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());

很简单的一个例子，和大家分享一下

mybatis实现继承映射