XML template engine

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A XML template engine (or XML template processor) is a specialized template processor for XML input and/or output, working in a XML template system context. There are two main types:

"XML-suite standards" compliant engines:
- XSLT engines, named also XSLT processors
- XQuery engines, named also XQuery processors
Others, like Web template engines

[edit] XSLT processors

XSLT processors may be delivered as standalone applications, or as software components or libraries intended for use by applications. Many web browsers and web server software have XSLT processor components built into them.

Most current operating systems have an XSLT processor installed. For example, Windows XP comes with the MSXML3 library, which includes an XSLT processor.

Early XSLT processors had very few optimizations; stylesheet documents were read using the Document Object Model and the processor would act on them directly. XPath engines were also not optimized.

By 2000, however, implementors saw optimization opportunities in both XPath evaluation and template rule processing. For example, the Java programming language's Transformation API for XML (TrAX), later subsumed into the Java API for XML Processing (JAXP), acknowledged one such optimization: before processing, the XSLT processor could condense the template rules and other stylesheet tree information into a single, compact Templates object, free from the constraints and bloat of standard DOMs, in an implementation-specific manner. This intermediate representation of the stylesheet tree allows for more efficient processing by potentially reducing preparation time and memory overhead. Additionally, the formal API allows for the object to be cached and reused for multiple transformations, potentially providing higher performance if several input documents are to be processed with the same XSLT stylesheet. Parallels are often drawn between this optimization and the compilation of programming language source code to bytecode: the stylesheets are said to be "compiled", even though they don't usually produce native programming language bytecode; rather, they produce intermediate structures and routines that are stored and processed internally.^[1]

XPath evaluation also has room for significant optimizations, and most processor vendors have implemented at least some of them, for speed. For example, in <xsl:if test="/some/nodes"> the test will evaluate to true if /some/nodes identifies any nodes, so evaluation can stop as soon as the first matching node is found; continuing to look for the entire set of matching nodes would not change the result. Similar optimizations can be undertaken when processing xsl:when and xsl:value-of, as well as expressions relying on, either implicitly or explicitly, string(), boolean(), or number(), and those that use numeric and position()/last()-based predicates.

[edit] Implementations

Some of these are only libraries for specific programming languages, but some form the basis for command line or shell script utilities for one or more operating systems. Such utilities are either bundled with the libraries or independently maintained, and some are incorporated into other applications, such as database engines and web browsers, in order to add XSLT functionality to them. With the exception of web browsers, such utilities and applications are not listed here.

Implementations for Java: Xalan-Java; SAXON by Michael Kay; XT originally by James Clark; Oracle XSLT, in the Oracle XDK
Implementations for the .NET Framework: Saxon .NET Project Weblog, an IKVM.NET-based port of Dr. Michael Kay's and Saxonica's Saxon Processor provides XSLT 2.0, XPath 2.0, and XQuery 1.0 support on the .NET platform.
Implementations for C or C++: Xalan-C++; libxslt the XSLT C library for GNOME; Sablotron, which is integrated into PHP4
Implementations for Perl: XML::LibXSLT is a Perl interface to the libxslt C library; XML::Sablotron is a Perl interface to the Sablotron processor
Implementations for PHP: XSLT is the PHP4 interface to the Sablotron processor; XSL is the new interface to XSL introduced in PHP5. The extension uses the libxslt.
Implementations for Python: 4XSLT, in the 4Suite toolkit by Fourthought, Inc.; lxml by Martijn Faassen is a Pythonic wrapper of the libxslt C library
Implementations for Ruby: Ruby/XSLT is a simple XSLT class based on libxml and libxslt; Sablotron module for Ruby is a ruby interface to Sablotron
Implementations for JavaScript: Google AJAXSLT is an implementation of XSLT in JavaScript, intended for use in Ajax applications.
Implementations for specific operating systems: Microsoft's MSXML library may be used in various Microsoft Windows application development environments and languages, such as Visual Basic, C, and JScript.; Microsoft offers a new XSLT processor in the System.XML component of the .NET Framework.
Implementations integrated into web browsers: (Comparison of layout engines (XML)); Mozilla has native XSLT support based on TransforMiiX.; Safari 1.3+ has native XSLT support, but is unable to perform XSL transformations via JavaScript, a limitation that does not occur in Mozilla or Internet Explorer. This limits the capabilities of Ajax applications that would run in Safari. Safari's XML-parser is also not standards-compliant; it will parse XML strings according to HTML rules. Therefore, under certain circumstances, it will omit data from the DOM tree if it encounters malformed "HTML" — even though it actually encountered valid XML. These errors will propagate to XSL-processed DOM trees.; X-Smiles has native XSLT support.; Opera has partial native XSLT support since Version 9. Notable exceptions include the absence of the document() function.; Internet Explorer 6 supports XSLT 1.0 via the MSXML library (described above). IE5 and IE5.5 came with an earlier MSXML component that only supported an older, nonrecommended dialect of XSLT. A newer version of MSXML can be downloaded and installed separately to enable IE5 and IE5.5 to support XSLT 1.0 through scripting, and if certain Windows Registry keys are modified, the newer library will replace the older version as the default used by IE.