java新特性——读写锁ReadWriteLock

from URL ： http://hi.baidu.com/zhizhesky/blog/item/cabcbb515b9b6808377abe5f.html

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，如果没有写锁的情况下，读是无阻塞的,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。
ReentrantReadWriteLock 和 ReentrantLock 不是继承关系，但都是基于 AbstractQueuedSynchronizer 来实现。

lock方法 是基于CAS 来实现的

注意： 在同一线程中，持有读锁后，不能直接调用写锁的lock方法 ，否则会造成死锁。

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：
[java] view plain copy
import java.util.concurrent.ExecutorService; 
import java.util.concurrent.Executors; 
import java.util.concurrent.locks.ReadWriteLock; 
import java.util.concurrent.locks.ReentrantReadWriteLock; 
 
/** 
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(); 
         } 
} 
 
/** 
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(); 
                 } 
         } 
} 
 
/** 
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