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;

     }

 }