Java线程:新特征-锁(上)
 
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。
 
Condition ConditionObject 监视器方法(waitnotifynotifyAll)分解成截然不同的对象,以便通过将这些对象与任意 Lock 实现组合使用,为每个对象提供多个等待 set (wait-set)。
Lock Lock 实现提供了比使用 synchronized 方法和语句可获得的更广泛的锁定操作。
ReadWriteLock ReadWriteLock 维护了一对相关的锁定,一个用于只读操作,另一个用于写入操作。
 
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 +
                                '}';
        }
}
 
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

Process finished with exit code 0
 
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。