dtd.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!--引入dtd文档到xml文档中
外部dtd：将约束的规则定义在外部的dtd文件中
    本地：<!DOCTYPE 根标签名 SYSTEM "dtd文件的位置">
    <!DOCTYPE 根标签名 PUBLIC "dtd文件名字" "dtd文件的位置URL">
-->
<!DOCTYPE students SYSTEM "students.dtd">

<!--内部dtd：将约束规则定义在xml文档中-->
<!--<!DOCTYPE students [

		<!ELEMENT students (student+) >
		<!ELEMENT student (name,age,sex)>
		<!ELEMENT name (#PCDATA)>
		<!ELEMENT age (#PCDATA)>
		<!ELEMENT sex (#PCDATA)>
		<!ATTLIST student number ID #REQUIRED>
		]>-->

<students>
    <student number="s1">
        <name>cpu_code</name>
        <age>22</age>
        <sex>男</sex>
    </student>

    <student number="s2">
        <name>cpu</name>
        <age>25</age>
        <sex>女</sex>
    </student>
</students>