编程改变世界（6）-- mybatis入门学习

mybatis是一个ORM（Object Relational Mapping）框架，即对象关系映射框架。

简单来说，就是将数据库中的表对应成java程序中的bean实体类，数据库中的字段对应实体类中的属性

那么如何创建一个最基本的mybatis项目呢，这次我们使用idea集成开发工具，首先先建立一个maven工程，在pom.xml中添加相应的依赖

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>com.miracle</groupId>
    <artifactId>t</artifactId>
    <version>1.0-SNAPSHOT</version>

    <dependencies>
        <dependency>
            <groupId>org.mybatis</groupId>
            <artifactId>mybatis</artifactId>
            <version>3.5.0</version>
        </dependency>
        <!-- https://mvnrepository.com/artifact/mysql/mysql-connector-java -->
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>8.0.12</version>
        </dependency>
    </dependencies>

</project>

然后呢，mybatis需要一个核心配置文件，名字可以任意起，我将它命名为mybatis.xml,具体内容如下

<?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="jdbcConfig.properties"></properties>
    <typeAliases>
        <typeAlias type="com.pojo.Teacher" alias="Teacher"/>
    </typeAliases>
    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="${jdbc.driverClassName}"/>
                <property name="url" value="${jdbc.url}"/>
                <property name="username" value="${jdbc.username}"/>
                <property name="password" value="${jdbc.password}"/>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <mapper resource="mapper/teacherMapper.xml"/>
    </mappers>
</configuration>

jdbc.properties 数据库的配置如下

jdbc.driverClassName=com.mysql.cj.jdbc.Driver
jdbc.url=jdbc:mysql://localhost:3306/studb?serverTimezone=UTC&useSSL=false
jdbc.username=root
jdbc.password=root

我们需要在resource下建立一个mapper文件（teacherMapper）,因为java中的属性和数据库中的字段不同，所以返回值为resultMap

<?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.teacherMapper">
    <select id="findTeacher" resultMap="teacherMap">
        select * from tb_teacher
    </select>
    
    <resultMap id="teacherMap" type="Teacher">
        <id property="id" column="id"/>
        <result property="name" column="t_name"/>
        <result property="role" column="role"/>
    </resultMap>
</mapper>

然后还需要一个pojo的teacher实体类

package com.pojo;

public class Teacher {

    private int id;
    private String name;
    private String role;

    public Teacher() {
    }

    public Teacher(int id, String name, String role) {
        this.id = id;
        this.name = name;
        this.role = role;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getRole() {
        return role;
    }

    public void setRole(String role) {
        this.role = role;
    }

    @Override
    public String toString() {
        return "Teacher [id=" + id + ", name=" + name + ", role=" + role + "]";
    }
    
    
}

数据库的表如下

image.png

下一步，我们需要一个工具类，MybatisUtil，主要的功能就是获取到SqlSession

package com.util;

import com.pojo.Classes;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;

import java.io.IOException;
import java.io.InputStream;


public class MyBatisUtil {

    public static  SqlSession getSqlSession(){
        String resource = "mybatis.xml";
        InputStream is = null;
        try {
            is  = Resources.getResourceAsStream(resource);
        } catch (
                IOException e) {
            e.printStackTrace();
        }

        SqlSession sqlSession = new SqlSessionFactoryBuilder().build(is).openSession(true);
        return sqlSession;
    }
}

下一步我们就可以写Dao类了，命名为teacherDao

package com.dao;

import com.pojo.Teacher;
import com.util.MyBatisUtil;
import org.apache.ibatis.session.SqlSession;

import java.util.List;

public class TeacherDao {
    private SqlSession sqlSession = MyBatisUtil.getSqlSession();
    private String sql = "mapper.teacherMapper.";
    public List<Teacher> findTeacher(){
        sql +="findTeacher";
        List<Teacher> list = sqlSession.selectList(sql);
        return list;
    }
}

最后我们来测试一下结果

package com.test;


import com.dao.TeacherDao;
import com.pojo.Teacher;


import java.util.List;

public class Test {

    public static void main(String[] args) {

        TeacherDao teacherDao = new TeacherDao();
        List<Teacher> list = teacherDao.findTeacher();
        for (Teacher teacher : list) {
            System.out.println(teacher);
        }


    }
}

结果如下图所示

image.png