A good example of XSLT is in providing XML to HTML translation services when you want to specify how to present an XML document within
an HTML presentation (and thus readily presentable by a Web browser). The examples in the book are thorough but
I decided to continue with our lightning strike example here so you would have another realistic example of using
XSLT. XSLT includes elements you can use to sort, filter, and loop through an XML document. So, in order to show
a more robust example, I changed the XML document to include a new strike element so we can include multiple
strikes in the same XML document. Note the strike element is an immediate child of the lightning_strike element (the
root element). Before, we imagined the lightning strike XML documents were created for each strike. Now, we imagine
the documents are created every minute and include all strikes across the country during that minute. The example
below includes four strikes that occurred at 2:46 EST on April 26th. Notice again how repetitive XML becomes when
we include multiple strike elements (like the animal elements in the book) in the same document. Now, continue below
after you've noticed the simple change to our XML document.
The XML Document
<?xml version="1.0" standalone="no"?>
You name your XSLT document files with a .xls or .xlst extension. XSLT documents need to be run through an XLST
processor (software) in order to generate the HTML file. I'll continue talking about the XLST processor
after we've considered the XLST document example that follows here. Note that like XML Schema, XLST follows
the rules for XML syntax. You provide the usual XML processing instruction as the first line. You then create
an xsl:stylesheet element to validate your XSLT with a valid XML namespace.
You can present any portion of your XML document in an HTML format. If you want to present information from
the whole document, you set your xsl:template match attribute to the root "/", otherwise you set the
value to the point in your document hierarchy you wish to use. After you start a template, you can start to
inject your HTML tags in the flow of the HTML document you wish to generate. When you want to mix in data from
your XML document, you use xsl:value-of elements, pointing to the data items you want to present using
the hierarchy relative to your template match (in the case of date, the hierarchy refers to the root element,
lightning_strike, then its child element (strike), then its child element (date). The date comes from the first
strike element found in the document (which is fine since date is the same for all strikes.
Make sure you understand how you can reference an attribute as well using the xsl:attribute element. If
you want to reference an attribute in a hierarchy, preface the attribute name with an at-symbol (@). Make sure
you look at how the xsl:for-each element lets you loop through the strikes and how the xsl:sort
element lets you sort the data as you wish before presenting it in the HTML (in the example, we sort by the
lightning strike's intensity). Note which tags require closing tags and which just close with />. Ask questions
in class if you don't understand this example. Now, after reading the
XSLT document, look below it to see how the transform process works.
The XSLT Document
<!-- Need to use an XSLT processor like
<title>This Hour's Lightning Strikes</title>
<h3>Latest Strike Info for <xsl:value-of
<p>A lightning strike of <xsl:value-of
joules occured at <xsl:value-of
Supposedly there are lots of XSLT processors out there, but I have had a fine experience with the SAXON XLST processor that Elizabeth recommends in the book. If you want to try processing your own XML documents, I recommend you use version 9.2 from http://saxon.sourceforge.net/.
You download the instant SAXON archive and place it into the same directory where you have saved your .xml and .xslt files.
The Tranform In Action (using SAXON)
Before you create your HTML document, you should check for any errors in your XML or XSLT by validating both with the online validators you prefer. To use the XSLT translation services within the SAXON services, you supply the appropriate parameters to the java command that runs the SAXON Transform service. First you change your command prompt's or Terminal's current directory to be the directory where you've placed the ls.xml and ls.xslt files:
Then, you can perform the translation:
C:> java -mx512m -cp saxon9he.jar net.sf.saxon.Transform -s:ls.xml -xsl:ls.xslt -o:ls.html
Let's take a look at this Transform command in detail. The keyword java tells your system to use the Java programming language and the Java virtual machine to run the transform code.
The -mx512 switch (also called a flag) tells Java to allocate 512 megabytes of computer memory to the process.
The -cp switch (referring to Java classpath) tells Java that you are going to provide additional code libraries with which to run the code. You can pass as many different libraries as you want to the Java virtual machine, but we only need one in our case. We need to pass in the SAXON saxon9he.jar library we downloaded from the Web. The period (.) in front refers to the current directory and the colon (:) is the delimiter used to separate library names in the classpath.
Next we tell Java which code component (called a class in Java terminology which is a common term in object-oriented programming theory) from within the library we are supplying.
Finally, we provide the parameters the Transform code requires in order for it to do its job: the -s switch refers to the XML document we want to convert. the -xsl switch refers to the XSL Stylesheet we want to use to perform the transform, and the -o switch refers to the output file name we want to generate with the transformed detail. A lot of information to have to provide in order to do a stylesheet transformation, but we get the huge benefit of being able to run such a translation on any platform that supports Java!
The Transformed Output (using SAXON)
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>This Hour's Lightning Strikes</title>
<h3>Latest Strike Info for 2002-04-26</h3>
<p>A lightning strike of 8354
joules occured at 14:26:33.775.
<p>A lightning strike of 7746
joules occured at 14:26:39.357.
<p>A lightning strike of 6246
joules occured at 14:26:55.445.
<p>A lightning strike of 1666
joules occured at 14:26:14.069.
The created ls.html document is here in case you don't want to create it yourself (the creation is busy work as long as you understand why it is necessary to do so).
If you want to read the SAXON documentation regarding XSL stylesheet Transformations, I have put the whole SAXON documentation online.