Waiting the main thread till all child thread has completed

Question

I am in the process of developing a batch application in Java 5.0 which extensively uses the java.util.concurrency API. Here is a small description of what the batch process will do,
1. Retrieve values from DB and populate a blocking queue in main thread.
2. Instantiate a Threadpool by calling, Executors.newFixedThreadPool(2)
3. Invoking the following block of code from the main thread,
while(!iBlockingQueue.isEmpty()) {
    AbstractProcessor lProcessor = new  DefaultProcessor((BusinessObject)iBlockingQueue.remove());
    iThreadPool.execute(lProcessor);
}DefaultProcessor is a class that extends Thread.
4. Invoking the following block of code from the main thread,
iThreadPool.shutdown();
try {
     iThreadPool.awaitTermination(30, TimeUnit.SECONDS);
     } catch (InterruptedException interruptedException) {
          iLogger.debug("Error in await termination...", interruptedException);
Since, this is the first time I am using the java.util.concurrency API, I want to know whether this is the right way to wait for all the child threads to complete before executing further statements in the main (parent) thread. Or do I necessariliy have to call join() to ensure that the main thread waits for all the child threads to finish which can only happen when the queue is empty.
Please note here that as per the requirements of the application the blocking queue is filled only once at the very beginning.
I will appreciate any inputs on this.
Thanks.

Answer 1

looks like you would be waiting on a queue twice, once in the loop and again, under the hood, in the threadpool's execute()
the threadpool's internal queue is all that is needed
if your iBlockingQueue is also the threadpool's internal queue, you might have a problem when you remove() the BusinessObject
by making DefaultProcessor extend Thread you are, in effect, implementing your own threadpool without the pooling
DefaultProcessor need only implement Runnable, it will be wrapped in a thread within the pool and start() called
to implement a clean shutdown, I suggest writing DefaultProcessor.run() as an infinite loop around the blocking queue poll(timeout) and a stop flag that is checked before going back to poll
class DefaultProcessor implements Runnable {
  private BlockingQueue myQ;
  private boolean myStopFlag;
  DefaultProcessor( BlockingQueue bq ) { myQ = bq; }
  public void run() {
    BusinessObject bo = null;
    while( !myStopFlag && (bo=myQ.poll( 10, SECONDS )) ) {
      // business code here
  public void stop() { myStopFlag = true; }
} Now, after iThreadPool.shutdown(), either call stop() on all DefaultProcessors (or alternatively send "poison" messages to the queue), and give yourself enough time to allow processing to finish.