mybatis如何实现继承映射

mybatis 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

mybatis如何实现继承映射

有一个机动车父类，它有两个子类：Car和Bus

关系模型(t_vehicle)

mybatis如何实现继承映射

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 xml映射文件的继承问题

1、首先dao层mapper.java需要继承原来的接口　　

原dao层接口

				?

									public interface TagMapper {

									    /**

									     * This method was generated by MyBatis Generator.

									     * This method corresponds to the database table t_tag

									     *

									     * @mbg.generated

									     */

									    long countByExample(TagExample example);

									    /**

									     * This method was generated by MyBatis Generator.

									     * This method corresponds to the database table t_tag

									     *

									     * @mbg.generated

									     */

									    int deleteByExample(TagExample example);

									}

扩展后的dao层接口

				?

									public interface TagExtendMapper extends TagMapper {

									    ...

									}

2、继承原始mapper.xml的结果映射　　

原始mapper.xml的结果映射

				?

									<mapper namespace="com.xxx.dao.mapper.TagMapper">

									  <resultMap id="BaseResultMap" type="com.xxx.dao.Tag">

									    <!--

									      WARNING - @mbg.generated

									      This element is automatically generated by MyBatis Generator, do not modify.

									    -->

									    <id column="id" jdbcType="CHAR" property="id" />

									    <result column="tag_name" jdbcType="VARCHAR" property="tagName" />

									    <result column="tag_alias" jdbcType="VARCHAR" property="tagAlias" />

									  </resultMap>

									</mapper>

扩展mapper.xml的结果映射

				?

									<mapper namespace="com.xxx.dao.TagExtendMapper">

									    <select id="xxxxx" resultMap="com.xxx.dao.mapper.TagMapper.BaseResultMap"> <!-- 这里时原始命名空间加上结果集id -->

									    </select>

									</mapper>

或者是

				?

									<mapper namespace="com.xxx.dao.TagExtendMapper">

									   <resultMap id="ExtBaseResultMap" type="com.xxx.dao.Tag" extend="com.xxx.dao.mapper.TagMapper.BaseResultMap">

									       ...

									  </resultMap>

									</mapper>