How to get an existing websocket instance

Question

I'm working on an application that uses Websockets (Java EE 7) to send messages to all the connected clients asynchronously. The server (Websocket endpoint) should send these messages whenever a new article (an engagement modal in my app) is created.

Everytime a connection is established to the websocket endpoint, I'm adding the corresponding session to a list, which I could be able to access outside.

But the problem I had is, when I'm accessing this created websocket endpoint to which all the clients connected from outside (any other business class), I've get the existing instance (like a singleton).

So, can you please suggest me a way I can get an existing instance of the websocket endpoint, as I can't create it as new MyWebsocketEndPoint() coz it'll be created by the websocket internal mechanism whenever the request from a client is received.

For a ref:

private static WebSocketEndPoint INSTANCE = null;

public static WebSocketEndPoint getInstance() {
if(INSTANCE == null) {
// Instead of creating a new instance, I need an existing one
    INSTANCE = new WebSocketEndPoint ();
}
        return INSTANCE;
}

Thanks in advance.

Answer 1

The container creates a separate instance of the endpoint for every client connection, so you can't do what you're trying to do. But I think what you're trying to do is send a message to all the active client connections when an event occurs, which is fairly straightforward.

The javax.websocket.Session class has the getBasicRemote method to retrieve a RemoteEndpoint.Basic instance that represents the endpoint associated with that session.

You can retrieve all the open sessions by calling Session.getOpenSessions(), then iterate through them. The loop will send each client connection a message. Here's a simple example:

@ServerEndpoint("/myendpoint")
public class MyEndpoint {
  @OnMessage
  public void onMessage(Session session, String message) {
    try {  
      for (Session s : session.getOpenSessions()) {
        if (s.isOpen()) {
          s.getBasicRemote().sendText(message);
        }
    } catch (IOException ex) { ... }
  } 
} 

But in your case, you probably want to use CDI events to trigger the update to all the clients. In that case, you'd create a CDI event that a method in your Websocket endpoint class observes:

@ServerEndpoint("/myendpoint")
public class MyEndpoint {
  // EJB that fires an event when a new article appears
  @EJB
  ArticleBean articleBean;
  // a collection containing all the sessions
  private static final Set<Session> sessions = 
          Collections.synchronizedSet(new HashSet<Session>());

  @OnOpen
  public void onOpen(final Session session) {
    // add the new session to the set
    sessions.add(session);
    ...
  }

  @OnClose
  public void onClose(final Session session) {
    // remove the session from the set
    sessions.remove(session);
  }

  public void broadcastArticle(@Observes @NewArticleEvent ArticleEvent articleEvent) {
    synchronized(sessions) {
      for (Session s : sessions) {
        if (s.isOpen()) {
          try {
            // send the article summary to all the connected clients
            s.getBasicRemote().sendText("New article up:" + articleEvent.getArticle().getSummary());
          } catch (IOException ex) { ... }
        }
      }
    }
  }
}

The EJB in the above example would do something like:

...
@Inject
Event<ArticleEvent> newArticleEvent;

public void publishArticle(Article article) {
  ...
  newArticleEvent.fire(new ArticleEvent(article));
  ...
}

See the Java EE 7 Tutorial chapters on WebSockets and CDI Events.

Edit: Modified the @Observer method to use an event as a parameter.

Edit 2: wrapped the loop in broadcastArticle in synchronized, per @gcvt.

Edit 3: Updated links to Java EE 7 Tutorial. Nice job, Oracle. Sheesh.