java.util.concurrent.CountDownLatch类是一个同步计数器,构造时传入int参数,该参数就是计数器的初始值，每调用一次countDown()方法，计数器减1,计数器大于0 时，await()方法会阻塞程序继续执行。

package cn.baokx;import java.util.Random;import java.util.concurrent.CountDownLatch;import java.util.concurrent.TimeUnit;class SubThread implements Runnable{	private CountDownLatch downLatch;	private String name;		public SubThread(CountDownLatch downLatch,String name){		this.downLatch = downLatch;		this.name = name;	}	@Override	public void run() {		System.out.println(this.name+" is working...");		try {			TimeUnit.SECONDS.sleep(new Random().nextInt(10));			System.out.println(this.name + " has finished its working...");		} catch (InterruptedException e) {			e.printStackTrace();		}finally{			this.downLatch.countDown();		}	}}public class CountDownLatchTest {	public static void main(String[] args) {		CountDownLatch downLatch = new CountDownLatch(4);		SubThread st1 = new SubThread(downLatch,"subThread1");		SubThread st2 = new SubThread(downLatch,"subThread2");		SubThread st3 = new SubThread(downLatch,"subThread3");		SubThread st4 = new SubThread(downLatch,"subThread4");				System.out.println("MainThread is wating...");		new Thread(st1).start();		new Thread(st2).start();		new Thread(st3).start();		new Thread(st4).start();		try {			downLatch.await();		} catch (InterruptedException e) {			e.printStackTrace();		}		System.out.println("All subThread has finished,MainThread can begin to checking...");	}}