Read CountDownLatch source code

//Allow one or more threads to wait for the internal AQS before completing a set of operations

private static final class Sync extends AbstractQueuedSynchronizer {
        private static final long serialVersionUID = 4982264981922014374L;

        Sync(int count) {
            setState(count);
        }

        int getCount() {
            return getState();
        }

        protected int tryAcquireShared(int acquires) {
            return (getState() == 0) ? 1 : -1;
        }

        protected boolean tryReleaseShared(int releases) {
            // Decrement count; signal when transition to zero
            for (;;) {
                int c = getState();
                if (c == 0)
                    return false;
                int nextc = c-1;
                if (compareAndSetState(c, nextc))
                    return nextc == 0;
            }
        }
    }


// first look at the constructor
public CountDownLatch(int count) {
        if (count < 0) throw new IllegalArgumentException("count < 0");
	//Initialize sync.
        this.sync = new Sync(count);
    }


// block the current thread
public void await() throws InterruptedException {
        sync.acquireSharedInterruptibly(1);
    }

//Block the specified time timeout and return false
public boolean await(long timeout, TimeUnit unit)
        throws InterruptedException {
        return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
    }

//Decrement count.0 to wake up all waiting threads.
public void countDown() {
        sync.releaseShared(1);
    }

//Get the current counter count
public long getCount() {
        return sync.getCount();
    }

public String toString() {
        return super.toString() + "[Count = " + sync.getCount() + "]";
    }