public class CountDownLatchTest {
    private static final  int N = 4;
    public static void main(String[] args) {

        final CountDownLatch latch = new CountDownLatch(4);

        for(int i=0;i<N;i++)
        {
            new Thread(){
                public void run() {
                    try {
                        System.out.println("子線程"+Thread.currentThread().getName()+"正在執(zhí)行");
                        Thread.sleep(3000);
                        System.out.println("子線程"+Thread.currentThread().getName()+"執(zhí)行完畢");
                        latch.countDown();
                        System.out.println("剩余計數(shù)"+latch.getCount());
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                };
            }.start();
        }


        try {
            System.out.println("等待"+N+"個子線程執(zhí)行完畢...");
            latch.await();
            System.out.println(N+"個子線程已經(jīng)執(zhí)行完畢");
            System.out.println("繼續(xù)執(zhí)行主線程");
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

public CountDownLatch(int count) {
    if (count < 0) throw new IllegalArgumentException("count < 0");
    this.sync = new Sync(count);
}

public void countDown() {
        sync.releaseShared(1);
    }

   /**
     * Decrements the count of the latch, releasing all waiting threads if
     * the count reaches zero.
     *
     * <p>If the current count is greater than zero then it is decremented.
     * If the new count is zero then all waiting threads are re-enabled for
     * thread scheduling purposes.
     *
     * <p>If the current count equals zero then nothing happens.
     */
    public void countDown() {
        sync.releaseShared(1);
    }


    public final boolean releaseShared(int arg) {
        if (tryReleaseShared(arg)) {
            doReleaseShared();//釋放所有正在等待的線程節(jié)點
            return true;
        }
        return false;
    }

        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;
            }
        }
    private void doReleaseShared() {
        for (;;) {
            Node h = head;
            if (h != null && h != tail) {
                int ws = h.waitStatus;
                if (ws == Node.SIGNAL) {
                    if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
                        continue;            // loop to recheck cases
                    unparkSuccessor(h);
                }
                else if (ws == 0 &&
                         !compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
                    continue;                // loop on failed CAS
            }
            if (h == head)                   // loop if head changed
                break;
        }
    }

public void await() throws InterruptedException {
    sync.acquireSharedInterruptibly(1);
}

public boolean await(long timeout, TimeUnit unit)
    throws InterruptedException {
    return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
}

public void await() throws InterruptedException {
        sync.acquireSharedInterruptibly(1);
    }

    public final void acquireSharedInterruptibly(int arg)
            throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();

        if (tryAcquireShared(arg) < 0)
            doAcquireSharedInterruptibly(arg);
    }
    /*
        函數(shù)功能：根據(jù)AQS的狀態(tài)位state來返回值，
        如果為state=0，返回 1
        如果state=1，則返回-1
    */
    protected int tryAcquireShared(int acquires) {
        return (getState() == 0) ? 1 : -1;
    }

    /**
     * Acquires in shared interruptible mode.
     * @param arg the acquire argument
     */
    private void doAcquireSharedInterruptibly(int arg)
        throws InterruptedException {
        final Node node = addWaiter(Node.SHARED);
        boolean failed = true;
        try {
            for (;;) {
                final Node p = node.predecessor();
                if (p == head) {
                    int r = tryAcquireShared(arg);
                    if (r >= 0) {//如果大于零，則說明需要喚醒
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        failed = false;
                        return;
                    }
                }
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    throw new InterruptedException();
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }