ThreadPoolExecutor Block When Queue Is Full?

Question

I am trying to execute lots of tasks using a ThreadPoolExecutor. Below is a hypothetical example:

def workQueue = new ArrayBlockingQueue(3, f         


        

Answer 1

You could use a semaphore to block threads from going into the pool.

ExecutorService service = new ThreadPoolExecutor(
    3, 
    3, 
    1, 
    TimeUnit.HOURS, 
    new ArrayBlockingQueue<>(6, false)
);

Semaphore lock = new Semaphore(6); // equal to queue capacity

for (int i = 0; i < 100000; i++ ) {
    try {
        lock.acquire();
        service.submit(() -> {
            try {
              task.run();
            } finally {
              lock.release();
            }
        });
    } catch (InterruptedException e) {
        throw new RuntimeException(e);
    }
}

Some gotchas:

• Only use this pattern with a fixed thread pool. The queue is unlikely to be full often, thus new threads won't be created. Check out the java docs on ThreadPoolExecutor for more details: https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/ThreadPoolExecutor.html There is a way around this, but it is out of scope of this answer.

• Queue size should be higher than the number of core threads. If we were to make the queue size 3, what would end up happening is:

  • T0: all three threads are doing work, the queue is empty, no permits are available.
  • T1: Thread 1 finishes, releases a permit.
  • T2: Thread 1 polls the queue for new work, finds none, and waits.
  • T3: Main thread submits work into the pool, thread 1 starts work.

  The example above translates to thread the main thread blocking thread 1. It may seem like a small period, but now multiply the frequency by days and months. All of a sudden, short periods of time add up to a large amount of time wasted.