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.
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.
build.gradle
link:initial/build.gradle[role=include]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
link:complete/src/main/java/hello/HelloMessage.java[role=include]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
link:complete/src/main/java/hello/Greeting.java[role=include]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.
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
link:complete/src/main/java/hello/GreetingController.java[role=include]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.
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
link:complete/src/main/java/hello/WebSocketConfig.java[role=include]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.
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
link:complete/src/main/resources/static/index.html[role=include]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).
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
link:complete/src/main/java/hello/Application.java[role=include]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.
Logging output is displayed. The service should be up and running within a few seconds.
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.