Hibernate关联关系映射之继承映射
来源：程序员人生 发布时间：2014-10-10 08:00:01 阅读次数：2126次
首先有一个文章类（Article）类中有id、title、content、postTime等属性。
package entity;
import java.util.Date;
public class Article {
private Integer id;
private String title;
private String content;
private Date postTime;
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getTitle() {
return title;
}
public void setTitle(String title) {
this.title = title;
}
public String getContent() {
return content;
}
public void setContent(String content) {
this.content = content;
}
public Date getPostTime() {
return postTime;
}
public void setPostTime(Date postTime) {
this.postTime = postTime;
}
}
然后看他的子类，Topic类和Reply类。他们除了父类的属性外还有自己独特的属性。
package entity;
public class Topic extends Article{
private int type;
public int getType() {
return type;
}
public void setType(int type) {
this.type = type;
}
}
package entity;
public class Reply extends Article{
private int floor;
public int getFloor() {
return floor;
}
public void setFloor(int floor) {
this.floor = floor;
}
}
继承映射的方式有三种方式，我们一个个看：
方式一：将父类和子类的信息存放在同一个表中，然后在该数据表中有个字段用来表示该条记录的类型，其中子类独有的属性允许为空。我们看一下映射配置文件。
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="entity">
<class name="entity.Article" table="article" schema="MYHR" discriminator-value="Article">
<id name="id" type="int">
<column name="ID" />
<generator class="assigned" />
</id>
<!-- 用于鉴别是什么类型的一个列 -->
<discriminator type="string" column="t_class"/>
<property name="title"/>
<property name="content" type="clob" length="5000"/>
<property name="postTime" type="timestamp"/>
<subclass name="Topic" discriminator-value="Topic">
<property name="type" type="int"/>
</subclass>
<subclass name="Reply" discriminator-value="Reply">
<property name="floor" type="int"/>
</subclass>
</class>
</hibernate-mapping>
方式二：父类和子类不在同一张表中，且每一个类一张表，抽象类对应一张表，这是配置子类使用joined-subclass
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="entity">
<class name="entity.Article" table="article" schema="MYHR">
<id name="id" type="int">
<column name="ID" />
<generator class="assigned" />
</id>
<property name="title" />
<property name="content" type="clob" length="5000" />
<property name="postTime" type="timestamp" />
<joined-subclass name="Topic" table="topic">
<key column="id" />
<property name="type" type="int" />
</joined-subclass>
<joined-subclass name="Reply" table="reply">
<key column="id" />
<property name="floor" type="int" />
</joined-subclass>
</class>
</hibernate-mapping>
方式三：每一个类单独一张表，并且抽象类不对应一张表，子类对应的数据库表中对应全部的属性，包括从父类继承的信息。
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="entity">
<!-- 采用每个具体类一张表，抽象类不对应表，abstract默认为false，设为true表示为抽象的不对应表 -->
<class name="entity.Article" abstract="true" schema="MYHR">
<id name="id" type="int">
<column name="ID" />
<generator class="assigned" />
</id>
<property name="title" />
<property name="content" type="clob" length="5000" />
<property name="postTime" type="timestamp" />
<union-subclass name="Topic" table="topic">
<property name="type" type="int" />
</union-subclass>
<union-subclass name="Reply" table="reply">
<property name="floor" type="int" />
</union-subclass>
</class>
</hibernate-mapping>
然后看一下测试类，三种方式测试类相同，只有方式三的save方法中因为抽象类没有单独的数据表因此不能存储他们父类的信息。
package test;
import org.hibernate.Session;
import org.hibernate.Transaction;
import entity.Article;
import entity.Reply;
import entity.Topic;
import factory.HibernateSessionFactory;
public class Test {
private Session session = null;
private Transaction tran = null;
@org.junit.Test
public void save() {
session = HibernateSessionFactory.getSession();
tran = session.beginTransaction();
try{
Article article = new Article();
article.setId(1);
article.setTitle("这是一个Article");
Topic topic = new Topic();
topic.setId(2);
topic.setTitle("这是一个Topic");
Reply reply = new Reply();
reply.setId(3);
reply.setTitle("这是一个reply");
session.save(article);
session.save(topic);
session.save(reply);
tran.commit();
}catch(Exception e){
tran.rollback();
}
}
@org.junit.Test
public void Get() {
session = HibernateSessionFactory.getSession();
tran = session.beginTransaction();
try{
Article a = (Article)session.get(Article.class, 1);
Topic t = (Topic)session.get(Topic.class, 2);
Reply r = (Reply)session.get(Reply.class, 3);
System.out.println(a.getTitle());
System.out.println(t.getTitle());
System.out.println(r.getTitle());
tran.commit();
}catch(Exception e){
tran.rollback();
}
}
}
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