Make other threads wait until ScheduledExecutorService completes all tasks

Question

The main goal is to run a method using ScheduledExecutorService and wait until all its tasks complete before resuming the main thread.

I've created a utility method in custom Scheduler class that accepts any Runnable:

public void scheduleFunction(Runnable function) {
    final ScheduledExecutorService scheduler = Executors.newScheduledThreadPool(1);
    final ScheduledFuture<?> producerHandle = scheduler.scheduleAtFixedRate(function, initialDelay, interval, MILLISECONDS);
    scheduler.schedule(() -> { producerHandle.cancel(true); }, timeout, MILLISECONDS);
}

And use it like this in other class when I need execute its method in scheduled mode:

public void sendToKafka() {
  Scheduler.scheduleFunction(this::produce);
}

This work fine, except one thing. When the main thread reaches sendToKafka() it calls Scheduler to schedule a function. Dut the main thread keeps running, at the same time the Scheduled function starts to work.

Actual result: two threads running at the same time

Expected result: when scheduler thread starts the main thread stops and waits until scheduler completes execution

How can I achieve this?

In this case, Use Join https://www.baeldung.com/java-executor-wait-for-threads — Harsh Mishra, Apr 16 '21 at 09:51
https://stackoverflow.com/questions/7939257/wait-until-all-threads-finish-their-work-in-java — Harsh Mishra, Apr 16 '21 at 09:55

score 3 · Accepted Answer · answered Apr 16 '21 at 12:41

Since you are creating and abandoning a ScheduledExecutorService in this method, you should call shutdown() to support timely release of the resources. If you do that, you can call awaitTermination to wait for the completion of all pending jobs.

public void scheduleFunction(Runnable function) {
    final ScheduledExecutorService scheduler = Executors.newScheduledThreadPool(1);
    final ScheduledFuture<?> producerHandle
        = scheduler.scheduleAtFixedRate(function, initialDelay, interval, MILLISECONDS);
    scheduler.schedule(() -> {
        producerHandle.cancel(true);
        scheduler.shutdown();
    }, timeout, MILLISECONDS);
    try {
        scheduler.awaitTermination(Long.MAX_VALUE, MILLISECONDS);
    } catch (InterruptedException ex) {
        Thread.currentThread().interrupt();
    }
}

Note that when you don’t need interruption, you can simply use

public void scheduleFunction(Runnable function) {
    final ScheduledExecutorService scheduler = Executors.newScheduledThreadPool(1);
    scheduler.scheduleAtFixedRate(function, initialDelay, interval, MILLISECONDS);
    scheduler.schedule(() -> scheduler.shutdown(), timeout, MILLISECONDS);
    try {
        scheduler.awaitTermination(Long.MAX_VALUE, MILLISECONDS);
    } catch (InterruptedException ex) {
        Thread.currentThread().interrupt();
    }
}

as shutting down the ScheduledExecutorService implies stopping to reschedule the job; only if there’s an ongoing execution it will be completed and awaitTermination will wait for it.

Unfortunately, none of solutions doesn't work. The scheduled tasks never runs in this examples. main thread is getting frozen as well. — Serhii Kachan, Apr 16 '21 at 15:02
Since the way the tasks are scheduled doesn’t differ from your original code, they should run exactly the same. [This runs smoothly](https://ideone.com/MODkKX) — Holger, Apr 16 '21 at 15:34
Yeah, that works fine now, I used some synchronization - that's why it wasn't working. Thanks! — Serhii Kachan, Apr 20 '21 at 10:53

Make other threads wait until ScheduledExecutorService completes all tasks

1 Answers1