SSE and Servlet 3.0 - html

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:
it handles "browser tab / window closed" via out.checkError() that also calls flush()
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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