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 name:"+Thread.currentThread().getName()+",thread counts = " + (countDownLatch.getCount()));
            }
            countDownLatch.await();
            System.out.println("thread name:"+Thread.currentThread().getName()+",concurrency counts = " + (100 - countDownLatch.getCount()));
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

结果集：

thread name:pool-1-thread-1,thread counts = 99
thread name:pool-1-thread-5,thread counts = 98
thread name:pool-1-thread-4,thread counts = 97
thread name:pool-1-thread-6,thread counts = 96
thread name:pool-1-thread-3,thread counts = 95
thread name:pool-1-thread-7,thread counts = 94
thread name:pool-1-thread-2,thread counts = 93
thread name:pool-1-thread-8,thread counts = 92
thread name:pool-1-thread-9,thread counts = 91
thread name:pool-1-thread-10,thread counts = 90
thread name:pool-1-thread-11,thread counts = 89
thread name:pool-1-thread-12,thread counts = 88
thread name:pool-1-thread-13,thread counts = 87
...
thread name:pool-1-thread-98,thread counts = 2
thread name:pool-1-thread-99,thread counts = 1
thread name:pool-1-thread-100,thread counts = 0
thread name:pool-1-thread-100,concurrency counts = 100
thread name:pool-1-thread-1,concurrency counts = 100
thread name:pool-1-thread-4,concurrency counts = 100
thread name:pool-1-thread-5,concurrency counts = 100
...

5.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

6、CountDownLatch的简单使用

1）、定义线程

// 定义工作线程
class ThreadDown implements Runnable {

    private CountDownLatch latch = null;

    public ThreadDown(CountDownLatch latch) {
        this.latch = latch;
    }

    @Override
    public void run() {

        for (int index = 0; index < 10; index++) {
            System.out.println(Thread.currentThread().getName() + " : " + index);
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }

        this.latch.countDown();
    }
}

2）、测试CountDownLatch

    @Test
    public void testCountDown() throws InterruptedException {

        //--1 定义锁
        CountDownLatch countDownLatch = new CountDownLatch(10);

        //--2 创建线程
        for (int index = 0; index < 10; index++) {
            Thread thread = new Thread(new ThreadDown(countDownLatch));
            thread.start();
        }

        //--3 等待所有线程结束
        countDownLatch.await();

        //--4 继续执行主线程
        System.out.println("主线程："+Thread.currentThread().getName());

    }

参考文档

• https://www.cnblogs.com/Vincent-yuan/p/15004258.html