CountDownLatch用法

CountDownLatch被用来同步一个或多个任务，强制它们等待由其它任务执行的一组操作完成。

构造方法CountDownLatch(int count）  构造一个用给定计数初始化的 CountDownLatch。

任何在这个对象上调用await()的方法将会被阻塞，直至这个技术值为0。其他任务在结束其工作时，可以在该对象上调用countDown()来减小这个计数值。CountDownLatch被设计为只触发一次，计数值不能被重置。如果需要重置计数值的版本，则可以调用CyclicBarrier。调用countDown()的任务在产生这个调用时并没有被阻塞，只有对await()的调用会被阻塞，直至计数值到达0。CountDownLatch的典型用法是将一个程序分为n个互相独立的可解决任务，并创建值为n的CountDownLatch。当每个任务完成时，都会在锁存器上调用countDown()。等待问题被解决的任务在这个锁存器上调用await(),将它们自己拦住，直至锁存器计数结束。

示例：

package thread;

import java.util.Random;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;

public class CountDownLatchDemo {
	static final int SIZE = 100;
	public static void main(String[] args) {
		ExecutorService exec = Executors.newCachedThreadPool();
		CountDownLatch latch = new CountDownLatch(100);
		for(int i=0;i<10;i++)
			exec.execute(new WaitingTask(latch));
		for(int i=0;i

输出：

Launched all tasks
37  completed
45  completed
99  completed
95  completed
94  completed
11  completed
23  completed
77  completed
7   completed
9   completed
75  completed
10  completed
79  completed
39  completed
25  completed
36  completed
96  completed
63  completed
30  completed
40  completed
55  completed
90  completed
28  completed
88  completed
6   completed
50  completed
8   completed
12  completed
0   completed
27  completed
13  completed
98  completed
72  completed
71  completed
2   completed
43  completed
35  completed
15  completed
91  completed
18  completed
5   completed
33  completed
97  completed
21  completed
69  completed
34  completed
4   completed
68  completed
38  completed
48  completed
87  completed
70  completed
84  completed
86  completed
66  completed
54  completed
42  completed
29  completed
46  completed
74  completed
57  completed
1   completed
20  completed
65  completed
80  completed
16  completed
60  completed
89  completed
14  completed
51  completed
53  completed
62  completed
58  completed
76  completed
92  completed
22  completed
19  completed
56  completed
85  completed
61  completed
31  completed
26  completed
24  completed
59  completed
67  completed
47  completed
41  completed
32  completed
3   completed
52  completed
93  completed
81  completed
78  completed
73  completed
44  completed
49  completed
17  completed
82  completed
64  completed
83  completed
Latch barrier passed for java.util.concurrent.CountDownLatch@189e7143[Count = 0]
Latch barrier passed for java.util.concurrent.CountDownLatch@189e7143[Count = 0]
Latch barrier passed for java.util.concurrent.CountDownLatch@189e7143[Count = 0]
Latch barrier passed for java.util.concurrent.CountDownLatch@189e7143[Count = 0]
Latch barrier passed for java.util.concurrent.CountDownLatch@189e7143[Count = 0]
Latch barrier passed for java.util.concurrent.CountDownLatch@189e7143[Count = 0]
Latch barrier passed for java.util.concurrent.CountDownLatch@189e7143[Count = 0]
Latch barrier passed for java.util.concurrent.CountDownLatch@189e7143[Count = 0]
Latch barrier passed for java.util.concurrent.CountDownLatch@189e7143[Count = 0]
Latch barrier passed for java.util.concurrent.CountDownLatch@189e7143[Count = 0]

从中可以看出WaitingTask最后执行，count=0;