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
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
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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
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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
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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】
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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
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<?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
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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
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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
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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
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<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
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<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
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<mapper namespace="com.xxx.dao.TagExtendMapper"> <resultMap id="ExtBaseResultMap" type="com.xxx.dao.Tag" extend="com.xxx.dao.mapper.TagMapper.BaseResultMap"> ... </resultMap></mapper> |
以上为个人经验，希望能给大家一个参考，也希望大家多多支持服务器之家。
原文链接：https://blog.csdn.net/wzwenhuan/article/details/36031021
