Free all waiting threads

Question

I'm writing this class that simulates a barrier point. When a thread reaches this barrier point it cannot proceed until the other threads have also reached this point. I am using a counter to keep track of the number of threads that have arrived at this point. Assume that the class is expecting N+1 threads, but is only given N threads. In this case the program will keep all the threads waiting because it thinks that there is still one more thread to arrive.

I want to write a method that will allow me to free all of the waiting threads regardless of whether or not the program thinks there is still more threads to arrive at the barrier point.

My program to wait for all threads,

public volatile int count;
public static boolean cycle = false;

public static Lock lock = new ReentrantLock();
public static Condition cv = lock.newCondition();

public void barrier() throws InterruptedException {
    boolean cycle;
    System.out.println("lock");
    lock.lock();
    try {
        cycle = this.cycle;
        if (--this.count == 0) {
            System.out.println("releasing all threads");
            this.cycle = !this.cycle;
            cv.signalAll();
        } else {
            while (cycle == this.cycle) {
                System.out.println("waiting at barrier");
                cv.await(); // Line 20
            }
        }
    } finally {
        System.out.println("unlock");
        lock.unlock();
    }
}

I was thinking I could simply create a method that calls the signalAll() method and all the threads would be free. However, a problem I am having is that if the program is expecting more threads it will maintain a lock because it will be waiting at line 20.

Is there a way to get around this lock? How should I approach this problem?

Answer 1

Better idea - use standard java.util.concurrent primitive - CyclicBarrier with method 'reset':

/**
 * Resets the barrier to its initial state.  If any parties are
 * currently waiting at the barrier, they will return with a
 * {@link BrokenBarrierException}. Note that resets <em>after</em>
 * a breakage has occurred for other reasons can be complicated to
 * carry out; threads need to re-synchronize in some other way,
 * and choose one to perform the reset.  It may be preferable to
 * instead create a new barrier for subsequent use.
 */
public void reset()