Thread sleeping in a Thread-Pool

Question

Let's say we have a thread-pool with a limited number of threads.

Executor executor = Executors.newFixedThreadPool(3);

Now let's say one of the active tasks must sleep for 3 seconds (for whatever reason).

executor.execute(() -> {
    try {
        Thread.sleep(3000L);
    } catch (InterruptedException ignore) {}
});

How can we implement such a thread-pool in way that, when a task sleeps (or waits on a monitor/condition), the thread¹ can be used effectively to run another task?

¹By thread I do not mean the "physical" Java thread, because that would be impossible while the thread is asleep. What I mean is, the thread-pool to have an abstract implementation which virtually seems to allow a thread to run another task during sleeping. The key point is that there are always N simultaneously running (non-sleeping) tasks.

Somewhat similar to the way a monitor handles access to a critical region:

• If a thread waits on a resource, the resource can be used by another thread.
• If the thread is notified, it is placed into the waiting set to (re-)gain access to that resource.

Answer 1

What you are asking for is essentially implementing coroutines/fibers on top of JVM/OS thread. Nice talk was given by Sanhong Li about the way how Alibaba's engineers implemented such construction - the idea is instead of relying on OS thread scheduler you need to rely on your own Selector.

See also Loom project for fibers (user-land green threads).