Hibernate（十一）：映射继承关系的三种方案

2022-10-26 18:28:54

背景：

　　在一些项目中，会采用集成的关系来定义数据库实体类，比如:人（Person）与学生(Student)，学生来源与人，所以人的基本属性学生也拥有；但学生有的一些属性，人就不具有。人与学生之间很显然就拥有了继承关系------学生继承于人，人是父类，学生是子类。

　　那么，这种继承关系在hibernate是如何映射呢？

　　对于面向对象的程序设计语言而言，继承和多态是两个最基本的概念。hibernate的集成映射可以理解为持久化类之间的继承关系。在上边的例子中，学生集成了人，可以认为学生是一个特殊的人，如果对人进行查询，学生的实例也将被得到。

　　在hibernate中支持三种继承映射策略：

　　1）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。

　　2）使用jioned-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。

　　3）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。

使用subclass进行映射

新建工程Hibernate08，导入hibernate开发包及mysql驱动包。

在src下新建hibernate.cfg.xml配置文件：

 <?xml version="1.0" encoding="UTF-8"?>

 <!DOCTYPE hibernate-configuration PUBLIC

         "-//Hibernate/Hibernate Configuration DTD 3.0//EN"

         "http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">

 <hibernate-configuration>

     <session-factory>

         <property name="hibernate.connection.username">root</property>

         <property name="hibernate.connection.password">123456</property>

         <property name="hibernate.connection.driver_class">com.mysql.jdbc.Driver</property>

         <property name="hibernate.connection.url">jdbc:mysql://localhost/hibernate_01</property>

         <!-- <property name="hibernate.dialect">org.hibernate.dialect.MySQLDialect</property>

             <property name="hibernate.dialect">org.hibernate.dialect.MySQLInnoDBDialect</property> -->

         <property name="hibernate.dialect">org.hibernate.dialect.MySQL5InnoDBDialect</property>

         <property name="hibernate.show_sql">true</property>

         <property name="hibernate.format_sql">true</property>

         <property name="hibernate.hbm2ddl.auto">update</property>

         <property name="hibernate.current_session_context_class">thread</property>

         <property name="hibernate.c3p0.max_size">500</property>

         <property name="hibernate.c3p0.min_size">20</property>

         <property name="hibernate.c3p0.max_statements">10</property>

         <property name="hibernate.c3p0.timeout">2000</property>

         <property name="hibernate.c3p0.idle_test_period">2000</property>

         <property name="hibernate.c3p0.acquire_increment">10</property>

         <mapping resource="com/dx/hibernate06/extend/Person.hbm.xml" />

     </session-factory>

 </hibernate-configuration>

在src下新建com.dx.hibernate06.extend包，在该包下新建Person.java：

 package com.dx.hibernate06.extend;

 public class Person {

     private Integer id;

     private String name;

     private Integer age;

     public Person() {

     }

     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 Integer getAge() {

         return age;

     }

     public void setAge(Integer age) {

         this.age = age;

     }

 }

新建Student.java（继承自Person）：

 package com.dx.hibernate06.extend;

 public class Student extends Person {

     private String className;

     private String schoolName;

     public Student() {

     }

     public String getClassName() {

         return className;

     }

     public void setClassName(String className) {

         this.className = className;

     }

     public String getSchoolName() {

         return schoolName;

     }

     public void setSchoolName(String schoolName) {

         this.schoolName = schoolName;

     }

 }

添加Person.hbm.xml配置文件：

 <?xml version="1.0"?>

 <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"

 "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

 <!-- Generated 2017-6-8 15:16:00 by Hibernate Tools 3.5.0.Final -->

 <hibernate-mapping package="com.dx.hibernate06.extend">

     <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="java.lang.Integer">

             <column name="AGE" />

         </property>

         <subclass name="Student" discriminator-value="student">

             <property name="className" column="CLASS_NAME" type="string"></property>

             <property name="schoolName" column="SCHOOL_NAME" type="string"></property>

         </subclass>

     </class>

 </hibernate-mapping>

注意：

1）在这里并没有定义student.hbm.xml配置文件，而是只需要在Person.hbm.xml配置文件中设置subclass配置项就可以；

2）除了设置subclass节点，还需要添加discriminator（辨别列）节点配置，且需要在table和subclass节点中设置discriminator-value（辨别列值）。

添加测试类TestMain.java：

 package com.dx.hibernate06.extend;

 import org.hibernate.Session;

 import org.hibernate.SessionFactory;

 import org.hibernate.Transaction;

 import org.hibernate.boot.Metadata;

 import org.hibernate.boot.MetadataSources;

 import org.hibernate.boot.model.naming.ImplicitNamingStrategyComponentPathImpl;

 import org.hibernate.boot.registry.StandardServiceRegistry;

 import org.hibernate.boot.registry.StandardServiceRegistryBuilder;

 import org.junit.After;

 import org.junit.Before;

 import org.junit.Test;

 public class TestMain {

     private SessionFactory sessionFactory = null;

     private Session session = null;

     private Transaction transaction = null;

     @Before

     public void init() {

         StandardServiceRegistry standardRegistry = new StandardServiceRegistryBuilder().configure().build();

         Metadata metadata = new MetadataSources(standardRegistry).getMetadataBuilder().applyImplicitNamingStrategy(ImplicitNamingStrategyComponentPathImpl.INSTANCE).build();

         sessionFactory = metadata.getSessionFactoryBuilder().build();

         session = sessionFactory.getCurrentSession();

         transaction = session.beginTransaction();

     }

     @After

     public void destory() {

         transaction.commit();

         session.close();

         sessionFactory.close();

     }

 }

测试：

在TestMain.java中添加测试函数1：

     @Test

     public void testInsert() {

         Person person = new Person();

         person.setName("person1");

         person.setAge(27);

         Student student = new Student();

         student.setName("student1");

         student.setAge(22);

         student.setClassName("class-1");

         student.setSchoolName("浙江大学");

         session.save(person);

         session.save(student);

     }

测试执行sql:

 Hibernate: 

     create table PERSONS (

        ID integer not null auto_increment,

         TYPE varchar(255) not null,

         NAME varchar(255),

         AGE integer,

         CLASS_NAME varchar(255),

         SCHOOL_NAME varchar(255),

         primary key (ID)

     ) engine=InnoDB

 Hibernate:

     insert

     into

         PERSONS

         (NAME, AGE, TYPE)

     values

         (?, ?, 'person')

 Hibernate:

     insert

     into

         PERSONS

         (NAME, AGE, CLASS_NAME, SCHOOL_NAME, TYPE)

     values

         (?, ?, ?, ?, 'student')

数据库查寻结果：

添加测试函数2：

    @Test

    public void testSelect() {

        List<Person> persons = session.createQuery("FROM Person").list();

        System.out.println(persons.size());

        List<Student> students = session.createQuery("FROM Student").list();

        System.out.println(students.size());

    }

执行sql及结果:

 Hibernate:

     select

         person0_.ID as ID1_0_,

         person0_.NAME as NAME3_0_,

         person0_.AGE as AGE4_0_,

         person0_.CLASS_NAME as CLASS_NA5_0_,

         person0_.SCHOOL_NAME as SCHOOL_N6_0_,

         person0_.TYPE as TYPE2_0_

     from

         PERSONS person0_

 2

 Hibernate:

     select

         student0_.ID as ID1_0_,

         student0_.NAME as NAME3_0_,

         student0_.AGE as AGE4_0_,

         student0_.CLASS_NAME as CLASS_NA5_0_,

         student0_.SCHOOL_NAME as SCHOOL_N6_0_

     from

         PERSONS student0_

     where

         student0_.TYPE='student'

 1

使用jioned-subclass进行映射

在工程中复制包com.dx.hibernate06.extend，并命名新包名称为：com.dx.hibernate06.joined.subclass

修改Person.hbm.xml:

 <?xml version="1.0"?>

 <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"

 "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

 <!-- Generated 2017-6-8 15:16:00 by Hibernate Tools 3.5.0.Final -->

 <hibernate-mapping package="com.dx.hibernate06.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="java.lang.Integer">

             <column name="AGE" />

         </property>

         <joined-subclass name="Student" table="STUDENTS">

             <key>

                 <column name="STUDENT_ID"></column>

             </key>

             <property name="className" column="CLASS_NAME" type="string"></property>

             <property name="schoolName" column="SCHOOL_NAME" type="string"></property>

         </joined-subclass>

     </class>

 </hibernate-mapping>

注意：

1）这里不需要添加discriminator（辨别列）节点配置，也不需要在table和joined-subclass节点中设置discriminator-value（辨别列值）；

2）但需要在joined-subclass节点中指定表名，及在节点内部指定key column。

修改hibernate.cfg.xml，修改mapping节点指定文件路径：

<?xml version="1.0" encoding="UTF-8"?>

<!DOCTYPE hibernate-configuration PUBLIC

        "-//Hibernate/Hibernate Configuration DTD 3.0//EN"

        "http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">

<hibernate-configuration>

    <session-factory>

        。。。

        <mapping resource="com/dx/hibernate06/joined/subclass/Person.hbm.xml" />

    </session-factory>

</hibernate-configuration>

测试：

在com.dx.hibernate06.joined.subclass.TestMain.java中执行测试函数testInsert()(备注：该测试函数与上边测试函数一致),执行sql及结果:

 Hibernate: 

     create table PERSONS (

        ID integer not null auto_increment,

         NAME varchar(255),

         AGE integer,

         primary key (ID)

     ) engine=InnoDB

 Hibernate: 

     create table STUDENTS (

        STUDENT_ID integer not null,

         CLASS_NAME varchar(255),

         SCHOOL_NAME varchar(255),

         primary key (STUDENT_ID)

     ) engine=InnoDB

 Hibernate: 

     alter table STUDENTS

        add constraint FK3md9kn7axci4c8qrnaav8ybo

        foreign key (STUDENT_ID)

        references PERSONS (ID)

 Hibernate:

     insert

     into

         PERSONS

         (NAME, AGE)

     values

         (?, ?)

 Hibernate:

     insert

     into

         PERSONS

         (NAME, AGE)

     values

         (?, ?)

 Hibernate:

     insert

     into

         STUDENTS

         (CLASS_NAME, SCHOOL_NAME, STUDENT_ID)

     values

         (?, ?, ?)

数据库查寻结果：

执行testSelect()测试函数,执行结果及sql:

 Hibernate:

     select

         person0_.ID as ID1_0_,

         person0_.NAME as NAME2_0_,

         person0_.AGE as AGE3_0_,

         person0_1_.CLASS_NAME as CLASS_NA2_1_,

         person0_1_.SCHOOL_NAME as SCHOOL_N3_1_,

         case

             when person0_1_.STUDENT_ID is not null then 1

             when person0_.ID is not null then 0

         end as clazz_

     from

         PERSONS person0_

     left outer join

         STUDENTS person0_1_

             on person0_.ID=person0_1_.STUDENT_ID

 2

 Hibernate:

     select

         student0_.STUDENT_ID as ID1_0_,

         student0_1_.NAME as NAME2_0_,

         student0_1_.AGE as AGE3_0_,

         student0_.CLASS_NAME as CLASS_NA2_1_,

         student0_.SCHOOL_NAME as SCHOOL_N3_1_

     from

         STUDENTS student0_

     inner join

         PERSONS student0_1_

             on student0_.STUDENT_ID=student0_1_.ID

 1

使用union-subclass进行映射

在工程中复制包com.dx.hibernate06.extend，并命名新包名称为：com.dx.hibernate06.union.subclass

修改Person.hbm.xml:

 <?xml version="1.0"?>

 <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"

 "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

 <!-- Generated 2017-6-8 15:16:00 by Hibernate Tools 3.5.0.Final -->

 <hibernate-mapping package="com.dx.hibernate06.union.subclass">

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

         <id name="id" type="java.lang.Integer">

             <column name="ID" />

             <generator class="increment" />

         </id>

         <property name="name" type="java.lang.String">

             <column name="NAME" />

         </property>

         <property name="age" type="java.lang.Integer">

             <column name="AGE" />

         </property>

         <union-subclass name="Student" table="STUDENTS">

             <property name="className" column="CLASS_NAME" type="string"></property>

             <property name="schoolName" column="SCHOOL_NAME" type="string"></property>

         </union-subclass>

     </class>

 </hibernate-mapping>

注意：

1）这里只需要指定union-subclass对应的表，及指定其独有的列；

2）Person的ID生成类型不能为identity,不能为native,这里采用的increment。

修改hibernate.cfg.xml，修改mapping节点指定文件路径：

 <?xml version="1.0" encoding="UTF-8"?>

 <!DOCTYPE hibernate-configuration PUBLIC

         "-//Hibernate/Hibernate Configuration DTD 3.0//EN"

         "http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">

 <hibernate-configuration>

     <session-factory>

         。。。

         <mapping resource="com/dx/hibernate06/union/subclass/Person.hbm.xml" />

     </session-factory>

 </hibernate-configuration>

测试：

在com.dx.hibernate06.union.subclass.TestMain.java中执行测试函数testInsert()(备注：该测试函数与上边测试函数一致),执行sql及结果:

 Hibernate: 

     create table PERSONS (

        ID integer not null,

         NAME varchar(255),

         AGE integer,

         primary key (ID)

     ) engine=InnoDB

 Hibernate: 

     create table STUDENTS (

        ID integer not null,

         NAME varchar(255),

         AGE integer,

         CLASS_NAME varchar(255),

         SCHOOL_NAME varchar(255),

         primary key (ID)

     ) engine=InnoDB

 Hibernate:

     select

         max(ids_.mx)

     from

         ( select

             max(ID) as mx

         from

             STUDENTS

         union

         select

             max(ID) as mx

         from

             PERSONS

     ) ids_

 Hibernate:

     insert

     into

         PERSONS

         (NAME, AGE, ID)

     values

         (?, ?, ?)

 Hibernate:

     insert

     into

         STUDENTS

         (NAME, AGE, CLASS_NAME, SCHOOL_NAME, ID)

     values

         (?, ?, ?, ?, ?)

数据库查寻结果：

执行testSelect()测试函数,执行结果及sql:

 Hibernate:

     select

         person0_.ID as ID1_0_,

         person0_.NAME as NAME2_0_,

         person0_.AGE as AGE3_0_,

         person0_.CLASS_NAME as CLASS_NA1_1_,

         person0_.SCHOOL_NAME as SCHOOL_N2_1_,

         person0_.clazz_ as clazz_

     from

         ( select

             ID,

             NAME,

             AGE,

             null as CLASS_NAME,

             null as SCHOOL_NAME,

             0 as clazz_

         from

             PERSONS

         union

         select

             ID,

             NAME,

             AGE,

             CLASS_NAME,

             SCHOOL_NAME,

             1 as clazz_

         from

             STUDENTS

     ) person0_

 2

 Hibernate:

     select

         student0_.ID as ID1_0_,

         student0_.NAME as NAME2_0_,

         student0_.AGE as AGE3_0_,

         student0_.CLASS_NAME as CLASS_NA1_1_,

         student0_.SCHOOL_NAME as SCHOOL_N2_1_

     from

         STUDENTS student0_

 1

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