ScheduledExecutorService 未按预期运行

Question

I'm trying to execute a task at midnight after a fixed amount of days using ScheduleExecutorService . My method runs inside of my tomcat 8 and looks like this:

public void schedule (int aInterval)
    {
        String timezone = TimeZone.getDefault().getID();
        ZoneId z = ZoneId.of(timezone);
        ZonedDateTime now = ZonedDateTime.now( z );

        LocalDate tomorrow = now.toLocalDate().plusDays(1);    
        ZonedDateTime tomorrowStart = tomorrow.atStartOfDay( z );
        Duration duration = Duration.between( now , tomorrowStart );
        long millisecondsUntilTomorrow = duration.toMillis();
        long interval;

        if (aInterval * 24 * 60 * 60 > Long.MAX_VALUE)
        {
            interval = Long.MAX_VALUE;
        }
        else
        {
            interval = aInterval * 24 * 60 * 60;
        }

        ScheduledExecutorService scheduler = Executors.newScheduledThreadPool(1, new ThreadFactory() {
            public Thread newThread(Runnable r) {
                Thread thread = new Thread(r);
                // allow the JVM to kill the scheduled task
                thread.setDaemon(true);
                return thread;
            });            
        scheduler.scheduleAtFixedRate(new Runnable()
            {
                public void run() {
                    System.out.println(String.format("schedule::run() at  %1$Td.%1$tm.%1$tY %1$tH:%1$tM:%1$tS \n", System.currentTimeMillis() ) );

                    doTask();
                }
            }, 
            delay,
            interval, 
            TimeUnit.SECONDS);
    }

Now when the method is first run it seems like is not executed as specifed by delay and interval . Eg when i set delay=60 and interval=5 in my stacktrace it looks like this:

...
schedule::run() at  10.08.2017 17:57:09 
schedule::run() at  10.08.2017 17:57:15 
schedule::run() at  10.08.2017 17:57:21 
schedule::run() at  10.08.2017 17:57:27 
schedule::run() at  10.08.2017 17:57:27 
schedule::run() at  10.08.2017 17:57:33 
schedule::run() at  10.08.2017 17:57:33 
schedule::run() at  10.08.2017 17:57:34 
...

So the intervals somehow are becoming shorter and shorter over time. What is going on here? Is there a problem in my method?

Answer 1

Now when the method is first run it seems like is not executed as specified by delay and interval. Eg when i set delay=60 and interval=5 in my stacktrace it looks like this...

I try to be very explicit with my time variables. In this case, you should be dealing with milliseconds so delayMillis , intervalMillis , aIntervalMillis , etc. should be in your code. If they are seconds then use delaySecs , etc. but you will need to multiple them by 1000 when you pass them to the scheduleAtFixedRate(...) method which is expecting millis.

So the intervals somehow are becoming shorter and shorter over time. What is going on here? Is there a problem in my method?

What's probably going on is that the task is trying to schedule it every 5 milliseconds so the random delays are just showing you how long your doTask() takes to run. If you want to set them to 60 and 5 seconds respectively then you should use 60000 and 5000 instead.

When i try that and my schedule() method is running i get a message from my eclipse (neon 3) saying tomcat is not responding and my tomcat is not shutting down properly.

Not sure about this but I suspect that your tomcat is waiting for your scheduled task to finish which is never will on its own. You could use a ThreadFactory and create a daemon thread instead which the JVM will not wait for when it shuts down. Note that if it is a daemon thread then it might get terminated right in the middle of running.

scheduler = Executors.newScheduledThreadPool(1, new ThreadFactory() {
    public Thread newThread(Runnable r) {
       Thread thread = new Thread(r);
       // allow the JVM to kill the scheduled task
       thread.setDaemon(true);
       return thread;
    }
});

Also, you should call scheduler.shutdown() on your thread-pool after you submit your fixed task. It will continue to run but no other jobs will be submitted. That's a good pattern.

How can I stop my execution task in a proper way?

The daemon thread mode about may be "proper" in your case but you can also cancel it. The scheduler.scheduleAtFixedRate(...) method returns a ScheduledFuture object that you can call cancel(...) on to stop it. Once your doTask() finishes it won't be scheduled again. You can also call cancel(true) on it to set the interrupt flag on the thread but your code is going to have to handle that specifically.

Of course you are going to need to detect that your JVM is shutting down so you can cancel your task. You could set a shutdown hook which is a bit of a hack. Maybe there is some way for tomcat to notify that it's coming down?

Answer 2

the intervals somehow are becoming shorter and shorter over time

We need to understand how ScheduledExecutorService:scheduleAtFixedRate is supposed to work. According to docs, https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/ScheduledExecutorService.html#scheduleAtFixedRate , we can conclude the following:

1. The time of each run is pre-determined.
2. This pre-calculated time may not tally when the previous run of the same task takes longer than point no 1. In such case, the next run will happen immediately after the longer running previous run has finished.
3. Same task can never run concurrently; even if the expected start time of the next run has exceeded. As mentioned by point no 2, the next run will wait till the previous longer run finishes.

Now, coming to your case, you had expected to see your task getting run at equal intervals. But, instead the intervals between them are getting shorter with each run. This could be due to any previous run that took considerable longer time, piling up other run(s) which have exceeded their expected start time. Further piled up runs ( doTask method) finish quickly. So, all the piled up runs are running at closer intervals. If you really want the task to run at equal intervals, you could instead use ScheduledExecutorService: scheduleWithFixedDelay