1、mybatis入门

项目中一直使用mybatis，一直在用，但是原理不甚了解，最近抽时间深入学习下，准备写一系列mybatis文章，记录下。

一、mybatis是什么？

mybatis是持久化框架。啥是持久化？可以简单的理解：把内存某一刻的状态持久化到文件（照片就是把某某一刻的状态记录下来）。框架就是把重复性工作抽象出来，大家开发只关注业务逻辑即可。具体详细的解释也不唠叨，大家可以关注官网mybatis；其实学习新的一门课程还是建议大家先去官网学习，不过有些官网的文档实在不敢恭维。

二、mybatis入门

任何一门技术的学习我们都先用它实现下helloworld。本人使用的是idea(2019.1.3） + maven(3.6.0）+ jdk（1.8）构建的项目。
github

使用mybatis有两种方式：1、使用xml配置。2、纯java配置（无xml）
pom配置如下就可以运行项目了：

    <!--mysql驱动-->
    <dependency>
      <groupId>mysql</groupId>
      <artifactId>mysql-connector-java</artifactId>
      <version>8.0.17</version>
    </dependency>
    <!--mybatis-->
    <dependency>
      <groupId>org.mybatis</groupId>
      <artifactId>mybatis</artifactId>
      <version>3.5.4</version>
    </dependency>

1、使用xml配置

上面图片可知使用mybatis代码主要有那些核心的组成：
①model；②Mapper；③Mapper.xml(非必须)；④mybatis-config.xml(核心配置文件)
mybatis-config.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>
    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.cj.jdbc.Driver"/>
                <property name="url" value="jdbc:mysql://127.0.0.1:3306/test?serverTimezone=UTC"/>
                <property name="username" value="root"/>
                <property name="password" value="root"/>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <mapper resource="mapper/UserMapper.xml"/>
        <mapper class="org.imzdong.study.mybatis.domain.AnnotationUserMapper" />
    </mappers>
</configuration>

启动程序：

package org.imzdong.study.mybatis.demo;

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 org.imzdong.study.mybatis.domain.AnnotationUserMapper;
import org.imzdong.study.mybatis.domain.User;
import org.imzdong.study.mybatis.domain.XmlUserMapper;

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

/**
 * @description: xml配置mybatis
 * @author: Winter
 * @time: 2020/4/6
 */
public class XmlMyBatisDemo {

    public static void main(String[] args) {
        String resource = "mybatis-config.xml";
        try {
            InputStream resourceAsStream = Resources.getResourceAsStream(resource);
            SqlSessionFactory build = new SqlSessionFactoryBuilder().build(resourceAsStream);
            try (SqlSession session = build.openSession()) {
                //User user = session.selectOne("org.imzdong.study.mybatis.domain.UserMapper.selectUserById", 1);
                XmlUserMapper mapper = session.getMapper(XmlUserMapper.class);
                User user = mapper.selectUserById(1);
                System.out.println("user: "+user);
                AnnotationUserMapper annotationMapper = session.getMapper(AnnotationUserMapper.class);
                User userAnnotation = annotationMapper.selectUserById(1);
                System.out.println("userAnnotation: "+userAnnotation);
            }

        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

就是这么简单。

2、纯java配置更加简单，贴出代码大家感受下：

package org.imzdong.study.mybatis.demo;

import com.mysql.cj.jdbc.MysqlDataSource;
import org.apache.ibatis.mapping.Environment;
import org.apache.ibatis.session.Configuration;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import org.apache.ibatis.transaction.TransactionFactory;
import org.apache.ibatis.transaction.jdbc.JdbcTransactionFactory;
import org.imzdong.study.mybatis.domain.AnnotationUserMapper;
import org.imzdong.study.mybatis.domain.User;


/**
 * @description: 非xml配置mybatis
 * @author: Winter
 * @time: 2020/4/6
 */
public class JavaConfigMyBatisDemo {

    public static void main(String[] args) {
        //1、获取mysql数据库连接
        String url = "jdbc:mysql://127.0.0.1:3306/test?serverTimezone=UTC";
        String userName = "root";
        String password = "root";
        MysqlDataSource dataSource = new MysqlDataSource();
        dataSource.setUrl(url);
        dataSource.setUser(userName);
        dataSource.setPassword(password);
        //2、构建事务管理工厂
        TransactionFactory transactionFactory = new JdbcTransactionFactory();
        //3、构造环境
        Environment environment = new Environment("development", transactionFactory, dataSource);
        //4、初始化配置
        Configuration configuration = new Configuration(environment);
        configuration.addMapper(AnnotationUserMapper.class);
        //configuration.addMapper(XmlUserMapper.class);
        //5、获取SqlSessionFactory
        SqlSessionFactory build = new SqlSessionFactoryBuilder().build(configuration);
        try (SqlSession session = build.openSession()) {
            AnnotationUserMapper annotationMapper = session.getMapper(AnnotationUserMapper.class);
            User userAnnotation = annotationMapper.selectUserById(1);
            System.out.println("userAnnotation: "+userAnnotation);
            /*XmlUserMapper mapper = session.getMapper(XmlUserMapper.class);
            User user = mapper.selectUserById(1);
            System.out.println("user: "+user);*/
        }
    }
}

3、小结

从java配置demo，详细的看到，mybatis步骤数据库连接、事务控制、配置初始化、获取SqlSessionFactory，就可以操作数据库了，非常简单。mybatis封装了重复的jdbc操作，比如启动连接，关闭连接，事务等，让开发人员仅通过配置搞定这些重复的工作。