Java多线程--让主线程等待所有子线程执行完毕

摘要: CountDownLatch，一个同步辅助类，在完成一组正在其他线程中执行的操作之前，它允许一个或多个线程一直等待。

主要方法
public CountDownLatch(int count);
public void countDown();
public void await() throws InterruptedException

构造方法参数指定了计数的次数
countDown方法，当前线程调用此方法，则计数减一
awaint方法，调用此方法会一直阻塞当前线程，直到计时器的值为0

例子

public class CountDownLatchDemo {

final static SimpleDateFormat sdf=new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");

public static void main(String[] args) throws InterruptedException {

CountDownLatch latch=new CountDownLatch(2);//两个工人的协作

Worker worker1=new Worker("zhang san", 5000, latch);

Worker worker2=new Worker("li si", 8000, latch);

worker1.start();//

worker2.start();//

latch.await();//等待所有工人完成工作

System.out.println("all work done at "+sdf.format(new Date()));

}

static class Worker extends Thread{

String workerName;

int workTime;

CountDownLatch latch;

public Worker(String workerName ,int workTime ,CountDownLatch latch){

this.workerName=workerName;

this.workTime=workTime;

this.latch=latch;

}

public void run(){

System.out.println("Worker "+workerName+" do work begin at "+sdf.format(new Date()));

doWork();//工作了

System.out.println("Worker "+workerName+" do work complete at "+sdf.format(new Date()));

latch.countDown();//工人完成工作，计数器减一

}

private void doWork(){

try {

Thread.sleep(workTime);

} catch (InterruptedException e) {

e.printStackTrace();

}

}

}

}

输出：
Worker zhang san do work begin at 2011-04-14 11:05:11
Worker li si do work begin at 2011-04-14 11:05:11
Worker zhang san do work complete at 2011-04-14 11:05:16
Worker li si do work complete at 2011-04-14 11:05:19
all work done at 2011-04-14 11:05:19