abstractqueuedsynchronizer详解(二)——countdownlatch源码分析

AbstractQueuedSynchronizer 详解（二） CountDownLatch 源码分析 CountDownLatch 的使用 CountDownLatch 是一个工具类，用于使一个或多个线程等待另一系列线程完成操作，也就 是说一些线程在另外一些完成之后才能继续执行，类似线性。 初始 CountDownLatch 时需要提供一个 count 参数，await 方法将会使线程阻塞，直到这个 值变为 0，countDown()方法可以使 count 值减 1，一旦到达 0 后，调用 await 阻塞的线程将 会得到释放。 CountDown 一般有两个用法。 一个线程等待多个线程 一个线程等待多个线程完成后再执行，比如一个不限时的考试，10 个学生考试，交卷时间 不同，当 10 个学生都交卷后，老师才认为考试结束。下面是该例子： public class ExamDemo public static class Student implements Runnable private int num;/学号 private CountDownLatch countDownLatch; public Student(int num, CountDownLatch countDownLatch) this.num = um; this.countDownLatch = countDownLatch; Override public void run() Random random = new Random(); try TimeUnit.SECONDS.sleep(random.nextInt(10) + 1); catch (InterruptedException e) e.printStackTrace(); System.out.println(“学生“ + num + “交卷了“); /完成工作，将 count-1 countDownLatch.countDown(); public static class Teacher implements Runnable private CountDownLatch countDownLatch; public Teacher(CountDownLatch countDownLatch) this.countDownLatch = countDownLatch; Override public void run() System.out.println(“考试开始，不限时间!“); try /等待 count 变为 0 countDownLatch.await(); catch (InterruptedException e) e.printStackTrace(); System.out.println(“考试结束“); public static void main(String args) Executor executor = Executors.newCachedThreadPool(); CountDownLatch countDownLatch = new CountDownLatch(10); executor.execute(new Teacher(countDownLatch); for (int i = 1; i = 0) setHeadAndPropagate(node, r); p.next = null; / help GC failed = false; return; /检查是否需要将当前线程挂起 if (shouldParkAfterFailedAcquire(p, node) finally if (failed) cancelAcquire(node); 队列的结构如下： 其中 Head 是个空节点，所以一旦前驱节点是头节点，并且要是 count 变为了 0，那么就调 用 setHeadAndPropagate 方法，然后释放头节点。 setHeadAndPropagate 方法主要完成两件 事，第一更改头节点，第二由于此时等待线程可以执行了，将该事件传播给后面的节点， 实现如下： private void setHeadAndPropagate(Node node, int propagate) Node h = head; / Record old head for check below /更改头节点 setHead(node); /* * Try to signal next queued node if: * Propagation was indicated by caller, * or was recorded (as h.waitStatus either before * or after setHead) by a previous operation * (note: this uses sign-check of waitStatus because * PROPAGATE status may transition to SIGNAL.) * and * The next node is waiting in shared mode, * or we dont know, because it appears null * * The conservatism in both of these checks may cause * unnecessary wake-ups, but only when there are multiple * racing acquires/releases, so most need signals now or soon * anyway. */ /如果需要传播 if (propagate 0 | h = null | h.waitStatus 0 | (h = head) = null | h.waitStatus 0) Node s = de.next; if (s = null | s.isShared() doReleaseShared(); 可以看到如果需要传播的话，将会调用 doReleasShared 方法 ,方法如下： private void doReleaseShared() for (;) Node h = head; if (h != null if (ws = Node.SIGNAL) if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0) /重置状态，成功 后，将后继节点唤醒 continue; / loop to recheck cases unparkSuccessor(h); else if (ws = 0 / loop on failed CAS if (h = head) / loop if head changed break; 至此，await 方法就使线程挂起了，下面再分析 countDown 方法。 countDown 方法 countDown 方法用于将 count 值减 1,如果 count 变为 0 则释放所有等待线程，方法的实现如 下： public void countDown() sync.releaseShared(1); releaseShared 方法如下 : public final boolean releaseShared(int arg) /尝试释放共享资源，如果需要释放节点，则释放节点 if (tryReleaseShared(arg) doReleaseShared(); return true; return false; tryReleaseShared 需要在子类中实现，Sync 中的实现如下： 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; 一开始设置了 count，每次 countDown 会使 AQS 的 state-1，一旦变成 0，则返回 true。而 如果变成了 0 后，又调用 countDown 则无效。doReleaseShared 方法用于释