Hi,
one-to-many:
Users.hbm.xml:
<id name="id" column="Userid" type="java.lang.String">
<generator class="assigned"/>
</id>
<property name="regtime" column="Regtime" type="java.util.Calendar" />
<set name="bbsTopicSet" lazy="false" cascade="all" inverse="true">
<key column="Creator"/>
<one-to-many class="BbsTopic"/>

bbsTopicSet.hbm.xml:
<many-to-one name="Users" column="Creator" class="SysUsers" update="true" insert="true"/>

bbstopic.getSysUsers().getId() will be ok,but if bbstopic.getSysUsers().getRegtime(),it can not work:
org.hibernate.LazyInitializationException: could not initialize proxy - the owning Session was closed

why?

It looks like the association of many-to-one is not defined as eager and so loads a proxy for SysUsers, which holds the id of the SysUsers entity in order to lazily load it later.
Maybe adding a lazy="false" to the many-to-one association will solve your problem?