MyBatis测试resultMap，分步查询以及延迟加载

2024-12-06 技术教程

什么是resultMap？

resultMap即自定义结果集映射规则，之前我们使用的resultType是MyBatis为我们提供的默认映射规则，使用方法如下：

<mapper namespace="com.zgz.MyBatis.dao.DeptMapper"> <select id="getDeptById" resultType="com.zgz.MyBatis.bean.Dept"> select id, dept_name deptName from tbl_dept where id = #{id} </select></mapper>

那么resultMap的作用是什么呢？之前我们在处理数据库中字段与javabean中的属性不一致的问题时，采用的方法是起别名或者开启mybatis的自动驼峰映射，现在可以使用resultMap来做，另外可以使用解决级联查询的问题

如何使用resultMap？

我们现在新建一张员工部门表，要求是在我们查询员工的同时查询出员工对应的部门？
首先给出对应的JavaBean：

public class Dept { private Integer id; private String deptName; //get，set，tostring()。。。}

写上相应的mapper：

<mapper namespace="com.zgz.MyBatis.dao.DeptMapper"> <select id="getDeptById" resultType="com.zgz.MyBatis.bean.Dept"> select id, dept_name deptName from tbl_dept where id = #{id} </select></mapper>

在SQL映射文件中进行配置：

<?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.zgz.MyBatis.dao.EmployeeMapperPlus">  <resultMap type="com.zgz.MyBatis.bean.Employee" id="MyEmp">  <id column="id" property="id"/>  <result column="last_name" property="lastName"/>  <result column="email" property="email"/> <result column="gender" property="gender"/> </resultMap>  <select id="getEmployeeByGender" resultMap="MyEmp"> select * from tbl_employee where gender = #{gender} </select>   <resultMap type="com.zgz.MyBatis.bean.Employee" id="MySecond"> <id column="id" property="id"/> <result column="last_name" property="lastName"/> <result column="gender" property="gender"/> <result column="email" property="email"/> <result column="did" property="dept.id"/> <result column="dept_name" property="dept.deptName"/> </resultMap>  <resultMap type="com.zgz.MyBatis.bean.Employee" id="MyThird"> <id column="id" property="id"/> <result column="last_name" property="lastName"/> <result column="gender" property="gender"/> <result column="email" property="email"/>  <association property="dept" javaType="com.zgz.MyBatis.bean.Dept"> <id column="did" property="id"/> <result column="dept_name" property="deptName"/> </association> </resultMap>  <resultMap type="com.zgz.MyBatis.bean.Employee" id="MyFourth"> <id column="id" property="id"/> <result column="last_name" property="lastName"/> <result column="gender" property="gender"/> <result column="email" property="email"/>  <association property="dept" select="com.zgz.MyBatis.dao.DeptMapper.getDeptById" column="d_id"> </association> </resultMap>  <select id="getEmpAndDept" resultMap="MyThird"> SELECT e.id id, e.last_name last_name,e.email email, e.gender gender, e.d_id d_id, d.id did, d.dept_name dept_name FROM tbl_employee e, tbl_dept d WHERE e.d_id = d.id AND e.id = #{id} </select>  <select id="getEmpByIdStep" resultMap="MyFourth"> select * from tbl_employee where id = #{id} </select> </mapper>

在主配置文件中配置：

<configuration>  <settings>  <setting name="lazyLoadingEnabled" value="true"/> <setting name="aggressiveLazyLoading" value="false"/> </settings> <environments default="development"> <environment id="test"> <transactionManager type=""></transactionManager> <dataSource type=""></dataSource> </environment> <environment id="development"> <transactionManager type="JDBC" /> <dataSource type="POOLED"> <property name="driver" value="com.mysql.jdbc.Driver" /> <property name="url" value="jdbc:mysql://localhost:3306/mybatis?useSSL=false" /> <property name="username" value="root" /> <property name="password" value="123456" /> </dataSource> </environment> </environments>  <mappers> <mapper resource="testEmployeeMapperPlus.xml" /> <mapper resource="testDeptMapper.xml" /> </mappers></configuration>注意事项(填了好长时间的坑)

1》千万不要忘记将我们写好的sql映射文件注册到全局配置文件中，注意是所有的sql映射文件

<mappers> <mapper resource="testEmployeeMapperPlus.xml" /> <mapper resource="testDeptMapper.xml" /> </mappers>

2》configuration里的标签必须按顺序写，标签是有顺序的