UserDao userDao = sqlSession.getMapper(UserDao.class);

<?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.itheibai.dao.UserDao">
    <!--查询用户-->
    <select id="findAll" resultType="user">
        select * from user
    </select>
    <!--插入用户-->
    <insert id="save" parameterType="user">
        insert into user values (#{id},#{username},#{password})
    </insert>
    <!--更新用户-->
    <update id="updateById" parameterType="user">
        update user set username=#{username},password=#{password} where id=#{id}
    </update>
    <!--删除用户-->
    <delete id="deleteById" parameterType="Integer">
        delete from user where id=#{id}
    </delete>
</mapper>

package com.itheibai.service;
import com.itheibai.dao.UserDao;
import com.itheibai.entity.User;
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 java.io.IOException;
import java.io.InputStream;
import java.util.List;
public class ServiceDemo {
    public static void main(String[] args) throws IOException {
        InputStream resourceAsStream = Resources.getResourceAsStream("sqlMapConfig.xml");
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resourceAsStream);
        SqlSession sqlSession = sqlSessionFactory.openSession();
        UserDao userDao = sqlSession.getMapper(UserDao.class);
        List<User> all = userDao.findAll();
        System.out.println(all);
    }
}