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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).
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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
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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>
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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.
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