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id
  private 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());