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

I guess the answer to this behavior could be rooted in:

A ThreadPoolExecutor will automatically adjust the pool size (see getPoolSize()) according to the bounds set by corePoolSize (see getCorePoolSize()) and maximumPoolSize (see getMaximumPoolSize()). When a new task is submitted in method execute(java.lang.Runnable), and fewer than corePoolSize threads are running, a new thread is created to handle the request, even if other worker threads are idle. If there are more than corePoolSize but less than maximumPoolSize threads running, a new thread will be created only if the queue is full.

( from ThreadPoolExecutor javadoc ).

Thing is: how are sleeping threads coming into this equation. My suggestion: change the corePoolSize from 0 to 10; and set the max pool size to 10, too.