How to stop a task in ScheduledThreadPoolExecutor once I think it's completed

Question

I have a ScheduledThreadPoolExecutor with which I schedule a task to run at a fixed rate. I want the task to be running with a specified delay for a maximum of say 10 times until it "succeeds". After that, I will not want the task to be retried. So basically I'll need to stop running the scheduled task when I want it to be stopped, but without shutting down the ScheduledThreadPoolExecutor. Any idea how I'd do that?

Here's some pseudocode -

public class ScheduledThreadPoolExecutorTest
{
  public static ScheduledThreadPoolExecutor executor = new ScheduledThreadPoolExecutor(15);  // no multiple instances, just one to serve all requests

  class MyTask implements Runnable
  {
    private int MAX_ATTEMPTS = 10;
    public void run()
    {
      if(++attempt <= MAX_ATTEMPTS)
      {
        doX();
        if(doXSucceeded)
        {
          //stop retrying the task anymore
        }
      }
      else
      { 
        //couldn't succeed in MAX attempts, don't bother retrying anymore!
      }
    }
  }

  public void main(String[] args)
  {
    executor.scheduleAtFixedRate(new ScheduledThreadPoolExecutorTest().new MyTask(), 0, 5, TimeUnit.SECONDS);
  }
}

Solution 1

run this test, it prints 1 2 3 4 5 and stops

public class ScheduledThreadPoolExecutorTest {
    static ScheduledThreadPoolExecutor executor = new ScheduledThreadPoolExecutor(15); // no
    static ScheduledFuture<?> t;

    static class MyTask implements Runnable {
        private int attempt = 1;

        public void run() {
            System.out.print(attempt + " ");
            if (++attempt > 5) {
                t.cancel(false);
            }
        }
    }

    public static void main(String[] args) {
        t = executor.scheduleAtFixedRate(new MyTask(), 0, 1, TimeUnit.SECONDS);
    }
}

Solution 2

Nicely cancelled outside of thread:

public class ScheduleTest {

    @Test
    public void testCancel() throws Exception {
        final ScheduledThreadPoolExecutor EXECUTOR = (ScheduledThreadPoolExecutor) Executors.newScheduledThreadPool(2);
        ScheduledFuture f1 = EXECUTOR.scheduleAtFixedRate(new Runnable() {
            @Override
            public void run() {
                System.out.println("Im alive 1");
            }
        }, 0, 1, TimeUnit.SECONDS);
        ScheduledFuture f2 = EXECUTOR.scheduleAtFixedRate(new Runnable() {
            @Override
            public void run() {
                System.out.println("Im alive 2");
            }
        }, 0, 2, TimeUnit.SECONDS);

        Thread.sleep(10000);
        f1.cancel(true);
        System.out.println("f1 cancel");
        Thread.sleep(10000);
        f2.cancel(false);
        System.out.println("f2 cancel");
        Thread.sleep(10000);
    }
}

Sometimes thread couldn't be cancelled, this solved usually via volatile boolean isCancelled;

Solution 3

CountDownLatch is an alternative approach. When the thread completes, call countDown() on the latch. The calling thread calls latch.await() until all threads complete. At that point call ExecutorService.shutdownNow() so that your main thread doesn't turn into a zombie.

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;

public class ScheduledThreadPoolExecutorTest {

  static int i = 0;

  public static void main(String[] args) throws Exception {
    final ScheduledExecutorService executor = Executors.newScheduledThreadPool(1);
    final CountDownLatch latch = new CountDownLatch(1);
    executor.scheduleAtFixedRate(() -> {
        System.out.println(++i);
        if (i > 4) {
          latch.countDown();
        }
    }, 0, 100, TimeUnit.MILLISECONDS);
    latch.await();
    executor.shutdownNow();
  }
}

Comments

[mystarrocks]

Perfect thanks! I did know about the scheduledfuture but I was trying to use this way: t = executor.scheduleAtFixedRate(new MyTask(), 0, 1, TimeUnit.SECONDS); t.cancel(true); which would never have worked for obvious reasons. Canceling the task inside the task implementation seems about right.

[Luffy]

Nice Example looking this from a while facing lots of issues because of this.... Thanks :)

[Kirby]

Thanks. This helped. Interesting to note that this example never exits. I think you need an executor shutdown

[Kirby]

Or rather you need a t.get() and an executor shutdown inside a catch block of CancellationException

[Sergey Podolsky]

Is this approach thread safe? Task execution may reach t.cancel() before it is assigned in the main thread causing NullPointerException.