这个问题在网上看到有人提出来过,却没有回答。经过几天的思考,终于有了答案。
问题描述如下:
尝试如下代码:
Transaction tx=userdao.getSession().beginTransaction();
resultUser=(User)userdao.getSession().load(User.class,new Integer(1));
resultUser.setPassword("passwordchanged");
tx.commit();
程序会抛出异常:
Exception in thread "main" java.lang.NullPointerException
at org.hibernate.tuple.AbstractEntityTuplizer.createProxy(AbstractEntityTuplizer.java:372)
at org.hibernate.persister.entity.AbstractEntityPersister.createProxy(AbstractEntityPersister.java:3121)
at org.hibernate.event.def.DefaultLoadEventListener.createProxyIfNecessary(DefaultLoadEventListener.java:232)
at org.hibernate.event.def.DefaultLoadEventListener.proxyOrLoad(DefaultLoadEventListener.java:173)
at org.hibernate.event.def.DefaultLoadEventListener.onLoad(DefaultLoadEventListener.java:87)
at org.hibernate.impl.SessionImpl.fireLoad(SessionImpl.java:862)
at org.hibernate.impl.SessionImpl.load(SessionImpl.java:781)
at org.hibernate.impl.SessionImpl.load(SessionImpl.java:774)
at databean.dao.UserDAO.main(UserDAO.java:30)
原因是因为hibernate3里lazy的属性默认为“true”。需要添加hbm配置文件中hibernate-mapping的属性default-lazy="false"