java多线程 等待多个并发事件的完成

java API中提供了CountDownLatch类，它允许线程一直等待，知道等待操作结束。下面以一个视频会议等待人数为例。
1.视频会议类，实现Runnable接口。

import java.util.concurrent.CountDownLatch;

public class Videoconference implements Runnable {
private final CountDownLatch controller;
public Videoconference(int number){
        controller = new CountDownLatch(number);
    }
//每一个视频会议者到达是都执行此方法
public void arrive(String name){
        System.out.printf("%s has arrived.",name);
        controller.countDown();
//还没有到达的人数
        System.out.printf("Videoconferencr has waitinf for %d participants\n",controller.getCount());
    }
public void run() { 
    System.out.printf("Videoconference initialization:%d participants.\n",controller.getCount());
try {
//等待所有人到达，如果没有这句，则会在参与人没到达之前开始会议
        controller.await();
        System.out.printf("Videoconference:ALL participants has come\n");
        System.out.print("Videoconference:Let is start...\n");
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
    }
}

2.与会者类

import java.util.concurrent.TimeUnit;

public class Participant implements Runnable{
private Videoconference conference;
public String name;
public Participant(Videoconference conference,String name){
this.conference = conference;
this.name = name;
}
@Override
public void run() {
long duration = (long)(Math.random()*10);
try {
        TimeUnit.SECONDS.sleep(duration);
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
    conference.arrive(name);
}

}

3.主方法

package Thread03semaphone;

public class Main2 {
public static void main(String[] args){
    Videoconference videoConference = new Videoconference(10);
    Thread threadconference = new Thread(videoConference);
    threadconference.start();
for(int i =0;i<10;i++){
        Participant p = new Participant(videoConference,"Participant"+i);
        Thread t = new Thread(p);
        t.start();
    }
}
}

当创建CountDownLatch对象时，通过构造器来初始化内部计数器（本例中为10），当countdown()方法被调用时，计数器将减1，当计数器为0时，countdownlatch将唤醒所有等待的线程。

注意点：

1. CountDownLatch对象计数器一旦被初始化之后，就不可以修改。
2. 他并不是用来保护临界资源或者同步代码块，只是用来同步多个任务的多个线程。
3. CountDownLatch对象只能被使用一次，一旦计数器为0时，如果再需要使用，则需要重新New一个CountDownLatch对象。