How to stop a ScheduledExecutorService?

Question

The program finishes after nine prints:

class BeeperControl {

    private final ScheduledExecutorService scheduler = Executors.newScheduledThreadPool(1);

          


        

Answer 1

The issue you have is that the scheduler keeps a live thread around after you have cancelled the beep task.

If there is a live non-daemon thread, the JVM stays alive.

The reason that it keeps this thread around is that you have told it to do so in this line:

private final ScheduledExecutorService scheduler
        = Executors.newScheduledThreadPool(1);

Note the documentation of newScheduledThreadPool(int corePoolSize):

corePoolSize - the number of threads to keep in the pool, even if they are idle.

So, you have two possible ways to cause the JVM to terminate:

1. Pass 0 to newScheduledThreadPool instead of 1. The scheduler will not keep a live thread, and the JVM will terminate.

2. Shut down the scheduler. You are supposed to do so anyway to release its resources. So change the run in your anonymous Runnable to:

   public void run() {
       beeperHandle.cancel(true);
       scheduler.shutdown();
   }
   

(In fact, you don't need the cancel there - the shutdown will take effect as soon as the next "beep" is completed.)