Executor with limited thread count on demand

Question

I am looking for an ExecutorService that creates threads on demand up to a predefined limit and destroys idle threads after a keep alive time.

The following constructor creates a ThreadPoolExecutor with fixed thread count:

// taken from Executors.newFixedThreadPool()
new ThreadPoolExecutor(nThreads, nThreads,
                                  0L, TimeUnit.MILLISECONDS,
                                  new LinkedBlockingQueue<Runnable>());

So I tried to create a ExecutorService this way:

// taken from Executors.newCachedThreadPool()
new ThreadPoolExecutor(0, nThreads,
    CACHED_POOL_SHUTDOWN_DELAY, TimeUnit.SECONDS,
    new SynchronousQueue<Runnable>());

But it doesn't work as expected, when nThreads are in use, the Executor does not enqueue new tasks but throws an RejectedExecutionException. I know I could implement a handler for that, but it didn't help me.

How can I create the Executor described ahead?

Answer 1

I found an approach on that post that does exactly what I need.
@assylias I recognized your answer and changed the queue implementation.

Now my code looks like this:

parallelExecutor = new ThreadPoolExecutor(nThreads, nThreads,
    CACHED_POOL_SHUTDOWN_DELAY, TimeUnit.SECONDS,
    new LinkedBlockingQueue<Runnable>());

parallelExecutor.allowCoreThreadTimeOut(true);   // this is the magic

It works like a fixed thread pool but these core thats are allowed to time out.