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Hibernate: insert into users (name, room_id) values (?, ?) select scope_identity()

org.hibernate.TransientObjectException: object references an unsaved transient instance - save the transient instance before flushing: po.Room

    at org.hibernate.engine.ForeignKeys.getEntityIdentifierIfNotUnsaved(ForeignKeys.java:219)

    at org.hibernate.type.EntityType.getIdentifier(EntityType.java:397)

    at org.hibernate.type.ManyToOneType.isDirty(ManyToOneType.java:242)

    at org.hibernate.type.TypeFactory.findDirty(TypeFactory.java:563)

    at org.hibernate.persister.entity.AbstractEntityPersister.findDirty(AbstractEntityPersister.java:3071)

    at org.hibernate.event.def.DefaultFlushEntityEventListener.dirtyCheck(DefaultFlushEntityEventListener.java:472)

    at org.hibernate.event.def.DefaultFlushEntityEventListener.isUpdateNecessary(DefaultFlushEntityEventListener.java:197)

    at org.hibernate.event.def.DefaultFlushEntityEventListener.onFlushEntity(DefaultFlushEntityEventListener.java:120)

    at org.hibernate.event.def.AbstractFlushingEventListener.flushEntities(AbstractFlushingEventListener.java:196)

    at org.hibernate.event.def.AbstractFlushingEventListener.flushEverythingToExecutions(AbstractFlushingEventListener.java:76)

    at org.hibernate.event.def.DefaultFlushEventListener.onFlush(DefaultFlushEventListener.java:26)

    at org.hibernate.impl.SessionImpl.flush(SessionImpl.java:1000)

    at org.hibernate.impl.SessionImpl.managedFlush(SessionImpl.java:338)

    at org.hibernate.transaction.JDBCTransaction.commit(JDBCTransaction.java:106)

    at dao.imp.UsersDAL.SaveUser(UsersDAL.java:22)

    at dao.imp.test.main(test.java:21)
这句异常的意思是：我们在使用一个临时的对象room，按常理我们应先保存room，然后保存users信息。解决此问题我们可以采取以下2种方式将room列入session保持其持久化状态：
  先save room再save users，即保证users与room共用一个session，因为dal中代码已封装好，因此采用第二种方式即cascade关键字
  使用cascade关键字（取值：none、save-update、all、delete）


按照以上方法配置过后出现

log4j:WARN No appenders could be found for logger (org.hibernate.cfg.Environment).

log4j:WARN Please initialize the log4j system properly.

Hibernate: insert into room (name) values (?) select scope_identity()

Exception in thread "main" java.lang.NullPointerException

    at dao.imp.RoomDAL.SaveRoom(RoomDAL.java:20)

    at dao.imp.test.main(test.java:23)
配置方法如下：

<

hibernate-mapping

>

    <class name="po.Users" table="users">

        <id name="id" type="integer">

            <column name="id"></column>

            <generator class="native"></generator>

        </id>

        <property name="name" type="string">

            <column name="name" not-null="true" length="30"></column>

        </property>

        <many-to-one name="room" class="po.Room" cascade="all">

            <column name="room_id"></column>

        </many-to-one>

    </class>

</

hibernate-mapping

>