java同步工具类之CountDownLatch



    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：http://www.blogjava.net/freeman1984/archive/2011/07/04/353654.html

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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