Hibernate（十一）：映射继承关系的三种方案

• 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