ReentrantReadWriteLock读写锁的使用2

本文可作为传智播客《张孝祥-Java多线程与并发库高级应用》的学习笔记。

这一节我们做一个缓存系统。

第一版

public class CacheDemo {

    private Map<String, Object> cache = new HashMap<String, Object>();
    public static void main(String[] args) {
        CacheDemo cd = new CacheDemo();
        System.out.println("ss   "+cd.getData2("ss"));
        System.out.println("ss   "+cd.getData2("ss"));
        System.out.println("mm   "+cd.getData2("mm"));
        System.out.println("mm   "+cd.getData2("mm"));
    }

    public Object getData2(String key){
        Object o=cache.get(key);
        if (o==null) {        //标识1
            System.out.println("第一次查  没有"+key);
            o=Math.random();     //实际上从数据库中获得
            cache.put(key, o);
        }
        return o;
    }
}

第二版 synchronized

    }

第三版 锁

    private Lock l=new ReentrantLock(); //l作为CacheDemo的成员变量

    public Object getData3(String key) {
        l.lock();
        Object o=null;
        try {
            o = cache.get(key);
            if (o == null) { // 标识1
                System.out.println("第一次查  没有" + key);
                o = Math.random(); // 实际上从数据库中获得
                cache.put(key, o);
            }
        } finally {
            l.unlock();
        }
        return o;
    }

为什么,自己想。

对同一个线程,可以在加了读锁后,没有解开读锁前,再加写锁吗?

换句话说,下面的代码会停吗?

public class CacheDemo {

    private Map<String, Object> cache = new HashMap<String, Object>();
    private ReadWriteLock rwl = new ReentrantReadWriteLock();
    public static void main(String[] args) {
        CacheDemo cd = new CacheDemo();
        cd.getData2();
    }
        
    public void getData2(){
        rwl.readLock().lock();
        System.out.println(1);
        rwl.writeLock().lock();
        System.out.println(2);
        rwl.readLock().unlock();
        System.out.println(3);
        rwl.writeLock().unlock();
        System.out.println(4);
    }
}

第四版

public static void main(String[] args) {
        CacheDemo cd = new CacheDemo();
        System.out.println("ss   "+cd.getData4("ss"));
        System.out.println("ss   "+cd.getData4("ss"));
        
        System.out.println("mm   "+cd.getData4("mm"));
        System.out.println("mm   "+cd.getData4("mm"));
    }

    public Object getData4(String key){
        rwl.readLock().lock();
        Object o=null;
        try {
            o=cache.get(key);
            if (o==null) {
                System.out.println("第一次查  没有"+key);
                rwl.readLock().unlock();  //标识4
                rwl.writeLock().lock();   
                try {
                    if(value==null){  //标识0
                       value = "aaaa";//实际调用 queryDB();
                       cache.put(key,value);
                    }
                } finally {
                    rwl.writeLock().unlock();  
                }
                rwl.readLock().lock(); //标识1
            }
        }finally {
            rwl.readLock().unlock();  //标注2
        }
        return o;
    }                 

mm   0.8534424949014686

这个还有三个问题

class CachedData {
   Object data;
   volatile boolean cacheValid;
   final ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();

   void processCachedData() {
     rwl.readLock().lock();
     if (!cacheValid) {
        // Must release read lock before acquiring write lock
        rwl.readLock().unlock();
        rwl.writeLock().lock();
        try {
          // Recheck state because another thread might have
          // acquired write lock and changed state before we did.
          if (!cacheValid) {
            data = ...
            cacheValid = true;
          }
          // Downgrade by acquiring read lock before releasing write lock
          rwl.readLock().lock();
        } finally {
          rwl.writeLock().unlock(); // Unlock write, still hold read
        }
     }

     try {
       use(data);
     } finally {
       rwl.readLock().unlock();
     }
   }
 }

ReentrantReadWriteLock读写锁的使用2

原文：http://blog.csdn.net/dlf123321/article/details/42933331