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 >