Error building SqlSession.
### Cause: org.apache.ibatis.builder.BuilderException: Error creating document instance.  Cause: org.xml.sax.SAXParseException; lineNumber: 2; columnNumber: 16; 文档根元素 "configuration" 必须匹配 DOCTYPE 根 "null"。

解决方法

src/main/resources/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>
    <properties resource="db.properties"/>
    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <!--                JDBC 驱动-->
                <property name="driver" value="${mysql.driver}"/>
                <!--                url数据库的 JDBC URL地址。-->
                <property name="url" value="${mysql.url}"/>
                <property name="username" value="${mysql.username}"/>
                <property name="password" value="${mysql.password}"/>
            </dataSource>
        </environment>
    </environments>

    <mappers>
        <!--引入映射文件-->
        <mapper resource="Mapper/UserMapper.xml"/>
    </mappers>

</configuration>

<?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">

注意一定要从第一行开始写，如果不从第一行会报错

src\main\resources\Mapper\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="org.example.POJO.User">
    <!-- id ="接口中的方法名"
    　　parameterType="传入的参数类型"
    　　resultType = "返回实体类对象，使用包.类名" -->
    <select id="findById" parameterType="int" resultType="org.example.POJO.User"> select * from users where uid = #{id} </select>
</mapper>

<?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">

 注意一定要从第一行开始写，如果不从第一行会报错

然后Rebuld Project