读写锁ReadWriteLock

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：

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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