XML XSLT
XSLT (Extensible Stylesheet Language Transformations) is a rule-based language that transforms XML documents into HTML, plain text, or other XML vocabularies by matching templates against the source tree.
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What Is XSLT?
XSLT works by pairing an XML source document with a stylesheet full of template rules. Each rule says, in effect, 'when you find a node that looks like this, output this instead.' An XSLT processor walks the source tree, matches nodes against your templates, and writes the result to a new document — most commonly HTML for display in a browser, but it could just as easily be plain text, CSV, or a different XML vocabulary entirely. Because the transformation logic lives outside the data, the same XML file can be rendered a dozen different ways just by swapping stylesheets.
Anatomy of a Stylesheet
A Minimal XSLT Stylesheet
<!DOCTYPE html>
<html>
<body>
<pre>
<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:template match="/">
<html>
<body>
<h2>Product Catalog</h2>
<xsl:apply-templates select="catalog/product"/>
</body>
</html>
</xsl:template>
<xsl:template match="product">
<p>
<b><xsl:value-of select="name"/></b> —
$<xsl:value-of select="price"/>
</p>
</xsl:template>
</xsl:stylesheet>
</pre>
</body>
</html>Every stylesheet starts with the xsl:stylesheet (or xsl:transform — the two are interchangeable) root element, declared against the XSLT namespace http://www.w3.org/1999/XSL/Transform and tagged with a version, almost always 1.0 or 2.0. Inside it, xsl:template elements each carry a match attribute holding an XPath pattern; the template matching '/' fires first, since it matches the document root, and typically builds the outer HTML skeleton before handing off to more specific templates with xsl:apply-templates.
Looping and Selecting Values
<!DOCTYPE html>
<html>
<body>
<pre>
<xsl:template match="/catalog">
<table border="1">
<tr><th>Name</th><th>Category</th><th>Price</th></tr>
<xsl:for-each select="product">
<tr>
<td><xsl:value-of select="name"/></td>
<td><xsl:value-of select="@category"/></td>
<td><xsl:value-of select="price"/></td>
</tr>
</xsl:for-each>
</table>
</xsl:template>
</pre>
</body>
</html>- xsl:template — defines a rule for transforming nodes that match a given XPath pattern
- xsl:value-of — outputs the string value of an XPath expression
- xsl:for-each — iterates over a node-set without needing a separate template
- xsl:apply-templates — hands control to whichever template best matches the selected nodes
- xsl:if — a single-branch conditional with a test attribute
- xsl:choose / xsl:when / xsl:otherwise — multi-branch conditional logic
- xsl:sort — reorders nodes inside a for-each or apply-templates
- xsl:attribute — writes a computed attribute onto an output element
Conditionals and Sorting
Sorting and Branching with xsl:choose
<!DOCTYPE html>
<html>
<body>
<pre>
<xsl:template match="/catalog">
<xsl:apply-templates select="product">
<xsl:sort select="price" data-type="number" order="ascending"/>
</xsl:apply-templates>
</xsl:template>
<xsl:template match="product">
<p>
<xsl:choose>
<xsl:when test="price &gt; 100">
<span class="premium"><xsl:value-of select="name"/></span>
</xsl:when>
<xsl:otherwise>
<xsl:value-of select="name"/>
</xsl:otherwise>
</xsl:choose>
</p>
</xsl:template>
</pre>
</body>
</html>Exercise: XML XSLT
What does XSLT fundamentally do to an XML document?
