WebSocket is a thin, lightweight layer above TCP. This makes it suitable for using “subprotocols” to embed messages. In this guide, we use STOMP messaging with Spring to create an interactive web application. STOMP is a subprotocol operating on top of the lower-level WebSocket.
implementation 'org.webjars:webjars-locator-core'
implementation 'org.webjars:sockjs-client:1.0.2'
implementation 'org.webjars:stomp-websocket:2.3.3'
implementation 'org.webjars:bootstrap:3.3.7'
implementation 'org.webjars:jquery:3.1.1-1'
getName() methodUpon receiving the message and extracting the name, the service will process it by creating a greeting and publishing that greeting on a separate queue to which the client is subscribed. The greeting will also be a JSON object, which as the following listing shows:
{
"content": "Hello, Fred!"
}
getContent() methodSpring will use the Jackson JSON library to automatically marshal instances of type Greeting into JSON.
In Spring’s approach to working with STOMP messaging, STOMP messages can be routed to @Controller classes. For example, the GreetingController
package com.example.messagingstompwebsocket;
import org.springframework.messaging.handler.annotation.MessageMapping;
import org.springframework.messaging.handler.annotation.SendTo;
import org.springframework.stereotype.Controller;
import org.springframework.web.util.HtmlUtils;
@Controller
public class GreetingController {
@MessageMapping("/hello")
@SendTo("/topic/greetings")
public Greeting greeting(HelloMessage message) throws Exception {
Thread.sleep(1000); // simulated delay
return new Greeting("Hello, " + HtmlUtils.htmlEscape(message.getName()) + "!");
}
}
This controller is concise and simple, but plenty is going on. We break it down step by step.
The @MessageMapping annotation ensures that, if a message is sent to the /hello destination, the greeting() method is called.
The payload of the message is bound to a HelloMessage object, which is passed into greeting().
Internally, the implementation of the method simulates a processing delay by causing the thread to sleep for one second. This is to demonstrate that, after the client sends a message, the server can take as long as it needs to asynchronously process the message. The client can continue with whatever work it needs to do without waiting for the response.
After the one-second delay, the greeting() method creates a Greeting object and returns it. The return value is broadcast to all subscribers of /topic/greetings, as specified in the @SendTo annotation. Note that the name from the input message is sanitized, since, in this case, it will be echoed back and re-rendered in the browser DOM on the client side.
Create a Java class named WebSocketConfig that resembles the following listing
package com.example.messagingstompwebsocket;
import org.springframework.context.annotation.Configuration;
import org.springframework.messaging.simp.config.MessageBrokerRegistry;
import org.springframework.web.socket.config.annotation.EnableWebSocketMessageBroker;
import org.springframework.web.socket.config.annotation.StompEndpointRegistry;
import org.springframework.web.socket.config.annotation.WebSocketMessageBrokerConfigurer;
@Configuration
@EnableWebSocketMessageBroker
public class WebSocketConfig implements WebSocketMessageBrokerConfigurer {
@Override
public void configureMessageBroker(MessageBrokerRegistry config) {
config.enableSimpleBroker("/topic");
config.setApplicationDestinationPrefixes("/app");
}
@Override
public void registerStompEndpoints(StompEndpointRegistry registry) {
registry.addEndpoint("/gs-guide-websocket").withSockJS();
}
}
WebSocketConfig is annotated with @Configuration to indicate that it is a Spring configuration class. It is also annotated with @EnableWebSocketMessageBroker. As its name suggests, @EnableWebSocketMessageBroker enables WebSocket message handling, backed by a message broker.
The configureMessageBroker() method implements the default method in WebSocketMessageBrokerConfigurer to configure the message broker. It starts by calling enableSimpleBroker() to enable a simple memory-based message broker to carry the greeting messages back to the client on destinations prefixed with /topic. It also designates the /app prefix for messages that are bound for methods annotated with @MessageMapping. This prefix will be used to define all the message mappings. For example, /app/hello is the endpoint that the GreetingController.greeting() method is mapped to handle.
The registerStompEndpoints() method registers the /gs-guide-websocket endpoint, enabling SockJS fallback options so that alternate transports can be used if WebSocket is not available. The SockJS client will attempt to connect to /gs-guide-websocket and use the best available transport (websocket, xhr-streaming, xhr-polling, and so on).
Create an index.html file similar to the following listing
<!DOCTYPE html>
<html>
<head>
<title>Hello WebSocket</title>
<link href="/webjars/bootstrap/css/bootstrap.min.css" rel="stylesheet">
<link href="/main.css" rel="stylesheet">
<script src="/webjars/jquery/jquery.min.js"></script>
<script src="/webjars/sockjs-client/sockjs.min.js"></script>
<script src="/webjars/stomp-websocket/stomp.min.js"></script>
<script src="/app.js"></script>
</head>
<body>
<noscript><h2 style="color: #ff0000">Seems your browser doesn't support Javascript! Websocket relies on Javascript being
enabled. Please enable
Javascript and reload this page!</h2></noscript>
<div id="main-content" class="container">
<div class="row">
<div class="col-md-6">
<form class="form-inline">
<div class="form-group">
<label for="connect">WebSocket connection:</label>
<button id="connect" class="btn btn-default" type="submit">Connect</button>
<button id="disconnect" class="btn btn-default" type="submit" disabled="disabled">Disconnect
</button>
</div>
</form>
</div>
<div class="col-md-6">
<form class="form-inline">
<div class="form-group">
<label for="name">What is your name?</label>
<input type="text" id="name" class="form-control" placeholder="Your name here...">
</div>
<button id="send" class="btn btn-default" type="submit">Send</button>
</form>
</div>
</div>
<div class="row">
<div class="col-md-12">
<table id="conversation" class="table table-striped">
<thead>
<tr>
<th>Greetings</th>
</tr>
</thead>
<tbody id="greetings">
</tbody>
</table>
</div>
</div>
</div>
</body>
</html>
This HTML file imports the SockJS and STOMP javascript libraries that will be used to communicate with our server through STOMP over websocket. We also import app.js, which contains the logic of our client application.
var stompClient = null;
function setConnected(connected) {
$("#connect").prop("disabled", connected);
$("#disconnect").prop("disabled", !connected);
if (connected) {
$("#conversation").show();
}
else {
$("#conversation").hide();
}
$("#greetings").html("");
}
function connect() {
var socket = new SockJS('/gs-guide-websocket');
stompClient = Stomp.over(socket);
stompClient.connect({}, function (frame) {
setConnected(true);
console.log('Connected: ' + frame);
stompClient.subscribe('/topic/greetings', function (greeting) {
showGreeting(JSON.parse(greeting.body).content);
});
});
}
function disconnect() {
if (stompClient !== null) {
stompClient.disconnect();
}
setConnected(false);
console.log("Disconnected");
}
function sendName() {
stompClient.send("/app/hello", {}, JSON.stringify({'name': $("#name").val()}));
}
function showGreeting(message) {
$("#greetings").append("<tr><td>" + message + "</td></tr>");
}
$(function () {
$("form").on('submit', function (e) {
e.preventDefault();
});
$( "#connect" ).click(function() { connect(); });
$( "#disconnect" ).click(function() { disconnect(); });
$( "#send" ).click(function() { sendName(); });
});