We're using Root Persistent Class pattern and DAO pattern 
together,while our root persistent class have only define some common 
attribute such as id, version, createTime, removed, etc, without 
persistent method.

"why load() instead of refresh()" -- please check hibernate 
documentation for detail. In most usecases, refresh() should be avoid, 
load() is better. load() will load data from db and refresh the 
persistent object with same id in session, too.