Hibernate关联关系映射之继承映射
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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