using ScheduledExecutorService to start and stop timer

Question

From my readings, it seems that ScheduledExecutorService is the right way to start and stop timers in Java.

I need to port some code that starts and stops a timer. This is not a periodic timer. This code, stops the timer before starting it. So, effectively every start is really a restart(). I am looking for the right way to do this using the ScheduledExecutorService. Here is what I came up with. Looking for comments and insight on things I am missing:

ScheduledExecutorService _Timer = Executors.newScheduledThreadPool(1);
ScheduledFuture<?> _TimerFuture = null;

private boolean startTimer() {
    try {
        if (_TimerFuture != null) {
            //cancel execution of the future task (TimerPopTask())
            //If task is already running, do not interrupt it.
            _TimerFuture.cancel(false);
        }

        _TimerFuture = _Timer.schedule(new TimerPopTask(), 
                                       TIMER_IN_SECONDS, 
                                       TimeUnit.SECONDS);
        return true;
    } catch (Exception e) {
        return false;
    }
}

private boolean stopTimer() {
    try {
        if (_TimerFuture != null) {
            //cancel execution of the future task (TimerPopTask())
            //If task is already running, interrupt it here.
            _TimerFuture.cancel(true);
        }

        return true;
    } catch (Exception e) {
        return false;
    }
}

private class TimerPopTask implements Runnable  {  
    public void run ()   {  
        TimerPopped();
    }  
}

public void TimerPopped () {
    //Do Something
}

tia, rouble

Answer 1

This looks like a problem:

private boolean startTimer() {
    // ......
        if (_TimerFuture != null) {
            _TimerFuture.cancel(false);
        }

        _TimerFuture = _Timer.schedule(new TimerPopTask(), 
                                       TIMER_IN_SECONDS, 
                                       TimeUnit.SECONDS);
    // ......
}

Since you're passing a false to cancel, the old _TimerFuture may not get cancelled if the task is already running. A new one gets created anyway (but it won't run concurrently because your ExecutorService has a fixed thread pool size of 1). In any case, that doesn't sound like your desired behavior of restarting a timer when startTimer() is called.

I would rearchitect a bit. I would make the TimerPopTask instance be the thing you "cancel", and I would leave the ScheduledFutures alone once they are created:

private class TimerPopTask implements Runnable  {
    //volatile for thread-safety
    private volatile boolean isActive = true;  
    public void run ()   {  
        if (isActive){
            TimerPopped();
        }
    }  
    public void deactivate(){
        isActive = false;
    }
}

then I would retain the instance of TimerPopTask rather than the instance of ScheduledFuture and rearrange startTimer method thusly:

private TimerPopTask timerPopTask;

private boolean startTimer() {
    try {
        if (timerPopTask != null) {
            timerPopTask.deactivate();
        }

        timerPopTask = new TimerPopTask();
        _Timer.schedule(timerPopTask, 
                        TIMER_IN_SECONDS, 
                        TimeUnit.SECONDS);
        return true;
    } catch (Exception e) {
        return false;
    }
}

(Similar modification to stopTimer() method.)

You may want to crank up the number of threads if you truly anticipate needing to 'restart' the timer before the current timer expires:

private ScheduledExecutorService _Timer = Executors.newScheduledThreadPool(5);

You may want to go with a hybrid approach, keeping references to both the current TimerPopTask as I described and also to the current ScheduledFuture and make the best effort to cancel it and free up the thread if possible, understanding that it's not guaranteed to cancel.

(Note: this all assumes startTimer() and stopTimer() method calls are confined to a single main thread, and only the TimerPopTask instances are shared between threads. Otherwise you'll need additional safeguards.)