class.dtd
<?xml version="1.0" encoding="UTF-8"?>
<!ELEMENT classrooms (classroom+)>
<!ELEMENT classroom (grade,classname,students)>
<!ATTLIST classroom id ID #REQUIRED>
<!ELEMENT classname (#PCDATA)>
<!ELEMENT grade (#PCDATA)>
<!ELEMENT students (student+)>
<!ELEMENT student (id,studentname,age)>
<!ELEMENT id (#PCDATA)>
<!ELEMENT studentname (#PCDATA)>
<!ELEMENT age (#PCDATA)>
class.xml
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE classrooms SYSTEM "class.dtd">
<classrooms>
<classroom id="c1">
<grade>2010</grade>
<classname>10级计算机应用技术一班</classname>
<students>
<student>
<id>1</id>
<studentname>宋发准</studentname>
<age>12</age>
</student>
<student>
<id>1</id>
<studentname>宋发准</studentname>
<age>12</age>
</student>
</students>
</classroom>
<classroom id="c2">
<grade>2010</grade>
<classname>10级计算机应用技术二班</classname>
<students>
<student>
<id>2</id>
<studentname>李四</studentname>
<age>22</age>
</student>
<student>
<id>2</id>
<studentname>李四</studentname>
<age>22</age>
</student>
</students>
</classroom>
</classrooms>