如何在xml文件中引入约束

1、先搞清楚为啥要对xml编写进行约束？

<?xml version="1.0" encoding="utf-8" ?>
<?xml-stylesheet type="text/css" href="a.css" ?>

<users>

    <user id="zhangsan">
        <name>张三</name>
        <age>21</age>
        <gender>男</gender>
        <gender>女</gender>
    </user>

</users>

2、言归正传，如何导入约束呢？

2.1、DTD约束

<?xml version="1.0" encoding="UTF-8" ?>
<!--方式一：外部dtd文件-->
<!DOCTYPE students SYSTEM "D:\Idea-workspace1\JavaWeb\06_xml\src\com\lin\dtd\student.dtd">

<!--方式二：内部dtd约束文档-->
<!--<!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="s001">
		<name>张三</name>
		<age>23</age>
		<sex>男</sex>
	</student>

	<student number="s002">
		<name>张三</name>
		<age>23</age>
		<sex>男</sex>
	</student>
</students>

2.2、Schema约束

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE students SYSTEM "D:\Idea-workspace1\JavaWeb\06_xml\src\com\lin\dtd\student.dtd">


<students>

	<student number="s001">
		<name>张三</name>
		<age>10000</age>
		<sex>男</sex>
	</student>

</students>

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

 <!--导入Schema约束：-->
 <students   xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
			 xsi:schemaLocation="http://www.itcast.cn/xml  student.xsd"
			  xmlns="http://www.itcast.cn/xml"
 		    >
 	<student number="heima_0001">
 		<name>tom</name>
 		<age>18</age>
 		<sex>male</sex>
 	</student>
	
	<student number="heima_0002">
 		<name>jane</name>
 		<age>22</age>
 		<sex>female</sex>
 	</student>

 </students>

导入Schema约束步骤解析：

 <students   xmlns="http://www.itcast.cn/xml"
 		    >


 </students>

 <students   xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
			 xsi:schemaLocation="http://www.itcast.cn/xml  student.xsd"
 		    >


 </students>

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

 <!--导入Schema约束：-->
 <a:students   xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
			 xsi:schemaLocation="
                    http://www.itcast.cn/xml  student.xsd
                    http://www.itcast.cn/xml2  D:\...\student.xsd"
			 xmlns:a="http://www.itcast.cn/xml"
             xmlns:a="http://www.itcast.cn/xml2"
 		    >
 	<a:student number="heima_0001">
 		<a:name>tom</a:name>
 		<a:age>18</a:age>
 		<a:sex>male</a:sex>
 	</a:student>
	
	<b:student number="heima_0002">
 		<b:name>jane</b:name>
 		<b:age>22</b:age>
 		<b:sex>female</b:sex>
 	</b:student>

 </a:students>