Mapper代理开发需要遵循的规则：

接口与对应的映射配置文件同名同目录；
映射配置文件中的namespace值等于接口的全路径；
映射配置文件中SQL声明的ID与接口中的函数名相同；
映射配置文件中SQL声明中的parameterType类型与接口函数的参数类型一致；
映射配置文件中SQL声明中的resultType类型与接口函数的返回值类型一致。

过程

创建pojo类对象并生成getter、setter、无参、有参、toString方法
导入对应Jar包

创建config的source文件夹

文件夹内创建MyBatis配置文件mybatis-config.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
  PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
  "http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
  <properties resource="db.properties"></properties>
  <!-- 数据库连接配置文件路径 -->
  <environments default="development">
    <environment id="development">
      <transactionManager type="JDBC"/>
      <dataSource type="POOLED">
        <property name="driver" value="${driver}"/>
        <property name="url" value="${url}"/>
        <property name="username" value="${username}"/>
        <property name="password" value="${password}"/>
      </dataSource>
    </environment>
  </environments>
  <mappers>
    <mapper resource="com/demo/mapper/UserMapper.xml"/> 
    <!-- 类实现接口的xml配置文件路径 -->
  </mappers>
</configuration>

创建数据库连接配置文件db.properties

driver=com.mysql.cj.jdbc.Driver	#驱动名
url=jdbc:mysql://127.0.0.1:3306/test?useUnicode=True&characterEncoding=UTF-8	#数据库连接url
username=root	#用户名
password=root	#密码

创建pojo类所需的接口方法UserMapper.java

package com.demo.mapper;

import java.util.List;

import com.demo.pojo.User;

public interface UserMapper {

	public User findUserById(String userid);
	public List<User> findUserByName(String username);
	public void insertUser(User user);
	public void deleteUser(String userid);
	public void updateUser(User user);
}

User类配置文件实现接口方法UserMapper.xml

<!DOCTYPE mapper
    PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
    "http://mybatis.org/dtd/mybatis-3-mapper.dtd">

<mapper namespace="com.demo.mapper.UserMapper">
  <!--  -->
	<select id="findUserById" parameterType="java.lang.String" resultType="com.demo.pojo.User">
    <!--parameterType为键入参数类型，resultType为结果参数类型 -->
	  SELECT * FROM user WHERE userid = #{userid}
	</select>

	<select id="findUserByName" parameterType="java.lang.String" resultType="com.demo.pojo.User">
	  SELECT * FROM user WHERE username like #{username}
	</select>

	<insert id="insertUser" parameterType="com.demo.pojo.User">
		  INSERT INTO user (userid, username, gender, birthday, height, phone, address)
  VALUES (#{userid},#{username},#{gender},#{birthday},#{height},#{phone},#{address})
	</insert>

	<update id="updateUser" parameterType="com.demo.pojo.User">
		  UPDATE user SET username = #{username}, gender = #{gender}, birthday = #{birthday}, 
		  height = #{height}, phone = #{phone}, address = #{address} WHERE userid = #{userid}
	</update>

	<delete id="deleteUser" parameterType="java.lang.String">
		DELETE FROM user WHERE userid = #{userid}
	</delete>

</mapper>

创建测试类

package com.demo.test;

import static org.junit.jupiter.api.Assertions.*;

import java.io.InputStream;
import java.util.Date;
import java.util.List;

import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import org.junit.jupiter.api.AfterAll;
import org.junit.jupiter.api.AfterEach;
import org.junit.jupiter.api.BeforeAll;
import org.junit.jupiter.api.BeforeEach;
import org.junit.jupiter.api.Test;

import com.demo.mapper.UserMapper;
import com.demo.pojo.User;

class test {

  //资源名
	static String resource = null;
  //输入流
	static InputStream inputStream = null;
  //sql会话工厂对象
	static SqlSessionFactory sqlSessionFactory = null;
  //sql会话对象
	static SqlSession sqlSession = null;
  //mapper映射对象
	static UserMapper mapper = null;

	@BeforeAll
	static void setUpBeforeClass() throws Exception {

		resource = "mybatis-config.xml";
		inputStream = Resources.getResourceAsStream(resource);
    //new SqlSessionFactoryBuilder().build(inputStream);
		sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);

	}

	@AfterAll
	static void tearDownAfterClass() throws Exception {
	}

	@BeforeEach
	void setUp() throws Exception {
		sqlSession = sqlSessionFactory.openSession();
		mapper = sqlSession.getMapper(UserMapper.class);	//接口的字节码文件

	}

	@AfterEach
	void tearDown() throws Exception {
		sqlSession.close();
	}

	@Test
	void findUserById(String userid) {
		User user = mapper.findUserById(userid);
		System.out.println(user);
	}

	@Test
	void findUserByName() {
		List<User> users = mapper.findUserByName("%王%");
		for(User user :users) {
			System.out.println(user);
		}
	}

	@Test
	void insertUser() {
		Date birthday = new Date();

		User user = new User();
		user.setUserid("110");
		user.setUsername("test");
		user.setGender("男");
		user.setBirthday(birthday);
		user.setHeight(190.0);
		user.setPhone("110");
		user.setAddress("广东");

		mapper.insertUser(user);
		sqlSession.commit();

	}

	@Test
	void updateUser() {
		Date birthday = new Date();

		User user = new User();
		user.setUserid("110");
		user.setUsername("done");
		user.setGender("男");
		user.setBirthday(birthday);
		user.setHeight(200.0);
		user.setPhone("120");
		user.setAddress("广东");

		mapper.updateUser(user);
		sqlSession.commit();

	}

	@Test
	void deleteUser() {
		mapper.deleteUser("110");
		sqlSession.commit();
	}

}

踩过的坑

mybatis-config.xml中针对UserMapper.xml的配置，包路径用"/"分割。
config已设置为source文件夹，因此可以直接在测试类中用resource = "mybatis-config.xml";
mapper = sqlSession.getMapper(UserMapper.class); 此处应该用接口文件名+class字节码后缀（编译后）。
接口名需与xml中id名一致。

参考资料

MyBatis