第一大坑：Caused by: java.sql.SQLException: Error setting driver on UnpooledDataSource. Cause: java.lang.ClassNotFoundException: Cannot find class: com.mysql.jdbc.Driver image.png

在排错时，发现Mysql 5.x版本8.X连接DB的环境配置有很大差别，所以我们要首先匹配版本号，具体连接区别可以参考： https://www.cnblogs.com/jyroy/p/11276152.html

连接好后，我的环境是：
Mysql5.7.19
mysql-connector-java 5.1.47
jdk 1.8

网上搜到的关于Cannot find class: com.mysql.jdbc.Driver问题一般都是：

1，dependcy version不匹配，比如Mysql8.0的用了mysql-connector-java5.1的驱动；
2，mybatis-config.xml里的url字符串配错，比如多了一个空格
3，或者你的XXXMapper.xml的namespace路径错误。

我仔细检查了这三个问题，发现都是正确。配置文件如下： pom.xml

<?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">
    <parent>
        <artifactId>mybatis-sky</artifactId>
        <groupId>com.kuang</groupId>
        <version>1.0-SNAPSHOT</version>
    </parent>
    <modelVersion>4.0.0</modelVersion>

    <artifactId>mybatis-2</artifactId>

    <properties>
        <maven.compiler.source>8</maven.compiler.source>
        <maven.compiler.target>8</maven.compiler.target>
    </properties>
    <!--资源引入标签-->
    <build>
        <resources>
            <resource>
                <directory>src/main/java</directory>
                <includes>
                    <include>**/*.properties</include>
                    <include>**/*.xml</include>
                </includes>
                <filtering>true</filtering>
            </resource>
            <resource>
                <directory>src/main/resources</directory>
                <includes>
                    <include>**/*.properties</include>
                    <include>**/*.xml</include>
                </includes>A
                <filtering>true</filtering>
            </resource>
        </resources>
    </build>

</project>

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.jdbc.Driver"/>
                <property name="url" value="jdbc:mysql://localhost:3306/mybatis?useSSL=true&amp;useUnicode=true&amp;characterEncoding=utf8"/>
                <property name="username" value="root"/>
                <property name="password" value="123456"/>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <mapper resource="com/kuang/mapper/UserMapper.xml"/>
    </mappers>

</configuration>

UserMapper.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="com.kuang.mapper.UserMapper">
    <select id="selectUser" resultType="com.kuang.pojo.User">
        select * from user
    </select>
</mapper>

可以发现都是正确的，但是进行测试依然会出现找不到驱动的场面。

我也是偶然发现解决方法，应该是属于IDE的问题，我用的是InteliJ Idea。其实我们进到UserMapper.xml里

image-20210729214151997.png

会发现select标签是黄色的，有一个黄色warning，点开小灯泡：

image-20210729214244126.png

选configure data source：

image-20210729214353837.png

会看到Drivers里面的Mysql5.1，如果这里你的驱动没装，会显示红色，你直接选择安装即可。

等你回去运行，你会发现，之前的driver缺失问题解决了！

接下来就会遇到第二个大坑：

第二个大坑：error parsing sql mapper configuration. cause: java.io.ioexception: could not find resource com/kuang/dao/usermapper.xml

我已经在mybatis-config.xml里配置率mappers，他依然说找不到，其实就是没有加资源，等加入后就能解决了，这串代码也能解决resources文件夹里资源不被识别的问题。

<build>
        <resources>
            <resource>
                <directory>src/main/java</directory>
                <includes>
                    <include>**/*.properties</include>
                    <include>**/*.xml</include>
                </includes>
                <filtering>true</filtering>
            </resource>
            <resource>
                <directory>src/main/resources</directory>
                <includes>
                    <include>**/*.properties</include>
                    <include>**/*.xml</include>
                </includes>A
                <filtering>true</filtering>
            </resource>
        </resources>
    </build>

注意一点：filtering标签里面的属性值是false。

本文视频参考：https://www.bilibili.com/read/cv5702420

课堂笔记参考：https://www.cnblogs.com/renxuw/p/13047424.html