Does ScheduledExecutorService guarantee order when pool size is one?

Question

I have a ScheduledExecutorService that has a pool size of 1 threads.

If I schedule many tasks using that service with the same delay, is the order of scheduling preserved during the execution?

Answer 1

Yes, as long as the scheduler implementation used will follow the interface specification. For example, new ScheduledThreadPoolExecutor(1) will use a DelayedWorkQueue which will preserve the order.

As per javadoc all ScheduledExecutorService implementations should preserve order:

Tasks scheduled for exactly the same execution time are enabled in first-in-first-out (FIFO) order of submission.

One can test the implementation with the example below:

import com.google.code.tempusfugit.concurrency.IntermittentTestRunner;
import com.google.code.tempusfugit.concurrency.annotations.Intermittent;
import org.junit.Test;
import org.junit.runner.RunWith;

import java.util.concurrent.*;
import java.util.concurrent.atomic.AtomicInteger;

import static org.assertj.core.api.Assertions.assertThat;


@RunWith(IntermittentTestRunner.class)
public class ScheduledExecutorServiceTest {
    @Test
    @Intermittent(repetition = 20)
    public void preservesOrderOfTasksScheduledWithSameDelay() throws InterruptedException {
        ScheduledExecutorService scheduledExecutorService = new ScheduledThreadPoolExecutor(1);

        AtomicInteger atomicInteger = new AtomicInteger(0);
        int numTasks = 1_000;
        CountDownLatch countDownLatch = new CountDownLatch(numTasks);
        for (int i = 0; i < numTasks; i++) {
            int finalI = i;
            scheduledExecutorService.schedule(() -> {
                atomicInteger.compareAndSet(finalI, finalI + 1);
                countDownLatch.countDown();
            }, 10, TimeUnit.MILLISECONDS);
        }
        countDownLatch.await();

        assertThat(atomicInteger.get()).isEqualTo(numTasks);
    }
}

Answer 2

Yes, the order is preserved. From the javadocs

Delayed tasks execute no sooner than they are enabled, but without any real-time guarantees about when, after they are enabled, they will commence. Tasks scheduled for exactly the same execution time are enabled in first-in-first-out (FIFO) order of submission.

You can see this in action too

    public static void main(String args[]) {
        ScheduledExecutorService e = Executors.newScheduledThreadPool(1);
        
        e.schedule(delay("delay for 1 second", 10), 1, TimeUnit.SECONDS);
        e.schedule(delay("delay for 5 second", 0), 5, TimeUnit.SECONDS);
        e.schedule(delay("delay for 3 second", 0), 3, TimeUnit.SECONDS);
        e.schedule(delay("delay for 7 second", 0), 7, TimeUnit.SECONDS);
        e.schedule(delay("delay for 2 second", 0), 2, TimeUnit.SECONDS);
    }
    
    private static Runnable delay(String message, int initialDelay) {
        return () -> {
            Thread.sleep(initialDelay);
            System.out.println(message);
        };
    }

prints

delay for 1 second
delay for 2 second
delay for 3 second
delay for 5 second
delay for 7 second