hibernate 继承映射

• 采用 joined-subclass 元素的继承映射可以实现每个子类一张表
• 采用这种映射策略时，父类实例保存在父类表中，子类实例由父类表和子类表共同存储。因为子类实例也是一个特殊的父类实例，因此必然也包含了父类实例的属性。于是将子类和父类共有的属性保存在父类表中，子类增加的属性，则保存在子类表中。
• 在这种映射策略下，无须使用鉴别者列，但需要为每个子类使用 key 元素映射共有主键。
• 子类增加的属性可以添加非空约束。因为子类的属性和父类的属性没有保存在同一个表中
• 
  
  

• 采用 union-subclass 元素可以实现将每一个实体对象映射到一个独立的表中。
  
• 子类增加的属性可以有非空约束 --- 即父类实例的数据保存在父表中，而子类实例的数据保存在子类表中。
  
• 子类实例的数据仅保存在子类表中, 而在父类表中没有任何记录
  
• 在这种映射策略下，子类表的字段会比父类表的映射字段要多,因为子类表的字段等于父类表的字段、加子类增加属性的总和
• 在这种映射策略下，既不需要使用鉴别者列，也无须使用 key 元素来映射共有主键.
• 使用 union-subclass 映射策略是不可使用 identity 的主键生成策略, 因为同一类继承层次中所有实体类都需要使用同一个主键种子, 即多个持久化实体对应的记录的主键应该是连续的. 受此影响, 也不该使用 native 主键生成策略, 因为 native 会根据数据库来选择使用 identity 或 sequence.
  
  
  
• 

package com.atguigu.hibernate.subclass;

public class Person {
	
	private Integer id;
	private String name;
	private int age;
	public Integer getId() {
		return id;
	}
	public void setId(Integer id) {
		this.id = id;
	}
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	public int getAge() {
		return age;
	}
	public void setAge(int age) {
		this.age = age;
	}
}

package com.atguigu.hibernate.subclass;

public class Student extends Person{

	private String school;

	public String getSchool() {
		return school;
	}

	public void setSchool(String school) {
		this.school = school;
	}
}

<?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="com.atguigu.hibernate.subclass">

    <class name="Person" table="PERSONS" discriminator-value="PERSON">

        <id name="id" type="java.lang.Integer">
            <column name="ID" />
            <generator class="native" />
        </id>
        
        <!-- 配置辨别者列 -->
		<discriminator column="TYPE" type="string"></discriminator>

        <property name="name" type="java.lang.String">
            <column name="NAME" />
        </property>
        
        <property name="age" type="int">
            <column name="AGE" />
        </property>
        
        <!-- 映射子类 Student, 使用 subclass 进行映射 -->
        <subclass name="Student" discriminator-value="STUDENT">
        	<property name="school" type="string" column="SCHOOL"></property>
        </subclass>
        
    </class>
</hibernate-mapping>

package com.atguigu.hibernate.subclass;

import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.sql.Blob;
import java.sql.Connection;
import java.sql.SQLException;
import java.util.Date;
import java.util.List;
import java.util.Set;

import org.hibernate.Hibernate;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.cfg.Configuration;
import org.hibernate.jdbc.Work;
import org.hibernate.service.ServiceRegistry;
import org.hibernate.service.ServiceRegistryBuilder;
import org.junit.After;
import org.junit.Before;
import org.junit.Test;

public class HibernateTest {

	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 destroy(){
		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. 
	 */
	
	/**
	 * 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(){
		List<Person> persons = session.createQuery("FROM Person").list();
		System.out.println(persons.size()); 
		
		List<Student> stus = session.createQuery("FROM Student").list();
		System.out.println(stus.size()); 
	}
	
	/**
	 * 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.setAge(11);
		person.setName("AA");
		
		session.save(person);
		
		Student stu = new Student();
		stu.setAge(22);
		stu.setName("BB");
		stu.setSchool("ATGUIGU");
		
		session.save(stu);
		
	}
}

package com.atguigu.hibernate.joined.subclass;

public class Person {
	
	private Integer id;
	private String name;
	private int age;
	public Integer getId() {
		return id;
	}
	public void setId(Integer id) {
		this.id = id;
	}
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	public int getAge() {
		return age;
	}
	public void setAge(int age) {
		this.age = age;
	}
}

package com.atguigu.hibernate.joined.subclass;

public class Student extends Person{

	private String school;

	public String getSchool() {
		return school;
	}

	public void setSchool(String school) {
		this.school = school;
	}

}

<?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="com.atguigu.hibernate.joined.subclass">

    <class name="Person" table="PERSONS">

        <id name="id" type="java.lang.Integer">
            <column name="ID" />
            <generator class="native" />
        </id>
        
        <property name="name" type="java.lang.String">
            <column name="NAME" />
        </property>
        
        <property name="age" type="int">
            <column name="AGE" />
        </property>
        
        <joined-subclass name="Student" table="STUDENTS">
        	<key column="STUDENT_id"></key>
        	<property name="school" type="string" column="SCHOOL"></property>
        </joined-subclass>
        
    </class>
</hibernate-mapping>

package com.atguigu.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 HibernateTest {

	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 destroy(){
		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 testQuery(){
		List<Person> persons = session.createQuery("FROM Person").list();
		System.out.println(persons.size()); 
		
		List<Student> stus = session.createQuery("FROM Student").list();
		System.out.println(stus.size()); 
	}
	
	/**
	 * 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 testSave(){
		
		Person person = new Person();
		person.setAge(11);
		person.setName("AA");
		
		session.save(person);
		
		Student stu = new Student();
		stu.setAge(22);
		stu.setName("BB");
		stu.setSchool("ATGUIGU");
		
		session.save(stu);
		
	}

}