XSLT


XSLT is a language for transforming XML documents into other XML documents, or other formats such as HTML for web pages, plain text or XSL Formatting Objects, which may subsequently be converted to other formats, such as PDF, PostScript and PNG. XSLT 1.0 is widely supported in modern web browsers.
The original document is not changed; rather, a new document is created based on the content of an existing one. Typically, input documents are XML files, but anything from which the processor can build an XQuery and XPath Data Model can be used, such as relational database tables or geographical information systems.
Although XSLT is designed as a special-purpose language for XML transformation, the language is Turing-complete, making it theoretically capable of arbitrary computations.

History

XSLT is influenced by functional languages, and by text-based pattern matching languages like SNOBOL and AWK. Its most direct predecessor is DSSSL, which did for SGML what XSLT does for XML.
The XSLT processor takes one or more XML source documents, plus one or more XSLT stylesheets, and processes them to produce an output document. In contrast to widely implemented imperative programming languages like C, XSLT is declarative. The basic processing paradigm is pattern matching. 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 comprise functional expressions 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 source tree 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.

XPath

XSLT uses XPath to identify subsets of the source document tree and perform calculations. XPath also provides a range of functions, 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.

XQuery compared

XSLT functionalities overlap with those of XQuery, 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 within W3C, working together to ensure a common approach where appropriate. They share the same data model, type system, and function library, and both include XPath 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, on the web, or on paper. XQuery was primarily conceived as a database query language in the tradition of SQL.
Because the two languages originate in different communities, XSLT is stronger in its handling
of narrative documents with more flexible structure, while XQuery is stronger in its data handling, for example when performing relational joins.

Media types

The <output> element can optionally take the attribute media-type, which allows one to set the media type for the resulting output, for example: . The XSLT 1.0 recommendation recommends the more general attribute types text/xml and application/xml since for a long time there was no registered media type for XSLT. During this time text/xsl became the de facto standard. In XSLT 1.0 it was not specified how the media-type values should be used.
With the release of the XSLT 2.0, the W3C recommended the registration of the MIME media type application/xslt+xml and it was later registered with the Internet Assigned Numbers Authority.
Pre-1.0 working drafts of XSLT used text/xsl in their embedding examples, and this type was implemented and continues to be promoted by Microsoft in Internet Explorer and MSXML. It is also widely recognized in the xml-stylesheet processing instruction by other browsers. In practice, therefore, users wanting to control transformation in the browser using this processing instruction are obliged to use this unregistered media type.

Examples

These examples use the following incoming XML document



John
Smith


Morka
Ismincius


Example 1 (transforming XML to XML)

This XSLT stylesheet provides templates to transform the XML document:















Its evaluation results in a new XML document, having another structure:



John
Morka

Example 2 (transforming XML to XHTML)

Processing the following example XSLT file


version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns="http://www.w3.org/1999/xhtml">



Testing XML Example

Persons











  • ,



    • with the XML input file shown above results in the following XHTML :



    Testing XML Example

    Persons



    • Ismincius, Morka
    • Smith, John



    This XHTML generates the output below when rendered in a web browser.
    In order for a web browser to be able automatically 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:



    In this example, text/xsl is technically incorrect according to the W3C specifications, but it is the only media type that is widely supported across browsers as of 2009.

    Processor implementations

    Most early XSLT processors were interpreters. More recently, code generation is increasingly common, using portable intermediate languages 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. This separation is reflected in the design of XSLT processing APIs.
    Early XSLT processors had very few optimizations. Stylesheet documents were read into Document 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, and lazy pipelined evaluation to reduce the memory footprint of intermediate results. Many processors also use tree representations that are significantly more efficient than general-purpose DOM implementations.
    In June 2014, Debbie Lockett and Michael Kay introduced an open-source benchmarking framework for XSLT processors called XT-Speedo.