countDownLatch

1.背景:

  • countDownLatch是在java1.5被引入,跟它一起被引入的工具类还有CyclicBarrier、Semaphore、concurrentHashMap和BlockingQueue。
  • 存在于java.util.cucurrent包下。

2.概念

  • countDownLatch这个类使一个线程等待其他线程各自执行完毕后再执行。
  • 是通过一个计数器来实现的,计数器的初始值是线程的数量。每当一个线程执行完毕后,计数器的值就-1,当计数器的值为0时,表示所有线程都执行完毕,然后在闭锁上等待的线程就可以恢复工作了。

3.源码

  • countDownLatch类中只提供了一个构造器:
//参数count为计数值
public CountDownLatch(int count) {  };  
  • 类中有三个方法是最重要的:
//调用await()方法的线程会被挂起,它会等待直到count值为0才继续执行
public void await() throws InterruptedException { };   
//和await()类似,只不过等待一定的时间后count值还没变为0的话就会继续执行
public boolean await(long timeout, TimeUnit unit) throws InterruptedException { };  
//将count值减1
public void countDown() { };  

4.示例

普通示例:

public class CountDownLatchTest {

    public static void main(String[] args) {
        final CountDownLatch latch = new CountDownLatch(2);
        System.out.println("主线程开始执行…… ……");
        //第一个子线程执行
        ExecutorService es1 = Executors.newSingleThreadExecutor();
        es1.execute(new Runnable() {
            @Override
            public void run() {
                try {
                    Thread.sleep(3000);
                    System.out.println("子线程:"+Thread.currentThread().getName()+"执行");
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                latch.countDown();
            }
        });
        es1.shutdown();

        //第二个子线程执行
        ExecutorService es2 = Executors.newSingleThreadExecutor();
        es2.execute(new Runnable() {
            @Override
            public void run() {
                try {
                    Thread.sleep(3000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println("子线程:"+Thread.currentThread().getName()+"执行");
                latch.countDown();
            }
        });
        es2.shutdown();
        System.out.println("等待两个线程执行完毕…… ……");
        try {
            latch.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("两个子线程都执行完毕,继续执行主线程");
    }
}

结果集:

主线程开始执行…… ……
等待两个线程执行完毕…… ……
子线程:pool-1-thread-1执行
子线程:pool-2-thread-1执行
两个子线程都执行完毕,继续执行主线程

模拟并发示例:

public class Parallellimit {
    public static void main(String[] args) {
        ExecutorService pool = Executors.newCachedThreadPool();
        CountDownLatch cdl = new CountDownLatch(100);
        for (int i = 0; i < 100; i++) {
            CountRunnable runnable = new CountRunnable(cdl);
            pool.execute(runnable);
        }
    }
}

 class CountRunnable implements Runnable {
    private CountDownLatch countDownLatch;
    public CountRunnable(CountDownLatch countDownLatch) {
        this.countDownLatch = countDownLatch;
    }
    @Override
    public void run() {
        try {
            synchronized (countDownLatch) {
                /*** 每次减少一个容量*/
                countDownLatch.countDown();
                System.out.println("thread counts = " + (countDownLatch.getCount()));
            }
            countDownLatch.await();
            System.out.println("concurrency counts = " + (100 - countDownLatch.getCount()));
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

*CountDownLatch和CyclicBarrier区别:
1.countDownLatch是一个计数器,线程完成一个记录一个,计数器递减,只能只用一次
2.CyclicBarrier的计数器更像一个阀门,需要所有线程都到达,然后继续执行,计数器递增,提供reset功能,可以多次使用

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