CountDownLatch

CountDownLatch用来同步一个或多个 任务，强制他们等待由其他任务执行的一组操作完成。

用法：

给CountDownLatch对象设置一个初值，任何在这个对象上调用await()的方法都将阻塞，等待这个计数值为0，

可以由其他线程在这个对象调用countDown()来减少这个计数值，计数值不能被重置。

countDown()不会造成阻塞。

示例：

/**
 * 完成初始化工作的对象
 */
class TaskPortion implements Runnable{
    private static int counter=0;
    private final int id=counter++;
    private static Random rand=new Random(47);
    private final CountDownLatch latch;
    TaskPortion(CountDownLatch latch){
        this.latch=latch;
    }
    @Override
    public void run() {
        try {
            doWork();
            latch.countDown();//减少计数值
        } catch (InterruptedException e) {

        }
    }
    public void doWork() throws InterruptedException{
        TimeUnit.MILLISECONDS.sleep(rand.nextInt(2000));//随机等待一段时间，模拟任务进行
        System.out.println(this+"completed");
    }
    public String toString(){
        return String.format("%1$-3d ",id);
    }
}

/**
 * 等待任务，必须等所有初始化任务完成之后才可以进行
 */
class WaitingTask implements Runnable{
    private static int counter=0;
    private final int id=counter++;
    private final CountDownLatch latch;
    WaitingTask(CountDownLatch latch){
        this.latch=latch;
    }

    @Override
    public void run() {
        try {
            latch.await();//使当前任务等待，直到计数值为0
            System.out.println("Latch barrier passed for "+this);
        } catch (InterruptedException e) {
            System.out.println(this+" interrupted");
        }
    }
    public String toString(){
        return String.format("WaitingTask %1$-3d",id);
    }
}
public class CountDownLatchDemo {
    static final int SIZE=100;
    public static void main(String[] args) throws Exception{
        ExecutorService exec= Executors.newCachedThreadPool();
        CountDownLatch latch=new CountDownLatch(SIZE);//初始化计数值为100
        for(int i=0;i<10;i++)
            exec.execute(new WaitingTask(latch));//10个等待任务
        for(int i=0;i<SIZE;i++)
            exec.execute(new TaskPortion(latch));//100个初始化任务
        System.out.println("Launched all tasks");
        exec.shutdown();
    }
}

Launched all tasks

43  completed

98  completed

37  completed

96  completed

94  completed

11  completed

21  completed

77  completed

6   completed

9   completed

75  completed

79  completed

10  completed

40  completed

95  completed

64  completed

23  completed

34  completed

29  completed

38  completed

55  completed

90  completed

88  completed

28  completed

4   completed

50  completed

8   completed

12  completed

0   completed

27  completed

99  completed

13  completed

72  completed

71  completed

45  completed

2   completed

91  completed

31  completed

14  completed

17  completed

7   completed

97  completed

35  completed

69  completed

20  completed

32  completed

5   completed

68  completed

36  completed

47  completed

87  completed

70  completed

84  completed

86  completed

66  completed

54  completed

42  completed

41  completed

46  completed

74  completed

57  completed

65  completed

80  completed

1   completed

19  completed

59  completed

15  completed

89  completed

51  completed

25  completed

53  completed

62  completed

58  completed

92  completed

76  completed

22  completed

56  completed

18  completed

85  completed

61  completed

30  completed

60  completed

67  completed

24  completed

26  completed

48  completed

39  completed

33  completed

52  completed

3   completed

93  completed

81  completed

78  completed

73  completed

44  completed

83  completed

49  completed

63  completed

82  completed

16  completed

Latch barrier passed for WaitingTask 0  

Latch barrier passed for WaitingTask 6  

Latch barrier passed for WaitingTask 5  

Latch barrier passed for WaitingTask 4  

Latch barrier passed for WaitingTask 1  

Latch barrier passed for WaitingTask 3  

Latch barrier passed for WaitingTask 2  

Latch barrier passed for WaitingTask 7  

Latch barrier passed for WaitingTask 8  

Latch barrier passed for WaitingTask 9