需求:根据用户id查询用户信息

一.开发步骤

1.mybatis.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>
	    <!-- 环境配置,和spring整合后environments配置将废除-->
	    <environments default="development">
	        <environment id="development">
	            <transactionManager type="JDBC"/>
	            <!-- 1.配置数据源 -->
	            <dataSource type="POOLED">
	                <property name="driver" value="com.mysql.jdbc.Driver" />
	                <property name="url" value="jdbc:mysql://localhost:3306/mybatis?useSSL=false" />
	                <property name="username" value="root" />
	                <property name="password" value="admin123" />
	            </dataSource>
	        </environment>
	    </environments>
	
	    <!-- 2.在全局配置文件中加载sql映射文件 -->
	    <mappers>
	        <mapper resource="mapper/UserMapper.xml"/>
	    </mappers>
	</configuration>

2.mapper接口

public interface UserMapper {
    User findUserById(Integer id) throws Exception;
}

3.mapper.xml

<?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="mapper.UserMapper">
        <!-- 根据用户id查找用户 -->
        <select id="findUserById" resultType="po.User" parameterType="int">
            select * from user where id=#{id}
        </select>

        <!-- 添加用户 -->
        <insert id="insertUser" parameterType="po.User">
              insert into user(id,username,birthday,sex,address) values(#{id},#{username},#{birthday},#{sex},#{address})
        </insert>

        <!-- 根据id删除用户 -->
        <delete id="deleteUser" parameterType="java.lang.Integer">
            delete from user where id=#{id}
        </delete>

        <!-- 根据id更新用户 -->
        <update id="updateUser" parameterType="po.User">
            update user set username=#{username},birthday=#{birthday},sex=#{sex},address=#{address}
            where id=#{id}
        </update>
    </mapper>

4.po类编写

public class User {
    //属性名要和数据库表的字段对应
    private int id;
    private String username;// 用户姓名
    private String sex;// 性别
    private String birthday;// 生日
    private String address;// 地址
    //get和set方法......
}

5.测试类

// 1.获取mybatis配置文件的路径
String resource = "config/mybatis.xml";
// 2.得到配置文件流
InputStream inputStream = Resources.getResourceAsStream(resource);
// 3.根据mybatis配置文件的信息创建会话工厂
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
SqlSession sqlSession = sqlSessionFactory.openSession();
UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
User user = userMapper.findUserById(1);
System.out.println(user);

二.总结

1.实现原理
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。
2.Mapper接口开发需要遵循以下规范：
a.Mapper.xml文件中的namespace与mapper接口的类路径相同。
b.Mapper接口方法名和Mapper.xml中定义的每个statement的id相同 。
c.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。
d.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。