[原]Hibernate继承映射-具体类映射为数据库表

[标题]:[原]Hibernate继承映射-具体类映射为数据库表
[时间]:2009-6-21
[摘要]:每一个具体子类映射成单个数据库表，而抽象基类不参与映射。优点:数据操作实现简单，每个表中都包含自己所需要的具体子类的所有信息，减少了多表关联操作时的性能消耗。缺点:类的修改会导致相对应的表及其子类所对应表的更改。不支持多态查询。应用:适合在类层次结构上有一定数量的抽象类的情况下使用。
[关键字]:Hibernate，ORM，关联，继承，持久化，映射，Abstract
[环境]:MyEclipse7，Hibernate3.2，MySQL5.1
[作者]:Winty ([e-mail]) http://www.blogjava.net/wintys

[正文]:
    每一个具体子类映射成单个数据库表，而抽象基类不参与映射。
优点:数据操作实现简单，每个表中都包含自己所需要的具体子类的所有信息，减少了多表关联操作时的性能消耗。

缺点:
    类的修改会导致相对应的表及其子类所对应表的更改。不支持多态查询。

应用:
    适合在类层次结构上有一定数量的抽象类的情况下使用。
    
    例:学校管理系统中的实体关系:

 
    【图:department.jpg】

1、概述
a.实体类
public class Department {
    ......
    //由于Person类没有被映射，所以无法实现Department到Person的关联
    ......
}

public abstract class Person {
    ......
    private Department department;
    ......
}

public class Student extends Person {
    ......
}

public class Teacher extends Person {
    ......
}

b.数据库表
    具体子类Student和Teacher分别映射成一张表，各自的表中都包含从父类继承来的属性。

c.配置文件
由于Person类没有被映射，所以无法映射Person到Department的关联，只能由子类Student和Teacher实现映射。

Student.hbm.xml:
包含父类属性的映射
......
<many-to-one name="department"
            unique="true"
            column="dept"
            class="wintys.hibernate.inheritance.concrete.Department"/>
......


Teacher.hbm.xml:
包含父类属性的映射
......
<many-to-one name="department"
            unique="true"
            column="dept"
            class="wintys.hibernate.inheritance.concrete.Department"/>
......

2、实体类:
Department.java:

package wintys.hibernate.inheritance.concrete;

/**
 *
 * @version 2009-06-20
 * @author Winty ([e-mail])
 *
 */
public class Department {
    private Integer id;
    private String name;
    private String desc;
    
    
    public Department(){
        
    }
    
    public Department(String name , String desc){
        this.name = name;
        this.desc = desc;
    }
    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 String getDesc() {
        return desc;
    }
    public void setDesc(String desc) {
        this.desc = desc;
    }
}

Person.java:

package wintys.hibernate.inheritance.concrete;

/**
 *
 * @version 2009-06-20
 * @author Winty ([e-mail])
 *
 */
public abstract class Person {
    private Integer id;
    private String name;
    private Department department;
    
    public Person(){
    }
    
    public Person(String name){
        this.name = name;
    }
    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 Department getDepartment() {
        return department;
    }
    public void setDepartment(Department department) {
        this.department = department;
    }
}

Student.java:

package wintys.hibernate.inheritance.concrete;

/**
 *
 * @version 2009-06-20
 * @author Winty ([e-mail])
 *
 */
public class Student extends Person {
    private String studentMajor;
    
    public Student(){
        
    }
    
    public Student(String name , String major){
        super(name);
        this.studentMajor = major;
    }

    public String getStudentMajor() {
        return studentMajor;
    }

    public void setStudentMajor(String studentMajor) {
        this.studentMajor = studentMajor;
    }
}

Teacher.java:

package wintys.hibernate.inheritance.concrete;

/**
 *
 * @version 2009-06-20
 * @author Winty ([e-mail])
 *
 */
public class Teacher extends Person {
    private Float teacherSalary;

    public Teacher(){
        
    }
    
    public Teacher(String name , Float teacherSalary){
        super(name);
        this.teacherSalary = teacherSalary;
    }
    
    public Float getTeacherSalary() {
        return teacherSalary;
    }

    public void setTeacherSalary(Float teacherSalary) {
        this.teacherSalary = teacherSalary;
    }
}

3、数据库表:

 
【图:concret_db.jpg】

db.sql:

-- Author:Winty ([e-mail])
-- Date:2009-06-20
-- http://www.blogjava.net/wintys

-- Department
CREATE TABLE mydepartment(
    id      INT(4) NOT NULL,
    name    VARCHAR(100),
    descs  VARCHAR(100),
    PRIMARY KEY(id)
);

-- Student
CREATE TABLE mystudent(
    id              INT(4) NOT NULL,
    name            VARCHAR(100),
    dept            INT(4),-- 从Person继承了与Department的关联
    studentMajor    VARCHAR(100),
    PRIMARY KEY(id),
    CONSTRAINT FK_dept_s FOREIGN KEY(dept) REFERENCES mydepartment(id)
);

-- Teacher
CREATE TABLE myteacher(
    id              INT(4) NOT NULL,
    name            VARCHAR(100),
    dept            INT(4), -- 从Person继承了与Department的关联
    teacherSalary   FLOAT(7,2),
    PRIMARY KEY(id),
    CONSTRAINT FK_dept_t FOREIGN KEY(dept) REFERENCES mydepartment(id)
);

4、映射文件:

 
【图:concrete_mapping.jpg】

Department.hbm.xml:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<!--
    Mapping file autogenerated by MyEclipse Persistence Tools
-->

<hibernate-mapping>
    <class name="wintys.hibernate.inheritance.concrete.Department" table="mydepartment" catalog="db">
        <id name="id" type="int">
            <column name="id" not-null="true"/>
            <generator class="increment" />
        </id>
        <property name="name" />
        <property name="desc" type="string" column="descs"/>
    </class>
</hibernate-mapping>

Student.hbm.xml:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<!--
    Mapping file autogenerated by MyEclipse Persistence Tools
-->

<hibernate-mapping>
    <class name="wintys.hibernate.inheritance.concrete.Student" table="mystudent" catalog="db">
        <id name="id" type="int">
            <column name="id" not-null="true"/>
            <generator class="increment" />
        </id>
        <property name="name" />
        <property name="studentMajor" />
        
        <many-to-one name="department"
                    unique="true"
                    column="dept"
                    class="wintys.hibernate.inheritance.concrete.Department"/>
    </class>
</hibernate-mapping>

Teacher.hbm.xml:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<!--
    Mapping file autogenerated by MyEclipse Persistence Tools
-->

<hibernate-mapping>
    <class name="wintys.hibernate.inheritance.concrete.Teacher" table="myteacher" catalog="db">
        <id name="id" type="int">
            <column name="id" not-null="true"/>
            <generator class="increment" />
        </id>
        <property name="name" />
        <property name="teacherSalary" type="float"/>
        
        <many-to-one name="department"
                    unique="true"
                    column="dept"
                    class="wintys.hibernate.inheritance.concrete.Department"/>
    </class>
</hibernate-mapping>

hibernate.cfg.xml:

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

<!-- Generated by MyEclipse Hibernate Tools.                   -->
<hibernate-configuration>

<session-factory>
    <property name="connection.username">root</property>
    <property name="connection.url">
        jdbc:mysql://localhost:3306/db?useUnicode=true&amp;characterEncoding=utf-8
    </property>
    <property name="dialect">
        org.hibernate.dialect.MySQLDialect
    </property>
    <property name="myeclipse.connection.profile">MySQLDriver</property>
    <property name="connection.password">root</property>
    <property name="connection.driver_class">
        com.mysql.jdbc.Driver
    </property>
    <property name="show_sql">true</property>
    <mapping
        resource="wintys/hibernate/inheritance/concrete/Department.hbm.xml" />
    <mapping
        resource="wintys/hibernate/inheritance/concrete/Student.hbm.xml" />
    <mapping
        resource="wintys/hibernate/inheritance/concrete/Teacher.hbm.xml" />

</session-factory>

</hibernate-configuration>

4、使用测试:
DAO.java:

package wintys.hibernate.inheritance.concrete;

import java.util.List;

public interface DAO {
    public void insert();
    public <T> List<T> select(String hql);
}

DAOBean.java:

package wintys.hibernate.inheritance.concrete;

import java.util.List;

import org.hibernate.HibernateException;
import org.hibernate.Query;
import org.hibernate.Session;
import org.hibernate.Transaction;

/**
 *
 * @version 2009-06-20
 * @author Winty ([e-mail])
 *
 */
public class DAOBean implements DAO {

    @Override
    public void insert() {
        Transaction tc = null;
        try{
            Department dept = new Department("college of math" , "the top 3 college");
            Person p1,p2;
            p1 = new Student("Sam" , "Math");
            p1.setDepartment(dept);
            p2 = new Teacher("Martin" , new Float(15000f));
            p2.setDepartment(dept);
            
            Session session = HibernateUtil.getSession();
            tc = session.beginTransaction();
                        
            session.save(dept);
            //多态保存
            session.save(p1);
            session.save(p2);
        
            tc.commit();
        }catch(HibernateException e){
            try{
                if(tc != null)
                    tc.rollback();
            }catch(Exception ex){
                System.err.println(ex.getMessage());
            }
            System.err.println(e.getMessage());
        }finally{
            HibernateUtil.closeSession();            
        }    
    }

    @SuppressWarnings("unchecked")
    @Override
    public <T> List<T> select(String hql) {
        List<T> items = null;
        Transaction tc = null;
        try{
            Session session = HibernateUtil.getSession();
            tc = session.beginTransaction();
                        
            Query query = session.createQuery(hql);
            items = query.list();
            
            tc.commit();
        }catch(HibernateException e){
            try{
                if(tc != null){
                    tc.rollback();
                    items = null;
                }
            }catch(Exception ex){
                System.err.println(ex.getMessage());
            }
            System.err.println(e.getMessage());
        }finally{
            //HibernateUtil.closeSession();            
        }
        
        return items;
    }
}

HibernateUtil.java:

package wintys.hibernate.inheritance.concrete;

import org.hibernate.HibernateException;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;

/**
 * Hibernate Session管理
 * @author Winty
 */
public class HibernateUtil {
    private static SessionFactory factory = null;
    private static ThreadLocal<Session> threadLocal;
        
    static {    
        threadLocal = new ThreadLocal<Session>();
    }
    
    private HibernateUtil(){    
    }
    
    //Should be call only once.
    //设置Hibernate.cfg.xml的位置
    public static void setConfigFile(String hibernate_cfg_xml){
        try{
            factory = new Configuration()
                    .configure(hibernate_cfg_xml)
                    .buildSessionFactory();
        }catch(HibernateException e){
            System.err.println(e.getMessage());
        }
    }
    
    public static Session getSession()throws HibernateException{
        Session session = threadLocal.get();
        if(session == null){
            session = factory.openSession();
            threadLocal.set(session);
        }
        
        return session;
    }
    
    public static void closeSession()throws HibernateException{
        Session session = threadLocal.get();
        if(session != null){
            session.close();
        }
        threadLocal.set(null);
    }
}

Test.java:

package wintys.hibernate.inheritance.concrete;

import java.util.Iterator;
import java.util.List;

public class Test {

    public static void main(String[] args) {
        String config = "wintys/hibernate/inheritance/concrete/hibernate.cfg.xml";
        HibernateUtil.setConfigFile(config);
        
        DAO dao = new DAOBean();
        dao.insert();
        //不支持多态查询
        //Error:Person is not mapped
        //List<Person> ps = dao.select("from Person");
        //System.out.println(printStudentOrTeacher(ps));
        
        List<Student> students = dao.select("from Student");
        System.out.println(printStudentOrTeacher(students));
        
        List<Student> teachers = dao.select("from Teacher");
        System.out.println(printStudentOrTeacher(teachers));        
    }
    
    public static String printStudentOrTeacher(List<? extends Person> students){
        String str = "";
        Iterator<? extends Person> it = students.iterator();
        while(it.hasNext()){
            Person person = it.next();
            int id = person.getId();
            String name = person.getName();
            Department dept = person.getDepartment();
                
            int deptId = dept.getId();
            String deptName = dept.getName();
            String deptDesc = dept.getDesc();
            
            str += "id:" +id + ""n";
            str += "name:" + name + ""n";
            if(person instanceof Student)
                str += "major:"  + ((Student) person).getStudentMajor() + ""n";
            if(person instanceof Teacher)
                str += "salary:" + ((Teacher) person).getTeacherSalary().toString() + ""n";
            str += "dept:" + ""n";
            str += "  deptId:" + deptId + ""n";
            str += "  deptName:" + deptName + ""n";
            str += "  deptDesc:" + deptDesc + ""n";    
            str += ""n";
        }
        return str;
    }
}

5、运行结果
控制台显示:

......
Hibernate: select max(id) from mydepartment
Hibernate: select max(id) from mystudent
Hibernate: select max(id) from myteacher
Hibernate: insert into db.mydepartment (name, descs, id) values (?, ?, ?)
Hibernate: insert into db.mystudent (name, studentMajor, dept, id) values (?, ?, ?, ?)
Hibernate: insert into db.myteacher (name, teacherSalary, dept, id) values (?, ?, ?, ?)
Hibernate: select student0_.id as id1_, student0_.name as name1_, student0_.studentMajor as studentM3_1_, student0_.dept as dept1_ from db.mystudent student0_
Hibernate: select department0_.id as id0_0_, department0_.name as name0_0_, department0_.descs as descs0_0_ from db.mydepartment department0_ where department0_.id=?
id:1
name:Sam
major:Math
dept:
  deptId:1
  deptName:college of math
  deptDesc:the top 3 college


Hibernate: select teacher0_.id as id2_, teacher0_.name as name2_, teacher0_.teacherSalary as teacherS3_2_, teacher0_.dept as dept2_ from db.myteacher teacher0_
id:1
name:Martin
salary:15000.0
dept:
  deptId:1
  deptName:college of math
  deptDesc:the top 3 college

[参考资料]:
《J2EE项目实训--Hibernate框架技术》-杨少波 : 清华大学出版社

原创作品，转载请注明出处。
作者:Winty ([e-mail])
博客:http://www.blogjava.net/wintys