CountDownLatch的简单使用

from https://www.jianshu.com/p/cef6243cdfd9

1.CountDownLatch是什么？

CountDownLatch是一个同步工具类，它允许一个或多个线程一直等待，直到其他线程的操作执行完后再执行。

典型的用法是将一个程序分为n个互相独立的可解决任务，并创建值为n的CountDownLatch。当每一个任务完成时，都会在这个锁存器上调用countDown，等待问题被解决的任务调用这个锁存器的await，将他们自己拦住，直至锁存器计数结束。

2.一个简单的例子

在主线程中，开5个子线程来执行20个任务，只有等任务全部完成后，主线程才能再去做其它事。

测试代码：

public class CountDownLatchTest {

    private int threadNum = 5;//执行任务的子线程数量
    private int workNum = 20;//任务数量
    private ExecutorService service;
    private ArrayBlockingQueue<String> blockingQueue;
    private CountDownLatch latch;

    @Before
    public void setUp() {
        service = Executors.newFixedThreadPool(threadNum, new ThreadFactoryBuilder().setNameFormat("WorkThread-%d").build());
        blockingQueue = new ArrayBlockingQueue<>(workNum);
        for (int i = 0; i < workNum; i++) {
            blockingQueue.add("任务-" + i);
        }
        latch = new CountDownLatch(workNum);//计数器的值为任务的数量
    }

    @Test
    public void test() throws InterruptedException {
        SoutUtil.print("主线程开始运行");
        for (int i = 0; i < workNum; i++) {
            service.execute(new WorkRunnable());
        }
        latch.await();//等待子线程的所有任务完成
        SoutUtil.print("主线程去做其它事");
    }

    //用blockQueue中的元素模拟任务
    public String getWork() {
        return blockingQueue.poll();
    }

    class WorkRunnable implements Runnable {

        public void run() {
            String work = getWork();
            performWork(work);
            latch.countDown();//完成一个任务就调用一次
        }
    }

    private void performWork(String work) {
        SoutUtil.print("处理任务：" + work);
        try {
            //模拟耗时的任务
            Thread.currentThread().sleep(60);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

}

执行结果为：

2016/12/09 22:23:02.860 main (CountDownLatchTest.java:36).test: 主线程开始运行
2016/12/09 22:23:02.862 WorkThread-0 (CountDownLatchTest.java:59).performWork: 处理任务：任务-0
2016/12/09 22:23:02.862 WorkThread-1 (CountDownLatchTest.java:59).performWork: 处理任务：任务-1
2016/12/09 22:23:02.862 WorkThread-2 (CountDownLatchTest.java:59).performWork: 处理任务：任务-2
2016/12/09 22:23:02.862 WorkThread-3 (CountDownLatchTest.java:59).performWork: 处理任务：任务-3
2016/12/09 22:23:02.863 WorkThread-4 (CountDownLatchTest.java:59).performWork: 处理任务：任务-4
2016/12/09 22:23:02.926 WorkThread-1 (CountDownLatchTest.java:59).performWork: 处理任务：任务-7
2016/12/09 22:23:02.926 WorkThread-0 (CountDownLatchTest.java:59).performWork: 处理任务：任务-5
2016/12/09 22:23:02.926 WorkThread-2 (CountDownLatchTest.java:59).performWork: 处理任务：任务-8
2016/12/09 22:23:02.926 WorkThread-3 (CountDownLatchTest.java:59).performWork: 处理任务：任务-6
2016/12/09 22:23:02.926 WorkThread-4 (CountDownLatchTest.java:59).performWork: 处理任务：任务-9
2016/12/09 22:23:02.992 WorkThread-0 (CountDownLatchTest.java:59).performWork: 处理任务：任务-11
2016/12/09 22:23:02.992 WorkThread-1 (CountDownLatchTest.java:59).performWork: 处理任务：任务-12
2016/12/09 22:23:02.992 WorkThread-3 (CountDownLatchTest.java:59).performWork: 处理任务：任务-13
2016/12/09 22:23:02.992 WorkThread-4 (CountDownLatchTest.java:59).performWork: 处理任务：任务-14
2016/12/09 22:23:02.992 WorkThread-2 (CountDownLatchTest.java:59).performWork: 处理任务：任务-10
2016/12/09 22:23:03.057 WorkThread-1 (CountDownLatchTest.java:59).performWork: 处理任务：任务-16
2016/12/09 22:23:03.057 WorkThread-2 (CountDownLatchTest.java:59).performWork: 处理任务：任务-18
2016/12/09 22:23:03.057 WorkThread-0 (CountDownLatchTest.java:59).performWork: 处理任务：任务-15
2016/12/09 22:23:03.057 WorkThread-3 (CountDownLatchTest.java:59).performWork: 处理任务：任务-17
2016/12/09 22:23:03.057 WorkThread-4 (CountDownLatchTest.java:59).performWork: 处理任务：任务-19
2016/12/09 22:23:03.120 main (CountDownLatchTest.java:41).test: 主线程去做其它事

参考目录：

1. 什么时候使用CountDownLatch
2. JDK1.8源码分析之CountDownLatch
3. CountDownLatch源码的理解
4. countdownlatch源码分析

作者：天然鱼
链接：https://www.jianshu.com/p/cef6243cdfd9
來源：简书
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