一、规范
- XML映射文件的名称要与Mapper接口一致,并且同包同名
- XML映射文件的namespace属性要与Mapper接口的全限定名一致
- XML映射文件中sql语句的id要与Mapper接口中的方法名称一致,并且返回一致的类型
二、实践
首先,同包同名

其次,namespace
最后,sql语句的id要与方法一致,还要返回值
java
//最上面是直接复制就行
<?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="org.example.springbootmybatisqiuckstart2.mapper.UserMapper">
<select id="finduser" resultType="org.example.springbootmybatisqiuckstart2.pojo.User">
select * from user where username=#{username} and age=#{age}
</select>
</mapper>
测试,得出结果
java
@Test
public void testFinduser(){
List<User> userList = userMapper.finduser("xiaoqiao", 18);
System.out.println(userList);
}