Re: Making one or more threads wait for another to produce a value or fail

On 5/31/2011 7:00 AM, Tom Anderson wrote:


Just popping in quickly. First, everyone's all about CountDownLatch.
Does this really work? It requires a fixed number of threads supplied
on creation to "fire," iirc.

Second what about other java.util.concurrent classes? Both locks (esp
ReadWriteLock) and AbstractQueuedSynchronizer look promising.

<http://download.oracle.com/javase/6/docs/api/java/util/concurrent/locks/AbstractQueuedSynchronizer.html>

Especially the second example for AQS, which is "a latch class that is
like a CountDownLatch except that it only requires a single signal to
fire. Because a latch is non-exclusive, it uses the shared acquire and
release methods."

Lastly, I must be dense, I have a simple implementation but I think it's
wrong. What is wrong with it? I'm sure I've missed something in the
original specification, but I'm drawing a blank. How is the exceptional
verdict delivered? There's also no way to set a verdict for one thread
vs setting a return value for all threads. I think your initial
specification of using just "await()" and "deliver()" might be a bit too
simple for the problem.

This is just a simple class that sends a single message (the verdict) to
all threads (i.e., it broadcasts the message). Note this is untested.

public class BroadcastSynchronizer<V, E extends Throwable> {

     private volatile V verdict;
     private final Object lock = new Object();

     public V await() throws E, InterruptedException {
         while( verdict == null ) {
             synchronized( lock ) {
                 if( verdict == null )
                     lock.wait();
             }
         }
         return verdict;
     }

     public void deliver( V verdict ) {
         this.verdict = verdict;
         synchronized( lock ) {
            lock.notifyAll();
         }
     }
}