gs-messaging-stomp-websocket

<#assign project_id="gs-rest-service">

Getting Started: Messaging with WebSocket and STOMP

This guide walks you through creating a "hello world" STOMP messaging server with Spring.

What you'll build

The server will accept a message carying the user's name. In response, it will push a greeting into a queue that the client is subscribed to.

What you'll need

• About 15 minutes
• Tomcat 8 (Tomcat 8.0.0-RC1 is available)
• <@prereq_editor_jdk_buildtools/>

<@how_to_complete_this_guide/>

Set up the project

<@build_system_intro/>

<@create_directory_structure_hello/>

Create a Maven POM

<@snippet path="pom.xml" prefix="initial"/>

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:

<@snippet path="src/main/java/hello/HelloMessage.java" prefix="complete"/>

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 another plain old Java object with a content property and corresponding getContent() method:

<@snippet path="src/main/java/hello/Greeting.java" prefix="complete"/>

Note: As you see in steps below, Spring uses the Jackson JSON library to automatically marshal instances of type Greeting into JSON.

Next, you'll create the 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 handled by a controller. These components are easily identified by the [@Controller][AtController] annotation, and the GreetingController below is mapped to handle messages published on the "/app/hello" destination.

<@snippet path="src/main/java/hello/GreetingController.java" prefix="complete"/>

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

The @MessageMapping annotation ensures that if a message is published on the "/app/hello" destination, then the greeting() method will called.

@RequestBody binds the payload of the message 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 new Greeting object, setting its content to say "Hello" to the name from the HelloMessage. It then calls convertAndSend() on the injected SimpMessageSendingOperations to send the Greeting on the "/queue/greetings" destination.

The Greeting object must be converted to JSON. Thanks to Spring's HTTP message converter support, you don't need to do this conversion manually. When you configure Spring for STOMP messaging, you'll inject SimpMessagingTemplate with an instance of MappingJackson2MessageConverter. It will be used to convert the Greeting instance to JSON.

Configure Spring for STOMP messaging

TODO: What follows is the configuration for the STOMP/WebSocket-specific piece.

<@snippet path="src/main/java/hello/StompConfig.java" prefix="complete"/>

TODO: This is extremely ugly at the moment, with many beans in play. Rossen says that SPR-10835 will be resolved in time for RC1. There's no need to write this section to describe all of these beans now. It's better to hold off and wait for SPR-10835 to be resolved and then adjust the configuration accordingly and write about that. At that time, assuming the solution is simple enough, the configuration in StompConfig.java can be merged back into WebConfig.java to have only a single configuration class.

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:

<@snippet path="src/main/webapp/index.html" prefix="complete"/>

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 "/queue/greetings" destination, where the server will publish greeting messages. When a greeting appears on that queue, 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

In order to deploy the application to Tomcat, you'll need to add a bit more configuration.

First, you'll need to configure Spring's DispatcherServlet to serve static resources so that it will serve index.html. This can be done by creating a configuration class that overrides the configureDefaultServletHandling() method of WebMvcConfigurerAdapter and calls enable() on the given DefaultServletHandlerConfigurer:

<@snippet path="src/main/java/hello/WebConfig.java" prefix="complete"/>

You'll also need to configure DispatcherServlet. This is most easily done by creating a class that extends AbstractAnnotationConfigDispatcherServletInitializer:

<@snippet path="src/main/java/hello/HelloServletInitializer.java" prefix="complete"/>

Here, DispatcherServlet is mapped to "/". Also, WebConfig and EndpointConfig are specified, respectively, as the servlet and root configuration classes.

Now you're ready to build and deploy the application to Tomcat 8. Start by building the WAR file:

mvn package

Then copy the WAR file to Tomcat 8's trunk/output/webapps directory.

Finally, restart Tomcat 8:

output/bin/shutdown.sh
output/bin/startup.sh

After the application starts, point your browser at http://localhost:8080/gs-messaging-stomp-websocket and click the "Connect" button.

Upon opening a connection, you will be asked for your name. Enter your name and click "Send". Your name will be sent to the server as a JSON message over STOMP. The server will send a message back with a "Hello" greeting that will be displayed on the page. At this point, you may choose to 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.