缓存代理

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User$Proxy extends User{

private Integer id;

User realUser=null;

getName(){

if(realUser==null){

realUser.session.get(i)

if(realUser==null) throw Exception

}

return realUser.getName();

}

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//j2se api中ReentrantReadWriteLock类的示例,下面的代码展示了如何利用重入来执行升级缓存后的锁降级

class CachedData {

   Object data;
   volatile boolean cacheValid;
   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();
        // 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();
        rwl.writeLock().unlock(); // Unlock write, still hold read
     }

     use(data);
     rwl.readLock().unlock();
   }
}

-----------------------------------------------

public class CacheDemo//缓存
{
   private Map<String,Object> cache=new HashMap<String,Object>();
   ReadWriteLock rwl=new ReentrantReadWriteLock();
   public void process(String key){
       rwl.readLock().lock();
       Object value=null;
       try
       {
           value=cache.get(key);
           if(value==null){
               rwl.readLock().unlock();
               rwl.writeLock().lock();
               if(value==null){
                   value="aaa";//queryDB 去数据库中查询
                   if(value!=null){
                       cache.put(key,value);
                   }
               }
               try
               {
                   rwl.writeLock().unlock();
               }
               catch (Exception e)
               {
                   e.printStackTrace();
               }finally{
                   rwl.writeLock().unlock();
               }
               rwl.readLock().lock();
           }
       }
       catch (Exception e)
       {
           e.printStackTrace();
       }finally{
           rwl.readLock().unlock();
       }

   }
}

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