SSE and Servlet 3.0

Question

I have registered a typical SSE when page loads:

Client:

sseTest: function(){

var source = new EventSource('mySSE');
source.onopen = function(event){
console.log("eventsource opened!");
};

source.onmessage = function(event){
var data = event.data;
console.log(data);
document.getElementById('sse').innerHTML+=event.data + "<br />";
};
}

My Javascript-Debugger says, that "eventsource opened!" was successfully.

My Server Code is a Servlet 3.0:

import java.io.IOException;
import java.io.PrintWriter;
import java.util.Random;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

@WebServlet(urlPatterns={"/mySSE"}, name = "hello-sse", asyncSupported=true)
public class MyServletSSE extends HttpServlet {

@Override
public void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

resp.setContentType("text/event-stream");
resp.setCharacterEncoding("UTF-8");

Random random = new Random();
PrintWriter out = resp.getWriter();

//AsyncContext aCtx = req.startAsync(req, resp);
//ServletRequest sReq = aCtx.getRequest();

String next = "data: " + String.valueOf(random.nextInt(100) + 1) + "\n\n";
//out.print("retry: 600000\n"); //set the timeout to 10 mins in milliseconds
out.write(next);
out.flush();
// do not close the stream as EventSource is listening
//out.close();
//super.doGet(req, resp);
}
}

The code works! The Client-Code triggers the doGet()-Method every 3 seconds and retrieves the new data.

Questions: However, I wonder how I can make this code better by using new Servlet 3.0 Futures such as Async-Support or asyncContext.addListener(asyncListener) or something else which I do not know. As I never closes the stream, I wonder how my server will scale?

Theoretically, the best approach would be to trigger the doGet()-Method via server-side-code explicitly when new data is there, so the client does not need to trigger the client-side "onmessage()"-Method and therefore the server side "doGet()"-Method every 3 seconds for new data.

Answer 1

This is an excellent question, here is a full working example (Servlet 3.0 / Java EE 6)

Some notes:

1. it handles "browser tab / window closed" via out.checkError() that also calls flush()
2. I wrote it quickly, so I'm sure it can be improved, just a POC, don't use in production before testing

Servlet: (omitted imports for brevity, will update a full gist soon)

@WebServlet(urlPatterns = {"/mySSE"}, asyncSupported = true)
public class MyServletSSE extends HttpServlet {

  private final Queue<AsyncContext> ongoingRequests = new ConcurrentLinkedQueue<>();
  private ScheduledExecutorService service;

  @Override
  public void init(ServletConfig config) throws ServletException {
    final Runnable notifier = new Runnable() {
      @Override
      public void run() {
        final Iterator<AsyncContext> iterator = ongoingRequests.iterator();
        //not using for : in to allow removing items while iterating
        while (iterator.hasNext()) {
          AsyncContext ac = iterator.next();
          Random random = new Random();
          final ServletResponse res = ac.getResponse();
          PrintWriter out;
          try {
            out = res.getWriter();
            String next = "data: " + String.valueOf(random.nextInt(100) + 1) + "num of clients = " + ongoingRequests.size() + "\n\n";
            out.write(next);
            if (out.checkError()) { //checkError calls flush, and flush() does not throw IOException
              iterator.remove();
            }
          } catch (IOException ignored) {
            iterator.remove();
          }
        }
      }
    };
    service = Executors.newScheduledThreadPool(10);
    service.scheduleAtFixedRate(notifier, 1, 1, TimeUnit.SECONDS);
  }

  @Override
  public void doGet(HttpServletRequest req, HttpServletResponse res) {
    res.setContentType("text/event-stream");
    res.setCharacterEncoding("UTF-8");

    final AsyncContext ac = req.startAsync();
    ac.setTimeout(60 * 1000);
    ac.addListener(new AsyncListener() {
      @Override public void onComplete(AsyncEvent event) throws IOException {ongoingRequests.remove(ac);}
      @Override public void onTimeout(AsyncEvent event) throws IOException {ongoingRequests.remove(ac);}
      @Override public void onError(AsyncEvent event) throws IOException {ongoingRequests.remove(ac);}
      @Override public void onStartAsync(AsyncEvent event) throws IOException {}
    });
    ongoingRequests.add(ac);
  }
}

JSP:

<%@page contentType="text/html" pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
    <head>
        <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
        <title>JSP Page</title>
        <script>
            function test() {
                var source = new EventSource('mySSE');
                source.onopen = function(event) {
                    console.log("eventsource opened!");
                };

                source.onmessage = function(event) {
                    var data = event.data;
                    console.log(data);
                    document.getElementById('sse').innerHTML += event.data + "<br />";
                };
            }
            window.addEventListener("load", test);
        </script>
    </head>
    <body>
        <h1>Hello SSE!</h1>
        <div id="sse"></div>
    </body>
</html>

Answer 2

Useful example.

People might get "IllegalStateException: Not supported" for startAsync(), in which case either don't forget:

@WebServlet(urlPatterns = "/Sse", asyncSupported=true)

or use

request.setAttribute("org.apache.catalina.ASYNC_SUPPORTED", true);

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