How Server Sent Event send response to a specific client

Question

In Server Sent Event, it always send the same response to all the client. but what i want to know is, How to send response to an only one client using java.

this is my event which define inside sw.js (SSE)

var eventSource = new EventSource("HelloServlet");
eventSource.addEventListener('up_vote',function(event){
    console.log("data from s" , event.data);
    var title = event.data;
    self.registration.showNotification(title, {
          'body': event.data,
          'icon': 'images/icon.png'
        })
});

I want to show this notification only to an specific user. not for everyboday. HelloServlet is my servlet and it contain this,

response.setContentType("text/event-stream");   
        response.setCharacterEncoding("UTF-8");
        PrintWriter writer = response.getWriter();
        String upVote = "u";
        String downVote = "d";
        for(int i=0; i<10; i++) {
            writer.write("event:up_vote\n");
            writer.write("data: "+ upVote +"\n\n");
            writer.write("event:down_vote\n");
            writer.write("data: "+ downVote +"\n\n");
            writer.flush();
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        writer.close();

Answer 1

The reason that you are only sending the same message to all clients is because you only have one channel that they are all connected to. The EventSource is just a GET request that you are leaving open. Like any get request you can make it bespoke to a particular user in a couple of ways.

var eventSource = new EventSource("HelloServlet?username=their-user-name");

In this example you are using the query string to create a unique channel for each person. You would then need logic on the server-side to send different content depending on the username variable.

You could also use sessions. So you could keep the current code on the client.

var eventSource = new EventSource("HelloServlet");

But on the server side you would need to examine the session and then have logic to send different content depending on the session information.

Does that help?

Answer 2

I know the post is very old but it still worth mentioning a solution to this problem. I had more or less same use case where I need to publish notifications to users and each user will have their own notifications. I used a simple trick, EventSource listen to events and I appended userId as a part of event type to distinguish among users. So instead of on up_vote, I used up_vote_userId1. And our backend based on which user has an update publish the events to these event type. And our problem solved.

Answer 3

I recommend you the JEaSSE library (Java Easy Server-Sent Events): http://mvnrepository.com/artifact/info.macias/jeasse

You can find some usage examples here: https://github.com/mariomac/jeasse

Specifically, for your use case:

@WebServlet(asyncSupported = true)
public class ExampleServlet1 extends HttpServlet {

    SseDispatcher dispatcher;;

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp)
                                      throws ServletException, IOException {
       dispatcher = new SseDispatcher(req).ok().open();
    }

    public void onGivenEvent(String info) {
       dispatcher.send("givenEvent",info);
    }
}