Why threadpool creates only one thread?

Question

I have wrote code sample:

class Test {
    public static void main(String[] args) throws InterruptedException {
        ThreadPoolExecutor executorService = new ThreadPoolExecutor(0, 100,
                2L, TimeUnit.SECONDS,
                new LinkedBlockingQueue<Runnable>());
        executorService.allowCoreThreadTimeOut(true);

        CountDownLatch countDownLatch = new CountDownLatch(20);
        long l = System.currentTimeMillis();
        for (int i = 0; i < 20; i++) {
            Thread.sleep(100);
            executorService.submit(new Runnable() {
                @Override
                public void run() {
                    try {
                        countDownLatch.countDown();
                        Thread.sleep(500);
                    } catch (Exception e) {
                        System.out.println(e);
                    }


                }
            });
        }
        executorService.shutdown();
        countDownLatch.await();

        System.out.println((System.currentTimeMillis() - l) / 100);
    }
}

Each 100 ms submits new task(overall task quantity - 20). Each task duration - 0.5 sec. thus 5 task can be executed in parallel and best execution time will be: 20*100+500 = 2.5 sec and pool should create 5 threads

But my experiment shows 9.6 sec.
I opened the jsvisualvm to see how many threads pool creates and I see that only one thread was created:

please correct where my threadPooll configuration incorrect.

Answer 1

The reason can be found from the ThreadPoolExecutor javadoc.

Any BlockingQueue may be used to transfer and hold submitted tasks.

The use of this queue interacts with pool sizing: If fewer than corePoolSize threads are running, the Executor always prefers adding a new thread rather than queuing.

If corePoolSize or more threads are running, the Executor always prefers queuing a request rather than adding a new thread.

If a request cannot be queued, a new thread is created unless this would exceed maximumPoolSize, in which case, the task will be rejected.

Therefore since your corePoolSize is 0, the first option is not used and since your queue is unlimited, the last option is not used. Therefore the remaining strategy of queuing is what you get.

Different results can be seen by modifying corePoolSize or the size of the workQueue.