JUC CountDownLach原理是什么

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();     }}

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