CountDownLatch是Java提供的线程递减锁，为开发人员封装出来针对特殊场景使用的工具类，主要用在某一个动作的执行以来其他动作对应线程的完成。

使用步骤：

1.拿到一个CountDownLatch实例，初始化大小为依赖的动作线程数

2.将CountDownLatch实例传递给依赖线程

3.在依赖线程执行完对应的动作后调用CountDownLatch.countDown方法

4.在主线程中调用CountDownLatch.await方法

package com.jv.parallel.feature;import java.util.concurrent.CountDownLatch;/* * 线程递减锁，用来控制某一个动作的执行依赖其他动作线程的完成 *  * 例如：开始钓鱼之前先要做主线、拌饵料、打窝子 */public class TestCountDownLatch {	public static void main(String[] args) throws InterruptedException {		// 1.声明一个线程递减锁		// 因为在钓鱼		CountDownLatch cdl = new CountDownLatch(3);		new Thread(new TieLine(cdl)).start();		new Thread(new MixedFood(cdl)).start();		new Thread(new Feed(cdl)).start();		cdl.await();		System.out.println("开始愉快的钓鱼了");	}		/*	 * 制作钓鱼的主线	 */	static class TieLine implements Runnable{		CountDownLatch cdl;		public TieLine(CountDownLatch cdl) {			this.cdl = cdl;		}		@Override		public void run() {			System.out.println("制作4.5的主线");			cdl.countDown();		}			}		/*	 * 拌饵料	 */	static class MixedFood implements Runnable{		CountDownLatch cdl;		public MixedFood(CountDownLatch cdl) {			this.cdl = cdl;		}		@Override		public void run() {			System.out.println("拌饵料");			cdl.countDown();		}	}		/*	 * 做窝子	 */	static class Feed implements Runnable{		CountDownLatch cdl;		public Feed(CountDownLatch cdl) {			this.cdl = cdl;		}		@Override		public void run() {			System.out.println("打窝子");			cdl.countDown();		}	}}

CountDownLatch核心是使用了一个继承AbstractQueuedSynchronizer的同步器，这种设计模式称之为模板方法（在抽象类中定义算法的骨架，由子类实现骨架中具体的方法），模板方法设计模式可以参阅

CountDownLatch.await方法

public void await() throws InterruptedException {        sync.acquireSharedInterruptibly(1);    }

同步器的acquireSharedInterruptibly方法其实是CountDownLatch.Syn父类AbstractQueuedSynchronizer的，它就是一个算法骨架

public final void acquireSharedInterruptibly(int arg)            throws InterruptedException {        if (Thread.interrupted())            throw new InterruptedException();        if (tryAcquireShared(arg) < 0)            doAcquireSharedInterruptibly(arg);    }

acquireSharedInterruptibly的核心就是调用子类实现的tryAcquireShared

再看CountDownLatch.Syn中实现的抽象方法tryAcquireShared，tryReleaseShared

protected int tryAcquireShared(int acquires) {            return (getState() == 0) ? 1 : -1;        }        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;            }        }

CountDownLatch.countDown方法

public void countDown() {        sync.releaseShared(1);    }

同步器的releaseShared方法其实是CountDownLatch.Syn父类AbstractQueuedSynchronizer的，它也是一个算法骨架

public final boolean releaseShared(int arg) {        if (tryReleaseShared(arg)) {            doReleaseShared();            return true;        }        return false;    }

releaseShared的核心就是调用子类实现的tryReleaseShared