How to schedule Callable within itself in Executor

Question

I am trying to schedule a thread that wait for some condition to happen. If condition is true, return some result else again schedule itself to execute after some delay. To achieve this, I am using Executors to schedule Callable<Boolean> but it hangs after second rescheduling.

 final ExecutorService executor = Executors.newFixedThreadPool(2);
    final MutableBoolean status = new MutableBoolean(false);

    new Thread(new Runnable() {
        @Override
        public void run() {
            System.out.println("Waiting now to set status");
            try {
                Thread.sleep(5*1000);
                System.out.println("status is now true");
                status.setValue(true);
            } catch (InterruptedException e) {
            }

        }
    }).start();

    Future<Boolean> future = executor.submit(new Callable<Boolean>() {
        public int count=0;
        @Override
        public Boolean call() throws Exception {
            if(status.isTrue()){
                System.out.println("Condition has become true now");
                return status.booleanValue();
            } else {
                System.out.println("Not true yet, count" + count++ + " Rescheduling");
                Future<Boolean> future = executor.submit(this);
                return future.get();
            }
        }
    });

    boolean finalStatus = future.get();
    System.out.println("Final status" + finalStatus);

Output:

Waiting now to set status
Not true yet, count0 Rescheduling
Not true yet, count1 Rescheduling
status is now true

Any suggestions on what might be going wrong?

Thanks

See also http://stackoverflow.com/questions/23294437/submitting-to-an-executor-from-a-running-task — Raedwald, Jun 03 '14 at 01:19

score 1 · Answer 1 · answered Jun 02 '14 at 23:58

1

you created a thread pool of size 2. you then submit tasks to that pool from jobs running within the pool (recursively) and wait for those to complete (call future.get()).

answered Jun 02 '14 at 23:58

Brett Okken

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consider using a scheduled executor service to control the retrying logic. use the returned future to cancel the retry work when the condition is met. – Brett Okken Jun 03 '14 at 00:00

score 1 · Answer 2 · answered Jun 03 '14 at 14:08

With executor.submit(this) a new task is submitted to the thread-pool each time, the thread-pool does not (and should not) check if the same task is already being executed by a thread in the thread-pool. Since the thread-pool only has two threads, both threads will be in use very quickly and waiting indefinitely.

A dirty fix is the following:

    @Override
    public Boolean call() throws Exception {
        while (true) {
            if(status.get()){
                System.out.println("Condition has become true now");
                return status.get();
            }
        }
    }

But that will cause very high CPU usage. As Brett Okken suggests, use a scheduled executor service. Or, if the status should always change to true in the near future, use a wait-barrier like CountDownLatch to react immediately to a status change.

How to schedule Callable within itself in Executor

2 Answers2