继承映射

父类：电脑（Computer）
子类：笔记本（Notebook）、台式机（PC）
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实体类：
电脑类

package entity;

public class Computer {
	private int oid;
	private String logo;
	private double price;
	
	public int getOid() {
		return oid;
	}
	public void setOid(int oid) {
		this.oid = oid;
	}
	public String getLogo() {
		return logo;
	}
	public void setLogo(String logo) {
		this.logo = logo;
	}
	public double getPrice() {
		return price;
	}
	public void setPrice(double price) {
		this.price = price;
	}
	
}

笔记本类

package entity;

public class NoteBook extends Computer {
	private double duration;

	public double getDuration() {
		return duration;
	}

	public void setDuration(double duration) {
		this.duration = duration;
	}
	
}

台式机类

package entity;

public class PC extends Computer {
	private double component;

	public double getComponent() {
		return component;
	}

	public void setComponent(double component) {
		this.component = component;
	}
	
}

1）所有字段（属性）放在同一张表
      需要用到得关键字<descriminator> <subclass>
   映射文件Computer.hbm.xml写法如下：
<?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="entity">
   <!-- 继承映射，方案一，所有对象属性存放在同一张表 -->
   <class name="Computer" table="t_computer">
       <id name="oid" column="computer_id">
           <generator class="native"></generator>
       </id>
       <!-- 不同记录用鉴别字段来区分类型 -->
       <discriminator column="computer_type"></discriminator>
       <property name="logo"></property>
       <property name="price"></property>
      
       <subclass name="PC" discriminator-value="P">
           <property name="component"></property>
       </subclass>  
</class>
<!-- subclass 写在class 外面要用关键字extends="父类名">
   <subclass name="NoteBook" extends="Computer" discriminator-value="N">
    <property name="duration"></duration>
   </subclass>
</hibernate-mapping>

2)每个类各自一张表
要用到得关键字 <joined-subclass>
映射文件Computer.hbm.xml写法如下：
<?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="entity">
<!-- 继承映射，方案二，所有对象各自存放一张表 -->
<class name="Computer" table="t_computer">
<id name="oid" column="computer_id">
<generator class="native"></generator>
</id>
  <property name="logo"></property>
<property name="price"></property>
</class>
<joined-subclass name="PC" extends="Computer"
table="t_pc" >
<key column="computer_id"></key>
<property name="component"></property>
</joined-subclass>
<joined-subclass name="NoteBook" extends="Computer"
table="t_notebook">
<key column="computer_id"></key>
<property name="duration"></property>
</joined-subclass>
</hibernate-mapping>

3）每个具体类各自一张表，没有父类表
要用到的关键字<union-subclass>注意主键id不能重复
               <class abstract="true"> -->不建父类表
映射文件Computer.hbm.xml写法如下：

<?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="entity">
<!-- 继承映射，方案三，每个具体子类各自存放一张表 -->
<class name="Computer" abstract="true">
<id name="oid" column="computer_id">
<generator class="assigned"></generator>
</id>
  <property name="logo"></property>
<property name="price"></property>
</class>

<union-subclass name="PC" extends="Computer"
table="t_pc" >
<property name="component"></property>
</union-subclass>
<union-subclass name="NoteBook" extends="Computer"
table="t_notebook">
<property name="duration"></property>
</union-subclass></hibernate-mapping>