文档根元素 "mapper" 必须匹配 DOCTYPE 根 "configuration"

解决方法：

把下面的 DOCTYPE 的头：configuration 改为 mapper 即可：（一般是由于直接从某个地方拷过来的，没有更改 configuration 造成的）

<?xml version="1.0" encoding="utf-8"?><!DOCTYPE configurationPUBLIC "-//mybatis.org//DTD Config 3.0//EN"  "http://mybatis.org/dtd/mybatis-3-mapper.dtd"><mapper namespace="test"><select id="findUserById" parameterType="int" resultType="com.mx.dao.user">select * from user where id =#{id}</select></mapper>

改为如下形式：

<?xml version="1.0" encoding="utf-8"?><!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Config 3.0//EN"  "http://mybatis.org/dtd/mybatis-3-mapper.dtd"><mapper namespace="test"><select id="findUserById" parameterType="int" resultType="com.mx.dao.user">select * from user where id =#{id}</select></mapper>