Error creating document instance. Cause: org.xml.sax.SAXParseException; lineNumber: 2; columnNumber: 40; 文档根元素 "mapper" 必须匹配 DOCTYPE 根 "null"

创建xml的时候没有这句代码，加上这串代码就可以了

<?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">