mybatis实现继承映射

ORM框架的优势在于能让我们利用面向对象的思维去操作数据库，hibernate作为重量级的ORM框架对面向对象的支持很强大。作为半自动化的mybatis，对面向对象的支持也是很完备的。这篇文章就来讨论一下如何利用mybatis实现继承映射。

类图

有一个机动车父类，它有两个子类：Car和Bus

 

关系模型(t_vehicle)

ORM映射有一个原则：对象模型细粒度，关系模型粗粒度。所以我们将所有的车都存储一张表里（t_vehicle），通过鉴别字段vType来区分车的类型（"c"代表Car，"b"代表Bus）

三个实体类的代码

Vehicle

package com.tgb.mybatis.model;public class Vehicle {//主键idprivate String id;//车的名字private String name;public String getId() {return id;}public void setId(String id) {this.id = id;}public String getName() {return name;}public void setName(String name) {this.name = name;}}

Car

package com.tgb.mybatis.model;public class Car extends Vehicle {//车门的数量private int carDoor;//车的牌子private String band;public int getCarDoor() {return carDoor;}public void setCarDoor(int carDoor) {this.carDoor = carDoor;}public String getBand() {return band;}public void setBand(String band) {this.band = band;}}

Bus

package com.tgb.mybatis.model;public class Bus extends Vehicle {//公共汽车的容量private int capacity;public int getCapacity() {return capacity;}public void setCapacity(int capacity) {this.capacity = capacity;}}

看看对“车”进行操作的Mapper接口【只关注查询】

package com.tgb.mybatis.data;import com.tgb.mybatis.model.Bus;import com.tgb.mybatis.model.Car;import com.tgb.mybatis.model.Vehicle;public interface VehicleMapper {//根据id查询机动车Vehicle getVechicleById(String id);//根据名字查询小汽车Car getCarByName(String name);}

xml方式ORM映射

<?xml version="1.0" encoding="UTF-8" ?>  <!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd"> <mapper namespace="com.tgb.mybatis.data.VehicleMapper">    <select id="getVechicleById" resultMap="vehicleMap">        SELECT * FROM TB_VEHICLE WHERE VID = #{id}    </select>    <select id="getCarByName" resultMap="vehicleMap">        SELECT * FROM TB_VEHICLE WHERE VTYPE='c' AND VName = #{id}    </select>   <resultMap type="vehicle" id="vehicleMap">       <id property="id" column="vId"/>       <result property="name" column="vName"/>       <discriminator javaType="string" column="vType">           <case value="c" resultType="car">               <result property="carDoor" column="cardoor"/>               <result property="band" column="band"/>           </case>           <case value="b" resultType="bus">               <result property="capacity" column="capacity"/>           </case>       </discriminator>   </resultMap></mapper>

分析

其中最为关键的就是<discriminator>标签中的内容，根据鉴别字段的值自动映射成对应的子类

 

客户端测试代码

VehicleMapper mapper = session.getMapper(VehicleMapper.class);Vehicle vehicle = mapper.getVechicleById("1");System.out.println(vehicle.getName());Car car = mapper.getCarByName("路虎007");System.out.println(car.getBand());

很简单的一个例子，和大家分享一下