實例代碼

public static void main(String[] args) throws InterruptedException {
    ExecutorService executorService = Executors.newFixedThreadPool(3);
    //規(guī)定幾步
    CountDownLatch countDownLatch = new CountDownLatch(3);//構(gòu)造函數(shù)
    executorService.submit(()->{
        System.out.println("飯煮好了");
        countDownLatch.countDown();
    });
    executorService.submit(()->{
        System.out.println("菜炒好了");
        countDownLatch.countDown();
    });
    executorService.submit(()->{
        System.out.println("湯煮好了");
        countDownLatch.countDown();
    });
    countDownLatch.await();
    //只有上面規(guī)定幾步都執(zhí)行完成了才會執(zhí)行下面語句武鲁，否則繼續(xù)等待
    System.out.println("出來吃飯了");
}

構(gòu)造函數(shù)new CountDownLatch(3)

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

Sync代碼繼承AQS
    private static final class Sync extends AbstractQueuedSynchronizer {
//將AQS的state設(shè)置為count
        Sync(int count) {
            setState(count);
        }
}

countDownLatch.countDown()方法

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

    public final boolean releaseShared(int arg) {
//判斷是否達到臨界點达舒，能否釋放鎖
        if (tryReleaseShared(arg)) {
            doReleaseShared();
            return true;
        }
        return false;
    }

//主要是利用cas獲取
        protected boolean tryReleaseShared(int releases) {
            for (;;) {
                int c = getState();
                if (c == 0)
                    return false;
                int nextc = c-1;

                if (compareAndSetState(c, nextc))
                    return nextc == 0;
            }
        }

/*
* 釋放鎖
*/

    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;
        }
    }

countDownLatch.await();

    public void await() throws InterruptedException {
        sync.acquireSharedInterruptibly(1);
    }
    public final void acquireSharedInterruptibly(int arg)
            throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();
//判斷state是否大于0牡直，如果其他的countdown方法已經(jīng)執(zhí)行完了這邊會返回1橄维，這個時候這邊不會進入該方法武契，也不會中斷線程钾虐，直接執(zhí)行下面的業(yè)務(wù)邏輯
        if (tryAcquireShared(arg) < 0)
            doAcquireSharedInterruptibly(arg);
    }

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

    private void doAcquireSharedInterruptibly(int arg)
        throws InterruptedException {
//添加一個waiter梳码，設(shè)置模式為SHARED(包含初始化waiter隊列，可參考AQS文章)
        final Node node = addWaiter(Node.SHARED);
        boolean failed = true;
        try {
            for (;;) {
//找到頭結(jié)點
                final Node p = node.predecessor();
                if (p == head) {
                    int r = tryAcquireShared(arg);
//如果state為0則返回1(getState() == 0) ? 1 : -1
                    if (r >= 0) {
//當(dāng)state為0智什，則表示waiter可以執(zhí)行了
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        failed = false;
                        return;
                    }
                }
//停止該線程,喚醒線程也是這邊喚醒的
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    throw new InterruptedException();
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }