hibernate继承映射
一.
Hibernate支持三种继承映射策略：
使用 subclass 进行映射：将域模型中的每一个实体对象映射到一个独立的表中，也就是说不用在关系数据模型中考虑域模型中的继承关系和多态。
使用 joined-subclass 进行映射： 对于继承关系中的子类使用同一个表，这就需要在数据库表中增加额外的区分子类类型的字段。
使用 union-subclass 进行映射：域模型中的每个类映射到一个表，通过关系数据模型中的外键来描述表之间的继承关系。这也就相当于按照域模型的结构来建立数据库中的表，并通过外键来建立表之间的继承关系。
注意区别:
二.采用 subclass 元素的继承映射
代码实现：
1.Person
package cn.edu.sdut.hibernate.subclass;
public class Person{
private int id;
private String name;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
package cn.edu.sdut.hibernate.subclass;
public class Student extends Person {
private String school;
public String getSchool() {
return school;
}
public void setSchool(String school) {
this.school = school;
}
}
3.Person.hbm.xml
discriminator-value="person">
package cn.edu.sdut.hibernate.subclass;
import java.util.List;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.cfg.Configuration;
import org.hibernate.service.ServiceRegistry;
import org.hibernate.service.ServiceRegistryBuilder;
import org.junit.After;
import org.junit.Before;
import org.junit.Test;
public class TestSubClass {
private SessionFactory sessionFactory;
private Session session;
private Transaction transaction;
@Before
public void init(){
Configuration configuration = new Configuration().configure();
ServiceRegistry serviceRegistry =
new ServiceRegistryBuilder().applySettings(configuration.getProperties())
.buildServiceRegistry();
sessionFactory = configuration.buildSessionFactory(serviceRegistry);
session = sessionFactory.openSession();
transaction = session.beginTransaction();
}
@After
public void destory(){
transaction.commit();
session.close();
sessionFactory.close();
}
/**
* 缺点:
* 1. 使用了辨别者列.
* 2. 子类独有的字段不能添加非空约束.
* 3. 若继承层次较深, 则数据表的字段也会较多.
*/
/**
* 插入操作:
* 1. 对于子类对象只需把记录插入到一张数据表中.
* 2. 辨别者列有 Hibernate 自动维护.
*/
@Test
public void testSave(){
Person person = new Person();
person.setName("aa");
session.save(person);
Student student = new Student();
student.setSchool("jianqiao");
session.save(student);
}
/**
* 查询:
* 1. 查询父类记录, 只需要查询一张数据表
* 2. 对于子类记录, 也只需要查询一张数据表
*/
@Test
public void testQuery(){ //Person是类
List list = session.createQuery("from Person").list();
System.out.println(list);
List list2 = session.createQuery("from Student").list();
System.out.println(list2);
}
}
三.采用 joined-subclass 元素的继承映射
1.Person
package cn.edu.sdut.hibernate.joined_subclass;
public class Person{
private int id;
private String name;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
2.Student
package cn.edu.sdut.hibernate.joined_subclass;
public class Student extends Person {
private String school;
public String getSchool() {
return school;
}
public void setSchool(String school) {
this.school = school;
}
}
4.TestSubClass
package cn.edu.sdut.hibernate.joined_subclass;
import java.util.List;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.cfg.Configuration;
import org.hibernate.service.ServiceRegistry;
import org.hibernate.service.ServiceRegistryBuilder;
import org.junit.After;
import org.junit.Before;
import org.junit.Test;
public class TestSubClass {
private SessionFactory sessionFactory;
private Session session;
private Transaction transaction;
@Before
public void init(){
Configuration configuration = new Configuration().configure();
ServiceRegistry serviceRegistry =
new ServiceRegistryBuilder().applySettings(configuration.getProperties())
.buildServiceRegistry();
sessionFactory = configuration.buildSessionFactory(serviceRegistry);
session = sessionFactory.openSession();
transaction = session.beginTransaction();
}
@After
public void destory(){
transaction.commit();
session.close();
sessionFactory.close();
}
/**
* 优点:
* 1. 不需要使用了辨别者列.
* 2. 子类独有的字段能添加非空约束.
* 3. 没有冗余的字段.
*/
/**
* 查询:
* 1. 查询父类记录, 做一个左外连接查询
* 2. 对于子类记录, 做一个内连接查询.
*/
@Test
public void testSave(){
Person person = new Person();
person.setName("aa");
session.save(person);
Student student = new Student();
student.setSchool("jianqiao");
session.save(student);
}
/**
* 插入操作:
* 1. 对于子类对象至少需要插入到两张数据表中.
*/
@Test
public void testQuery(){
List list = session.createQuery("from Person").list();
System.out.println(list);
List list2 = session.createQuery("from Student").list();
System.out.println(list2);
}
}
四.采用 union-subclass 元素的继承映射
1.Person,Student代码与上面相同
2.Person.hbm.xml
package cn.edu.sdut.hibernate.union_subclass;
import java.util.List;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.cfg.Configuration;
import org.hibernate.service.ServiceRegistry;
import org.hibernate.service.ServiceRegistryBuilder;
import org.junit.After;
import org.junit.Before;
import org.junit.Test;
import sun.reflect.generics.tree.VoidDescriptor;
public class TestSubClass {
private SessionFactory sessionFactory;
private Session session;
private Transaction transaction;
@Before
public void init(){
Configuration configuration = new Configuration().configure();
ServiceRegistry serviceRegistry =
new ServiceRegistryBuilder().applySettings(configuration.getProperties())
.buildServiceRegistry();
sessionFactory = configuration.buildSessionFactory(serviceRegistry);
session = sessionFactory.openSession();
transaction = session.beginTransaction();
}
@After
public void destory(){
transaction.commit();
session.close();
sessionFactory.close();
}
/**
* 优点:
* 1. 无需使用辨别者列.
* 2. 子类独有的字段能添加非空约束.
*
* 缺点:
* 1. 存在冗余的字段
* 2. 若更新父表的字段, 则更新的效率较低
*/
/**
* 查询:
* 1. 查询父类记录, 需把父表和子表记录汇总到一起再做查询. 性能稍差.
* 2. 对于子类记录, 也只需要查询一张数据表
*/
@Test
public void testSave(){
Person person = new Person();
person.setName("aa");
session.save(person);
Student student = new Student();
student.setSchool("jianqiao");
session.save(student);
}
/**
* 插入操作:
* 1. 对于子类对象至少需要插入到一张数据表中.
*/
@Test
public void testQuery(){
List list = session.createQuery("from Person").list();
System.out.println(list);
List list2 = session.createQuery("from Student").list();
System.out.println(list2);
}
}