1.概述
关系数据库的表之间不存在继承关系，为了将面向对象中的继承关系映射到关系数据库中，可使用三种映射策略：

1、每个继承层次一张表
2、每个具体类一张表
3、每个类一张表

2.每个继承层次一张表
即用一张表表示一个继承层次，因此需要增加一个字段用于区别

public class Person {
    private int id;
    private String name;
    private String sex;
    private int age;
}
public class Student extends Person {
    private String sno;
    private String school;
}
public class Worker extends Person{
    private String no;
    private double salary;
}

image.png

映射语法：

1、定义一个以父类命名的映射文件
2、<id>标记对后面添加<discriminator>标记，并指定column属性来定义鉴别字段
3、父类属性放在<class>标记对之间，子类属性放在<subclass>标记对之间
4、在各类标记中使用discriminator-value属性指定各类对应的鉴别字段的值

示例：

1、数据库连接
2、为项目添加Hibernate支持
3、创建持久化类
4、创建映射文件
5、修改hibernate配置文件
6、创建工具类获取Session实例
7、编写Dao接口及其实现类
8、测试

项目结构：

image.png

Person类：

public class Person {
    private int id;
    private String name;
    private String sex;
    private int age;

    public Person() {}

    public Person(String name, String sex, int age) {
        this.name = name;
        this.sex = sex;
        this.age = age;
    }

    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 String getSex() {
        return sex;
    }

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

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }
}

Worker类：

public class Worker extends Person{
    private String no;
    private double salary;

    public Worker() {}
    public Worker(String name, String sex, int age, String no, double salary) {
        super(name, sex, age);
        this.no = no;
        this.salary = salary;
    }

    public String getNo() {
        return no;
    }

    public void setNo(String no) {
        this.no = no;
    }

    public double getSalary() {
        return salary;
    }

    public void setSalary(double salary) {
        this.salary = salary;
    }
}

Student类：

public class Student extends Person {
    private String sno;
    private String school;

    public Student() {
    }

    public Student(String name, String sex, int age, String sno, String school) {
        super(name, sex, age);
        this.sno = sno;
        this.school = school;
    }

    public String getSno() {
        return sno;
    }

    public void setSno(String sno) {
        this.sno = sno;
    }

    public String getSchool() {
        return school;
    }

    public void setSchool(String school) {
        this.school = school;
    }
}

工具类：

public class HibernateUtil {
    private static SessionFactory sf;

    // 工具类不允许实例化
    private HibernateUtil() {
    }
    // 静态代码块，只执行一次，提高性能
    static {
        Configuration cf = new Configuration().configure();
        sf = cf.buildSessionFactory();
    }
    public static Session getSession() {
        Session session = sf.getCurrentSession();
        return session;
    }
}

Dao及其实现类：

public interface HibernateDao {
    public void addPerson(Person person);
    @SuppressWarnings("unchecked")
    public List getAllPerson(String hql);
}

public class HibernateDaoImpl implements HibernateDao {
    @Override
    public void addPerson(Person person) {
        Session session = HibernateUtil.getSession();
        Transaction tx = session.beginTransaction();
        try {
            session.save(person);
            tx.commit();
        } catch (Exception e) {
            if (null != tx) {
                tx.rollback();
            }
            e.printStackTrace();
        }
    }

    @Override
    @SuppressWarnings("unchecked")
    public List getAllPerson(String hql) {
        Session session = HibernateUtil.getSession();
        Transaction tx = session.beginTransaction();
        Query query = session.createQuery(hql);
        List persons = query.list();
        tx.commit();
        return persons;
    }
}

测试类：

public class HibernateTest {
    public static void main(String[] args) {
        Person person = new Person("张三", "男", 20);
        Worker worker = new Worker("李四", "女", 28, "P0001", 6000);
        Student student = new Student("王五", "女", 19, "SD001", "广金");
        HibernateDao hibernateDao = new HibernateDaoImpl();
        hibernateDao.addPerson(person);
        hibernateDao.addPerson(student);
        hibernateDao.addPerson(worker);
        String hql = "from Person";
        List<Person> persons = hibernateDao.getAllPerson(hql);
        System.out.println("========所有人员信息=======");
        for (Person p : persons) {
            System.out.println("姓名：" + p.getName() + " 性别：" + p.getSex() + " 年龄：" + p.getAge());
        }
        hql = "from Worker";
        List<Worker> workers = hibernateDao.getAllPerson(hql);
        System.out.println("========工人信息=======");
        for (Worker w : workers) {
            System.out.println("姓名：" + w.getName() + " 性别：" + w.getSex() + " 年龄：" + w.getAge() +
                    " 工号：" + w.getNo() + "薪水：" + w.getSalary());
        }
        hql = "from Student";
        List<Student> students = hibernateDao.getAllPerson(hql);
        System.out.println("========学生信息=======");
        for (Student s : students) {
            System.out.println("姓名：" + s.getName() + " 性别：" + s.getSex() + " 年龄：" + s.getAge() +
                    " 学号：" + s.getSno() + "学校：" + s.getSchool());
        }
    }
}

Person.hbm.xml

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
        "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="这里写你的Person类所在的包">
    <!--ORM对象、关系数据库映射-->
    <class name="Person" table="person" discriminator-value="person">
        <id name="id" type="integer">
            <generator class="native"/>
        </id>
        <!--设置鉴别字段-->
        <discriminator column="type" type="string" length="15"/>
        <!--从父类继承下来的共享属性-->
        <property name="name" type="string" column="username" length="12" not-null="true"/>
        <property name="sex" type="string" length="1"/>
        <property name="age" type="integer"/>
        <subclass name="Student" discriminator-value="student">
            <!--子类新增属性-->
            <property name="sno" type="string" length="10"/>
            <property name="school" type="string" length="50"/>
        </subclass>
        <subclass name="Worker" discriminator-value="worker">
            <!--子类新增属性-->
            <property name="no" column="wno" type="string" length="10"/>
            <property name="salary" type="double"/>
        </subclass>
    </class>
</hibernate-mapping>

hibernate配置文件：

<?xml version='1.0' encoding='utf-8'?>
<!DOCTYPE hibernate-configuration PUBLIC
    "-//Hibernate/Hibernate Configuration DTD//EN"
    "http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
  <session-factory>
    <property name="connection.driver_class">com.mysql.jdbc.Driver</property>
    <property name="connection.url">jdbc:mysql://localhost:3306/hibernate?useSSL=true</property>
    <property name="connection.username">root</property>
    <property name="connection.password">这里填你的密码</property>
    <!--连接池设置-->
    <property name="connection.pool_size">2</property>
    <!--数据库方言设置-->
    <property name="dialect">org.hibernate.dialect.MySQL8Dialect</property>
    <!--向控制台显示执行的SQL语句-->
    <property name="show_sql">false</property>
    <!--格式化SQL语句后输出-->
    <property name="hibernate.format_sql">true</property>
    <!--事务配置-->
    <property name="current_session_context_class">thread</property>
    <!--创建SessionFactory对象时自动创建数据库表,可取create、create-drop、update等值-->
    <property name="hibernate.hbm2ddl.auto">update</property>
    <!--配置映射文件-->
    <mapping resource="Person.hbm.xml"/>
  </session-factory>
</hibernate-configuration>

运行结果：

image.png

数据库中：

image.png

补充说明：需要先建立数据库hibernate，此项目为maven项目。也可以建立普通java项目或web项目。

一个继承层次一张表的优缺点如下：

1、最简单，执行效率最高（无需关联）
2、存在冗余字段，需要加入区分各类的字段；不允许子类属性为not null约束（允许为空）。