CountDownLatch的两种常用场景

淡淡的烟草味﹌ 2023-02-17 14:27 146阅读 0赞

#### CountDownLatch的两种使用场景 ####

先来看看 CountDownLatch 的源码注释；

/**
     * A synchronization aid that allows one or more threads to wait until
     * a set of operations being performed in other threads completes.
     *
     * @since 1.5
     * @author Doug Lea
     */
    public class CountDownLatch {
    }

描述如下：它是一个同步工具类，允许一个或多个线程一直等待，直到其他线程运行完成后再执行。

通过描述，可以清晰的看出，CountDownLatch的两种使用场景：

*  场景1：让多个线程等待
 *  场景2：和让单个线程等待。

###### 场景1 让多个线程等待：模拟并发，让并发线程一起执行 ######

为了模拟高并发，让一组线程在指定时刻(秒杀时间)执行抢购，这些线程在准备就绪后，进行等待(CountDownLatch.await())，直到秒杀时刻的到来，然后一拥而上；  
这也是本地测试接口并发的一个简易实现。

在这个场景中，CountDownLatch充当的是一个`发令枪`的角色；  
就像田径赛跑时，运动员会在起跑线做准备动作，等到发令枪一声响，运动员就会奋力奔跑。和上面的秒杀场景类似，代码实现如下：

CountDownLatch countDownLatch = new CountDownLatch(1);
    for (int i = 0; i < 5; i++) {
        new Thread(() -> {
            try {
                //准备完毕……运动员都阻塞在这，等待号令
                countDownLatch.await();
                String parter = "【" + Thread.currentThread().getName() + "】";
                System.out.println(parter + "开始执行……");
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }).start();
    }
    
    Thread.sleep(2000);// 裁判准备发令
    countDownLatch.countDown();// 发令枪：执行发令

运行结果：

【Thread-0】开始执行……
    【Thread-1】开始执行……
    【Thread-4】开始执行……
    【Thread-3】开始执行……
    【Thread-2】开始执行……

我们通过CountDownLatch.await()，让多个参与者线程启动后阻塞等待，然后在主线程 调用CountDownLatch.countdown(1) 将计数减为0，让所有线程一起往下执行；  
以此实现了多个线程在同一时刻并发执行，来模拟并发请求的目的。

###### 场景2 让单个线程等待：多个线程(任务)完成后，进行汇总合并 ######

很多时候，我们的并发任务，存在前后依赖关系；比如数据详情页需要同时调用多个接口获取数据，并发请求获取到数据后、需要进行结果合并；或者多个数据操作完成后，需要数据check；  
这其实都是：在多个线程(任务)完成后，进行汇总合并的场景。

代码实现如下：

CountDownLatch countDownLatch = new CountDownLatch(5);
    for (int i = 0; i < 5; i++) {
        final int index = i;
        new Thread(() -> {
            try {
                Thread.sleep(1000 + ThreadLocalRandom.current().nextInt(1000));
                System.out.println("finish" + index + Thread.currentThread().getName());
                countDownLatch.countDown();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }).start();
    }
    
    countDownLatch.await();// 主线程在阻塞，当计数器==0，就唤醒主线程往下执行。
    System.out.println("主线程:在所有任务运行完成后，进行结果汇总");

运行结果：

finish4Thread-4
    finish1Thread-1
    finish2Thread-2
    finish3Thread-3
    finish0Thread-0
    主线程:在所有任务运行完成后，进行结果汇总

在每个线程(任务) 完成的最后一行加上CountDownLatch.countDown()，让计数器-1；  
当所有线程完成-1，计数器减到0后，主线程往下执行汇总任务。

> 示例源码：https://github.com/ljheee/JavaConcurrencyInPractice/blob/master/src/main/java/com/ljheee/juc/CountDownLatchUsage.java

从上面两个例子的代码，可以看出 CountDownLatch 的API并不多；

*  CountDownLatch的构造函数中的count就是闭锁需要等待的线程数量。这个值只能被设置一次，而且不能重新设置；
 *  await()：调用该方法的线程会被阻塞，直到构造方法传入的 N 减到 0 的时候，才能继续往下执行；
    
    调用了await()进行阻塞等待的线程，它们阻塞在Latch门闩/栅栏上；只有当条件满足的时候（countDown() N次，将计数减为0），它们才能同时通过这个栅栏；以此能够实现，让所有的线程站在一个起跑线上。  
    ![format_png][]
 *  countDown()：使 CountDownLatch 计数值 减 1；

#### CountDownLatch 工作原理 ####

CountDownLatch是通过一个计数器来实现的，计数器的初始值为线程的数量；  
调用await()方法的线程会被阻塞，直到计数器 减到 0 的时候，才能继续往下执行；  
countDown()方法则是将计数器减1；

在CountDownLatch的构造函数中，指定的线程数量，只能指定一次；由于CountDownLatch采用的是减计数，因此当计数减为0时，计数器不能被重置。  
这是和CyclicBarrier的一个重要区别。

CountDownLatch 的源码在JUC并发工具中，也相对算是简单的；  
底层基于 AbstractQueuedSynchronizer 实现，CountDownLatch 构造函数中指定的count直接赋给AQS的state；每次countDown()则都是release(1)减1，最后减到0时unpark阻塞线程；这一步是由最后一个执行countdown方法的线程执行的。  
而调用await()方法时，当前线程就会判断state属性是否为0，如果为0，则继续往下执行，如果不为0，则使当前线程进入等待状态，直到某个线程将state属性置为0，其就会唤醒在await()方法中等待的线程。

#### CountDownLatch与Thread.join ####

CountDownLatch的作用就是允许一个或多个线程等待其他线程完成操作，看起来有点类似join() 方法，但其提供了比 join() 更加灵活的API。  
CountDownLatch可以手动控制在n个线程里调用n次countDown()方法使计数器进行减一操作，也可以在一个线程里调用n次执行减一操作。  
而 join() 的实现原理是不停检查join线程是否存活，如果 join 线程存活则让当前线程永远等待。所以两者之间相对来说还是CountDownLatch使用起来较为灵活。

#### CountDownLatch与CyclicBarrier ####

CountDownLatch和CyclicBarrier都能够实现线程之间的等待，只不过它们侧重点不同：

*  CountDownLatch一般用于一个或多个线程，等待其他线程执行完任务后，再才执行
 *  CyclicBarrier一般用于一组线程互相等待至某个状态，然后这一组线程再同时执行  
    另外，CountDownLatch是减计数，计数减为0后不能重用；而CyclicBarrier是加计数，可置0后复用。

更多CyclicBarrier的内容，请见下篇。

阅读原文：[https://mp.weixin.qq.com/s/RhK\_BrYrooGGYbN5OvyXow][https_mp.weixin.qq.com_s_RhK_BrYrooGGYbN5OvyXow]

--------------------

**推荐阅读**

*  JUC源码
    
    [面试题拷问Java线程池][Java]
    
    [吃透线程池ThreadPoolExecutor源码][ThreadPoolExecutor]
    
    [手写简易版Java线程池][Java 1]
    
    [FutureTask源码解析][FutureTask]
    
    [手写FutureTask][FutureTask 1]
    
    [AtomicXxxx原子类全解析][AtomicXxxx]
    
    [ReentrantLock之Condition源码][ReentrantLock_Condition]
    
    [细聊ReentrantLock之Condition][ReentrantLock_Condition 1]

> 本文首发于 公众号 架构道与术(ToBeArchitecturer)，欢迎关注、学习更多干货~  
> ![在这里插入图片描述][20200614153511238.jpg_pic_center]

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