JavaCountDownLatch的源码硬核解析

@Slf4j(topic = "a.CountDownLatchTest")
public class CountDownLatchTest {

    public static void main(String[] args) throws InterruptedException {
        // 创建一个倒时器，默认10个数量
        CountDownLatch latch = new CountDownLatch(10);
        ExecutorService service = Executors.newFixedThreadPool(10);
        // 设置进度数据
        String[] personProcess = new String[10];
        Random random = new Random();

        for (int i = 0; i < 10; i++) {
            int finalJ = i;
            service.submit(() -> {
                // 模拟10个人的进度条
                for (int j = 0; j <= 100; j++) {
                    // 模拟网速快慢，随机生成
                    try {
                        Thread.sleep(random.nextInt(100));
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    // 设置进度数据
                    personProcess[finalJ] = j + "%";
                   log.info("{}", Arrays.toString(personProcess));
                }

                // 运行结束，倒时器 - 1
                latch.countDown();
            });
        }
        // 打开"阀门"
        latch.await();
       log.info("王者峡谷到了");
        service.shutdown();
    }
}

// CountDownLatch#await
public void await() throws InterruptedException {
    // 调用AbstractQueuedSynchronizer的acquireSharedInterruptibly方法
    sync.acquireSharedInterruptibly(1);
}
// AbstractQueuedSynchronizer#acquireSharedInterruptibly
public final void acquireSharedInterruptibly(int arg) throws InterruptedException {
    // 判断线程是否被打断，抛出打断异常
    if (Thread.interrupted())
        throw new InterruptedException();
    // 尝试获取共享锁
    // 条件成立说明 state > 0，此时线程入队阻塞等待，等待其他线程获取共享资源
    // 条件不成立说明 state = 0，此时不需要阻塞线程，直接结束函数调用
    if (tryAcquireShared(arg) < 0)
        // 阻塞当前线程的逻辑
        doAcquireSharedInterruptibly(arg);
}
// CountDownLatch.Sync#tryAcquireShared
protected int tryAcquireShared(int acquires) {
    return (getState() == 0) ? 1 : -1;
}

// AbstractQueuedSynchronizer#doAcquireSharedInterruptibly
private void doAcquireSharedInterruptibly(int arg) throws InterruptedException {
    // 将调用latch.await()方法的线程 包装成 SHARED 类型的 node 加入到 AQS 的阻塞队列中
    final Node node = addWaiter(Node.SHARED);
    boolean failed = true;
    try {
        for (;;) {
            // 获取当前节点的前驱节点
            final Node p = node.predecessor();
            // 前驱节点时头节点就可以尝试获取锁
            if (p == head) {
                // 再次尝试获取锁，获取成功返回 1
                int r = tryAcquireShared(arg);
                if (r >= 0) {
                    // 获取锁成功，设置当前节点为 head 节点，并且向后传播
                    setHeadAndPropagate(node, r);
                    p.next = null; // help GC
                    failed = false;
                    return;
                }
            }
            // 阻塞在这里
            if (shouldParkAfterFailedAcquire(p, node) && parkAndCheckInterrupt())
                throw new InterruptedException();
        }
    } finally {
        // 阻塞线程被中断后抛出异常，进入取消节点的逻辑
        if (failed)
            cancelAcquire(node);
    }
}

private final boolean parkAndCheckInterrupt() {
    	// 阻塞线程
        LockSupport.park(this);
        return Thread.interrupted();
    }

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

public final boolean releaseShared(int arg) {
    // 尝试释放共享锁
    if (tryReleaseShared(arg)) {
        // 释放锁成功开始唤醒阻塞节点
        doReleaseShared();
        return true;
    }
    return false;
}

protected boolean tryReleaseShared(int releases) {
    for (;;) {
        int c = getState();
        // 条件成立说明前面【已经有线程触发唤醒操作】了，这里返回 false
        if (c == 0)
            return false;
        // 计数器减一
        int nextc = c-1;
        if (compareAndSetState(c, nextc))
            // 计数器为 0 时返回 true
            return nextc == 0;
    }
}

private void doReleaseShared() {
    for (;;) {
        Node h = head;
        // 判断队列是否是空队列
        if (h != null && h != tail) {
            int ws = h.waitStatus;
            // 头节点的状态为 signal，说明后继节点没有被唤醒过
            if (ws == Node.SIGNAL) {
                // cas 设置头节点的状态为 0，设置失败继续自旋
                if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
                    continue;
                // 唤醒后继节点
                unparkSuccessor(h);
            }
            // 如果有其他线程已经设置了头节点的状态，重新设置为 PROPAGATE 传播属性
            else if (ws == 0 && !compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
                continue;
        }
        // 条件不成立说明被唤醒的节点非常积极，直接将自己设置为了新的head，
        // 此时唤醒它的节点（前驱）执行 h == head 不成立，所以不会跳出循环，会继续唤醒新的 head 节点的后继节点
        if (h == head)
            break;
    }
}