Java线程：新特征-锁（上）

古城微笑少年丶 2024-02-17 21:16 55阅读 0赞

本文转载至：[http://lavasoft.blog.51cto.com/62575/222084][http_lavasoft.blog.51cto.com_62575_222084]

在Java5中，专门提供了锁对象，利用锁可以方便的实现资源的封锁，用来控制对竞争资源并发访问的控制，这些内容主要集中在java.util.concurrent.locks 包下面，里面有三个重要的接口Condition、Lock、ReadWriteLock。

<table style="padding:0px; margin:0px auto 10px; font-size:12px; border-collapse:collapse; color:rgb(85,85,85); font-family:宋体,&quot;Arial Narrow&quot;,arial,serif; background-color:rgb(255,250,250)"> 
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 <td style="padding:5px 10px; margin:0px; border:1px solid rgb(221,221,221)"> <a title="java.util.concurrent.locks" style="padding:0px; margin:0px; color:rgb(67,67,67)" rel="nofollow">Condition</a></td> 
 <td style="padding:5px 10px; margin:0px; border:1px solid rgb(221,221,221)">Condition&nbsp;将&nbsp;Object&nbsp;监视器方法（<a style="padding:0px; margin:0px; color:rgb(67,67,67)" rel="nofollow"><code style="padding:0px; margin:0px">wait</code></a>、<a style="padding:0px; margin:0px; color:rgb(67,67,67)" rel="nofollow"><code style="padding:0px; margin:0px">notify</code></a>&nbsp;和&nbsp;<a style="padding:0px; margin:0px; color:rgb(67,67,67)" rel="nofollow"><code style="padding:0px; margin:0px">notifyAll</code></a>）分解成截然不同的对象，以便通过将这些对象与任意&nbsp;<a title="java.util.concurrent.locks" style="padding:0px; margin:0px; color:rgb(67,67,67)" rel="nofollow"><code style="padding:0px; margin:0px">Lock</code></a>&nbsp;实现组合使用，为每个对象提供多个等待 set （wait-set）。</td> 
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 <td style="padding:5px 10px; margin:0px; border:1px solid rgb(221,221,221)"> <a title="java.util.concurrent.locks" style="padding:0px; margin:0px; color:rgb(67,67,67)" rel="nofollow">Lock</a></td> 
 <td style="padding:5px 10px; margin:0px; border:1px solid rgb(221,221,221)">Lock&nbsp;实现提供了比使用&nbsp;synchronized&nbsp;方法和语句可获得的更广泛的锁定操作。</td> 
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 <td style="padding:5px 10px; margin:0px; border:1px solid rgb(221,221,221)"> <a title="java.util.concurrent.locks" style="padding:0px; margin:0px; color:rgb(67,67,67)" rel="nofollow">ReadWriteLock</a></td> 
 <td style="padding:5px 10px; margin:0px; border:1px solid rgb(221,221,221)">ReadWriteLock&nbsp;维护了一对相关的<a title="java.util.concurrent.locks" style="padding:0px; margin:0px; color:rgb(67,67,67)" rel="nofollow"><code style="padding:0px; margin:0px">锁定</code></a>，一个用于只读操作，另一个用于写入操作。</td> 
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有关锁的介绍,API文档解说很多，看得很烦，还是看个例子再看文档比较容易理解。

import java.util.concurrent.ExecutorService; 
    import java.util.concurrent.Executors; 
    import java.util.concurrent.locks.Lock; 
    import java.util.concurrent.locks.ReentrantLock; 
    
    /** 
    * Java线程：锁 
    * 
    * @author leizhimin 2009-11-5 10:57:29 
    */ 
    public class Test { 
            public static void main(String[] args) { 
                    //创建并发访问的账户 
                    MyCount myCount = new MyCount("95599200901215522", 10000); 
                    //创建一个锁对象 
                    Lock lock = new ReentrantLock(); 
                    //创建一个线程池 
                    ExecutorService pool = Executors.newCachedThreadPool(); 
                    //创建一些并发访问用户，一个信用卡，存的存，取的取，好热闹啊 
                    User u1 = new User("张三", myCount, -4000, lock); 
                    User u2 = new User("张三他爹", myCount, 6000, lock); 
                    User u3 = new User("张三他弟", myCount, -8000, lock); 
                    User u4 = new User("张三", myCount, 800, lock); 
                    //在线程池中执行各个用户的操作 
                    pool.execute(u1); 
                    pool.execute(u2); 
                    pool.execute(u3); 
                    pool.execute(u4); 
                    //关闭线程池 
                    pool.shutdown(); 
            } 
    } 
    
    /** 
    * 信用卡的用户 
    */ 
    class User implements Runnable { 
            private String name;                //用户名 
            private MyCount myCount;        //所要操作的账户 
            private int iocash;                 //操作的金额，当然有正负之分了 
            private Lock myLock;                //执行操作所需的锁对象 
    
            User(String name, MyCount myCount, int iocash, Lock myLock) { 
                    this.name = name; 
                    this.myCount = myCount; 
                    this.iocash = iocash; 
                    this.myLock = myLock; 
            } 
    
            public void run() { 
                    //获取锁 
                    myLock.lock(); 
                    //执行现金业务 
                    System.out.println(name + "正在操作" + myCount + "账户，金额为" + iocash + "，当前金额为" + myCount.getCash()); 
                    myCount.setCash(myCount.getCash() + iocash); 
                    System.out.println(name + "操作" + myCount + "账户成功，金额为" + iocash + "，当前金额为" + myCount.getCash()); 
                    //释放锁，否则别的线程没有机会执行了 
                    myLock.unlock(); 
            } 
    } 
    
    /** 
    * 信用卡账户，可随意透支 
    */ 
    class MyCount { 
            private String oid;         //账号 
            private int cash;             //账户余额 
    
            MyCount(String oid, int cash) { 
                    this.oid = oid; 
                    this.cash = cash; 
            } 
    
            public String getOid() { 
                    return oid; 
            } 
    
            public void setOid(String oid) { 
                    this.oid = oid; 
            } 
    
            public int getCash() { 
                    return cash; 
            } 
    
            public void setCash(int cash) { 
                    this.cash = cash; 
            } 
    
            @Override 
            public String toString() { 
                    return "MyCount{" + 
                                    "oid='" + oid + '\'' + 
                                    ", cash=" + cash + 
                                    '}'; 
            } 
    }

运行结果：

张三正在操作MyCount{oid='95599200901215522', cash=10000}账户，金额为-4000，当前金额为10000 
    张三操作MyCount{oid='95599200901215522', cash=6000}账户成功，金额为-4000，当前金额为6000 
    张三他爹正在操作MyCount{oid='95599200901215522', cash=6000}账户，金额为6000，当前金额为6000 
    张三他爹操作MyCount{oid='95599200901215522', cash=12000}账户成功，金额为6000，当前金额为12000 
    张三他弟正在操作MyCount{oid='95599200901215522', cash=12000}账户，金额为-8000，当前金额为12000 
    张三他弟操作MyCount{oid='95599200901215522', cash=4000}账户成功，金额为-8000，当前金额为4000 
    张三正在操作MyCount{oid='95599200901215522', cash=4000}账户，金额为800，当前金额为4000 
    张三操作MyCount{oid='95599200901215522', cash=4800}账户成功，金额为800，当前金额为4800

从上面的输出可以看到，利用锁对象太方便了，比直接在某个不知情的对象上用锁清晰多了。  
  
但一定要注意的是，在获取了锁对象后，用完后应该尽快释放锁，以便别的等待该锁的线程有机会去执行。

[http_lavasoft.blog.51cto.com_62575_222084]: http://lavasoft.blog.51cto.com/62575/222084