Document root element "mapper", must match DOCTYPE root "null"报凑

最近遇到项目中用mybatis，项目一直报错，Document root element "mapper", must match DOCTYPE root "null"，反复检查了mapper配置，一直找不到哪里配置错了，最后发现少了mapper的配置头。


其实在configuration的配置文件中，头是这样的：

<?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">

而在mapper.xml中配置文件头是这样的，而且不能缺：
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">