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本篇内容主要讲解"JUC CountDownLach原理是什么"，感兴趣的朋友不妨来看看。本文介绍的方法操作简单快捷，实用性强。下面就让小编来带大家学习"JUC CountDownLach原理是什么"吧!

CountDownLach闭锁

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。

在java.util.cucurrent包下。

概念

CountDownLatch这个类使一个线程等待其它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来。

源码

countDownLatch类中只提供了一个构造器

public CountDownLatch(int count) { if (count < 0) throw new IllegalArgumentException("count < 0"); this.sync = new Sync(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 { sync.acquireSharedInterruptibly(1); }//和await()方法类似，只不过等待一定的时间后count值还没变为0的化就会继续执行 public boolean await(long timeout, TimeUnit unit) throws InterruptedException { return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout)); }//将count值减1 public void countDown() { sync.releaseShared(1); }

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(); } }}

源码分析

public class CountDownLatch { //继承AQS来实现他的模板方法（tryAcquireShared，tryReleaseShared） private static final class Sync extends AbstractQueuedSynchronizer { //计数个数Count Sync(int count) { setState(count); } int getCount() { return getState(); } //AQS方法getState()，返回同步状态，这里指计数器值 protected int tryAcquireShared(int acquires) { return (getState() == 0) ? 1 : -1; } //循环+cas重试 直到计数器为0 跳出，则release（实现aqs共享模式释放方法） protected boolean tryReleaseShared(int releases) { // Decrement count; signal when transition to zero for (;;) { int c = getState(); if (c == 0) return false; int nextc = c-1; if (compareAndSetState(c, nextc)) return nextc == 0; } } } private final Sync sync; //实例化 public CountDownLatch(int count) { if (count < 0) throw new IllegalArgumentException("count < 0"); this.sync = new Sync(count); } public void await() throws InterruptedException { sync.acquireSharedInterruptibly(1); } //带有一个超时时间的awit public boolean await(long timeout, TimeUnit unit) throws InterruptedException { return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout)); } public void countDown() { sync.releaseShared(1); } public long getCount() { return sync.getCount(); }}到此，相信大家对"JUC CountDownLach原理是什么"有了更深的了解，不妨来实际操作一番吧!这里是网站，更多相关内容可以进入相关频道进行查询，关注我们，继续学习!

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