SynchronousQueue does not block when offered task by ThreadPoolExecutor

Question

I use a pretty much default newCachedThreadPool but I want to limit the thread creation so I create ExecutorService like this

new ThreadPoolExecutor(0, Runtime.getRuntime().availableProcessors() * 2,
            60L, TimeUnit.SECONDS,
            new SynchronousQueue<>());

After reading javadocs I expected it to work in way that when I submit Runnable, taskExecutor blocks when offering SynchronousQueue a new task, untill there is available thread to execute it and then a handoff occurs. Unfortunately after reaching the thread pool capacity and when all threads are busy, taskExecutor throws RejectedExecutionException. I know I can pass a RejectedExecutionHandler that will block, but I'm just suprise that it seems I have to. Can someone explain if it is really working as intended or am I doing something wrong?

This code reproduces my case:

public static void main(String[] args) {
    ThreadPoolExecutor executor = new ThreadPoolExecutor(0, Runtime.getRuntime().availableProcessors() * 2,
            60L, TimeUnit.SECONDS,
            new SynchronousQueue<>());

    while (true) {
        executor.submit(() -> System.out.println("bla"));
    }
}

Answer 1

This is in accordance with ThreadPoolExecutor API:

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

As to why SynchronousQueue does not block - because ThreadPoolExecutor uses queue.offer() instead of put()