XML Tutorial
Uses of XML :
XML stands for Extensible Markup Language. XML can be used in many ways and one of which is 'data storage'. This is the one we will be exploring in this article. XML along with XSL ( Extensible Stylesheet Language ) can by used to present data on the web pages. XML provides the data and XSL allows us to present it the way we want. Remember though that not all browsers support XML on the client side, only Micrsoft Internet Explorer 5.0 and above support XML. XML can also be used to perform RPC ( Remote Procedure Call ). Actually this capability of XML to allow communication between distant applications is so strong that Microsoft has developed SOAP ( Simple Object Access Protocol ) specification which uses XML to allow communication between remote applications. XML can be used for a lot more purposes which I haven't mentioned here. Also keep in mind that Microsoft's future .NET platform will make use of XML even more than any tool does today. ASP+, ADO+ and others will use XML to define and present data. So if you are comfortable with XML today, it will help you in the near future when you will be getting yourself ready to develop applications in the revolutionary platform called Microsoft .NET.
What are XML files ?
If you know HTML then you already know 70% XML. XML is tag based just like HTML. The major difference between HTML and XML is that in XML you can define your own tags while in HTML you make use of pre-defined tags. XML files most often than not begin with following tag on top of the page :
<?xml version="1.0"?>
This line tells the XML parser the version of XML we are using. You for now don't need to change it, just remember to add it on top of every XML page you create.
Every starting XML tag should have a corresponding end tag.
<name>Faisal Khan</name>
If you don't want to write end tag then simply add forward slash in the tag like this :
<br/>
Thus <br> tag of HTML will be written as <br/> in XML.
Elements :
The tags in XML are known as 'elements'. 'elements' may or may not contain any content, thus following are all correct :
<name>Faisal Khan</name>
<name></name>
<name/>
Attributes :
Attributes are the same name / value pairs that you use in HTML. Elements may or may not contain attributes. Following is an example of an element containing an attribute :
<name language="US-EN">Faisal Khan</name>
This was a very simple and basic introduction to XML, on the next page we will see what is the difference between well formed and valid XML documents.
Creating 'Page.xml' file :
We will now create a simple XML file; 'page.xml'. Open notepad and create a new file 'page.xml' and then copy paste the following code into it :
<?xml version="1.0"?>
<main>
<title>Our Page.xml file</title>
<heading>This is a test heading</heading>
<paragraph>This is our paragraph and you can write whatever
you want to in this space.</paragraph>
<testHTML><![CDATA[We will enclose some HTML code in CDATA section now :<br>
<table width="60%" border="1" bordercolor="silver" cellspacing="2"
cellpadding="3">
<tr>
<td>
You can write any HTML code or for that matter any type of text inside
the CDATA section without a fear of getting any error.<br><br>
Note if we write this without the CDATA tags, the xml parser will
raise an error and won't show the document.
</td>
</tr>
</table>]]></testHTML>
</main>
There are certain points to be noted in the code above. Notice that after writing the xml version line we have enclosed our three elements (title, heading, paragraph, testHTML) inside a single tag (main). This is so because in XML after the processing instructions (xml version tag in the beginning) all the elements and sub-elements that we define have to be enclosed within a single element whatever you name it. Thus in our case we defined that single element and named it 'main' which encloses our other 4 elements.
Notice that in the 'testHTML' tag we have put lot of our normal HTML code. XML parser will raise an error when it encounters HTML code which doesn't abide to XML rules. To get through this we make use of <![CDATA[....]]> markup to tell the XML parser that the following text is character data and should not be parsed and evaluated like other XML data is done. CDATA actually stands for 'character data' and allows any kind of characters / text to be written inside it. So remember that whenever you want to enclose HTML inside your XML elements, enclose it between the CDATA markup tags.
Rest of the code is easier to understand. In the first line we defined the processing instruction by telling the XML version to the XML parser. Then we created a 'main' tag element to enclose rest of our XML document. In the 'main' element we created 'title', 'heading', 'paragraph' and 'testHTML' tags to contain some text which will be read by our ASP page.
On the next page we will create the ASP page to read this XML file and show the data to us.
Creating 'showxml.asp' page :
We will now create a very simple ASP page and code it to show the XML data that we created in the 'page.xml' file. Open notepad and create a new ASP page. Then copy paste the following code into it and save it :
<%
Option Explicit
Response.Buffer = True
Dim xml
Set xml = Server.CreateObject("Microsoft.XMLDOM")
xml.async = False
xml.load (Server.MapPath("page.xml"))
Dim title, heading, paragraph, testHTML
title = xml.documentElement.childNodes(0).text
heading = xml.documentElement.childNodes(1).text
paragraph = xml.documentElement.childNodes(2).text
testHTML = xml.documentElement.childNodes(3).text
Set xml = Nothing
%>
<html>
<head>
<title><%= title %></title>
</head>
<body>
<h3 align="center"><%= heading %></h3>
<p align="center"><% = paragraph %></p>
<div align="center"><%= testHTML %></div>
</body>
</html>
Put 'showxml.asp' page in the same directory where you have kept the 'page.xml' file.
Explanation :
Dim xml
Set xml = Server.CreateObject("Microsoft.XMLDOM")
We create the XML Document Object in the 'xml' variable.
xml.async = False
By setting '.async' option to False, we are saying to the XML parser to show the data as soon as it begins to read it. Note that by setting '.async' to False, retrieving of XML data is speeded up.
xml.load (Server.MapPath("page.xml"))
Next we load the 'page.xml' file. The 'load()' method asks for complete physical path to the XML file. We give it the complete physical path by using 'Server.MapPath' method to convert the relative path to complete physical path.
Dim title, heading, paragraph, testHTML
title = xml.documentElement.childNodes(0).text
heading = xml.documentElement.childNodes(1).text
paragraph = xml.documentElement.childNodes(2).text
testHTML = xml.documentElement.childNodes(3).text
Next we create four variables to hold the data which we retrieve from the XML file. xml.documentElement takes us to the 'main' tag of our XML file. Then .childNodes(n) takes us to that child node within the super node which in our case is the 'main' node. Since there are 4 child nodes within the 'main' node we can use values from 0 - 3 to get to the node we want. Once we have got to the node we want, we can use .text to retrieve it's value as text.
In the rest of the code we show the retrieved values which we got from the page.xml file.
Running 'showxml.asp' page :
Put 'page.xml' and 'showxml.asp' pages in the same directory where you can run ASP pages. For example, you can put both of them in the c:/inetpub/wwwroot directory. Then use http://127.0.0.1/showxml.asp to see it running. Otherwise add directory path to the URL depending on where you have kept it e.g. if you have kept these pages in c:/inetpub/wwwroot/xml/ directory then use http://127.0.0.1/xml/showxml.asp page to view it.
Summary
Creating XML Files :
Always add <?xml version="1.0"?> on top of your XML page. Then enclose all elements and their sub-elements inside a main element, you can name this main element whatever you want but in our example we named it 'main'.
Every starting tag must have an end tag. Elements may or may not contain attributes, in our example we did not make use of attributes to keep things simple.
If you want to write HTML code inside an XML element, then enclose that HTML code within <![CDATA[..............]]> markup tags.
Reading XML Files :
Set .sync to False to speed up the retrieval of data from XML page.