线程：CountDownLatch同步工具

一个同步辅助类，在完成一组正在其他线程中执行的操作之前，它允许一个或多个线程一直等待。

类似计数器，当计数器的值为0时，继续往下执行。

 package ch03;

 import java.util.Random;
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;

 public class CountDownLatchTest {

     public static void main(String[] args) {
         ExecutorService threadPool = Executors.newCachedThreadPool();
         //初始值设置为2
         final CountDownLatch latch = new CountDownLatch(2);
         for(int i=0; i<2; i++){
             Runnable runnable = new Runnable() {

                 @Override
                 public void run() {
                     try {
                         Thread.sleep(new Random().nextInt(6000));
                         System.out.println("子线程执行。。。");
                         //计数器自减
                         latch.countDown();
                     } catch (Exception e) {
                         e.printStackTrace();
                     }
                 }
             };
             threadPool.execute(runnable);
         }

         try {
             //等待计数器自减到0，然后再往下执行。
             latch.await();
         } catch (InterruptedException e) {
             e.printStackTrace();
         }
         System.out.println("主线程....");
     }

 }