CountDownLatch 简介和例子

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，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： 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;
         }
    }
}