classroom.dtd文件：<?xml version="1.0" encoding="UTF-8"?>
<!ELEMENT classroom (claName,grade,students)>
<!ATTLIST classroom id ID #REQUIRED>
<!ELEMENT claName (#PCDATA)>
<!ELEMENT grade (#PCDATA)>
<!ELEMENT students (student+)>
<!ELEMENT student (id,stuName,age)>
<!ELEMENT id (#PCDATA)>
<!ELEMENT stuName (#PCDATA)>
<!ELEMENT age (#PCDATA)>
classroom.xml文件：<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE classroom SYSTEM "classroom.dtd">
<classroom id="c1">
<claName>10计算机应用技术</claName>
<grade>2010</grade>
<students>
<student>
<id>1</id>
<stuName>zhangsan</stuName>
<age>12</age>
</student>
<student>
<id>2</id>
<stuName>lisi</stuName>
<age>122</age>
</student>
</students>
</classroom>