hibernate继承映射策略之每个子类一张表

来源:互联网 发布:一句话自我介绍 知乎 编辑:程序博客网 时间:2024/03/29 21:10

以文章《hibernate单表继承映射》中的几个类为例,我们来看下“每个子类一张表”的映射策略。

与之不同的是Person.hbm.xml配置文件,代码如下:

<hibernate-mapping package="com.test.pojo"><class name="Person"><id name="id"><!-- assigned:自己指定主键 --><generator class="assigned"></generator></id><property name="name" /><property name="age" /><joined-subclass name="Student"><key column="id" /><property name="work" /></joined-subclass><joined-subclass name="Teacher"><key column="id" /><property name="salary" /></joined-subclass></class></hibernate-mapping>
测试类:

public class HibernateTest {@Testpublic void testCreateDB(){Configuration cfg=new Configuration().configure();SchemaExport se=new SchemaExport(cfg);//第一个参数表示是否生成ddl脚本,第二个参数表示是否执行到数据库中se.create(true, true);}/** * 保存数据 */@Testpublic void save(){Session session=null;Transaction tx=null;try{session=HibernateUtil.getSession();tx=session.beginTransaction();Teacher teacher=new Teacher();teacher.setId(1);teacher.setName("teacher");teacher.setAge(20);teacher.setSalary(5000);Student student=new Student();student.setId(2);student.setName("stu1");student.setAge(20);student.setWork("hello world");Student stu=new Student();stu.setId(3);stu.setName("stu2");stu.setAge(21);stu.setWork("Struts2");session.save(student);session.save(stu);session.save(teacher);tx.commit();}catch(Exception e){if(tx!=null)tx.rollback();e.printStackTrace();}finally{HibernateUtil.closeSession();}}}
执行testCreateDB,打印信息如下:

alter table Student         drop         foreign key FK_ohs43dct8k52ch2exlmf4bs3lalter table Teacher         drop         foreign key FK_g6jmt7fcm6gfd0jvhimb9xy84drop table if exists Person    drop table if exists Student    drop table if exists Teacher    create table Person (        id integer not null,        name varchar(255),        age integer,        primary key (id)    )    create table Student (        id integer not null,        work varchar(255),        primary key (id)    )    create table Teacher (        id integer not null,        salary integer,        primary key (id)    )    alter table Student         add constraint FK_ohs43dct8k52ch2exlmf4bs3l         foreign key (id)         references Person (id)    alter table Teacher         add constraint FK_g6jmt7fcm6gfd0jvhimb9xy84         foreign key (id)         references Person (id)

数据库表信息如下:


执行save方法,打印sql语句如下:

Hibernate:     insert     into        Person        (name, age, id)     values        (?, ?, ?)Hibernate:     insert     into        Student        (work, id)     values        (?, ?)Hibernate:     insert     into        Person        (name, age, id)     values        (?, ?, ?)Hibernate:     insert     into        Student        (work, id)     values        (?, ?)Hibernate:     insert     into        Person        (name, age, id)     values        (?, ?, ?)Hibernate:     insert     into        Teacher        (salary, id)     values        (?, ?)

数据库表信息如下:


总结:

三种继承映射比较:

1、所有数据存一张表,数据冗余比较多。效率比较高。---推荐使用

2、每个子类一张表,数据冗余比较少,查询效率低,主键不能自增。需要指明为assigned,uuid,hilo等。

3、每个类一张表,数据冗余比较少。查询效率比每个子类一张表稍高。维护的表个数多。







阅读全文
0 0