CountDownLatch学习与分享

CountDownLatch使用场景：

通常使用于多个线程异步执行，之后希望将返回结果转为同步处理，可以说是起到多个线程间的通信。

基于源码进行分析：

CountDownLatch有一下几个方法：

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

方法中count为传入的并发线程待执行个数，执行Sync(count)，初始化AbstractQueuedSynchronizer中state值，作为统计个数

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

执行过程中会调用wait()方法，对以下执行过程进行解释。

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

以上执行顺序中doAcquireSharedInterruptibly（）方法，会获取tryAcquireShared（）返回值，tryAcquireShared比较的是当前线程未执行完毕的个数，只有当线程全部执行完毕时，tryAcquireShared返回1，才会退出循环，否则阻塞在for循环中。

中间的参数state即待执行线程个数，通过调用coutDown()方法进行修改，每个线程在执行完毕时调用。

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) { // 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; } }

tryReleaseShared方法会获取当前state进行修改，当所有线程执行完毕时，state值为0，触发doAcquireSharedInterruptibly中循环，int r = tryAcquireShared(arg);r为1，进入if判断，退出循环，释放锁。if判断内容为AQS相关知识，就不详述了，有兴趣可以自行学习。CountDownLatch是AQS的一种实现，有兴趣的再看AQS。

本文梳理CountDownLatch执行流程，具体细节可以自行学习。