利用readwritelock简单模拟实现多线程下cache的系统

package cn.lyy.hibernate.many2one;

import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;

public class MultiCacheDemo {

    private Map<String, Object> cache = new HashMap<String,Object>();
    public static void main(String args[]){
        
    }
    
    private ReadWriteLock rwl = new ReentrantReadWriteLock();
    public Object getData(String key){
        rwl.readLock().lock();
        Object value = null;
        try{
            value = cache.get(key);
            if(value == null){
                rwl.readLock().unlock();
                rwl.writeLock().lock();
                try{
                    //因为如果同时又多个线程在读数据的话，可能会同时发现value
                    //为空，所以应该再在这里加一个if语句判断value是否为空
                    if(value == null){
                        
                    value = "aaaa";  //实际是去queryDB();
                    
                    }
                }finally{
                    rwl.writeLock().unlock();
                }
                rwl.readLock().lock();
            }
        }finally{
            rwl.readLock().unlock();
        }
        return value;
    }
}