Extensible Mark-up Language (XML) has very quickly established itself as a viable technology with a huge range of real-world applications. One of th...
Extensible Mark-up Language (XML) has very quickly established itself as a viable technology with a huge range of real-world applications. One of the main reasons for its importance and wide acceptance is the fact that it offers a working solution to one of the key problems faced by software developers and computer users alike: the exchange of incompatible data. Each software environment creates its own unique type of binary file which only it can understand. When data is exported in XML format, it becomes a known quantity, independent of the environment in which it was originated.
Adobe’s PDF format is another example of a platform-independent data format which has gained wide acceptance. When a document is saved as a PDF file, its format is set in stone, it can viewed and printed with its layout and formatting intact, without the need for the software which created the original file. However, whereas the PDF format concerns itself primarily with the presenting information, XML is used to describe and encapsulate the information itself.
Though XML itself is still fairly new, the idea behind it is not. Back in the 1970s, Standard Generalized Markup Language (SGML) was developed in an attempt to create an application-independent method of describing data. SGML is a text-based language which uses the concept of adding mark-up to data which describes the data itself. An SGML document contains both data and a set of rules defining the structure of the data. SGML is a pretty complex language and, unlike XML, has never become mainstream. In the early 1990s, SGML was used to develop HTML and in the late 1990s, SGML was also used as the basis for the development of XML. So, basically, XML is a restricted form of SGML.
XML has already proved itself to be an excellent medium for storing, describing and transporting data, particularly over the web. It offers developers flexibility, clarity and simplicity. An XML document resembles an HTML document and consists of the same human-readable tags. However, the tags used to markup an HTML document are predetermined: only a fixed set of tags can legitimately be used. XML allows you to create your own markup language and define the tags which are legitimate for your data. It does this via the mechanism of a schema document, which can itself be an XML document. The schema document defines the vocabulary and grammar which may be used within the XML document containing your data.
The fact that, when creating and generating XML documents, you can invent all the rules, means that you never have to force your data into a container which was not designed to hold it. You design tags which reflect the nature of your data; you create a schema document which defines the hierarchical structure of your information; and you specify the type of information each element within your document is permitted to contain. In short, if you end up with an XML documents which is unsuitable for holding your information, you have only yourself to blame!
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Extensible Stylesheet Language (XSL) is an XML technology which is used to transform XML data. XML file can be used to set the ground-rules for a given body of data and to describe the data itself. XSL allows us to take that input XML data and convert it into information which can be displayed in a web browser window or opened with a text editor. It can also be used to create an XML file which is a modified version of an input XML file.
When displaying elements in a browser window, the XSL elements which enable looping through an XML tree and decision-making really come into their own. One of the key methods used in XSL for looping through XML elements is the “For-each” element. This uses the “select” attribute to specify which element is to be targeted in the loop. The “For-each” element has both an opening and a closing tag and any lines of code placed between the opening and closing tags will be repeated each time the element specified in the “select” attribute is encountered in the XML file.
For example, let’s say we have an input XML file containing a list of companies and the addresses of their websites. Let’s also say that the element which we will be targeting in the XSL “For-each” statement is called “company” and that, for each company element, we have a “name”, “telephone” and “website” element. Before beginning our “For-each” loop, we could output the opening tag of an HTML “ul” element (an unordered or bulleted list).
Inside the “For-each” loop, we could output an HTML “li” element and, between its opening and closing tags, output the contents of the “name” and “telephone” elements from the XML file. The data in the “website” element in the input XML could be used to convert the “name” into a clickable link.
As to the appearance of the data in the resulting HTML output, this would be controlled by a linked CSS file. CSS can be used to format the output in any way we desire, so our bulleted list can take on pretty much any appearance we desire.
The The writer of this article is a trainer and developer with , an independent computer training company offering in London and throughout the UK.
XML seems to be popping up everywhere nowadays; but what exactly is it? Extensible Markup Language (XML) is a specification by the Worldwide Web Consortium (W3C) for creating markup languages such as XHTML. The XML specification is being used in an increasingly wide range of applications for exchanging data between different environments, platforms and media. One of the main reasons for its rapidly growing popularity is its flexibility. It allows developers to define their own hierarchy of tags which reflect the structure of their data. In addition to the core XML specification, a number of other technologies have been developed for working with XML projects.
XML Schema
Closely linked with XML documents, we have XML schema. XML schema documents and schema languages are used to validate and document XML markup languages. They allow developers to explicitly lay down the syntactical and structural ground rules for particular classes of XML documents. A number of schema languages have been developed but perhaps the two most important are those defined by the W3C: Document Type Definition (DTD) which was built into the original XML specification and the more XML schema language developed a few years later.
XSL, XSLT AND XSL-FO
Extensible Stylesheet language (XSL) comprises two parts: XSL Transformations (XSLT) and XSL Formatting Objects (XSL-FO). XSLT stylesheets are essentially XML documents which transform a given class of XML document (referred to as the source tree) into a different class of XML document (referred to as the output tree). XSLT documents can be compared to CSS documents but are much more powerful. XSL-FO documents are written in XML and control the formatting layout in an output document in a very detailed manner. It allows the definition of layout masters from which are derived page sequences for generating the final document pages. Flow objects are then used to bind data to the various regions of pages.
XPath, XLink, XPointer and XQuery
XPath, XML Linking Language (XLink), and XML Pointer Language (XPointer) are all provide solutions for linking and addressing when working with XML documents. XPath is a declarative language which uses expressions that resemble the directory paths used to describe locations in computer file systems. XPath expressions are used in XSLT documents to address specific nodes within the hierarchy of the XML source document.
XLink is essentially a method of creating hyperlinks within XML documents. It overcomes some of the limitations of the hyperlinks used in XHTML, such as the fact that each hyperlink has only one source and one destination. XPointer allows you to create links which point to very specific points within the hierarchy of an XML document by including XPath expressions in its syntax. XQuery is designed to query XML data in much the same way that SQL is used to query databases. It uses XPath expressions to extract elements and attributes within the structure of an XML document.
The author is a training consultant with , an independent computer training company offering in London and throughout the UK.
