继承映射策略分为三种类型 -
每个类层次结构的表
每个具体类的表
-
每个子类的表
在本文中，我们将讨论每个类的表层次结构。
每个类层次结构的表
在此，仅创建一个表用于继承映射。这种方法的缺点是表中存储了大量空值。
@Inheritance(strategy=InheritanceType.SINGLE_TABLE)、@DiscriminatorColumn 和 @DiscriminatorValue 是此策略中使用的注解。
@DiscriminatorColumn 用于创建一个附加列，用于标识层次结构类。
考虑以下示例来理解这一点 -
实施步骤 -
创建实体类并为其使用适当的注释。
编写 hibernate 配置文件并添加映射类。
编写代码来创建数据并将其存储在表中。
1.创建实体类
Car.java
package com.tutorialspoint;
@Entity
@Table(name = "car")
@Inheritance(strategy=InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(name="category",discriminatorType=DiscriminatorType.STRING)
@DiscriminatorValue(value="car")
public class Car {
@Id
@GeneratedValue(strategy=GenerationType.AUTO)
private int id;
@Column(name = "name")
private String name;
@Column(name = "color")
private String color;
//Getters
//Setters
}
Sports_Car.java
package com.tutorialspoint;
import javax.persistence.*;
@Entity
@DiscriminatorValue("sportscar")
public class Sports_Car extends Car{
@Column(name="mileage")
private int mileage;
@Column(name="cost")
private int cost;
//Getters
//Setters
}
Taxi_Car.java
package com.tutorialspoint;
import javax.persistence.*;
@Entity
@DiscriminatorValue("taxicar")
public class Taxi_Car extends Car{
@Column(name="farePerKm")
private int farePerKm;
@Column(name="available")
private boolean available;
//Getters
//Setters
}
2。 Hibernate配置文件（hibernate.cfg.xml）
<!DOCTYPE hibernate-configuration PUBLIC
"-//Hibernate/Hibernate Configuration DTD 3.0//EN"
"http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
<session-factory>
<!-- JDBC Database connection settings -->
<property name="connection.driver_class">com.mysql.cj.jdbc.Driver</property>
<property name="connection.url">jdbc:mysql://localhost:3306/demo?useSSL=false</property>
<property name="connection.username">root</property>
<property name="connection.password">root</property>
<!-- JDBC connection pool settings ... using built-in test pool -->
<property name="connection.pool_size">4</property>
<!-- Echo the SQL to stdout -->
<property name="show_sql">true</property>
<!-- Select our SQL dialect -->
<property name="dialect">org.hibernate.dialect.MySQL5Dialect</property>
<!-- Drop and re-create the database schema on startup -->
<property name="hbm2ddl.auto">create-drop</property>
<!-- name of annotated entity class -->
<mapping class="com.tutorialspoint.Car"/>
<mapping class="com.tutorialspoint.Sports_Car"/>
<mapping class="com.tutorialspoint.Taxi_Car"/>
</session-factory>
</hibernate-configuration>
3。创建表并存储数据的代码
package com.tutorialspoint;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.cfg.Configuration;
public class StoreTest {
public static void main(String args[]){
SessionFactory sessionFactory = new Configuration()
.configure("com/tutorialspoint/hibernate.cfg.xml")
.buildSessionFactory();
Session session=sessionFactory.openSession();
Transaction t=session.beginTransaction();
Car c1=new Car();
c1.setName("Mercedes");
c1.setColor("Black");
Sport_Car c2=new Sport_Car();
c2.setName("Porsche");
c2.setColor("Red");
c2.setMileage(20);
c2.setCost(5000000);
Taxi_Car c3=new Taxi_Car();
c3.setName("Innova");
c3.setColor("White");
c3.setFarePerKm(7);
c3.setAvailable(true);
session.persist(c1);
session.persist(c2);
session.persist(c3);
t.commit();
session.close();
}
}
MySQL 表
