Hibernate继承映射：每个子类一张表(joined-subclass)表结构
我在前面写了关于继承映射的整个继承树对应一张表的例子，但是他存在以一定的弊端。在数据表中会产生大量的null值。
为了避免这种情况的发生。我们可以这样去做：
<joined-subclass name="Skiller" table="skiller">
?? <key column="emp_id"/>
?? <property name="skill"></property>
?</joined-subclass>
?
?<joined-subclass name="Sales" table="sales">
?? <key column="emp_id"/>
?? <property name="sell"></property>
?</joined-subclass>
通过使用joined-subclass映射，对每一个子类生成一张表。
1、domain类：
package com.zhaosoft.domain;
public class Employee {
?private int id;
?private String name;
?private Department depart;
?public int getId() {
??return id;
?}
?public void setId(int id) {
??this.id = id;
?}
?public String getName() {
??return name;
?}
?public void setName(String name) {
??this.name = name;
?}
?public Department getDepart() {
??return depart;
?}
?public void setDepart(Department depart) {
??this.depart = depart;
?}
}
2、各个子类：
package com.zhaosoft.domain;
public class Sales extends Employee{
?private int sell;//销售额
?public int getSell() {
??return sell;
?}
?public void setSell(int sell) {
??this.sell = sell;
?}
}
---------------------------------------------
package com.zhaosoft.domain;
public class Skiller extends Employee{
?private String skill;
?public String getSkill() {
??return skill;
?}
?public void setSkill(String skill) {
??this.skill = skill;
?}
}
?
3、映射文件：
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
?"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
?"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="com.zhaosoft.domain">
???? <!-- 当discriminator-value值为0时，为普通员工 -->
?<class name="Employee" discriminator-value="0">??
??<id name="id">
????? <!-- 表示主键为自动增长 -->
???<generator type="string"/>
?<many-to-one name="depart" column="depart_id"></many-to-one>
?
?<joined-subclass name="Skiller" table="skiller">
?? <key column="emp_id"/>
?? <property name="skill"></property>
?</joined-subclass>
?
?<joined-subclass name="Sales" table="sales">
?? <key column="emp_id"/>
?? <property name="sell"></property>
?</joined-subclass>
?
?</class>
?
</hibernate-mapping>
?
4、测试文件：
public static void add() {
??Session s = null;
??Transaction t=null;
??try {
???s=HibernateUtil.getSession();
???t = s.beginTransaction();
???t.begin();
???Department d = new Department();
???d.setName("销售部");
???Employee employee1 = new Employee();
???employee1.setName("小三");
???employee1.setDepart(d);
???Skiller employee2 = new Skiller();
???employee2.setName("李斯");
???employee2.setSkill("skill");
???employee2.setDepart(d);
???
???Sales employee3 = new Sales();
???employee3.setName("王五");
???employee3.setSell(100);
???employee3.setDepart(d);
???Set<Employee> set=new HashSet<Employee>();
???set.add(employee1);
???set.add(employee2);
???set.add(employee3);
???d.setEmps(set);
???s.save(d);
???s.save(employee1);
???s.save(employee2);
???s.save(employee3);
???
???t.commit();
??} catch (Exception e) {
??} finally {
???if (s != null) {
????s.close();
???}
??}
?}
