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。

场景应用：

10个运动员准备赛跑，他们等待裁判一声令下就开始同时跑，当最后一个人通过终点的时候，比赛结束。10个运动相当于10个线程，这里关键是控制10个线程同时跑起来，还有怎么判断最后一个线程到达终点。可以用2个闭锁，第一个闭锁用来控制10个线程等待裁判的命令，第二个闭锁控制比赛结束。

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();	//裁判一声令下开始跑
		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));
		}
	}
}

输出

1thpeopleisready

2thpeopleisready

4thpeopleisready

6thpeopleisready

3thpeopleisready

10thpeopleisready

8thpeopleisready

5thpeopleisready

7thpeopleisready

9thpeopleisready

judgesay:run!

1thpeoplearrive

4thpeoplearrive

10thpeoplearrive

5thpeoplearrive

2thpeoplearrive

judgesay:allarrived!

9thpeoplearrive

7thpeoplearrive

8thpeoplearrive

3thpeoplearrive

6thpeoplearrive

spendtime:970933