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此行，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。public class CountdownLatchTest {public static void main(String[] args) {ExecutorService service = Executors.newCachedThreadPool();final CountDownLatch cdOrder = new CountDownLatch(1);final CountDownLatch cdAnswer = new CountDownLatch(3);for(int i=0;i<3;i++){Runnable runnable = new Runnable(){public void run(){try {System.out.println("线程" + Thread.currentThread().getName() +"正准备接受命令");cdOrder.await();System.out.println("线程" + Thread.currentThread().getName() +"已接受命令");Thread.sleep((long)(Math.random()*10000));System.out.println("线程" + Thread.currentThread().getName() +"回应命令处理结果");cdAnswer.countDown();} catch (Exception e) {e.printStackTrace();}}};service.execute(runnable);}try {Thread.sleep((long)(Math.random()*10000));System.out.println("线程" + Thread.currentThread().getName() +"即将发布命令");cdOrder.countDown();System.out.println("线程" + Thread.currentThread().getName() +"已发送命令，正在等待结果");cdAnswer.await();System.out.println("线程" + Thread.currentThread().getName() +"已收到所有响应结果");} catch (Exception e) {e.printStackTrace();}service.shutdown();}}
上面示例初始化两个CountDownLatch示例，分别传入初始值1和3，cdOrder.await()后面的代码，将会被阻塞，直到调用一次cdOrder.countDown()后才向下执行；cdAnswer.await()后面代码阻塞，调用三次cdAnswer.countDown()向下执行。
所以上面执行结果:
线程pool-1-thread-1正准备接受命令线程pool-1-thread-3正准备接受命令线程pool-1-thread-2正准备接受命令线程main即将发布命令线程main已发送命令，正在等待结果线程pool-1-thread-1已接受命令线程pool-1-thread-2已接受命令线程pool-1-thread-3已接受命令线程pool-1-thread-2回应命令处理结果线程pool-1-thread-3回应命令处理结果线程pool-1-thread-1回应命令处理结果线程main已收到所有响应结果
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。
下面例子来源：
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：
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;}}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;}}}
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