Java线程:新特征-锁(下)

作者: ldsea 分类: 程序生活 发布时间: 2011-08-19 15:41
Java线程:新特征-锁(下)
 
在上文中提到了Lock接口以及对象,使用它,很优雅的控制了竞争资源的安全访问,但是这种锁不区分读写,称这种锁为普通锁。为了提高性能,Java提供了读写锁,在读的地方使用读锁,在写的地方使用写锁,灵活控制,在一定程度上提高了程序的执行效率。
 
Java中读写锁有个接口java.util.concurrent.locks.ReadWriteLock,也有具体的实现ReentrantReadWriteLock,详细的API可以查看JavaAPI文档。
 
下面这个例子是在文例子的基础上,将普通锁改为读写锁,并添加账户余额查询的功能,代码如下:
 
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;

/**
* 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
                ReadWriteLock lock = new ReentrantReadWriteLock(false);
                //创建一个线程池
                ExecutorService pool = Executors.newFixedThreadPool(2);
                //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, false);
                User u2 = new User("张三他爹", myCount, 6000, lock, false);
                User u3 = new User("张三他弟", myCount, -8000, lock, false);
                User u4 = new User("张三", myCount, 800, lock, false);
                User u5 = new User("张三他爹", myCount, 0, lock, true);
                //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);
                pool.execute(u5);
                //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 ReadWriteLock myLock;                //执行操作所需的锁对象
        private boolean ischeck;        //是否查询

        User(String name, MyCount myCount, int iocash, ReadWriteLock myLock, boolean ischeck) {
                this.name = name;
                this.myCount = myCount;
                this.iocash = iocash;
                this.myLock = myLock;
                this.ischeck = ischeck;
        }

        public void run() {
                if (ischeck) {
                        //获取读锁
                        myLock.readLock().lock();
                        System.out.println("读:" + name + "正在查询" + myCount + "账户,当前金额为" + myCount.getCash());
                        //释放读锁
                        myLock.readLock().unlock();
                } else {
                        //获取写锁
                        myLock.writeLock().lock();
                        //执行现金业务
                        System.out.println("写:" + 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("写:" + name + "操作" + myCount + "账户成功,金额为" + iocash + ",当前金额为" + myCount.getCash());
                        //释放写锁
                        myLock.writeLock().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 +
                                '}';
        }
}

 
写:张三正在操作MyCount{oid='95599200901215522', cash=10000}账户,金额为-4000,当前金额为10000
写:张三操作MyCount{oid='95599200901215522', cash=6000}账户成功,金额为-4000,当前金额为6000
写:张三他弟正在操作MyCount{oid='95599200901215522', cash=6000}账户,金额为-8000,当前金额为6000
写:张三他弟操作MyCount{oid='95599200901215522', cash=-2000}账户成功,金额为-8000,当前金额为-2000
写:张三正在操作MyCount{oid='95599200901215522', cash=-2000}账户,金额为800,当前金额为-2000
写:张三操作MyCount{oid='95599200901215522', cash=-1200}账户成功,金额为800,当前金额为-1200
读:张三他爹正在查询MyCount{oid='95599200901215522', cash=-1200}账户,当前金额为-1200
写:张三他爹正在操作MyCount{oid='95599200901215522', cash=-1200}账户,金额为6000,当前金额为-1200
写:张三他爹操作MyCount{oid='95599200901215522', cash=4800}账户成功,金额为6000,当前金额为4800

Process finished with exit code 0

 
在实际开发中,最好在能用读写锁的情况下使用读写锁,而不要用普通锁,以求更好的性能。
 

本文出自 “熔 岩” 博客,请务必保留此出处http://lavasoft.blog.51cto.com/62575/222433

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