解读Mapper与Mapper.xml文件之间匹配的问题

public class User {
    private Integer id;
    private String username;
    private String password;
    private String email;
    private String phone;

<?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="wdc.dao.UserMapper">
    <resultMap id="BaseResultMap" type="wdc.pojo.User">
        <!-- column为数据库字段名  property 为实体类字段名-->
        <id column="id" property="id" jdbcType="INTEGER"/>
        <result column="user_name" property="username" jdbcType="VARCHAR"/>
        <result column="user_password" property="password" jdbcType="VARCHAR"/>
        <result column="user_email" property="email" jdbcType="VARCHAR"/>
        <result column="user_phone" property="phone" jdbcType="VARCHAR"/>

    </resultMap>
    <!-- 这里 id 是数据库字段名 #{id}是实体类字段名，sql语句后面不加封号-->
    <select id="selectUser" parameterType="java.lang.Integer" resultMap="BaseResultMap">
         select *from USER where id=#{id}
    </select>
    <!--此sql用于登录查询
    #{username} 是对应mapper接口文件中 @Param注解中的参数名（防止mapper之间不匹配）-->
    <select id="login" parameterType="java.lang.String" resultMap="BaseResultMap">
        select *from user where user_name=#{username} and user_password=#{password}
    </select>
</mapper>

  <!-- 这里 id 是数据库字段名 #{id}是实体类字段名，sql语句后面不加封号-->
  <select id="selectUser" parameterType="java.lang.Integer" resultMap="BaseResultMap">

 <!-- #{username} 是对应mapper接口文件中 @Param注解中的参数名（防止mapper之间不匹配）-->
   <select id="login" parameterType="java.lang.String" resultMap="BaseResultMap">
       select *from user where user_name=#{username} and user_password=#{password}
   </select>

package wdc.dao;

import org.apache.ibatis.annotations.Param;
import wdc.pojo.User;

public interface UserMapper {
    User selectUser(int id)throws Exception;
    //    登录
    //    @Param中的参数名，对应.xml文件中sql语句#{}中的参数名（防止mapper之间不匹配）
    User login(@Param("username") String username, @Param("password") String password)throws Exception;
}

@Test
    public void testLogin() throws Exception{
        ApplicationContext ac = new ClassPathXmlApplicationContext("classpath:config/spring-mybatis.xml");
        UserMapper userMapper = (UserMapper) ac.getBean("userMapper");
        User user = null;

            user = userMapper.login("张","123");
            if(user == null)
                System.out.println("没数据");
            else
                System.out.println(user);


    }