This guide walks you through the process of creating a "hello world" application that sends messages back and forth, between a browser and the server. WebSocket is a very thin, lightweight layer above TCP. It makes it very suitable to use "subprotocols" to embed messages. In this guide we’ll dive in and use STOMP messaging with Spring to create an interactive web application.

What you’ll build

You’ll build a server that will accept a message carrying a user’s name. In response, it will push a greeting into a queue that the client is subscribed to.

What you’ll need

How to complete this guide

Like most Spring Getting Started guides, you can start from scratch and complete each step, or you can bypass basic setup steps that are already familiar to you. Either way, you end up with working code.

To start from scratch, move on to Set up the project.

To skip the basics, do the following:

When you’re finished, you can check your results against the code in gs-messaging-stomp-websocket/complete.

Set up the project

First you set up a basic build script. You can use any build system you like when building apps with Spring, but the code you need to work with Gradle and Maven is included here. If you’re not familiar with either, refer to Building Java Projects with Gradle or Building Java Projects with Maven.

Create the directory structure

In a project directory of your choosing, create the following subdirectory structure; for example, with mkdir -p src/main/java/hello on *nix systems:

└── src
    └── main
        └── java
            └── hello

Create a Gradle build file

Below is the initial Gradle build file. But you can also use Maven. The pom.xml file is included right here. If you are using Spring Tool Suite (STS), you can import the guide directly.

build.gradle

buildscript {
    repositories {
        maven { url "http://repo.spring.io/libs-snapshot" }
        mavenLocal()
    }
    dependencies {
        classpath("org.springframework.boot:spring-boot-gradle-plugin:1.0.1.RELEASE")
    }
}

apply plugin: 'java'
apply plugin: 'eclipse'
apply plugin: 'idea'
apply plugin: 'spring-boot'

jar {
    baseName = 'gs-messaging-stomp-websocket'
    version =  '0.1.0'
}

repositories {
    mavenCentral()
    maven { url "http://repo.spring.io/libs-snapshot" }
}

dependencies {
    compile("org.springframework.boot:spring-boot-starter-websocket")
    compile("org.springframework:spring-messaging")
    testCompile("junit:junit")
}

task wrapper(type: Wrapper) {
    gradleVersion = '1.11'
}

The Spring Boot gradle plugin provides many convenient features:

  • It collects all the jars on the classpath and builds a single, runnable "über-jar", which makes it more convenient to execute and transport your service.

  • It searches for the public static void main() method to flag as a runnable class.

  • It provides a built-in dependency resolver that sets the version number to match Spring Boot dependencies. You can override any version you wish, but it will default to Boot’s chosen set of versions.

Create a resource representation class

Now that you’ve set up the project and build system, you can create your STOMP message service.

Begin the process by thinking about service interactions.

The service will accept messages containing a name in a STOMP message whose body is a JSON object. If the name given is "Fred", then the message might look something like this:

{
    "name": "Fred"
}

To model the message carrying the name, you can create a plain old Java object with a name property and a corresponding getName() method:

src/main/java/hello/HelloMessage.java

package hello;

public class HelloMessage {

    private String name;

    public String getName() {
        return name;
    }

}

Upon receiving the message and extracting the name, the service will process it by creating a greeting and publishing that greeting on a separate queue that the client is subscribed to. The greeting will also be a JSON object, which might look something like this:

{
    "content": "Hello, Fred!"
}

To model the greeting representation, you add another plain old Java object with a content property and corresponding getContent() method:

src/main/java/hello/Greeting.java

package hello;

public class Greeting {

    private String content;

    public Greeting(String content) {
        this.content = content;
    }

    public String getContent() {
        return content;
    }

}

Spring will use the Jackson JSON library to automatically marshal instances of type Greeting into JSON.

Next, you’ll create a controller to receive the hello message and send a greeting message.

Create a message-handling controller

In Spring’s approach to working with STOMP messaging, STOMP messages can be routed to @Controller classes. For example the GreetingController is mapped to handle messages to destination "/hello".

src/main/java/hello/GreetingController.java

package hello;

import org.springframework.messaging.handler.annotation.MessageMapping;
import org.springframework.messaging.handler.annotation.SendTo;
import org.springframework.stereotype.Controller;

@Controller
public class GreetingController {


    @MessageMapping("/hello")
    @SendTo("/topic/greetings")
    public Greeting greeting(HelloMessage message) throws Exception {
        Thread.sleep(3000); // simulated delay
        return new Greeting("Hello, " + message.getName() + "!");
    }

}

This controller is concise and simple, but there’s plenty going on. Let’s break it down step by step.

The @MessageMapping annotation ensures that if a message is sent to destination "/hello", then 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 3 seconds. This is to demonstrate that after the client sends a message, the server can take as long as it needs to process the message asynchronously. The client may continue with whatever work it needs to do without waiting on the response.

After the 3 second delay, the greeting() method creates a Greeting object and returns it. The return value is broadcast to all subscribers to "/topic/greetings" as specified in the @SendTo annotation.

Configure Spring for STOMP messaging

Now that the essential components of the service are created, you can configure Spring to enable WebSocket and STOMP messaging.

Create a Java class named WebSocketConfig that looks like this:

src/main/java/hello/WebSocketConfig.java

package hello;

import org.springframework.context.annotation.Configuration;
import org.springframework.messaging.simp.config.MessageBrokerRegistry;
import org.springframework.web.socket.config.annotation.AbstractWebSocketMessageBrokerConfigurer;
import org.springframework.web.socket.config.annotation.EnableWebSocketMessageBroker;
import org.springframework.web.socket.config.annotation.StompEndpointRegistry;

@Configuration
@EnableWebSocketMessageBroker
public class WebSocketConfig extends AbstractWebSocketMessageBrokerConfigurer {

	@Override
	public void configureMessageBroker(MessageBrokerRegistry config) {
		config.enableSimpleBroker("/topic");
		config.setApplicationDestinationPrefixes("/app");
	}

	@Override
	public void registerStompEndpoints(StompEndpointRegistry registry) {
		registry.addEndpoint("/hello").withSockJS();
	}

}

WebSocketConfig is annotated with @Configuration to indicate that it is a Spring configuration class. It is also annotated @EnableWebSocketMessageBroker. As its name suggests, @EnableWebSocketMessageBroker enables WebSocket message handling, backed by a message broker.

The configureMessageBroker() method overrides 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 @MessageMapping-annotated methods.

The registerStompEndpoints() method registers the "/hello" endpoint, enabling SockJS fallback options so that alternative messaging options may be used if WebSocket is not available. This endpoint, when prefixed with "/app", is the endpoint that the GreetingController.greeting() method is mapped to handle.

Create a browser client

With the server side pieces in place, now let’s turn our attention to the JavaScript client that will send messages to and receive messages from the server side.

Create an index.html file that looks like this:

src/main/resources/static/index.html

<!DOCTYPE html>
<html>
<head>
    <title>Hello WebSocket</title>
    <script src="sockjs-0.3.4.js"></script>
    <script src="stomp.js"></script>
    <script type="text/javascript">
        var stompClient = null;

        function setConnected(connected) {
            document.getElementById('connect').disabled = connected;
            document.getElementById('disconnect').disabled = !connected;
            document.getElementById('conversationDiv').style.visibility = connected ? 'visible' : 'hidden';
            document.getElementById('response').innerHTML = '';
        }

        function connect() {
            var socket = new SockJS('/hello');
            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() {
            stompClient.disconnect();
            setConnected(false);
            console.log("Disconnected");
        }

        function sendName() {
            var name = document.getElementById('name').value;
            stompClient.send("/app/hello", {}, JSON.stringify({ 'name': name }));
        }

        function showGreeting(message) {
            var response = document.getElementById('response');
            var p = document.createElement('p');
            p.style.wordWrap = 'break-word';
            p.appendChild(document.createTextNode(message));
            response.appendChild(p);
        }
    </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>
    <div>
        <button id="connect" onclick="connect();">Connect</button>
        <button id="disconnect" disabled="disabled" onclick="disconnect();">Disconnect</button>
    </div>
    <div id="conversationDiv">
        <label>What is your name?</label><input type="text" id="name" />
        <button id="sendName" onclick="sendName();">Send</button>
        <p id="response"></p>
    </div>
</div>
</body>
</html>

The main piece of this HTML file to pay attention to is the JavaScript code in the connect() and sendName() functions.

The connect() function uses SockJS and stomp.js to open a connection to "/gs-messaging-stomp-websocket/hello", which is where GreetingController is waiting for connections. Upon a successful connection, it subscribes to the "/topic/greetings" destination, where the server will publish greeting messages. When a greeting is received on that destination, it will append a paragraph element to the DOM to display the greeting message.

The sendName() function retrieves the name entered by the user and uses the STOMP client to send it to the "/app/hello" destination (where GreetingController.greeting() will receive it).

Make the application executable

Although it is possible to package this service as a traditional WAR file for deployment to an external application server, the simpler approach demonstrated below creates a standalone application. You package everything in a single, executable JAR file, driven by a good old Java main() method. Along the way, you use Spring’s support for embedding the Tomcat servlet container as the HTTP runtime, instead of deploying to an external instance.

src/main/java/hello/Application.java

package hello;

import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
import org.springframework.boot.SpringApplication;
import org.springframework.context.annotation.ComponentScan;

@ComponentScan
@EnableAutoConfiguration
public class Application {

    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);
    }
}

The main() method defers to the SpringApplication helper class, providing Application.class as an argument to its run() method. This tells Spring to read the annotation metadata from Application and to manage it as a component in the Spring application context.

The @ComponentScan annotation tells Spring to search recursively through the hello package and its children for classes marked directly or indirectly with Spring’s @Component annotation. This directive ensures that Spring finds and registers the GreetingController, because it is marked with @Controller, which in turn is a kind of @Component annotation.

The @EnableAutoConfiguration annotation switches on reasonable default behaviors based on the content of your classpath. For example, because the application depends on the embeddable version of Tomcat (tomcat-embed-core.jar), a Tomcat server is set up and configured with reasonable defaults on your behalf. And because the application also depends on Spring MVC (spring-webmvc.jar), a Spring MVC DispatcherServlet is configured and registered for you — no web.xml necessary! Auto-configuration is a powerful, flexible mechanism. See the API documentation for further details.

Build an executable JAR

You can build a single executable JAR file that contains all the necessary dependencies, classes, and resources. This makes it easy to ship, version, and deploy the service as an application throughout the development lifecycle, across different environments, and so forth.

./gradlew build

Then you can run the JAR file:

java -jar build/libs/gs-messaging-stomp-websocket-0.1.0.jar

If you are using Maven, you can run the application using mvn spring-boot:run. Or you can build the JAR file with mvn clean package and run the JAR by typing:

java -jar target/gs-messaging-stomp-websocket-0.1.0.jar
The procedure above will create a runnable JAR. You can also opt to build a classic WAR file instead.

Run the service

If you are using Gradle, you can run your service at the command line this way:

./gradlew clean build && java -jar build/libs/gs-messaging-stomp-websocket-0.1.0.jar
If you are using Maven, you can run your service by typing mvn clean package && java -jar target/gs-messaging-stomp-websocket-0.1.0.jar.

You can alternatively run the app directly from Gradle like this:

./gradlew bootRun
With mvn, you can run mvn spring-boot:run.

Logging output is displayed. The service should be up and running within a few seconds.

Test the service

Now that the service is running, point your browser at http://localhost:8080 and click the "Connect" button.

Upon opening a connection, you are asked for your name. Enter your name and click "Send". Your name is sent to the server as a JSON message over STOMP. After a 3-second simulated delay, the server sends a message back with a "Hello" greeting that is displayed on the page. At this point, you can send another name, or you can click the "Disconnect" button to close the connection.

Summary

Congratulations! You’ve just developed a STOMP-based messaging service with Spring.