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">
<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" />
<property name="phone" type="string" column="phone" />
<property name="email" type="string" column="email" />
</class>
除了polymorphism="explicit"这一句，和每个子类对应一个数据表的继承关系没有什么区别。正是这句设置，去除了对persondetail的隐式多态查询。
query query=getsession().createquery("from person");
query query=getsession().createquery("from persondetail");
上面两句中，第一句并不会查询到persondetail 对象，即查询字段不包括email和phone。
注：参考《深入浅出hibernate》一书
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