Implementing long polling in an asynchronous fashion

Question

Is it possible to take an HTTPServletRequest away from its thread, dissolve this thread (i.e. bring it back to the pool), but keep the underlying connection with the browser working, until I get the results from a time-consuming operation (say, processing an image)? When the return data are processed, another method should be called asynchronously, and be given the request as well as the data as parameters.

Usually, long pooling functions in a pretty blocking fashion, where the current thread is not dissolved, which reduces the scalability of the server-side app, in terms of concurrent connections.

Answer 1

Yes, you can do this with Servlet 3.0

Below is the sample to write the alert every 30 secs(not tested).

@WebServlet(async =“true”)
public class AsyncServlet extends HttpServlet {

Timer timer = new Timer("ClientNotifier");

public void doGet(HttpServletRequest req, HttpServletResponse res) {

    AsyncContext aCtx = request.startAsync(req, res);
    // Suspend request for 30 Secs
    timer.schedule(new TimerTask(aCtx) {

        public void run() {
            try{
                  //read unread alerts count
                 int unreadAlertCount = alertManager.getUnreadAlerts(username); 
                  // write unread alerts count
    response.write(unreadAlertCount); 
             }
             catch(Exception e){
                 aCtx.complete();
             }
        }
    }, 30000);
}
}

Below is the sample to write based on an event. The alertManager has to be implemented which notifies AlertNotificationHandler when client has to be alerted.

@WebServlet(async=“true”)
public class AsyncServlet extends HttpServlet {
 public void doGet(HttpServletRequest req, HttpServletResponse res) {
        final AsyncContext asyncCtx = request.startAsync(req, res);
        alertManager.register(new AlertNotificationHandler() {
                   public void onNewAlert() { // Notified on new alerts
                         try {
                               int unreadAlertCount =
                                      alertManager.getUnreadAlerts();
                               ServletResponse response = asyncCtx.getResponse();
                               writeResponse(response, unreadAlertCount);
                               // Write unread alerts count
                         } catch (Exception ex) {
                               asyncCtx.complete();
                               // Closes the response
                         }
                   }
        });
  }
}

Answer 2

Yes, it's possible using Servlet spec ver. 3.0. Implementation I can recommend is Jetty server. See here.