在一些应用场合中，某段程序需要等待某个条件达到要求后才能执行，或者等待一定长的时间后才行，从jdk1.5开始就可以使用CountDownLatch实现。CountDownLatch类是一个同步倒数计数器，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()方法会阻塞后面程序执行，直到计数器为0，await(long timeout, TimeUnit unit)，是等待一定时间，然后执行，不管计数器是否到0了。
下面举一个等车的例子：
10个同学上车，车等待同学上车，如果有等待时间限制，到时间就开走，不管学生上没上完。如果没有等待时间，学生上完了再开：

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.TimeUnit;
public class CountDownLatchTest {
public static int numberOfPeople = 10;//等车的学生数
public static boolean isGone = false;//车开的标志
public static int carWaitTime = 3;//车等的时间
public static void main(String[] args) throws InterruptedException {
CountDownLatch waitStudentsGetOn = new CountDownLatch(numberOfPeople);
new Thread(new GetOn(waitStudentsGetOn)).start();
waitStudentGetOn(waitStudentsGetOn);//等所有的学生上车
driveHome();//开车走
}
private static void waitStudentGetOn(CountDownLatch waitStudentsGetOn) throws InterruptedException {
System.out.println(“赶紧的,抓紧时间上车..”);
waitStudentsGetOn.await(carWaitTime, TimeUnit.SECONDS);//等5秒，还没上车，就开走。。
}
private static void driveHome() throws InterruptedException {
System.out.println(“开车，鞋儿破 帽儿破 身上的袈裟破 你笑我 他笑我 一把扇儿破”);
isGone = true;
}
}


import java.util.Random;
import java.util.concurrent.CountDownLatch;
class GetOn implements Runnable {
private CountDownLatch waitStudentsGetOn;
GetOn(CountDownLatch waitStudentsGetOn) {
this.waitStudentsGetOn = waitStudentsGetOn;
}
public void run() {
for (int i = 0; i < CountDownLatchTest.numberOfPeople; i++) { try { if (CountDownLatchTest.isGone) { System.out.println("妈的，还差：" + waitStudentsGetOn.getCount() + " 个没娃上车呢.怎么车走了"); break; } boolean goonSuccess = new Student(i + 1).getOn();//顺序上车 if (goonSuccess) waitStudentsGetOn.countDown(); } catch (InterruptedException e) { } if (waitStudentsGetOn.getCount() != 0l) { System.out.println("还差：" + (waitStudentsGetOn.getCount()) + " 个没上车"); } else { System.out.println("都上车了"); } } } class Student { private int myNum;//学生编号 public Student(int num) { this.myNum = num; } //上车 @SuppressWarnings("static-access") public boolean getOn() throws InterruptedException { Thread.currentThread().sleep(new Random().nextInt(2) * 1000);//上车使用的时间，随机 if (CountDownLatchTest.isGone) { return false;//不能上了，上车失败 } System.out.print("编号为:" + myNum + "的同学上车了.."); return true; } } }
[整理自网络]