XML concise tutorial (5)

Table of Contents

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Development History

##XMLComparison with HTML Extensible

XML Syntax details compared with HTML

XML validation DTD

XMLNamespace

XMLSyntax structure

DOM4JRead and write configuration files

About SLT

XML namespace

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A XML document can contain many elements and attributes. When we use others XML document or use multiple validation files (DTD or Schema ), it is very likely that the same element or attribute name will appear, but the meaning of these elements or attributes is different, similar to java where many classes have the same class name. The specific functions of these classes are different, so in the Java field, people introduced the concept of packages. Classes with the same name under different packages have different functions. , thus effectively distinguishing classes with the same name. Similarly, namespaces are introduced in XML to solve the problem of duplicate names. First look at the following example

<?xml version="1.0" encoding="gb2312" ?>
<books>
	<book>
		<title>JSP深入编程</title>
		<author>
			<name>张三</name>
			<title>作家</title>
		</author>
	</book>
	<book>
		<title>XML从入门到精通</title>
		<author>
			<name>李四</name>
			<title>教师</title>
		</author>
	</book>
</books>

This is A well-formatted XML document about a book, but there is a conflict in the elements. The title element represents both the book title and the Author's title, two elements with the same name but different meanings. This makes XML very confusing for the parser to read and unable to correctly XML validate. To solve this problem we use namespaces to distinguish identical elements. Before that, we first introduce the definition of XML namespace:

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元素名是指在该元素上声明命名空间，在这个元素上声明的名称空间适用于声明它的元素和属性，以及该元素内容中的所有元素及属性prefixname是名称空间的前缀，往往URI比较长所以可以用prefixname来代替URI比如下面的例子

<hr xmlns:hr="http://blog.csdn.net/beijiguangyong">

上面的句子声明了一个名称空间，它的名字是http://www.php.cn/。一般而言这个名称很长，于是就用了hr来表示,作为该名称空间下的元素或者属性可以其在前面加上前缀hr。例如3054b7184ac02298ea7ae5766c4f2c08就表示employee元素属于此命名空间下。所以上面的那个拥有title重复属性的xml为了区分歧义可以写成下面这样

<?xml version="1.0" encoding="utf-8" ?>
<books xmlns:people="http://blog.csdn.net/beijiguangyong">
	<book>
		<title>JSP深入编程</title>
		<author>
			<name>张三</name>
			<title>作家</title>
		</author>
	</book>
	<book>
		<title>XML从入门到精通</title>
		<author>
			<people:name>李四</name>
			<people:title>教师</title>
		</author>
	</book>
</books>

其中people是名称空间http://www.php.cn/的前缀，用于表示相应的元素是属于该命名空间下的。对于上面的XML而言解析器就很容易区分people:title和title的不同。

还有另一种定义名称空间的方式：

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这种声明形式没有给出名称空间的前缀名，我们可以认为URI所标识的是一个默认的名称空间。例如5f9e0a8344348d6297757aae2a36262b表明了一个默认的名称空间，除非元素中有其他的命名空间进行覆盖，否则hr元素及其内容中所有的没有前缀的元素都属于http://www.php.cn/所标识的名称空间

After understanding the namespace, it will be easier to learn to use Schema to verify the correctness of XML.

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