传统方式实现

1. 编写UserDao接口

public interface UserDao {
    List<User> findAll() throws IOException;
}

2. 编写映射文件

<?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="userMapper">
    <select id="findAll" resultType="user">
        select * from user
    </select>
</mapper>

3. 编写UserDaoImpl实现

public class UserDaoImpl implements UserDao {
    @Override
    public List<User> findAll() throws IOException {
        //加载核心配置文件
        InputStream inputStream = Resources.getResourceAsStream("sqlMapConfig.xml");
        //获得sqlSession工厂对象
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        //获得sqlSession对象
        SqlSession session = sqlSessionFactory.openSession();
        //执行sql语句
        List<User> userList = session.selectList("userMapper.findAll");

        session.close();
        
        return userList;
    }
}

使用代理的dao层实现

Mapper 接口开发方法只需要程序员编写Mapper 接口（相当于Dao 接口），由Mybatis框架根据接口定义创建接口的动态代理对象，代理对象的方法体同上边Dao接口实现类方法。
Mapper 接口开发需要遵循以下规范：

1. Mapper.xml文件中的namespace与mapper接口的全限定名相同
2. Mapper接口方法名和Mapper.xml中定义的每个statement的id相同
3. Mapper接口方法的输入参数类型和mapper.xml中定义的每个sql的parameterType的类型相同
4. Mapper接口方法的输出参数类型和mapper.xml中定义的每个sql的resultType的类型相同

==一一对应==

测试

    @Test
    public void proxyTest() throws IOException {
        InputStream inputStream = Resources.getResourceAsStream("SqlMapConfig.xml");
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);

        SqlSession sqlSession = sqlSessionFactory.openSession();
        //获得MyBatis框架生成的UserMapper接口的实现类
        UserMapper mapper = sqlSession.getMapper(UserMapper.class);
        List<User> userList = mapper.findAll();

        System.out.println(userList);
    }