1. Java Concurrency Utilities - java.util.concurrent
2. Java BlockingQueue
3. ArrayBlockingQueue
4. DelayQueue
5. LinkedBlockingQueue
6. PriorityBlockingQueue
7. SynchronousQueue
8. BlockingDeque
9. LinkedBlockingDeque
10. Java ConcurrentMap
11. ConcurrentNavigableMap
12. CountDownLatch
13. CyclicBarrier
14. Exchanger
15. Semaphore
16. Java ExecutorService
17. Java Callable
18. Java Future
19. ThreadPoolExecutor
20. ScheduledExecutorService
21. Java ForkJoinPool
22. Java Lock
23. ReadWriteLock
24. AtomicBoolean
25. AtomicInteger
26. AtomicLong
27. AtomicReference
28. AtomicStampedReference
29. AtomicIntegerArray
30. AtomicLongArray
31. AtomicReferenceArray

A java.util.concurrent.CountDownLatch is a concurrency construct that allows one or more threads to wait for a given set of operations to complete.

A CountDownLatch is initialized with a given count. This count is decremented by calls to the countDown() method. Threads waiting for this count to reach zero can call one of the await() methods. Calling await() blocks the thread until the count reaches zero.

Below is a simple example. After the Decrementer has called countDown() 3 times on the CountDownLatch, the waiting Waiter is released from the await() call.

CountDownLatch latch = new CountDownLatch(3);

Waiter      waiter      = new Waiter(latch);
Decrementer decrementer = new Decrementer(latch);

new Thread(waiter)     .start();
new Thread(decrementer).start();

Thread.sleep(4000);

public class Waiter implements Runnable{

    CountDownLatch latch = null;

    public Waiter(CountDownLatch latch) {
        this.latch = latch;
    }

    public void run() {
        try {
            latch.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        System.out.println("Waiter Released");
    }
}

public class Decrementer implements Runnable {

    CountDownLatch latch = null;

    public Decrementer(CountDownLatch latch) {
        this.latch = latch;
    }

    public void run() {

        try {
            Thread.sleep(1000);
            this.latch.countDown();

            Thread.sleep(1000);
            this.latch.countDown();

            Thread.sleep(1000);
            this.latch.countDown();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}