XML structure and wording dtd constraints

XML front two rows

The first row and the version number encoded
second line specifies constraints dtd

<?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">

<?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">

DTD constraints

*  >=0
+  >=1
?  <=1 

//表示最少一个mappe大标签   mapper大标签里有9种子标签
<!ELEMENT mapper (cache-ref | cache | resultMap* | parameterMap* | sql* | insert* | update* | delete* | select* )+>

//表示有且只有一个configuration大标签
<!ELEMENT configuration (properties?, settings?,  plugins?, environments?,  mappers?)>