MyBatis实现Mapper配置并查询数据

1.准备数据源

# 删除mybatis_demo数据库
drop database if exists mybatis_demo;

# 创建mybatis_demo数据库
create database mybatis_demo;

# 使用mybatis_demo数据库
use mybatis_demo;

# 创建account表
create table user (
    id int auto_increment primary key,
    username varchar(20),
    age int,
    score int
);

# 新增数据
insert into user (id, username, age, score) values
(1,'peter', 18, 100), (2,'pedro', 24, 200),
(3,'jerry', 28, 500), (4,'mike', 12, 300),
(5,'tom', 27, 1000);

2.mapper配置

在mybatis-config.xml配置文件中添加上对应的mapper配置

<!-- mapper配置 -->
<mappers>
    <mapper class="mapper.UserMapper"/>
</mappers>

3.新建mapper包，并在其下新建UserMapper.java类

package mapper;

public interface UserMapper {
    /**
     * 通过用户id查询用户名称
     *
     * @param id 用户id
     * @return 用户名称
     */
    String selectUsernameById(Integer id);
}

为该方法添加上对应的SQL语句

@Select("SELECT username FROM user WHERE id = #{id}")

4.新建User模块测试类：UserTest.java

@SuppressWarnings({"Duplicates"})
public class UserTest {
    public static void main(String[] args) throws IOException, SQLException {
        // 读取配置文件
        InputStream configuration = Resources.getResourceAsStream("mybatis-config.xml");
        // 得到 SqlSessionFactory 核心类
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(configuration);
        // 开始一个 sql 会话
        SqlSession session = sqlSessionFactory.openSession();
        // 得到 mapper
        UserMapper mapper = session.getMapper(UserMapper.class);
        // 调用注解的SQL
        String username = mapper.selectUsernameById(1);
        System.out.println("username: " + username);
        // 关闭会话
        session.close();
    }
}

5.XML方式使用Mapper
在mybatis-config.xml配置文件中再新增一个方法selectUserAgeById，

/**
 * 通过用户id查询用户年龄
 *
 * @param id 用户id
 * @return 用户年龄
 */
Integer selectUserAgeById(Integer id);

6.在资源文件夹下新建mapper包，并在其下新建名为UserMapper的XML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="mapper.UserMapper">
</mapper>