A DESCRIPTION OF THE REQUEST :
ThreadPoolExecutor is a very flexible Executor class to achieve multi threading in a simple way. However, due to the lack of blocking  customer , its use is limited.

I find 2 discussions related to this issue.

http://www. customer .com/developerworks/forums/message.jspa?messageID=13785440
http://forum.springframework.org/showthread.php?t=35869

JUSTIFICATION :
This is a typical requirement in any multi threaded enterprise application.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
It would be nice if ThreadPoolExecutor.submit() or ThreadPoolExecutor.execute() method block until a thread gets freed up for picking up a new task.
ACTUAL -
In the current implementation ThreadPoolExecutor.submit() and ThreadPoolExecutor.execute() methods throw RejectedExecutionException exception after the configured # of threads get busy.

---------- BEGIN SOURCE ----------
import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.RejectedExecutionException;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

public class BlockingThreadPoolExecutor {

	public static void main(String[] args) {
		ArrayBlockingQueue jQ = new ArrayBlockingQueue(3);
		ThreadPoolExecutor tpExe = new ThreadPoolExecutor(1, 3, 30, TimeUnit.SECONDS,jQ );
		int numJobs = 10;
		
		System.out.println("Starting application to add "+ numJobs + " jobs");
		
		for (int i=1; i<= numJobs; i++)
			try {
				tpExe.submit(new WorkerThread(i));
				System.out.println("Added job #" + (i));
			}
		   	catch (RejectedExecutionException e) {
		      System.err.println("RejectedExecutionException");
		   }
		}

	}
	class WorkerThread implements Runnable {
		int jobId;
		public WorkerThread (int jobId){
			this.jobId = jobId;
		}
		public void run (){
			try{
				Thread.sleep(1000);
			}catch (Exception excep){}
		}
	}
---------- END SOURCE ----------
Posted Date : 2008-01-08 17:06:13.0

N/A

Brian Goetz points out an excellent solution using a Semaphore
in 

http://www.ibm.com/developerworks/forums/message.jspa?messageID=13785440

so there seems to be no need for this feature.

I would like to see improvements to task submission for another reason,
namely to avoid creating a new Thread when a waiting idle thread is available.

6452337: ThreadPoolExecutor should prefer reusing idle threads

I see that the Prometheus project
http://prometheus.codehaus.org/overview.html
has a BlockingExecutor, but I am not sure this is a good design.
Perhaps best would be to add to the documentation of TPE explaining how to
layer on blocking.
Posted Date : 2008-01-09 17:56:15.0

-----------------------------------
A discussion of the need raised here, together with suggested solutions, appears in:
http://java.net/pub/a/today/2008/10/23/creating-a-notifying-blocking-thread-pool-executor.html
-----------------------------------

I'm not sure why something like the following cannot be added:

public class BlockingRejectedExecutionHandler implements RejectedExecutionHandler
{
	public void rejectedExecution(Runnable r, ThreadPoolExecutor executor)
	{
		try {
			executor.getQueue ().put (r);
		} catch (InterruptedException e) {
			throw new RejectedExecutionException ("Interrupted while waiting for queue slot to empty");
		}
	}
}

My initial tests suggest this works flawlessly, and would be a trivial addition to the class.