有三种类型的继承映射策略 -

  • 每个类层次结构的表
  • 每个混凝土等级的桌子
  • 每个子类的表
    在本文中，我们将讨论每个类层次结构的表。

每个类层次结构的表

  • 在此中，仅为继承映射创建一个表。此方法的缺点是表中存储了许多空值。
  • @Inheritance（策略=InheritanceType.SINGLE_TABLE）、@DiscriminatorColumn和@DiscriminatorValue是此策略中使用的注释。
  • @DiscriminatorColumn用于创建用于标识层次结构类的附加列。

考虑以下示例来理解这一点 -

Hibernate 中有哪些可用的继承映射策略？_java

实施步骤 -

  • 创建实体类并为其使用适当的批注。
  • 编写休眠配置文件并添加映射类。
  • 编写代码以在表中创建和存储数据。

1.创建实体类

车.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. 休眠配置文件（休眠.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. 创建表和存储数据的代码


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

Hibernate 中有哪些可用的继承映射策略？_mysql_02