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该文章转自：http://www.itzhai.com/the-introduction-and-use-of-a-countdownlatch.html
1、类介绍
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。用给定的计数 初始化 CountDownLatch。由于调用了 countDown() 方法，所以在当前计数到达零之前，await 方法会一直受阻塞。之后，会释放所有等待的线程，await 的所有后续调用都将立即返回。这种现象只出现一次——计数无法被重置。 一个线程(或者多个)， 等待另外N个线程完成某个事情之后才能执行
public void countDown()
如果当前计数等于零，则不发生任何操作。
public boolean await(long timeout,
TimeUnit unit)
throws InterruptedException
true 值。
如果当前计数大于零，则出于线程调度目的，将禁用当前线程，且在发生以下三种情况之一前，该线程将一直处于休眠状态：
countDown() 方法，计数到达零；或者如果计数到达零，则该方法返回 true 值。
如果当前线程：
则抛出 InterruptedException，并且清除当前线程的已中断状态。如果超出了指定的等待时间，则返回值为 false。如果该时间小于等于零，则此方法根本不会等待。
timeout - 要等待的最长时间unit - timeout 参数的时间单位。true；如果在计数到达零之前超过了等待时间，则返回 falseInterruptedException - 如果当前线程在等待时被中断1 public class CountDownLatchTest { 2 3 // 模拟了100米赛跑，10名选手已经准备就绪，只等裁判一声令下。当所有人都到达终点时，比赛结束。 4 public static void main(String[] args) throws InterruptedException { 5 6 // 开始的倒数锁 7 final CountDownLatch begin = new CountDownLatch(1); 8 9 // 结束的倒数锁 10 final CountDownLatch end = new CountDownLatch(10); 11 12 // 十名选手 13 final ExecutorService exec = Executors.newFixedThreadPool(10); 14 15 for (int index = 0; index < 10; index++) { 16 final int NO = index + 1; 17 Runnable run = new Runnable() { 18 public void run() { 19 try { 20 // 如果当前计数为零，则此方法立即返回。 21 // 等待 22 begin.await(); 23 Thread.sleep((long) (Math.random() * 10000)); 24 System.out.println("No." + NO + " arrived"); 25 } catch (InterruptedException e) { 26 } finally { 27 // 每个选手到达终点时，end就减一 28 end.countDown(); 29 } 30 } 31 }; 32 exec.submit(run); 33 } 34 System.out.println("Game Start"); 35 // begin减一，开始游戏 36 begin.countDown(); 37 // 等待end变为0，即所有选手到达终点 38 end.await(); 39 System.out.println("Game Over"); 40 exec.shutdown(); 41 } 42 }
5、输出结果
1 package com.cakushin.thread.countdownlatch; 2 3 import java.util.Random; 4 import java.util.concurrent.CountDownLatch; 5 6 public class TaskPortion implements Runnable { 7 private static int counter = 0; 8 private final int id = counter++; 9 private static Random rand = new Random(47); 10 private final CountDownLatch latch; 11 12 public TaskPortion(CountDownLatch latch){ 13 this.latch = latch; 14 } 15 16 @Override 17 public void run() { 18 try { 19 doWork(); 20 latch.countDown(); 21 } catch (InterruptedException e) { 22 e.printStackTrace(); 23 } 24 } 25 26 private void doWork() throws InterruptedException { 27 Thread.sleep(rand.nextInt(2000)); 28 System.out.println(this + " completed!"); 29 } 30 31 public String toString(){ 32 return String.format("%1$-3d", id); 33 } 34 35 }
代码2：
1 package com.cakushin.thread.countdownlatch; 2 3 import java.util.concurrent.CountDownLatch; 4 5 public class WaitingTask implements Runnable { 6 private static int counter = 0; 7 private final int id = counter++; 8 private final CountDownLatch latch; 9 10 public WaitingTask(CountDownLatch latch){ 11 this.latch = latch; 12 } 13 14 @Override 15 public void run() { 16 try { 17 latch.await(); 18 System.out.println("Latch barrier passed for " + this); 19 } catch (InterruptedException e) { 20 System.out.println(this + " interrupted"); 21 } 22 } 23 24 public String toString(){ 25 return String.format("WatingTask %1$-3d", id); 26 } 27 28 }
代码3：
1 package com.cakushin.thread.countdownlatch; 2 3 import java.util.concurrent.CountDownLatch; 4 import java.util.concurrent.ExecutorService; 5 import java.util.concurrent.Executors; 6 7 public final class CountDownLatchDemo { 8 9 static final int SIZE = 100; 10 11 /** 12 * @author Administrator 13 * @param args 14 */ 15 public static void main(String[] args) { 16 ExecutorService exec = Executors.newCachedThreadPool(); 17 CountDownLatch latch = new CountDownLatch(SIZE); 18 for(int i = 0; i < 10; i++){ 19 exec.execute(new WaitingTask(latch)); 20 } 21 for(int i = 0; i < 100; i++){ 22 exec.execute(new TaskPortion(latch)); 23 } 24 System.out.println("Launched all tasks"); 25 exec.shutdown(); 26 } 27 28 }
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原文地址：http://www.cnblogs.com/wubingshenyin/p/4486476.html
