快来看看！AQS 和 CountDownLatch 有怎么样的关系？

{"type":"doc","content":[{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"前言"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"CountDownLatch 一个同步辅助工具，同样是基于 AQS 实现，本篇文件主要是介绍 CountDownLatch 的使用，以及源码。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"公众号：liuzhihangs，记录工作学习中的技术、开发及源码笔记；时不时分享一些生活中的见闻感悟。欢迎大佬来指导！"}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"介绍"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"一个同步辅助工具，允许一个或多个线程等待，直到在其他线程中执行的一组操作完成为止"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"一个 CountDownLatch 初始化为给定计数。 在 await 方法阻塞，调用 countDown 方法会减少计数直到达到零，此后所有等待的线程被释放，任何后续调用 await 都会立即返回。 这是一次性的现象 - 计数不能复位。 如果你需要一个版本重置计数，请考虑使用CyclicBarrier 。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"CountDownLatch 是一种通用的同步工具，可用于多种用途。 "}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"numberedlist","attrs":{"start":"1","normalizeStart":1},"content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","text":"用作一个简单的开/关锁存器，或者门：所有线程调用await在门口等待，直到被调用 countDown 的线程打开。 "}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","text":"初始化计数为 N ，用一个线程等待，直到 N 个线程完成某项操作，或某些动作已经完成 N 次。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"CountDownLatch 一个有用的属性是，它不要求调用 countDown 线程等待计数到达零之前继续，它只是阻止任何线程通过await ，直到所有线程可以通过。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"基本使用"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在我之前 CAS 那篇文章《"},{"type":"link","attrs":{"href":"https://mp.weixin.qq.com/s/nSZW-bZ_snQ0ZYyzeEBJiw","title":""},"content":[{"type":"text","text":"从JUC源码看CAS，我做了个笔记 ......"}]},{"type":"text","text":"》中介绍 CAS 举例时使用了 CountDownLatch，其代码如下："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public class CasTest {\n\n private static final CountDownLatch LATCH = new CountDownLatch(10);\n\n private static int NUM_I = 0;\n private static volatile int NUM_J = 0;\n private static final AtomicInteger NUM_K = new AtomicInteger(0);\n\n public static void main(String[] args) throws InterruptedException {\n\n ExecutorService threadPool = Executors.newFixedThreadPool(10);\n for (int i = 0; i < 10; i++) {\n\n threadPool.execute(new Runnable() {\n public void run() {\n for (int j = 0; j < 10000; j++) {\n NUM_I++;\n NUM_J++;\n NUM_K.incrementAndGet();\n }\n LATCH.countDown();\n }\n });\n }\n LATCH.await();\n\n System.out.println(\"NUM_I = \" + NUM_I);\n System.out.println(\"NUM_J = \" + NUM_J);\n System.out.println(\"NUM_K = \" + NUM_K.get());\n threadPool.shutdown();\n }\n\n}"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"简单介绍下这段代码的主要逻辑及功能："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"numberedlist","attrs":{"start":"1","normalizeStart":1},"content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","text":"CountDownLatch 初始化计数为 10 。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","text":"开 10 个线程去处理业务逻辑，业务逻辑结束会调用 LATCH.countDown() 对计数进行 -1 操作。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":3,"align":null,"origin":null},"content":[{"type":"text","text":"在 LATCH.await() 处会阻塞等待，直到 LATCH 的值为 0 ，即 10 个线程业务都处理结束。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":4,"align":null,"origin":null},"content":[{"type":"text","text":"然后主线程继续执行。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"问题疑问"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"numberedlist","attrs":{"start":"1","normalizeStart":1},"content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","text":"CountDownLatch 和 AQS 有什么关系？"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","text":"CountDownLatch 的实现原理是什么？"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"源码分析"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"基本结构"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/49/49d697970c435d29fc7c8f84b392dcad.png","alt":"uml-VAGlMb","title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"通过类图可以看出，CountDownLatch 内部存在一个静态类 Sync，而 Sync 继承了 AbstractQueuedSynchronizer。具体内部是如何实现的，则下面通过源码和画图一步一步的进行介绍。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"初始化"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public CountDownLatch(int count) {\n if (count < 0) throw new IllegalArgumentException(\"count < 0\");\n this.sync = new Sync(count);\n}"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"通过初始化构造器可以看出，在 new 创建对象时必须传递一个 int 类型的非负数。实现逻辑可以看出，是创建了一个 Sync 对象。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"private static final class Sync extends AbstractQueuedSynchronizer {\n private static final long serialVersionUID = 4982264981922014374L;\n\n Sync(int count) {\n setState(count);\n }\n\n int getCount() {\n return getState();\n }\n\n} "}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"之前在介绍 AQS 源码中已经介绍了 state 的含义，state 在不同子类中代表不同的含义。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"numberedlist","attrs":{"start":"1","normalizeStart":1},"content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","text":"在 ReentrantLock 中 state 代表加锁状态，0 没有线程获得锁，大于等于 1 已经有线程获得锁，大于 1 说明该获得锁的线程多次重入。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","text":"在 ReentrantReadWriteLock 中 state 代表锁的状态。state 为 0 ，没有线程持有锁，state 的高 16 为代表读锁状态，低 16 为代表写锁状态。通过位运算可以获取读写锁的实际值。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":3,"align":null,"origin":null},"content":[{"type":"text","text":"而在这里 （CountDownLatch）则代表门闩或者说计数的值。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"countDown"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public void countDown() {\n sync.releaseShared(1);\n}"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"递减锁存器的计数："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如果当前计数大于零，则递减。 "}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如果计数到达零，则释放所有等待的线程。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如果那么当前计数等于零没有任何反应。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"此处调用的是 AQS 的 releaseShard() 方法，释放共享资源。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"// AQS 代码\npublic final boolean releaseShared(int arg) {\n if (tryReleaseShared(arg)) {\n doReleaseShared();\n return true;\n }\n return false;\n}\n"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在 AQS 释放共享资源方法中 "},{"type":"text","marks":[{"type":"strong"}],"text":"tryReleaseShared(arg)"},{"type":"text","text":" 部分是在 CountDownLatch 的内部类 Sync 中实现的，代码部分如下："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"protected boolean tryReleaseShared(int releases) {\n // Decrement count; signal when transition to zero\n for (;;) {\n int c = getState();\n if (c == 0)\n return false;\n int nextc = c-1;\n if (compareAndSetState(c, nextc))\n return nextc == 0;\n }\n}"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"递减更新 state ，如果 state 为 0 则返回 false，否则返回 true 。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"此时再对照上面 AQS 代码，发现：如果 tryReleaseShared 返回 true ，则会唤醒后续节点开始执行操作。所以也就是说，如果 state 不为 0，则不会唤醒后续节点，直到 state 为 0 。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"await"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public void await() throws InterruptedException {\n sync.acquireSharedInterruptibly(1);\n}"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"导致当前线程等待，直到锁存器倒计数至零，除非线程被中断。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如果当前计数为零，则此方法立即返回。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如果当前计数大于零，则当前线程用于线程调度目的，禁用并一直处于休眠状态的两件事情之一发生："}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":" - 因调用countDown方法使计数达到0; "}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":" - 其他某些线程中断当前线程。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public final void acquireSharedInterruptibly(int arg)\n throws InterruptedException {\n if (Thread.interrupted())\n throw new InterruptedException();\n if (tryAcquireShared(arg) < 0)\n doAcquireSharedInterruptibly(arg);\n}"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"AQS 定义了 tryAcquireShared 返回值分为 3 种："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"numberedlist","attrs":{"start":"1","normalizeStart":1},"content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","text":"小于 0: 表示失败；"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","text":"等于 0: 表示共享模式获取资源成功，但后续的节点不能以共享模式获取成功; "}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":3,"align":null,"origin":null},"content":[{"type":"text","text":"大于 0: 表示共享模式获取资源成功，后续节点在共享模式获取也可能会成功，在这种情况下，后续等待线程必须检查可用性。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"其中 tryAcquireShared 同样由 CountDownLatch 的内部类 Sync 中实现，内部逻辑主要是判断 state 的值，进行返回。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在内部实现中返回的值只有 1 和 -1 ，说明在 state == 0 时，返回 1 ，即唤醒后续节点。不等于 0 时，会阻塞。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"protected int tryAcquireShared(int acquires) {\n return (getState() == 0) ? 1 : -1;\n}"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"总结"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"Q: CountDownLatch 和 AQS 有什么关系？"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"A:"},{"type":"text","text":" CountDownLatch 是基于 AQS 的共享模式实现的。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"Q: CountDownLatch 的实现原理是什么？"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"A:"},{"type":"text","text":" 可以参考上面的源码解析，进行总结介绍。 CountDownLatch 是基于 AQS 共享模式实现的，在初始化时必须传入计数，该计数实际上是 AQS 的 state 值。在 countDown 时对 state 进行递减，在 当 state 为 0 时 会唤醒 AQS 队列中的所有等待的节点 （因为是共享模式）。而 await 方法是判断 state 的值，如果不是 0 ，则所有线程在队列中阻塞，等待唤醒。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"Q: state 在代表的含义是什么？"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"A:"},{"type":"text","text":" "}]},{"type":"numberedlist","attrs":{"start":"1","normalizeStart":1},"content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","text":"在 ReentrantLock 中 state 代表加锁状态，0 没有线程获得锁，大于等于 1 已经有线程获得锁，大于 1 说明该获得锁的线程多次重入。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","text":"在 ReentrantReadWriteLock 中 state 代表锁的状态。state 为 0 ，没有线程持有锁，state 的高 16 为代表读锁状态，低 16 为代表写锁状态。通过位运算可以获取读写锁的实际值。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":3,"align":null,"origin":null},"content":[{"type":"text","text":"而在这里 （CountDownLatch）则代表门闩或者说计数的值。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}}]}