CountDownlatch是一个多功能的同步工具，可以被用于各种目的。一个CountDownLatch通过一个值为1的count被初始化，来作为一个开/关的门或门闩：所有调用了await()的线程都会在门前等待，直到门被一个线程通过调用countDown()打开。一个被初始化为N的CountDownLatch可以被用来“在N个线程都完成了某种操作（或者一些操作已经被完成了N次）之后创建一个线程”。

CountDownLatch一个有用的属性就是它不需要线程们在继续执行之前，调用countDown来等待count被减到0。它简单地阻止了任何调用了await()的线程继续，直到所有的线程都能够通过。

import java.util.concurrent.CountDownLatch;/*** Created by cuijunyong on 2018/2/3.*/
public class Xunhuan {public static int i = 0;private static final CountDownLatch end = new CountDownLatch(2);private static final CountDownLatch start = new CountDownLatch(1);public static void main(String[] args) throws Exception{A a = new A();
//    A b = new A();MyThread myThread = new MyThread();Thread b = new Thread(myThread);
//    Thread[] threads = new Thread[10];
//    for(int x = 0; x < threads.length; x++){
//      threads[i] = new Thread(myThread);
//      threads[i].start();
//    }
    a.start();b.start();System.out.println("开始工作了\n");start.countDown();end.await();System.out.println("a:" + a.isAlive());System.out.println("b:" +b.isAlive());System.out.println("i=" + i);}static class MyThread implements Runnable{public void run() {
//      int i = 0;try {start.await();int j = 0;while (j < 100){j++;i++;System.out.println("B = " + i);}System.out.println("end.count = " + end.getCount());end.countDown();} catch (Exception e) {e.printStackTrace();}}}static class A extends Thread{@Overridepublic void run() {
//      int i = 0;try {start.await();int j = 0;while (j < 100){j++;i++;System.out.println("A = " + i);}System.out.println("end.count = " + end.getCount());end.countDown();} catch (Exception e) {e.printStackTrace();}}}}

A = 190
A = 191
A = 192
A = 193
A = 194
A = 195
B = 171
end.count = 2
B = 196
B = 197
B = 198
B = 199
B = 200
end.count = 1
a:false
b:false
i=200