How to shut down a Java ScheduledExecutorService based on a condition?

Question

I came across a way of stopping a Java ScheduledExecutorService which runs a periodic background task, after a time limit from this forum discussion.

In my case, I need to stop the ScheduledExecutorService when a certain condition is true. For example, I want to stop printing out "beep {count}", once the count exceeds 5. I have used the previously highlighted example for this purpose.

public class BeeperControl {
    private final ScheduledExecutorService scheduler = Executors.newScheduledThreadPool(1);

    private int count;

    public void beep() {
        final Runnable beeper = new Runnable() {
            public void run() {
                count = count + 1;
                System.out.println("beep " + count);

                if (count == 5) {
                    scheduler.shutdown();
                }
            }
        };
        final ScheduledFuture<?> beeperHandle = scheduler.scheduleAtFixedRate(
                beeper, 1, 1, SECONDS);
    }

    public static void main(String[] args) {
        BeeperControl bc = new BeeperControl();
        bc.beep();
    }
}

Here, I have checked if the count is equal to 5 and then has used scheduler.shutdown() method to shutdown the ScheduledExecutorService.

My question is whether this is a good practice for the given scenario (as the ScheduledExecutorService is shutdown by a running task) or is there a better alternative which can be used in such a scenario?

Answer 1

Using Mutable state (count variable) in a Multithreaded Environment is not recommended as it may result in stale value in count variable as a Read-Increment-Write (count = count + 1) is taking place.

It is good if you use a AtomicInteger instead of a plain old int for count variable.

public class BeeperControl {
    private final ScheduledExecutorService scheduler = Executors.newScheduledThreadPool(1);

    private final AtomicInteger count = new AtomicInteger(0);

    public void beep() {
        final Runnable beeper = new Runnable() {
            public void run() {
                count.getAndIncrement();
                System.out.println("beep " + count);

                if (count.get() == 5) {
                    scheduler.shutdown();
                }
            }
        };
        final ScheduledFuture<?> beeperHandle = scheduler.scheduleAtFixedRate(
                beeper, 1, 1, SECONDS);
    }

    public static void main(String[] args) {
        BeeperControl bc = new BeeperControl();
        bc.beep();
    }
}

Shutting down the ScheduledExecutorService by a running task is fine as in many scenarios that is what happens.