Detecting aborted requests in a HttpServlet

Question

Is there a way to find out if a HttpServletRequest is aborted?

I'm writing an instant browser application (some kind of chat): The clients asks for new events in a loop using AJAX-HTTP-Requests. The server (Tomcat) handles the requests in a HttpServlet. If there are no new events for this client, the server delays the reply until a new event arrives or a timeout occurs (30sec).

Now I want to identify clients that are no longer polling. Therefore, I start a kick-Timer at the end of a request which is stopped when a new request arrives. If the client closes the browser window the TCP-Connection is closed and the HTTP-Request is aborted.

Problem: The client does not run into the kick-Timeout because the Servlet still handles the event request - sleeping and waiting for an event or timeout.

It would be great if I could somehow listen for connection abort events and then notify the waiting request in order to stop it. But I couldn't find anything like that in the HttpServletRequest or HttpServletResponse...

Answer 1

This probably won't help the OP any more, but it might help others trying to detect aborted HTTP connections in HttpServlet in general, as I was having a similar problem and finally found an answer.

The key is that when the client cancels the request, normally the only way for the server to find out is to send some data back to the client, which will fail in that case. I wanted to detect when a client stops waiting for a long computation on server, so I ended up periodically writing a single character to response body through HttpServletResponse's writer. To force sending the data to the client, you must call HttpServletResponse.flushBuffer(), which throws ClientAbortException if the connection is aborted.

Answer 2

You are probably using some sort of thread-notification (Semaphores or Object.wait) to hold and release the Servlet threads. How about adding a timeout (~10s) to the wait, then somehow checking whether the connection is still alive and then continuing the wait for another 10s, if the connection is still there.

I don't know whether there are reliable ways to poll the "liveness" of the connection (e.g. resp.getOutputStream not throwing an Exception) and if so, which way is the best (most reliable, least CPU intense).