Hibernate的对象关系映射（5）

2.3多态性

2.3.1 3种继承策略

(1) 共享一个数据表

利用子类之间识别字段来区分。

<?xmlversion="1.0"?>
<!DOCTYPEhibernate-mappingPUBLIC
"-//Hibernate/HibernateMappingDTD3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mappingpackage="com.weportal.container">
<classname="Container"abstract="true"table="CONTAINER"where="size&lt;3000">
<idname="containerId">
<generatorclass="hilo"/>
</id>
<discriminatorcolumn="CONTAINER_TYPE"type="string"/>
<propertyname="size"/>
<propertyname="name"/>
<propertyname="description"/>

<subclassname="Box"discriminator-value="BOX">
<propertyname="height">
</property>
<propertyname="length"/>
<propertyname="width"/>
</subclass>

<subclassname="Bottle"discriminator-value="BOTTLE">
<propertyname="diameter"/>
<propertyname="height"/>
</subclass>

</class>
</hibernate-mapping>

(2) 每个类一个数据表

<?xmlversion="1.0"?>
<!DOCTYPEhibernate-mappingPUBLIC
"-//Hibernate/HibernateMappingDTD3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mappingpackage="com.weportal.container">
<classname="Container"table="container_type2">
<idname="containerId"type="string">
<generatorclass="hilo"></generator>
</id>
<propertyname="size"></property>
<propertyname="name"></property>
<propertyname="description"></property>
<joined-subclassname="Box"table="box_type2">
<keycolumn="containerId"></key>
<propertyname="height"></property>
<propertyname="length"></property>
<propertyname="width"></property>
</joined-subclass>
<joined-subclassname="Bottle"table="bottle_type2">
<keycolumn="containerId"></key>
<propertyname="diameter"></property>
<propertyname="height"></property>
</joined-subclass>
</class>
</hibernate-mapping>

(3) 每个子类一个数据表

<?xmlversion="1.0"?>
<!DOCTYPEhibernate-mappingPUBLIC
"-//Hibernate/HibernateMappingDTD3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mappingpackage="com.weportal.subselect">
<classname="Box"table="Box_type3">
<idname="id">
<generatorclass="hilo"/>
</id>
<propertyname="size"column="size"/>
<propertyname="name"column="name"/>
<propertyname="description"column="description"/>
<propertyname="width"column="width"/>
<propertyname="length"column="length"/>
<propertyname="height"column="height"/>
</class>
<classname="Bottle"table="Bottle_type3">
<idname="id">
<generatorclass="hilo"/>
</id>
<propertyname="size"column="size"/>
<propertyname="name"column="name"/>
<propertyname="description"column="description"/>
<propertyname="diameter"column="diameter"/>
<propertyname="height"column="height"/>
</class>
</hibernate-mapping>

2.3.2 多重继承

可用第一种和第二种策略实现。

2.3.3 联合子类

在hibernate3.0中，除了使用subclass和joined-subclass支持多态外，还可以使用union-subclass来建立联合子类。

<?xmlversion="1.0"?>
<!DOCTYPEhibernate-mappingPUBLIC
"-//Hibernate/HibernateMappingDTD3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mappingpackage="com.weportal.unionsubclass"default-access="field">
<classname="Person">
<idname="id">
<generatorclass="hilo"/>
</id>
<propertyname="name"/>

<union-subclassname="Programmer">
<propertyname="language"/>

</union-subclass>

<union-subclassname="Player">
<propertyname="style"/>
</union-subclass>

</class>
</hibernate-mapping>

union-subclass元素也可以嵌套来支持多重继承关系。例如dbprogrammer extends programmer。

<?xmlversion="1.0"?>
<!DOCTYPEhibernate-mappingPUBLIC
"-//Hibernate/HibernateMappingDTD3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mappingpackage="com.weportal.unionsubclass"default-access="field">
<classname="Person">
<idname="id">
<generatorclass="hilo"/>
</id>
<propertyname="name"/>

<union-subclassname="Programmer">
<propertyname="language"/>
<union-subclassname="DBDesigner">
<propertyname="platform"></property>
</union-subclass>
</union-subclass>

<union-subclassname="Player">
<propertyname="style"/>
</union-subclass>

</class>
</hibernate-mapping>