-websocket-sample/.