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 nature, its use is limited.

I find 2 discussions related to this issue.

http://www.ibm.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 ----------