How does maximumPoolSize of ThreadPoolExecutor works?

Question

I'm trying to understand ThreadPoolExecutor class. I have read this answer and the Javadoc. But my experimentation doesn't match with that description:

I initialize the threadpool with an factory for tracking the ids

int tcounter = 0;
ThreadPoolExecutor executor = new ThreadPoolExecutor(2, 4, 1, TimeUnit.MINUTES,
        new ArrayBlockingQueue<Runnable>(1000), new ThreadFactory() {

            @Override
            public Thread newThread(Runnable r) {
                return new mThread(tcounter++, r);
            }
        });

public class mThread extends Thread {
    int id;

    private mThread(int id, Runnable run) {
        super(run);
        GLog.e("created thread " + id);
        this.id = id;
    }

}

then the task:

public class mRunanble implements Runnable {
    int value = 0;

    private mRunanble(int value) {
        super();
        this.value = value;
    }

    @Override
    public void run() {
        SystemClock.sleep(3000);
        Thread t = Thread.currentThread();
        if (t instanceof mThread) {

            GLog.e("Say " + (value) + " on thread " + ((mThread) t).id);
        } 

    }

}

and assign a button the action:

executor.execute(new mRunanble(i++));

But I spam that button and the third thread is never created, so what is for the second paramether in the ThreadPoolExecutor constructor (maximumPoolSize=4). I was specting 4 threads to be created and 2 of them to be killed after 1 minute of the end of the execution

Answer 1

From the API for ThreadPoolExecutor:

If there are more than corePoolSize but less than maximumPoolSize threads running, a new thread will be created only if the queue is full.

Your queue never fills since it has a capacity of 1000. If you change the capacity to 1, you will see Threads being created.

The Executors class uses a SynchronousQueue for its newCachedThreadPool methods, so you might want to consider using it as well.

Answer 2

In ThreadPoolExecutor maximumPoolSize comes in picture when corePoolSize no is not sufficient to execute your tasks and if all no are ocupied by tasks then only one more tread is being created to execute task .this no can grow upto maxPoolSize.

Edit you missunderstood the maxPoolsize concept.please refer below link.

http://www.bigsoft.co.uk/blog/index.php/2009/11/27/rules-of-a-threadpoolexecutor-pool-size