java源码 - CountDownLatch

开篇

• CountDownLatch是一个同步工具类，用来协调多个线程之间的同步，或者说起到线程之间的通信（而不是用作互斥的作用）。

• CountDownLatch能够使一个线程在等待另外一些线程完成各自工作之后，再继续执行。使用一个计数器进行实现。计数器初始值为线程的数量。当每一个线程完成自己任务后，计数器的值就会减一。当计数器的值为0时，表示所有的线程都已经完成了任务，然后在CountDownLatch上等待的线程就可以恢复执行任务。

• CountDownLatch是一次性的，计数器的值只能在构造方法中初始化一次，之后没有任何机制再次对其设置值，当CountDownLatch使用完毕后，它不能再次被使用。

CountDownLatch的用法

• CountDownLatch典型用法1：某一线程在开始运行前等待n个线程执行完毕。将CountDownLatch的计数器初始化为n new CountDownLatch(n) ，每当一个任务线程执行完毕，就将计数器减1 countdownlatch.countDown()，当计数器的值变为0时，在CountDownLatch上 await() 的线程就会被唤醒。一个典型应用场景就是启动一个服务时，主线程需要等待多个组件加载完毕，之后再继续执行。

• CountDownLatch典型用法2：实现多个线程开始执行任务的最大并行性。注意是并行性，不是并发，强调的是多个线程在某一时刻同时开始执行。类似于赛跑，将多个线程放到起点，等待发令枪响，然后同时开跑。做法是初始化一个共享的CountDownLatch(1)，将其计数器初始化为1，多个线程在开始执行任务前首先 coundownlatch.await()，当主线程调用 countDown() 时，计数器变为0，多个线程同时被唤醒。

CountDownLatch的demo

public class CountDownLatchDemo { public static void main(String[] args) throws InterruptedException{ CountDownLatch countDownLatch = new CountDownLatch(2){ @Override public void await() throws InterruptedException { super.await(); System.out.println(Thread.currentThread().getName() + " count down is ok"); } }; Thread thread1 = new Thread(new Runnable() { @Override public void run() { //do something try { Thread.sleep(1000); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println(Thread.currentThread().getName() + " is done"); countDownLatch.countDown(); } }, "thread1"); Thread thread2 = new Thread(new Runnable() { @Override public void run() { try { Thread.sleep(2000); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println(Thread.currentThread().getName() + " is done"); countDownLatch.countDown(); } }, "thread2"); thread1.start(); thread2.start(); countDownLatch.await(); } 

CountDownLatch的类定义

• CountDownLatch内部包含Sync类。
• CountDownLatch内部包含Sync类的对象sync。
• Sync类继承自AQS(神奇的AQS)，构造函数设置AQS的state值为等待值。

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

CountDownLatch的等待过程

• CountDownLatch通过await()进入等待。
• CountDownLatch通过await(long timeout, TimeUnit unit)进入超时等待。

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

CountDownLatch的await()过程

• await()通过sync.acquireSharedInterruptibly()获锁。
• acquireSharedInterruptibly通过tryAcquireShared()尝试获锁。
• tryAcquireShared()判断获锁成功与否的依据是AQS的state的值是否为零。
• 获锁失败后通过doAcquireSharedInterruptibly()进入锁等待队列CLH。

 public void await() throws InterruptedException { sync.acquireSharedInterruptibly(1); } public final void acquireSharedInterruptibly(int arg) throws InterruptedException { if (Thread.interrupted()) throw new InterruptedException(); // 尝试获锁失败 if (tryAcquireShared(arg) < 0) // doAcquireSharedInterruptibly(arg); } protected int tryAcquireShared(int acquires) { return (getState() == 0) ? 1 : -1; } 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) { setHeadAndPropagate(node, r); p.next = null; // help GC failed = false; return; } } if (shouldParkAfterFailedAcquire(p, node) && parkAndCheckInterrupt()) throw new InterruptedException(); } } finally { if (failed) cancelAcquire(node); } } 

CountDownLatch的await(long timeout, TimeUnit unit)过程

• await(long timeout, TimeUnit unit)通过sync.tryAcquireSharedNanos()获锁。
• tryAcquireSharedNanos()通过doAcquireSharedNanos()尝试获锁。
• tryAcquireShared()判断获锁成功与否的依据是AQS的state的值是否为零。
• 获锁失败后通过doAcquireSharedNanos()进入锁等待队列CLH，和doAcquireSharedInterruptibly()方法相比增加了超时检测机制，通过LockSupport.parkNanos()实现超时。

 public boolean await(long timeout, TimeUnit unit) throws InterruptedException { return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout)); } public final boolean tryAcquireSharedNanos(int arg, long nanosTimeout) throws InterruptedException { if (Thread.interrupted()) throw new InterruptedException(); return tryAcquireShared(arg) >= 0 || doAcquireSharedNanos(arg, nanosTimeout); } private boolean doAcquireSharedNanos(int arg, long nanosTimeout) throws InterruptedException { if (nanosTimeout <= 0L) return false; final long deadline = System.nanoTime() + nanosTimeout; 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) { setHeadAndPropagate(node, r); p.next = null; // help GC failed = false; return true; } } nanosTimeout = deadline - System.nanoTime(); if (nanosTimeout <= 0L) return false; if (shouldParkAfterFailedAcquire(p, node) && nanosTimeout > spinForTimeoutThreshold) LockSupport.parkNanos(this, nanosTimeout); if (Thread.interrupted()) throw new InterruptedException(); } } finally { if (failed) cancelAcquire(node); } } 

CountDownLatch的唤醒过程

• CountDownLatch通过sync.releaseShared(1)释放锁实现state的递减
• tryReleaseShared()方法判断锁状态state==0，递减后值为0说明锁已经被释放。
• releaseShared()释放锁成功后通过doReleaseShared()方法唤醒所有等待线程。
• doReleaseShared()唤醒锁的过程是一个传播性的唤醒，通过线程A唤醒线程B，然后由线程B唤醒线程C的传播性依次唤醒所有等待线程。

 public void countDown() { sync.releaseShared(1); } public final boolean releaseShared(int arg) { if (tryReleaseShared(arg)) { doReleaseShared(); return true; } return false; } 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; } } 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; } } 

总结

CountDownLatch的工作原理，总结起来就两点（基于AQS实现）：

• 初始化锁状态的值为需要等待的线程数。
• 判断锁状态是否已经释放，如果锁未释放所有等待锁的线程就会进入等待的CLH队列。
• 如果锁状态已经释放，那么就会通过传播性唤醒所有的等待线程。