XML құжаттарды түрлендіру тілі: XSLT

Language for transforming XML documents

XSLT (Extensible Stylesheet Language Transformations) is a language originally designed 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. Support for JSON and plain text transformation was added in later updates to the XSLT 1.0 specification. as of August 2022, 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. For an XSLT document transformation, 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.

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. 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.

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.

The <output> element can optionally take the attribute media type, which allows one to set the media 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 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 in 2007 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 continued to be promoted by Microsoft in Internet Explorer and MSXML circa 2012. 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 were obliged to use this unregistered media type.

Most early XSLT processors were interpreters. More recently, code generation is increasingly common, using portable intermediate languages (such as Java bytecode or NET Common 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. This separation is reflected in the design of XSLT processing APIs (such as JAXP). 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 (e. g., to move calculations out of loops), and lazy pipelined evaluation to reduce the memory footprint of intermediate results (and allow "early exit" when the processor can evaluate an expression such as following 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) 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.