Implementation of barrier in Java

Question

How can a barrier be implemented with semaphores in Java. Will the following pseudo code work? How can it be written using java Semaphore class.

N is the number of threads to be waited for at the barrier. EveryoneHasReachedBarrier is a conditional variable.

Aquire(mutex)
m = m + 1;
if(m != N)
{ 
    Release(mutex);
    Aquire(EveryoneHasReachedBarrier);
}
else
{
   m = 0;
   Release(mutex);
   for(i=0; i<N; i++)
   {
       Release(EveryoneHasReachedBarrier);
   }
}

Answer 1

Just use a CountDownLatch or a CyclicBarrier.

Answer 2

1) Your pseudo-code doesn't use semaphores so it is not a solution.

2) It doesn't correspond to the way that Java primitive mutex / wait / notify work.

3) It probably wouldn't work anyway. Since you release the mutex before acquiring the condition there is potential for a race condition. (It is not entirely clear that this is the case because the semantics of your "primitives" are open to interpretation.)

Hint: What you need to do is read the javadocs for the Semaphore class thoroughly and then try to map them onto the problem you are trying to solve.