xml dtd的格式：
<?xml version="1.0" encoding="UTF-8"?>
<!ELEMENT classroom (className,grade,students)>
<!ATTLIST classroom id ID #REQUIRED>
<!ELEMENT className (#PCDATA)>
<!ELEMENT grade (#PCDATA)>
<!ELEMENT students (student+)>
<!ELEMENT student (id,name,age)>
<!ELEMENT id (#PCDATA)>
<!ELEMENT name (#PCDATA)>
<!ELEMENT age (#PCDATA)>
相应的xml文件：
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE classroom SYSTEM "classroom.dtd">
<!--<!DOCTYPE classroom PUBLIC "http://www.zizhu.com/classroom">-->
<classroom id="p1">
<className>2010计算机应用技术</className>
<grade>2010</grade>
<students>
<student>
<id>11</id>
<name>zizhu8</name>
<age>20</age>
</student>
<student>
<id>12</id>
<name>zizhu3</name>
<age>20</age>
</student>
</students>
</classroom>
