用dtd对xml文件进行约束

<?xml version="1.0" encoding="UTF-8"?>

<!DOCTYPE stus SYSTEM "stus.dtd">

<stus>
    <stu id="stu1">
        <name>张三</name>
        <age>23</age>
        <address>上海</address>
    </stu>
    
    <stu id="stu2">
        <name>李四</name>
        <age>19</age>
        <address>南京</address>
    </stu>
</stus>

<!ELEMENT stus (stu+)>
<!ELEMENT stu (name,age,address)>
<!ELEMENT name (#PCDATA)>
<!ELEMENT age (#PCDATA)>
<!ELEMENT address (#PCDATA)>
<!ATTLIST stu id ID #IMPLIED>