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08-07
08
Hibernate的继承关系
作者:Java伴侣 日期:2008-07-08
一、每个子类对应一个数据表(Table per concrete class)
学生表
create table `sample`.`student`(
`id` bigint not null auto_increment,
`name` varchar(20) default "" not null,
`score` float,
primary key (`id`)
);
create unique index `PRIMARY` on `sample`.`student`(`id`);
教师表
create table `sample`.`teacher`(
`id` bigint not null auto_increment,
`name` varchar(20) default "" not null,
`salary` float(6,2),
primary key (`id`)
);
create unique index `PRIMARY` on `sample`.`teacher`(`id`);
Person抽象基类
public abstract class Person implements java.io.Serializable {
private Long id;
private String name;
/**defaultconstructor*/
public Person() {
}
public Long getId() {
returnthis.id;
}
publicvoid setId(Long id) {
this.id = id;
}
public String getName() {
returnthis.name;
}
publicvoid setName(String name) {
this.name = name;
}
}
子类分别实现它，并添加额外的属性和相应gettter和setter方法。
如Student类：
public class Student extends Person {
private Float score;
public Student() {
super();
}
public Float getScore() {
returnscore;
}
publicvoid setScore(Float score) {
this.score = score;
}
}
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">
<hibernate-configuration>
<session-factory>
<property name="connection.username">root</property>
<property name="connection.url">
jdbc:mysql://localhost:3306/sample
</property>
<property name="dialect">
org.hibernate.dialect.MySQLDialect
</property>
<property name="connection.password">12345678</property>
<property name="connection.driver_class">
com.mysql.jdbc.Driver
</property>
<property name="show_sql">true</property>
<property name="current_session_context_class">thread</property>
<mapping resource="powerwind/bean/Student.hbm.xml" />
<mapping resource="powerwind/bean/Teacher.hbm.xml" />
</session-factory>
</hibernate-configuration>
由于Person抽象类没有对应数据库的表，也没有对应的映射文件，在HQL查询中也就不支持多态查询。感觉上，Person抽象类的作用只是减少JAVA代码的编写而已。
二、每个类对应一个表(Table per subclass)
人员表
create table `sample`.`person`(
`id` bigint not null auto_increment,
`name` varchar(20),
primary key (`id`)
);
学生表
create table `sample`.`student`(
`id` bigint default "" not null,
`score` float,
primarykey (`id`)
);
教师表
create table `sample`.`teacher`(
`id` bigint default "" not null,
`salary` float,
primary key (`id`)
);
两个子类的实现和前一种完全一样， Person类也只是去掉abstract修饰符而已。映射文件只需要Person.hbm.xml一个即可。
<hibernate-mapping>
<class name="powerwind.bean.Person" table="person"
catalog="sample">
<id name="id" type="java.lang.Long">
<column name="id" />
<generator class="native"></generator>
</id>
<property name="name" type="java.lang.String">
<column name="name" length="20" not-null="true" />
</property>
<joined-subclass name="powerwind.bean.Student"
table="student">
<key column="id" />
<property name="score" type="java.lang.Float">
<column name="score" precision="12" scale="0" />
</property>
</joined-subclass>
<joined-subclass name="powerwind.bean.Teacher"
table="teacher">
<key column="id" />
<property name="salary" type="java.lang.Float">
<column name="salary" precision="12" scale="0" />
</property>
</joined-subclass>
</class>
</hibernate-mapping>
这种方式是支持多态查询的。
多态查询语句：Query query=sdao.getSession().createQuery("from Person");
三、一个表对多个类(Table per class hierarchy)
人员表
create table `sample`.`person`(
`id` bigint not null auto_increment,
`name` varchar(20),
`score` float,
`salary` float,
`type` char(1),
primary key (`id`)
);
Person.hbm.xml
<hibernate-mapping>
<class name="powerwind.bean.Person" table="person"
catalog="sample">
<id name="id" type="java.lang.Long">
<column name="id" />
<generator class="native"></generator>
</id>
<discriminator column="type" type="java.lang.String" />
<property name="name" type="java.lang.String">
<column name="name" length="20" not-null="true" />
</property>
<subclass name="powerwind.bean.Student"
discriminator-value="S">
<property name="score" type="java.lang.Float" />
</subclass>
<subclass name="powerwind.bean.Teacher"
discriminator-value="T">
<property name="salary" type="java.lang.Float" />
</subclass>
</class>
</hibernate-mapping>
优点是单表查询，支持多态；缺点是要在表增加字段（type）用于区分子类。
附加：实体粒度设计
1、面向设计的细粒度
人员表
create table `sample`.`person`(
`id` bigint not null auto_increment,
`name` varchar(20),
`email` varchar(50),
`phone` varchar(20),
primary key (`id`)
);
Person类
publicclass Person implements java.io.Serializable {
private Long id;
private String name;
private Contact contact;
}
Contact类
publicclass Contact implements java.io.Serializable {
private String email;
private String phone;
}
注：以上两个类的代码省略了getter和setter方法。
Person.hbm.xml
<hibernate-mapping>
<class name="powerwind.bean.Person" table="person"
catalog="sample">
<id name="id" type="java.lang.Long">
<column name="id" />
<generator class="native"></generator>
</id>
<property name="name" type="string" column="name"/>
<component name="contact" class="powerwind.bean.Contact">
<property name="email" type="string" column="email"/>
<property name="phone" type="string" column="phone"/>
</component>
</class>
</hibernate-mapping>
这样的细粒度，有什么用呢？应该在处理比较复杂表结构才体现出来吧。
2、面向性能的细粒度
假如Contact类包含的字段是重量级的数据，如图片之类，而我们一般可能只需要一些简单的信息摘要。要怎么做呢？
create table `sample`.`person`(
`id` bigint not null auto_increment,
`name` varchar(20),
`gender` varchar(2),
`email` varchar(50),
`phone` varchar(20),
primary key (`id`)
);
首先定义个基类BasePerson
publicclass BasePerson {
private Long id;
private String name;
}
Person类继承BasePerson类，添加多一个gender属性。
PersonDetail类继承Person类，添加多一个email和phone属性。
Person.hbm.xml
<class name="powerwind.bean.Person" table="person"
catalog="sample">
<id name="id" type="java.lang.Long">
<column name="id" />
<generator class="native"></generator>
</id>
<property name="name" type="string" column="name"/>
<property name="gender" type="string" column="gender"/>
</class>
PersonDetail.hbm.xml
<class name="powerwind.bean.PersonDetail" table="person"
catalog="sample" polymorphism="explicit">