类和表的关系：

单独使用<subclass/>元素的类会和父类在同一张表里；

使用<subclass><join></join></subclass>这两个元素的类会单独占一个表。

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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;}@Overridepublic 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;}@Overridepublic 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;}@Overridepublic 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" discriminator-value="0"><id name="id" column="EMPLOYEE_ID"><generator class="native" /></id><discriminator column="type" type="int" force="false" /><property name="name" column="EMPLOYEE_NAME" /><subclass name="Sale" discriminator-value="1"><property name="sell" column="SELL"/></subclass><subclass name="Skill" discriminator-value="2"><join table="skill" ><key column="id" /><property name="skiller" column="SKILLER"/></join></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/unionextend</property><!-- ///表示连接本机的数据库//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() {}@BeforeClasspublic static void setUpClass() throws Exception {}@AfterClasspublic static void tearDownClass() throws Exception {}@Beforepublic void setUp() {}@Testpublic 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();}}}
}

生成的表结构：

employees

skill

插入语句：

Sale
Hibernate: insert into employees (EMPLOYEE_NAME, SELL, type) values (?, ?, 1)

Skill
Hibernate: insert into employees (EMPLOYEE_NAME, type) values (?, 2)
Hibernate: insert into skill (SKILLER, id) values (?, ?)

Employee
Hibernate: insert into employees (EMPLOYEE_NAME, type) values (?, 0)

查询语句：
Hibernate:
select
sale0_.EMPLOYEE_ID as EMPLOYEE1_0_0_,
sale0_.EMPLOYEE_NAME as EMPLOYEE3_0_0_,
sale0_.SELL as SELL0_0_
from employees sale0_
where sale0_.EMPLOYEE_ID=? and sale0_.type=1

Hibernate:
select skill0_.EMPLOYEE_ID as EMPLOYEE1_0_0_,
skill0_.EMPLOYEE_NAME as EMPLOYEE3_0_0_,
skill0_1_.SKILLER as SKILLER1_0_
from employees skill0_
inner join skill skill0_1_
on skill0_.EMPLOYEE_ID=skill0_1_.id
where skill0_.EMPLOYEE_ID=? and skill0_.type=2

Hibernate:
select employee0_.EMPLOYEE_ID as EMPLOYEE1_0_0_,
employee0_.EMPLOYEE_NAME as EMPLOYEE3_0_0_,
employee0_.SELL as SELL0_0_,
employee0_1_.SKILLER as SKILLER1_0_,
employee0_.type as type0_0_
from employees employee0_
left outer join skill employee0_1_
on employee0_.EMPLOYEE_ID=employee0_1_.id
where employee0_.EMPLOYEE_ID=?

查询结果：
Sale{id=1, name=sunliu, sell=300000}
Skill{id=2, name=wangwu, skiller=java}

Employee{id=3, name=lisi}

PS:这样联合使用subclass和join的好处是结合了单独使用subclass的高效率和joined-subclass的对于关系模型的理念。如果类的属性比较多则用join，如果属性少则直接用subclass即可（指用本案例的模型）。