Hibernate類的繼承使用joined-class實現

類與表的關係：

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Employee.java

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package blog.hibernate.domain;

public class Employee {
    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;
    }

    @Override
    public String toString() {
        return "Employee{" + "id=" + id + ", name=" + name + '}';
    }
}

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Sale.java

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package blog.hibernate.domain;

public class Sale extends Employee {
    private int sell;

    public int getSell() {
        return sell;
    }

    public void setSell(int sell) {
        this.sell = sell;
    }

    @Override
    public String toString() {
        return "Sale{"  + "id=" + this.getId() + ", name=" + this.getName() + "sell=" + sell + '}';
    }
}

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Skill.java

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package blog.hibernate.domain;

public class Skill extends Employee{
    private String skiller;

    public String getSkiller() {
        return skiller;
    }

    public void setSkiller(String skiller) {
        this.skiller = skiller;
    }

    @Override
    public String toString() {
        return "Skill{"  + "id=" + this.getId() + ", name=" + this.getName() + "skiller=" + skiller + '}';
    }
}

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Employee.hbm.xml

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<?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="blog.hibernate.domain">
    <class name="Employee" table="employees">
        <id name="id" column="EMPLOYEE_ID">
            <generator class="native" />
        </id>
        
        <property name="name" column="EMPLOYEE_NAME" />
        
        <joined-subclass name="Sale" table="sale" >
            <key column="SALE_ID" />
            <property name="sell" column="SELL"/>
        </joined-subclass>
        
        <joined-subclass name="Skill" table="skill" >
            <key column="SKILL_ID" />
            <property name="skiller" column="SKILLER"/>
        </joined-subclass>
    </class>
</hibernate-mapping>

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HibernateUtil.java

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package blog.hibernate;

import java.util.logging.Level;
import java.util.logging.Logger;
import org.hibernate.HibernateException;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.cfg.Configuration;

public final class HibernateUtil {
    
    private static SessionFactory sessionFactory;
    private HibernateUtil(){}
    
    static{
        Configuration cfg = new Configuration();
        sessionFactory =  cfg.configure("hibernate.cfg.xml").buildSessionFactory();
    }
    
    public static SessionFactory getSessionFactory(){
        return sessionFactory;
    }
    
    public static Session getSession(){
        return sessionFactory.openSession();
    }

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hibernate.cfg.xml

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<?xml version='1.0' encoding='utf-8'?>
<!DOCTYPE hibernate-configuration PUBLIC
    "-//Hibernate/Hibernate Configuration DTD//EN"
    "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">

<hibernate-configuration>
    <session-factory>
        <property name="hibernate.connection.driver_class">org.gjt.mm.mysql.Driver</property>
        <property name="hibernate.connection.url">jdbc:mysql://localhost:3306/joinextend</property><!-- ///表示連接本機的數據庫//localhost:3306 -->
        <property name="hibernate.connection.username">root</property>
        <property name="hibernate.connection.password">1234</property>
        <property name="hibernate.dialect">org.hibernate.dialect.MySQLDialect</property>
   		
        <property name="hibernate.hbm2ddl.auto">create</property>
        <property name="hibernate.show_sql">true</property>
                
        <mapping resource="blog/hibernate/domain/Employee.hbm.xml"/>
    </session-factory>	
</hibernate-configuration>

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junit test: JoinExtend.java

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package junit.test;

import java.util.logging.Logger;
import java.util.logging.Level;
import org.hibernate.Transaction;
import blog.hibernate.HibernateUtil;
import blog.hibernate.domain.Employee;
import blog.hibernate.domain.Sale;
import blog.hibernate.domain.Skill;
import org.hibernate.Session;
import org.junit.AfterClass;
import org.junit.Before;
import org.junit.BeforeClass;
import org.junit.Test;

public class JoinExtend {
    
    public JoinExtend() {
    }

    @BeforeClass
    public static void setUpClass() throws Exception {
    }

    @AfterClass
    public static void tearDownClass() throws Exception {
    }
    
    @Before
    public void setUp() {
    }
    
    @Test
    public void test(){
        add();
        query();
    }
    
    public void add(){
        Employee emp1 = new Employee();
        emp1.setName("lisi");
        
        Skill emp2 = new Skill();
        emp2.setName("wangwu");
        emp2.setSkiller("java");
        
        Sale emp3 = new Sale();
        emp3.setName("sunliu");
        emp3.setSell(300000);
        
        Session session = null;
        Transaction tx = null;
        try {
            session = HibernateUtil.getSession();
            tx = session.beginTransaction();
            session.save(emp3);
            session.save(emp2);
            session.save(emp1);
            tx.commit();
        } catch (Exception e) {
            Logger.getLogger(JoinExtend.class.getName()).log(Level.SEVERE, null, e);
            if (tx != null) {
                tx.rollback();
            }
        } finally {
            if (session != null) {
                session.close();
            }
        }
    }
    
    public void query(){
        Session session = null;
        try {
            session = HibernateUtil.getSession();
            Employee emp1 = (Employee)session.get(Sale.class, 1);
            Employee emp2 = (Employee)session.get(Skill.class, 2);
            Employee emp3 = (Employee)session.get(Employee.class, 3);
            System.out.println(emp1.toString());
            System.out.println(emp2.toString());
            System.out.println(emp3.toString());
        } catch (Exception e) {
            Logger.getLogger(JoinExtend.class.getName()).log(Level.SEVERE, null, e);
        } finally {
            if (session != null) {
                session.close();
            }
        }
    }        
}

用joined-class會生成三張表：

employees

sale

skill

Hibernate生成的sql語句爲：

Hibernate: insert into employees (EMPLOYEE_NAME) values (?)
Hibernate: insert into sale (SELL, SALE_ID) values (?, ?)
Hibernate: insert into employees (EMPLOYEE_NAME) values (?)
Hibernate: insert into SKILL (SKILLER, SKILL_ID) values (?, ?)
Hibernate: insert into employees (EMPLOYEE_NAME) values (?)

Hibernate:
select
sale0_.SALE_ID as EMPLOYEE1_0_0_,
sale0_1_.EMPLOYEE_NAME as EMPLOYEE2_0_0_,
sale0_.SELL as SELL1_0_
from sale sale0_
inner join
employees sale0_1_
on
sale0_.SALE_ID=sale0_1_.EMPLOYEE_ID
where
sale0_.SALE_ID=?


Hibernate:
select
skill0_.SKILL_ID as EMPLOYEE1_0_0_,
skill0_1_.EMPLOYEE_NAME as EMPLOYEE2_0_0_,
skill0_.SKILLER as SKILLER2_0_
from SKILL skill0_
inner join
employees skill0_1_
on
skill0_.SKILL_ID=skill0_1_.EMPLOYEE_ID
where
skill0_.SKILL_ID=?

測試結果爲：

Sale{id=1, name=sunliusell=300000}
Skill{id=2, name=wangwuskiller=java}
Employee{id=3, name=lisi}

Hibernate:
select
employee0_.EMPLOYEE_ID as EMPLOYEE1_0_0_,
employee0_.EMPLOYEE_NAME as EMPLOYEE2_0_0_,
employee0_1_.SELL as SELL1_0_,
employee0_2_.SKILLER as SKILLER2_0_,
case
when employee0_1_.SALE_ID is not null then 1
when employee0_2_.SKILL_ID is not null then 2
when employee0_.EMPLOYEE_ID is not null then 0
end
as clazz_0_
from
employees employee0_
left outer join
sale employee0_1_
on
employee0_.EMPLOYEE_ID=employee0_1_.SALE_ID
left outer join
SKILL employee0_2_
on
employee0_.EMPLOYEE_ID=employee0_2_.SKILL_ID
where
employee0_.EMPLOYEE_ID=?

PS:

採用這種方式的好處是數據庫的表結構符合關係模型的設計理念，子類對應的字段可以強制爲非空，如果要新增一個子類，只需要新增一個表即可；缺點是效率不高，而且表的個數比較多。