设计模式 Concurrency 之 Mutex 互斥锁

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1. 定义

不同线程通过竞争进入临界区（共享的数据和硬件资源），为了防止访问冲突，在有限的时间内只允许其中之一独占性的使用共享资源。

是Semaphore的特殊情况 当信号量的阈值设为1时就是互斥锁模式

2. 例子

Lock

package com.hqq.concurrency.mutex;/*** Lock* Created by heqianqian on 2017/7/29.*/
public interface Lock {void acquire() throws InterruptedException;void release();}

Mutex

package com.hqq.concurrency.mutex;/*** Mutex* 独占锁* Created by heqianqian on 2017/7/29.*/
public class Mutex implements Lock {/*** 当前锁的拥有者*/private Object owner;@Overridepublic synchronized void acquire() throws InterruptedException {while (owner != null) {wait();}owner = Thread.currentThread();}@Overridepublic synchronized void release() {if ( Thread.currentThread() == owner) {owner = null;notify();}}public Object getOwner() {return owner;}public void setOwner(Object owner) {this.owner = owner;}
}

Jar

package com.hqq.concurrency.mutex;/*** Jar* Mutex锁保证jar只可以被单个Thief线程访问* Created by heqianqian on 2017/7/29.*/
public class Jar {private final Lock lock;private int beans;public Jar(Lock lock, int beans) {this.lock = lock;this.beans = beans;}public boolean takeBean() {boolean success = false;try {lock.acquire();success = beans > 0;if (success) {beans -= 1;}} catch (InterruptedException e) {e.printStackTrace();} finally {lock.release();}return success;}
}

Thief

package com.hqq.concurrency.mutex;import org.slf4j.Logger;
import org.slf4j.LoggerFactory;/*** Thief* Created by heqianqian on 2017/7/29.*/
public class Thief extends Thread {private static Logger LOGGER = LoggerFactory.getLogger(Thief.class);private String name;private Jar jar;public Thief(String name, Jar jar) {this.name = name;this.jar = jar;}/*** 一直获取bean对象直到bean个数为0*/@Overridepublic void run() {int beans = 0;while (jar.takeBean()) {beans += 1;LOGGER.info("{} take a bean", name);}LOGGER.info("{} take {} beans ", name, beans);}
}

App

package com.hqq.concurrency.mutex;/*** Mutex 可以防止多个线程同时访问一个资源* 在这个例子里.我们开启两个线程Thief对象同时从jar对象中获取bean* 一次只有一个对象可以获取bean 获得Mutex的锁 而另一个对象则将阻塞* Created by heqianqian on 2017/7/29.*/
public class App {/*** main方法*/public static void main(String[] args) {Lock lock = new Mutex();Jar jar = new Jar(lock, 2);Thief jack = new Thief("Jack", jar);Thief john = new Thief("Join", jar);jack.start();john.start();}}

测试结果

INFO  [2017-08-09 02:08:41,674] com.hqq.concurrency.mutex.Thief: Join take a bean
INFO  [2017-08-09 02:08:41,674] com.hqq.concurrency.mutex.Thief: Jack take a bean
INFO  [2017-08-09 02:08:41,674] com.hqq.concurrency.mutex.Thief: Jack take 1 beans 
INFO  [2017-08-09 02:08:41,674] com.hqq.concurrency.mutex.Thief: Join take 1 beans 

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