JUC - CountDownLatch 源码分析
2016-10-11 10:58
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简介
CountDownLatch,一个同步辅助类,在完成一组正在其他线程中执行的操作之前,它允许一个或多个线程一直等待。源码分析
CountDownLatch的实现方式是在内部定义了一个实现AbstractQueuedSynchronizer(详见:JUC 源码分析 - AbstractQueuedSynchronizer(AQS))的内部类Sync,Sync主要实现了AbstractQueuedSynchronizer中共享模式的获取和释放方法tryAcquireShared和tryReleaseShared,在CountDownLatch中使用AQS的子类Sync,初始化的state表示一个计数器,每次countDown的时候计数器会减少1,直到减为0的时候或超时或中断,await方法从等待中返回。private static final class Sync extends AbstractQueuedSynchronizer { private static final long serialVersionUID = 4982264981922014374L; Sync(int count) {//初始化计数器数量,不可重置 setState(count); } int getCount() {//获取当前计数器数量 return getState(); } protected int tryAcquireShared(int acquires) {//await方法会调用,当前计数器为0是才返回,不为0时,会挂起当前线程,直到计数器减少为0的时候,才被countDown方法依次唤醒 return (getState() == 0) ? 1 : -1; } protected boolean tryReleaseShared(int releases) {//countDown方法会调用,每次调用都会使计数器减1,直到减少为0的时候,会返回true,依次唤醒等待中的线程,使从await方法返回。 // 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; } } }
public void await() throws InterruptedException {//该方法等待计数器减少为0,await系列方法分别调用AQS的共享模式的acquire系列方法 sync.acquireSharedInterruptibly(1); }
public void countDown() {//该方法减少计数器,调用AQS的共享模式的release方法 sync.releaseShared(1); }
使用方式
主线程等待子线程都执行完任务后才返回。public class CountDownLatchTest { public static void main(String[] args) throws InterruptedException { final CountDownLatch c = new CountDownLatch(3); Thread thread = new Thread(new Runnable() { @Override public void run() { try { Thread.sleep(1000); } catch (InterruptedException e) { e.printStackTrace(); } c.countDown(); System.out.println("countDown 1000 : " + c.getCount()); } }); thread.start(); Thread thread2 = new Thread(new Runnable() { @Override public void run() { try { Thread.sleep(2000); } catch (InterruptedException e) { e.printStackTrace(); } c.countDown(); System.out.println("countDown 2000 : " + c.getCount()); } }); thread2.start(); Thread thread3 = new Thread(new Runnable() { @Override public void run() { try { Thread.sleep(3000); } catch (InterruptedException e) { e.printStackTrace(); } c.countDown(); System.out.println("countDown 3000 : " + c.getCount()); } }); thread3.start(); System.out.println("await before : " + c.getCount()); c.await(); System.out.println("await after : " + c.getCount()); } }
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