person 表
       id;(主键) int;
       username  varchar;
       password  varchar;
         sex   varchar;
       birthday  DateTime;
*******************
student表  
    id;(主键)
   
    grade  varchar;

*******************
teacher表
    id;(主键)
   schoolage varchar;

/////////////////////******////////////////////////////////
再创建VO
Person 

package wsq.po;

import java.util.Date;
public class Person implements java.io.Serializable {
 private static final long serialVersionUID = -4431851578811178858L;

 private Integer id;

 private String username;

 private String password;

 private String sex;

 private Date birthday;
 public Person() {
 }
 public Person(String username, String password, String sex, Date birthday) {
  this.username = username;
  this.password = password;
  this.sex = sex;
  this.birthday = birthday;
 }

 public Integer getId() {
  return this.id;
 }

 public void setId(Integer id) {
  this.id = id;
 }

 public String getUsername() {
  return this.username;
 }

 public void setUsername(String username) {
  this.username = username;
 }

 public String getPassword() {
  return this.password;
 }

 public void setPassword(String password) {
  this.password = password;
 }

 public String getSex() {
  return this.sex;
 }

 public void setSex(String sex) {
  this.sex = sex;
 }

 public Date getBirthday() {
  return this.birthday;
 }

 public void setBirthday(Date birthday) {
  this.birthday = birthday;
 }

}

 Student 

package wsq.po;
public class Student extends Person implements java.io.Serializable {

 private static final long serialVersionUID = -2981420482383402755L;
 private String grade;
 public Student() {
 }

 public String getGrade() {
  return this.grade;
 }

 public void setGrade(String grade) {
  this.grade = grade;
 }

}

Teacher 
  

package wsq.po;

public class Teacher extends Person implements java.io.Serializable {

 private static final long serialVersionUID = -8404469876025711535L;

 private String schoolage;

  public Teacher() {
 }

 public String getSchoolage() {
  return this.schoolage;
 }

 public void setSchoolage(String schoolage) {
  this.schoolage = schoolage;
 }

}
////////////////////****************/////////////////////////
最关键的就是配置了
  <?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<!--
    Mapping file autogenerated by MyEclipse Persistence Tools
-->
<hibernate-mapping>
    <class name="wsq.po.Person" table="person" schema="dbo" catalog="J2EE">
        <id name="id" type="java.lang.Integer">
            <column name="id" />
            <generator class="increment" />
        </id>
        <property name="username" type="java.lang.String">
            <column name="username" length="20" not-null="true" />
        </property>
        <property name="password" type="java.lang.String">
            <column name="password" length="20" not-null="true" />
        </property>
        <property name="sex" type="java.lang.String">
            <column name="sex" length="2" not-null="true" />
        </property>
        <property name="birthday" type="java.util.Date">
            <column name="birthday" length="8" not-null="true" />
        </property>
     <joined-subclass name="wsq.po.Student" table="student">
         <key column="id"/>
         <property name="grade" type="java.lang.String" column="grade" />
     </joined-subclass>
     <joined-subclass name="wsq.po.Teacher" table="teacher">
         <key column="id"/>
        <property name="schoolage" type="java.lang.String" column="schoolage" />
     </joined-subclass>
    </class>
</hibernate-mapping>