shutdown only one task in ScheduledExecutorService

Question

I have a few tasks which are registered by

final ScheduledExecutorService ses = Executors.newScheduledThreadPool(10);
List<ScheduledFuture<?>> futures = new ArrayList<>();
        tasks.forEach(task->{
            var future = ses.scheduleWithFixedDelay(() -> run(task), 0, 3, TimeUnit.SECONDS);
            futures.add(future);
});

// and now cancel all tasks one for one after 10 seconds..
ses.scheduleWithFixedDelay(() ->
        {
            log.info("cancel task----");
            futures.get(0).cancel(false);
        }, 0, 10, TimeUnit.SECONDS);

As you can see, for each task the futures holds a task.getId() so I can obtain the ScheduledFuture of a task afterwards. I do not want to ses.shutdown() because this will shutdown the whole schedulings for the other tasks as well, which I want to avoid.

The only solution I actually see is to create one ScheduledExecutorService for each task to be able to shutdown it afterwards for a specified task, but then I cannot make use of the pooling.

How can I shutdown only a specified task within the pool?

Answer 1

Use

Future<?> future;
future.cancel(false);

Cancel will cancel the task and any further scheduling of it.¹ The Boolean parameter decides if you want to throw an interruption exception on the task if it is already running and blocking on a resource.

To ensure the task is removed from the queue immediately upon cancelling, use the setRemoveOnCancelPolicy method on your ScheduledThreadPoolExecutor and set the policy to true.²

final ScheduledThreadPoolExecutor ses = (ScheduledThreadPoolExecutor) Executors.newScheduledThreadPool(10);
ses.setRemoveOnCancelPolicy(true);

¹ https://docs.oracle.com/javase/8/docs/api/index.html?java/util/concurrent/Future.html

² https://stackoverflow.com/a/36748183/4425643 , https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/ScheduledThreadPoolExecutor.html#setRemoveOnCancelPolicy-boolean-