XML Basics

Selena Sol

March 8, 1999

XML Example

Okay, you are probably beginning to get a little bit dizzy with all of this theoretical stuff. If you are like me, by now you are quite ready to sink your teeth into the meat of XML.
So to conclude this section, we will run through a very simple XML example so that you can see how it all fits together. We'll keep it simple of course, in fact we will be sloppy and not include a DTD (for simplicity). But in the next few sections we will give you all the tools to start doing more advanced work.
Let's return to our contact document:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?> <DOCUMENT> <CONTACT> <NAME>Gunther Birznieks</NAME> <EMAIL>gunther@bobsfishstore.com</EMAIL> <PHONE>662-9999</PHONE> </CONTACT> <CONTACT> <NAME>Susan Czigany</NAME> <EMAIL>susan@eudora.org</EMAIL> <PHONE>555-1234</PHONE> </CONTACT> </DOCUMENT>

You could copy this text using your favorite word processor and save it as plain text, naming it something like contacts.xml or test.xml.
Notice that the first line is something we have not seen before. This line is called a processing instruction. We will talk much more about processing instructions and their attributes later. For now, just know that all XML documents need this first line much like HTML documents begin with <HTML>.
Other than that, you see a set of opening and closing tags with data (together, the tags and data are called XML Elements).

Unlike HTML, XML is VERY precise. If the syntax isn't exactly right, the parser will stop processing it and nothing (except an error message) will be displayed.
For example, the processing instruction is absolutely required for all XML documents. In contrast, most browsers will accept a missing tag at the beginning of an HTML document. This is because browsers have built-in "recovery" code to guess what's missing and to recover from invalid HTML. XML parsers, whether embedded in browsers or as standalone processors, are explicitly not allowed to recover. Much like compiling a program, an XML file is either correct, or it's toast. If this seems arbitrary, consider that XML is about transmitting structured data using tags that are usually non-standard. Parsers can't guess what's missing the way they can with HTML.
Ken Sall

XSL Basics

Okay, the next thing you will need to do is associate stylistic meanings to the tags so that a browser can display the document. As we said before, since XML allows you to create your own sets of tags, you must also create your own style guidelines which a browser can use to interpret your tags (which it has never seen before).
Because they are extracted from the data, style sheets can be shared by any number of XML documents. Also, they can be written in a number of style languages such as Cascading Style Sheet Language (CSS) or eXtensible Style Language (XSL). In this example, we will use XSL.
Let's take a look at a Style Sheet in XSL for our contacts.xml document

<xsl:stylesheet xmlns:xsl="http://www.w3.org/TR/WD-xsl"> <xsl:template pattern = "DOCUMENT"> <HTML> <HEAD> <TITLE>Contacts</TITLE> </HEAD> <BODY> <xsl:process-children> </BODY> </HTML> </xsl:template> <xsl:template pattern = "CONTACT"> <UL> <xsl:process-children> </UL> </xsl:template> <xsl:template pattern = "NAME"> <LI> <xsl:process-children> </LI> </xsl:template> <xsl:template pattern = "PHONE"> <LI> <xsl:process-children> </LI> </xsl:template> <xsl:template pattern = "EMAIL"> <LI> <xsl:process-children> </LI> </xsl:template> </xsl:stylesheet>

Putting it all Together

Once you have defined your XML and XSL documents, you can run them through a processor and display them. We talk a lot more about how you do this in later sections. For now, we only show you what the final converted document will look like.

<HTML> <HEAD> <TITLE>Contacts</TITLE> </HEAD> <BODY> <UL> <LI>Gunther Birznieks</LI> <LI>gunther@bobsfishstore.com</LI> <LI>662-9999</LI> </UL> <UL> <LI>Susan Czigany</LI> <LI>susan@eudora.org</LI> <LI>555-1234</LI> </UL> </BODY> </HTML>

In the next couple of sections we will delve more deeply into the syntax of the XML document, the XSL document and the DTD.

History of XML
Introduction to XML For Web Developers | Table of Contents
Introduction to XML For Web Developers Part 2

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