Java并发编程之闭锁简介

闭锁相当于一扇门，在闭锁到达结束状态之前，这扇门一直是关闭着的，没有任何线程可以通过，当到达结束状态时，这扇门才会打开并容许所有线程通过。它可以使一个或多个线程等待一组事件发生。闭锁状态包括一个计数器，初始化为一个正式，正数表示需要等待的事件数量。countDown方法递减计数器，表示一个事件已经发生，而await方法等待计数器到达0，表示等待的事件已经发生。CountDownLatch强调的是一个线程（或多个）需要等待另外的n个线程干完某件事情之后才能继续执行。

     import java.util.concurrent.CountDownLatch;             class Aworker implements Runnable {      private int num;      private CountDownLatch begin;      private CountDownLatch end;             public Aworker(int num, final CountDownLatch begin, final CountDownLatch end) {      this.num = num;      this.begin = begin;      this.end = end;      }             @Override      public void run() {      // TODO Auto-generated method stub      try {      System.out.println(num + "th people is ready");      begin.await();    //准备就绪      } catch (InterruptedException e) {      e.printStackTrace();      } finally {      end.countDown();    //计数器减一，到达终点      System.out.println(num + "th people arrive");      }      }      }             public class Race {      public static void main(String[] args) {      int num = 10;      CountDownLatch begin = new CountDownLatch(1);      CountDownLatch end = new CountDownLatch(num);             for (int i = 1; i <= num; i++) {      new Thread(new Aworker(i, begin, end)).start();      }             try {      Thread.sleep((long) (Math.random() * 5000));      } catch (InterruptedException e1) {      // TODO Auto-generated catch block      e1.printStackTrace();      }        System.out.println("judge say : run !");      begin.countDown(); //开始跑      long startTime = System.nanoTime();      try {      end.await(); //等待结束      } catch (InterruptedException e) {      // TODO Auto-generated catch block      e.printStackTrace();      } finally {      long endTime = System.nanoTime();      System.out.println("judge say : all arrived !");      System.out.println("spend time: " + (endTime - startTime));      }      }      }