（三）线程同步工具集_3---等待多个并发事件

等待多个并发事件

Java concurrent API 提供了一个类，可以使一个或多个线程去等待一系列操作完成，该类是 CountDownLatch，该类初始化一个整数，这个整数代表了线程要等待的操作个数，

当一个线程等待操作完成时调用await()方法，当一个操作结束后，调用countDown()方法；

在下面的例子中展示CounDownLatch的使用，这是一个非常有意思的例子，一个视频会议，多个参与者；一个线程等待参与者的到来，直到所有的参与者都到达后，该线程才继续执行；

动手实现

1.创建VideoConference

public class VideoConference implements Runnable {
    private final CountDownLatch controller;

    public VideoConference(int number) {
        this.controller = new CountDownLatch(number);
    }
    public void arrive(String name) {
        System.out.printf("%s has arrived.\n", name);
        controller.countDown();
        System.out.printf("VideoConference: Waiting for %d participants.\n",
                controller.getCount());
    }

    @Override
    public void run() {
        System.out.printf("VideoConference: Initialization: %d participants.\n",
                controller.getCount());
        try {
            controller.await();
            System.out.printf("VideoConference: All the participants have come\n");
            System.out.printf("VideoConference: Let's start...\n");
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

2.创建Participant

public class Participant implements Runnable {
    private VideoConference videoConference;
    private String name;

    public Participant(VideoConference videoConference, String name) {
        this.videoConference = videoConference;
        this.name = name;
    }

    @Override
    public void run() {
        long duration=(long)(Math.random()*10);
        try {
            TimeUnit.SECONDS.sleep(duration);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        videoConference.arrive(name);
    }
}

3.Main

public class Main {
    public static void main(String[] args) {
        VideoConference conference = new VideoConference(10);
        Thread threadConference = new Thread(conference);
        threadConference.start();

        for (int i = 0; i < 10; i++) {
            Participant p = new Participant(conference, "Participant " + i);
            Thread t = new Thread(p);
            t.start();
        }
    }
}

一次运行结果：

VideoConference: Initialization: 10 participants.
Participant 4 has arrived.
VideoConference: Waiting for 9 participants.
Participant 7 has arrived.
Participant 6 has arrived.
VideoConference: Waiting for 7 participants.
VideoConference: Waiting for 8 participants.
Participant 8 has arrived.
VideoConference: Waiting for 6 participants.
Participant 2 has arrived.
VideoConference: Waiting for 5 participants.
Participant 3 has arrived.
VideoConference: Waiting for 4 participants.
Participant 9 has arrived.
VideoConference: Waiting for 3 participants.
Participant 0 has arrived.
Participant 1 has arrived.
VideoConference: Waiting for 1 participants.
Participant 5 has arrived.
VideoConference: Waiting for 2 participants.
VideoConference: All the participants have come
VideoConference: Waiting for 0 participants.
VideoConference: Let's start...

要点

1.初始化操作序列个数

2.调用await()方法

3.调用countDown()方法

需要注意：当CounDownLatch中的内部计数为0时，将会唤醒所有被await()方法睡眠的线程；该内部计数器只能被初始化一次；当countDown()方法达到0时，在调用将不会其任何作用；