How to implement push to client using Java EE 7 WebSockets?

Question

I\'ve browsed a lot of Web Socket examples, presentation slides and they are mostly concentrated on a rather simple scenarios in which client-server communication is initiat

Answer 1

Store the active sessionList in another class SessionManager.

List<Session> socketSessions = new ArrayList<>();

Add incoming session in @OnOpen to list. Remove session from list in @OnClose

@OnClose
public void close(Session session) {
  sessionManager.removeSession(session);
}

To send a message to everyone,

public void broadcast(String message){
    for(Session session: sessionList){
        session.getBasicRemote().sendText(message);
    }
}

You can use the sessionManager.broadcast() method wherever the event triggers.

Here is a complete example of how websocket can be used to make push with HTML5 websocket API. https://metamug.com/article/java-push-notification-with-websocket.php

Answer 2

I do it this way (no client request is needed):

@ServerEndpoint("/hello")
public class HelloWebSocket {

   @OnOpen
   public void greetTheClient(Session session){
       try {
        session.getBasicRemote().sendText("Hello stranger");

    } catch (IOException ioe) {
        System.out.println(ioe.getMessage());
    }
   }
}

Answer 3

I have implemented a solution similar to your problem using omnifaces socket component. Notifications are triggered by JMS messages and recipients may be specific users or all of them. The blog post url https://konstpan.wordpress.com/2018/03/25/push-notifications-in-a-jee-web-application-secured-by-keycloak/

Answer 4

Probably this is not most elegant way but just to demonstrate idea. Method broadcast() will send message to all connected clients.

@ServerEndpoint("/echo")
public class ServerEndPoint {

    private static Set<Session> userSessions = Collections.newSetFromMap(new ConcurrentHashMap<Session, Boolean>());

    @OnOpen
    public void onOpen(Session userSession) {
        userSessions.add(userSession);
    }

    @OnClose
    public void onClose(Session userSession) {
        userSessions.remove(userSession);
    }

    @OnMessage
    public void onMessage(String message, Session userSession) {
        broadcast(message);
    }

    public static void broadcast(String msg) {
        for (Session session : userSessions) {
            session.getAsyncRemote().sendText(msg);
        }
    }

}