Hibernate 有哪些不同的继承映射策略

有三种继承映射策略:

类层次结构表
具体类表
子类表

在本文中，我们讨论类层次结构表。

类层次结构表

在这种情况下，只创建一个用于继承映射的表。这种方法的缺点是表中存储了很多空值。
@Inheritance(strategy=InheritanceType.SINGLE_TABLE)、@DiscriminatorColumn和@DiscriminatorValue是这个策略中使用的注释。
@DiscriminatorColumn用于创建一个额外的列，用于标识层次结构类。

考虑以下示例以理解此概念：

Hibernate 有哪些不同的继承映射策略

实现步骤如下：

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。
编写代码以创建和存储数据到表中。

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();
   }
}