2019独角兽企业重金招聘Python工程师标准>>> 
你是否遇到这这样一种情况，我们要举行一个视频会议，有若干的参会人员，需要等待所有的人员到齐后视频会议才能开始。
为了解决这个问题，Java API提供了一个线程同步辅助类CountDownLatch，使用这个辅助类可以让线程等待其它线程完成一组操作后才能执行，否则就一直等待。这个类使用一个整形参数来初始化，这个整形参数代表着等待其他线程的数量，使用await()方法让线程开始等待其他线程执行完毕，每一个线程执行完毕后后调用countDown()方法，这个方法会让CountDownLatch内部的计数器减1，当计数器变为0的时候，CountDownLatch类将唤醒所有调用await()方法并进入WAITING状态线程。
下面我们来完成这个视频会议的例子：
创建视频会议线程类VideoConference，并声明一个CountDownLatch属性controller来控制视频会议线程等待所有参会者到齐。会议线程启动后会调用await()方法并进入等待状态，每一个参会者到达后代用arrive()方法，并把controller中的计数器减1，当计数器等于0的时候会议线程继续执行。
public class VideoConference implements Runnable{
private final CountDownLatch controller;
public VideoConference(int number) {
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("VidwoConference: Initialization: %d participants.\n", controller.getCount());
try {
controller.await();
System.out.printf("VidwoConference: All the participants have come.\n");
System.out.printf("VidwoConference: Let's start...");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}创建参会者线程类Participant，这个类持有会议类的引用，启动后用一个随机休眠时间来模拟与会者到达所需的时间，休眠结束后代用会议类的arrive(name)方法告诉会议类，与会者到达并把CountDownLatch计数器减1。
public class Participant implements Runnable{
private String name;
private VideoConference videoConference;
public Participant(String name, VideoConference videoConference) {
this.name = name;
this.videoConference = videoConference;
}
@Override
public void run() {
long duration = (long) (Math.random() * 10);
try {
TimeUnit.SECONDS.sleep(duration);
} catch (InterruptedException e) {
e.printStackTrace();
}
videoConference.arrive(name);
}
}实现主方法类，这里我们创建并移动一个视频会议，规定参会者有10个人。然后我们启动10个参会者线程，当所有参会者都到达后，视频会议开始执行。
public class Main {
public static void main(String[] args) {
VideoConference videoConference = new VideoConference(10);
Thread threadConference = new Thread(videoConference);
threadConference.start();
Thread[] threads = new Thread[10];
for (int i = 0; i < 10; i++) {
threads[i] = new Thread(new Participant("P-" + i, videoConference));
}
for (int i = 0; i < 10; i++) {
threads[i].start();
}
}
}查看控制台日志，我们看到每次有一个参会者到达都可以调用getCount()方法来获取计数器的值。
VidwoConference: Initialization: 10 participants.
P-0 has arrived.
VideoConference: Waiting for 9 participants.
P-4 has arrived.
VideoConference: Waiting for 8 participants.
P-2 has arrived.
VideoConference: Waiting for 7 participants.
P-5 has arrived.
VideoConference: Waiting for 6 participants.
P-8 has arrived.
VideoConference: Waiting for 5 participants.
P-3 has arrived.
VideoConference: Waiting for 4 participants.
P-6 has arrived.
VideoConference: Waiting for 3 participants.
P-1 has arrived.
VideoConference: Waiting for 2 participants.
P-7 has arrived.
VideoConference: Waiting for 1 participants.
P-9 has arrived.
VideoConference: Waiting for 0 participants.
VidwoConference: All the participants have come.
VidwoConference: Let's start...注意，CountDownLatch并不是用来保护共享资源同步访问的，而是用来控制并发线程等待的。并且CountDownLatch只允许进入一次，一旦内部计数器等于0，再调用这个方法将不起作用，如果还有第二次并发等待，你还得创建一个新的CountDownLatch。