| Paradigm | Declarative |
|---|---|
| Developer | World Wide Web Consortium (W3C) |
| First appeared | 1998 |
| Stable release | 3.0 / June 8, 2017; 7 years ago (2017-06-08) |
| Filename extensions | .xslt |
| Website | www |
| Majorimplementations | |
| libxslt,Saxon,Xalan | |
| Influenced by | |
| DSSSL | |
| Filename extension | .xslt |
|---|---|
| Internet media type | application/xslt+xml |
| Uniform Type Identifier (UTI) | org.w3.xsl |
XSLT (Extensible Stylesheet Language Transformations) is a language originally designed fortransformingXML documents into other XML documents,[1] or other formats such asHTML forweb pages,plain text, orXSL Formatting Objects. These formats can be subsequently converted to formats such asPDF,PostScript, andPNG.[2] Support for JSON and plain-text transformation was added in later updates to the XSLT 1.0 specification.
As of August 2022[update], the most recent stable version of the language is XSLT 3.0, which achieved Recommendation status in June 2017.
XSLT 3.0 implementations support Java, .NET, C/C++, Python, PHP and NodeJS. An XSLT 3.0 JavaScript library can also be hosted within the web browser. Modern web browsers also include native support for XSLT 1.0.[3]
The XSLT document transformation specifies how to transform an XML document into new document (usually XML, but other formats, such as plain text are supported).[4] Typically, input documents are XML files, but anything from which the processor can build anXQuery and XPath Data Model can be used, such asrelational database tables orgeographical information systems.[1]
While XSLT was originally designed as a special-purpose language for XML transformation, the language isTuring-complete, making it theoretically capable of arbitrary computations.[5]
XSLT is influenced byfunctional languages,[6] and by text-based pattern matching languages likeSNOBOL andAWK. Its most direct predecessor isDSSSL, which did forSGML what XSLT does for XML.[7]
The XSLT processor takes one or more XML source documents, plus one or more XSLT stylesheets, and processes them to produce one or multiple output documents.[16][17] In contrast to widely implementedimperative programming languages likeC, XSLT isdeclarative.[18] The basic processing paradigm is pattern matching.[19] Rather than listing an imperative sequence of actions to perform in a stateful environment, template rules only define how to handle a node matching a particular XPath-like pattern, if the processor should happen to encounter one, and the contents of the templates effectively comprisefunctionalexpressions that directly represent their evaluated form: the result tree, which is the basis of the processor's output.
A typical processor behaves as follows. First, assuming a stylesheet has already been read and prepared, the processor builds a sourcetree from the input XML document. It then processes the source tree's root node, finds the best-matching template for that node in the stylesheet, and evaluates the template's contents. Instructions in each template generally direct the processor to either create nodes in the result tree, or to process more nodes in the source tree in the same way as the root node. Finally the result tree is serialized as XML or HTML text.
XSLT usesXPath to identify subsets of the source document tree and perform calculations. XPath also provides a range offunctions, which XSLT itself further augments.
XSLT 1.0 uses XPath 1.0, while XSLT 2.0 uses XPath 2.0. XSLT 3.0 will work with either XPath 3.0 or 3.1. In the case of 1.0 and 2.0, the XSLT and XPath specifications were published on the same date. With 3.0, however, they were no longer synchronized; XPath 3.0 became a Recommendation in April 2014, followed by XPath 3.1 in February 2017; XSLT 3.0 followed in June 2017.
XSLT functionalities overlap with those ofXQuery, which was initially conceived as a query language for large collections of XML documents.
The XSLT 2.0 and XQuery 1.0 standards were developed by separate working groups withinW3C, working together to ensure a common approach where appropriate. They share the same data model, type system, and function library, and both includeXPath 2.0 as a sublanguage.
The two languages, however, are rooted in different traditions and serve the needs of different communities. XSLT was primarily conceived as a stylesheet language whose primary goal was to render XML for the human reader on screen, onthe web (as aweb template language), or on paper. XQuery was primarily conceived as adatabase query language in the tradition ofSQL.
Because the two languages originate in different communities, XSLT is stronger in its handlingof narrative documents with more flexible structure, while XQuery is stronger in its data handling, for example when performing relational joins.[20]
The<output> element can optionally take the attributemedia-type, which allows one to set themedia type (or MIME type) for the resulting output, for example:<xsl:output output="xml" media-type="application/xml"/>. The XSLT 1.0 recommendation recommends the more general attribute typestext/xml andapplication/xml since for a long time there was no registered media type for XSLT. During this timetext/xsl became the de facto standard. In XSLT 1.0 it was not specified how themedia-type values should be used.
With the release of the XSLT 2.0, the W3C recommended in 2007 the registration of the MIME media typeapplication/xslt+xml[21] and it was later registered with theInternet Assigned Numbers Authority.[22]
Pre-1.0 working drafts of XSLT usedtext/xsl in their embedding examples, and this type was implemented and continued to be promoted by Microsoft in Internet Explorer[23] and MSXML circa 2012. It is also widely recognized in thexml-stylesheet processing instruction by other browsers. In practice, therefore, users wanting to control transformation in the browser using this processing instruction were obliged to use this unregistered media type.[24]
These examples use the following incoming XML document:
<?xml version="1.0" ?><persons><personusername="JS1"><name>John</name><family-name>Smith</family-name></person><personusername="MI1"><name>Morka</name><family-name>Ismincius</family-name></person></persons>
This XSLT stylesheet provides templates to transform the XML document:
<?xml version="1.0" encoding="UTF-8"?><xsl:stylesheetxmlns:xsl="http://www.w3.org/1999/XSL/Transform"version="1.0"><xsl:outputmethod="xml"indent="yes"/><xsl:templatematch="/persons"><root><xsl:apply-templatesselect="person"/></root></xsl:template><xsl:templatematch="person"><nameusername="{@username}"><xsl:value-ofselect="name"/></name></xsl:template></xsl:stylesheet>
Its evaluation results in a new XML document, having another structure:
<?xml version="1.0" encoding="UTF-8"?><root><nameusername="JS1">John</name><nameusername="MI1">Morka</name></root>
Processing the following example XSLT file
<?xml version="1.0" encoding="UTF-8"?><xsl:stylesheetversion="1.0"xmlns:xsl="http://www.w3.org/1999/XSL/Transform"xmlns="http://www.w3.org/1999/xhtml"><xsl:outputmethod="xml"indent="yes"encoding="UTF-8"/><xsl:templatematch="/persons"><html><head><title>TestingXMLExample</title></head><body><h1>Persons</h1><ul><xsl:apply-templatesselect="person"><xsl:sortselect="family-name"/></xsl:apply-templates></ul></body></html></xsl:template><xsl:templatematch="person"><li><xsl:value-ofselect="family-name"/><xsl:text>,</xsl:text><xsl:value-ofselect="name"/></li></xsl:template></xsl:stylesheet>
with the XML input file shown above results in the followingXHTML (whitespace has been adjusted here for clarity):
<?xml version="1.0" encoding="UTF-8"?><htmlxmlns="http://www.w3.org/1999/xhtml"><head><title>TestingXMLExample</title></head><body><h1>Persons</h1><ul><li>Ismincius,Morka</li><li>Smith,John</li></ul></body></html>
This XHTML generates the output below when rendered in a web browser.
In order for a web browser to be able to apply an XSL transformation to an XML document on display, an XML stylesheet processing instruction can be inserted into XML. So, for example, if the stylesheet in Example 2 above were available as "example2.xsl", the following instruction could be added to the original incoming XML:[25]
<?xml-stylesheet href="example2.xsl" type="text/xsl" ?>In this example,text/xsl is technically incorrect according to the W3C specifications[25] (which say the type should beapplication/xslt+xml), but it is the only media type that is widely supported across browsers as of 2009, and the situation is unchanged in 2021.
msxsl.exe.[38] The .NET runtime includes a separate built-in XSLT processor in itsSystem.Xml.Xsl library.Most early XSLT processors were interpreters. More recently, code generation is increasingly common, using portable intermediate languages (such asJava bytecode or .NETCommon Intermediate Language) as the target. However, even the interpretive products generally offer separate analysis and execution phases, allowing an optimized expression tree to be created in memory and reused to perform multiple transformations. This gives substantial performance benefits in online publishing applications, where the same transformation is applied many times per second to different source documents.[42] This separation is reflected in the design of XSLT processing APIs (such asJAXP).
Early XSLT processors had very few optimizations. Stylesheet documents were read intoDocument Object Models and the processor would act on them directly.XPath engines were also not optimized. Increasingly, however, XSLT processors use optimization techniques found in functional programming languages and database query languages, such as static rewriting of an expression tree (e.g., to move calculations out of loops), and lazy pipelined evaluation to reduce thememory footprint of intermediate results (and allow "early exit" when the processor can evaluate an expression such asfollowing-sibling::*[1] without a complete evaluation of all subexpressions). Many processors also use tree representations that are significantly more efficient (in both space and time)[43] than general-purpose DOM implementations.
In June 2014, Debbie Lockett andMichael Kay introduced an open-source benchmarking framework for XSLT processors called XT-Speedo.[44]
Example: Multiple Result Documents
XSLT is a very specialized language with a distinct declarative flavor.
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