Java并发编程与技术内幕:聊聊锁的技术内幕（中）

摘要：本文主要讲了读写锁。

一、读写锁ReadWriteLock

在上文中回顾了并发包中的可重入锁ReentrantLock，并且也分析了它的源码。从中我们知道它是一个单一锁（笔者自创概念），意思是在多人读、多人写、或同时有人读和写时。只能有一个人能拿到锁，执行代码。但是在很多场景。我们想控制它能多人同时读，但是又不让它多人写或同时读和写时。（想想这是不是和数据库的可重复读有点类型？），这时就可以使用读写锁：ReadWriteLock。

下面来看一个应用

[java] view plain copy

package com.lin;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;
public class ReadWriteLockTest {
public static void main(String[] args) {
//创建一个锁对象 ,非公平锁
ReadWriteLock lock = new ReentrantReadWriteLock(false);
//创建一个线程池
ExecutorService pool = Executors.newCachedThreadPool();
//设置一个账号，设置初始金额为10000
Account account = new Account(lock,"123456",10000);
//账号取钱10次，存钱10次，查询20次
for(int i=1;i<=10;i++) {
Operation operation1 = new Operation(account,"take");
Operation operation2 = new Operation(account,"query");
Operation operation3 = new Operation(account,"save");
Operation operation4 = new Operation(account,"query");
pool.execute(operation1);
pool.execute(operation2);
pool.execute(operation3);
pool.execute(operation4);
}
pool.shutdown();
while(!pool.isTerminated()){
//wait for all tasks to finish
}
System.out.println("账号"+ account.getAccoutNo() +",最后金额为："+account.getMoney());
}
}
class Operation implements Runnable{
private Account account;//账号
private String type;
Operation(Account account,String type){
this.account = account;
this.type = type;
}
public void run() {
if ("take".equals(type)) { //每次取100元
//获取写锁
account.getLock().writeLock().lock();
account.setMoney(account.getMoney() -100);
System.out.println( "取走100元,账号"+ account.getAccoutNo()+" 还有"+account.getMoney()+"元");
account.getLock().writeLock().unlock();
}
else if ("query".equals(type)) {
//获取写锁
account.getLock().readLock().lock();
System.out.println( "查询账号"+ account.getAccoutNo()+" 还有"+account.getMoney()+"元");
account.getLock().readLock().unlock();
}
else if ("save".equals(type)) {
//获取写锁
account.getLock().writeLock().lock();
account.setMoney(account.getMoney() + 100);
System.out.println( "存入100元,账号"+ account.getAccoutNo()+" 还有"+account.getMoney()+"元");
account.getLock().writeLock().unlock();
}
}
}
class Account {
private int money;//账号上的钱
private ReadWriteLock lock;//读写写
private String accoutNo;//账号
Account(ReadWriteLock lock,String accoutNo,int money) {
this.lock = lock;
this.accoutNo = accoutNo;
this.money = money;
}
public int getMoney() {
return money;
}
public void setMoney(int money) {
this.money = money;
}
public ReadWriteLock getLock() {
return lock;
}
public void setLock(ReadWriteLock lock) {
this.lock = lock;
}
public String getAccoutNo() {
return accoutNo;
}
public void setAccoutNo(String accoutNo) {
this.accoutNo = accoutNo;
}
}