Server-Sent Events on Java Web server

Question

I have a Java Web server using Spring Framework and I want to use Server Sent Events to send notifications to the web client at each second.

My Controller for these notifications looks like this:

@Controller
public class NotificationController {

    private static final String REST_PREFIX = "/rest/notifications";

    @RequestMapping(value = {REST_PREFIX}, method = {RequestMethod.GET})
    synchronized public void getMonitoringNotifications(HttpServletRequest request, HttpServletResponse response) {
        response.setContentType("text/event-stream;charset=UTF-8");
        response.setHeader("Cache-Control", "no-cache");
        response.setHeader("Connection", "keep-alive");

        try {
            PrintWriter out = response.getWriter();
            int i = 0;

            while (true) {
                out.print("id: " + "ServerTime" + "\n");
                out.print("data: " + (i++) + "\n\n");
                out.flush();

                Thread.currentThread().sleep(1000);
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

My problem is that instead of receiving each notification after 1 second, the client waits for all notifications to be sent. If I try to send a number of notifications, let's say 30, on the client I will receive them all at the end.

My client is a simple as it only listens to the specific endpoint to get notifications:

<!DOCTYPE html>
<html>
<body>
 <h1>Notification received  : </h1>

 <div id="ServerTime"></div>

 <script>
  if (typeof (EventSource) !== "undefined") {
   var source = new EventSource("https://10.241.53.185/rest/notifications");
   source.addEventListener('message', function(event) {
    console.log(event.data);
   });
  } else {
   document.getElementById("ServerTime").innerHTML = "Working, processing, getting info....";
  }
 </script>

</body>
</html>

Could you please help me with this issue?

Thanks

That is clearly NOT the way to send information to the client. Use an SseEmitter or a StreamingResponseBody as the result. — M. Deinum
– M. Deinum, Commented Dec 14, 2016 at 13:20
Thanks for the information. I have also found the problem with my code as i needed to write to the printer in a different thread. — Ancuta
– Ancuta, Commented Dec 14, 2016 at 13:37

rwfbc · Accepted Answer · 2017-11-10 23:23:32Z

2

You need to use SseEmitter to send events. See Server-Sent Events with Spring (blog).

You also should send the events from a separate thread (letting the request thread return).

answered Nov 10, 2017 at 23:23

rwfbc

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1 Comment

Ancuta Over a year ago

Thanks, that really helped.

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