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;
     }
 }