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 ----------