缓存空对象的使用

public class UserManager {
     UserDAO userDAO;
     LocalCache localCache;

     public UserDO getUser(String userNick) {
          UserDO user = (UserDO)localCache.get(userNick);
          if(user == null) {
               user = userDAO.getUser(userNick);
               if(user != null) {
                    localCache.put(userNick,user);
               }
          }
          return user;
     }          
}

public class NullValueResultDO implements Serializable{
     private static final long serialVersionUID = -6550539547145486005L;
}

public class UserManager {
     UserDAO userDAO;
     LocalCache localCache;

     public UserDO getUser(String userNick) {
          Object object = localCache.get(userNick);
          if(object != null) {
               if(object instanceof NullValueResultDO) {
                    return null;
               }
               return (UserDO)object;
          } else {
               User user = userDAO.getUser(userNick);
               if(user != null) {
                    localCache.put(userNick,user);
               } else {
                    localCache.put(userNick, new NullValueResultDO());
               }
               return user;
          }
     }          
}