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。

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC 
    "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="com.weportal.container">
    <class name="Container" abstract="true" table="CONTAINER" where="size &lt; 3000">
        <id name="containerId">
            <generator class="hilo"/>
        </id>
        <discriminator column="CONTAINER_TYPE" type="string"/>
        <property name="size"/>
        <property name="name"/>        
        <property name="description"/>

        <subclass name="Box" discriminator-value="BOX">
            <property name="height">
            </property>
            <property name="length"/>
            <property name="width"/>                
        </subclass>
        
        <subclass name="Bottle" discriminator-value="BOTTLE">
            <property name="diameter"/>
            <property name="height"/>
        </subclass>

    </class>
</hibernate-mapping>

(2) 每个类一个数据表

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
    "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="com.weportal.container">
  <class name="Container" table="container_type2">
      <id name="containerId" type="string">
          <generator class="hilo"></generator>
      </id>
      <property name="size"></property>
      <property name="name"></property>
      <property name="description"></property>
      <joined-subclass name="Box" table="box_type2">
          <key column="containerId"></key>
          <property name="height"></property>
          <property name="length"></property>
          <property name="width"></property>
      </joined-subclass>
      <joined-subclass name="Bottle" table="bottle_type2">
          <key column="containerId"></key>
          <property name="diameter"></property>
          <property name="height"></property>
      </joined-subclass>
  </class>
</hibernate-mapping>

(3) 每个子类一个数据表

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC 
    "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="com.weportal.subselect">
    <class name="Box" table="Box_type3">
        <id name="id">
            <generator class="hilo"/>
        </id>
        <property name="size" column="size"/>
        <property name="name" column="name"/>        
        <property name="description" column="description"/>
        <property name="width" column="width"/>
        <property name="length" column="length"/>
        <property name="height" column="height"/>
    </class>
    <class name="Bottle" table="Bottle_type3">
        <id name="id">
            <generator class="hilo"/>
        </id>
        <property name="size" column="size"/>
        <property name="name" column="name"/>        
        <property name="description" column="description"/>
        <property name="diameter" column="diameter"/>    
        <property name="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。

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC 
    "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="com.weportal.unionsubclass" default-access="field">
    <class name="Person">
        <id name="id">
            <generator class="hilo"/>
        </id>
        <property name="name"/>        

        <union-subclass name="Programmer">
            <property name="language"/>
            
        </union-subclass>
        
        <union-subclass name="Player">
            <property name="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。

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC 
    "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="com.weportal.unionsubclass" default-access="field">
    <class name="Person">
        <id name="id">
            <generator class="hilo"/>
        </id>
        <property name="name"/>        

        <union-subclass name="Programmer">
            <property name="language"/>
            <union-subclass name="DBDesigner">
                <property name="platform"></property>
            </union-subclass>
        </union-subclass>
        
        <union-subclass name="Player">
            <property name="style"/>
        </union-subclass>

    </class>
</hibernate-mapping>