Send Message to all clients via SimpMessagingTemplate in ServletContextListener

Question

I'm using the Spring framework and I have a working websocket controller that looks like this:

@Controller
public class GreetingController {

    @MessageMapping("/hello")
    @SendTo("/topic/greetings")
    public Greeting greeting(HelloMessage message) throws InterruptedException {
        return new Greeting("Hello, " + message.getName() + "!");
    }
}

I also have this configuration:

@Configuration
@EnableWebSocketMessageBroker
public class HelloWebSocketConfig extends AbstractWebSocketMessageBrokerConfigurer {

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

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

That part works great! I can successfully send and receive messages between two or more browsers using Stomp.js. Here's the part that doesn't work. I have implemented a ServletContextListener that contains a custom object that, for simplicity sake, I have called 'notifier'. The notifier listens for certain events to happen on the server-side. It then calls the 'notify' method, which is supposed to send details about the event to all clients. It doesn't work, though.

@WebListener
public class MessageListener implements ServletContextListener, Notifiable {

    private Notifier notifier;

    @Autowired
    private SimpMessagingTemplate messageSender;


    public MessageListener() {
        notifier = new Notifier(this);
    }

    public void contextInitialized(ServletContextEvent contextEvent) {
        WebApplicationContextUtils
        .getRequiredWebApplicationContext(contextEvent.getServletContext())
        .getAutowireCapableBeanFactory()
        .autowireBean(this);

        notifier.start();
    }

    public void contextDestroyed(ServletContextEvent contextEvent) {
        notifier.stop();
    }

    public void notify(NotifyEvent event) {
        messageSender.convertAndSend("/topic/greetings", new Greeting("Hello, " + event.subject + "!"));
    }
}

I do not get an exception. The SimpMessagingTemplate has been successfully injected by Spring, so it's not null. I have been able to step into the Spring code and have figured out that the SimpleBrokerMessageHandler's subscriptionRegistry is empty when using the SimpMessagingTemplate. So it must be a separate instance from the one that the controllers are using. How can I get the same subscriptionRegistry that is used by the controllers?

Answer 1

The solution was to use Spring's ApplicationListener class instead of a ServletContextListener, and to specifically listen for the ContextRefreshedEvent.

This is my working example:

@Component
public class MessagingApplicationListener implements ApplicationListener<ContextRefreshedEvent>, Notifiable {
    private final NotifierFactor notifierFactory;
    private final MessageSendingOperations<String> messagingTemplate;
    private Notifier notifier;

    @Autowired
    public MessagingApplicationListener(NotifierFactor notifierFactory, MessageSendingOperations<String> messagingTemplate) {
        this.notifierFactory = notifierFactory;
        this.messagingTemplate = messagingTemplate;
    }

    @Override
    public void onApplicationEvent(ContextRefreshedEvent event) {
        if (notifier == null) {
            notifier = notifierFactory.create(this);
            notifier.start();
        }
    }

    public void notify(NotifyEvent event) {
        messagingTemplate.convertAndSend("/topic/greetings", new Greeting("Hello, " + event.subject + "!"));
    }

    @PreDestroy
    private void stopNotifier() {
        if (notifier != null) {
            notifier.stop();
        }
    }
}

Answer 2

This solution worked great.

For anybody who wants a quick solution - strip out all the "Notifier" stuff and just inject MessagingApplicationListener into your class. Then call 'notify' with a String to send a message. Obviously, as @battmanz says you'll want some way to push events to the listener, but this class does the basics of what you need.