Terminating enqueued threads at synchronized block

Question

Hi!
I would really appreciate your opinions: :)
The application desing:
- I'm developing a web applicaction running on a Tomcat server.
- The app is being developed using Struts and Spring.
- Pages (presentation layer) are made of JSPs.
- There are Spring beans (Services) that execute the business logic.
- These Services are used by the Struts Actions.
My problem case:
1. When you submit myPage.jsp, MyAction.java's execute method is called.
2. MyAction.java uses MyService.java's "process()" method to do some file processing that takes 2 to 3 hours to finish.
3. This MyService.java is a singleton Spring bean.
4. The call to process() method is synchronized on MyAction.java like this:
public ActionForward execute(...) throws Exception {
     // Some code ...
     synchronized (this) {
          myService.process(...); // This method takes up to 3 hours to return
          // Some code ...
     return mapping.findForward("success");
}As you can imagine, many requests (threads) will be enqueued at the synchronized keyword line. Because of this, I need to provide a cancelation functionality. This would be fired by a cancel button on myPage.jsp.
So, if you tried to launch the so-long-process by the "Process" button on myPage.jsp and you received a "Waiting..." message at your screen (because the 3 hours process was launched by another client), you should be able to cancel your processing request (because you don't want to wait so much time to begin your file processing).
I would really appreciate your opinions on which would be the best approach to achive this cancelation functionality.
I'm sure there's a better way than the one I came up with: I could populate a class variable Map with "sessionId - Thread" (key - value) everytime a request arrives at the line before the synchronized block. Thus, when you clicked the Cancel button, MyAction.java would get the request's session id and then get the Thread it must terminate from the Map. Having the Thread instance, I could maybe call it's interrupt method.
I don't think that's the best approach, but it's the only one I could thought about.
Thank you very much for your time and opinions!

Answer 1

Well, first thing I'd say is that if you use Spring, you'd be better off using it's own Controller architecture which is, to my mind, a lot more developed than Struts, especially now you can do pretty much the whole thing with @RequestMapping rather than XML.
Having a web transaction wait for a long running server action is basically a bad idea. Rather you should have the web front end launch the file processing job and return immediately, probably showing a list of jobs to the user which may be queued, running or finished (for good or ill). One technique is to display a "background action in progress" page which submits a new transaction every few minutes to check if the job has finished yet (using a Javascript timer). If the job is still queued or running then this request displays the "in progress" page again, if finished it displays the result.
Such a page could have a "Stop" button to abort the running job if feasible.
I would suggest that you have a queue of background jobs, probably serviced by a ThreadPoolExecuter. The initial request simply builds a request object and queues it.
Use a ContextListener object to create the queue and start the executer when the application starts, and close it down when the application is stopped,