Timer: canceling running task

Question

I need to create async thread that runs once with a delay of 2 minutes and that can be killed at any moment. I saw several possible solutions:

1. ScheduledExecutorService and FutureTask allow me to interrupt a running task, but I will have to invoke shutdown() to terminate all the running threads, and this will block user until the processes were terminated. Also, I will have to frequently invoke Thread.interrupted() as described in Enno Shioji's answer.
2. Timer and TimerTask do not require to release running threads, but I have no way to interrupt a running timer thread (Timer.cancel() just cancels future scheduling)
3. Using Thread and sleep with thread interruption problem.

Is there a good solution? (I'm using tomcat 7)

thank you

Answer 1

For your scenario 2 with TimerTask, why not just return from the run() method after calling this.cancel()?

Here's a snippet from something I wrote. I use the same technique whenever the tool encounters a situation that would make further execution invalid, like a misconfiguration.

...
if ( count < 1 ) {
   logger.error("CANCELING THREAD FOR " + host + ":" + jmxPort + "! " +
                "- CONFIGURATION INCOMPLETE - DUMP_COUNT must be 1 or greater.");
   this.cancel();
   return;
}
...

Answer 2

After some tests and researches, FutureTask.cancel() and Threads need similar handling of interrupts, as stated in Enno Shioji's answer

1. Check interruption flag in your logic
2. Act upon Interrupted exception

An example that tests interruption flag:

private final class MyTask implements Runnable {

    public void run() {
        try{
            for(int j=0; j<100000000; j++) {
                for(int i=1; i<1000000000; i++){
                    if(Thread.interrupted()){ //Don't use Thread.interrupt()!
                        Log.debug("Thread was interrupted for" + cache);
                        return; //Stop doing what you are doing and terminate.
                    }
                    Math.asin(0.1565365897770/i);
                    Math.tan(0.4567894289/i);
                }
            }                            
        }catch(Throwable e){//if exception is uncaught, the scheduler may not run again 
            ...
        }
    }
}

As I understand, ScheduledExecutorService maybe be shutdown when application ends running

Answer 3

Using Option 1 you can use FutureTask.cancel() with mayInterruptIfRunning parameter set to true to cancel your tasks.

The ScheduledExecutorService.scheduleAtFixedRate() creates a ScheduledFuture, which still can be canceled trough the FutureTask api.

Here is a naive test I've used to verify that task gets canceled. I suppose the worker thread may not be interrupted if you do some blocking IO (network or disk), but I haven't tested it. If cancel is called while task is not running, it all stops nicely, but if task is running when cancel is called the executor will try to kill the thread.

public static void main(String[] args) throws InterruptedException {
    ScheduledThreadPoolExecutor executor = new ScheduledThreadPoolExecutor(2);

    ScheduledFuture<?> future = executor.scheduleAtFixedRate(new Runnable() {
        int i = 0;
        public void run() {
            int j = i++;
            System.err.println("Run " + j);

            try {
                Thread.sleep(5000L);
            } catch (InterruptedException e) {
                System.err.println("Interrupted " + j);
            }
        }

    }, 1000L, 2000L, TimeUnit.MILLISECONDS);

    Thread.sleep(10000L);
    System.err.println("Canceled " + future.cancel(true));

    Thread.sleep(20000L);

    executor.shutdownNow();
    System.err.println("Finished");
}