hibernate 继承映射
http://www.cnblogs.com/tongzhiyong/archive/2009/05/06/1451022.html
http://www.ibm.com/developerworks/cn/java/j-hibernate/index.html
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http://www.ibm.com/developerworks/cn/views/java/libraryview.jsp?view_by=search&search_by=Hibernate
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http://developer.51cto.com/art/200909/154182.htm
