ITEC 325 2012spring ibarland

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lect11b-ch01
XML overview

Chapter 1 - XML

In 1991, the first Web site was put online Twenty years later (not yet quite an adult), in 2011.Mar, one report reported 100 million active websites (e.g. onsite.bloggspot.com/), which (extrapolating from 2005 estimates of ~275 pages/site) would mean nearly 30billion (30,000million) pages. Another site even claims 20 trillion pages (that's 20,000,000 million); this might be trying to count the number of dynamic pages ever generated??

Most of the information available on the web is made available through HTML. HTML's simplicity has helped fuel the popularity of the Web. However, when faced with the Web's huge and growing quantity of information, it has presented real limitations. These limitations are evident when the task is to categorize, search or perform other operations on web available information.

XML (“eXtensible Markup Language”) is a general-purpose grammar/specification for structured (hierarchical) data. It can be thought of as a tree, where the leaves are strings, and the nodes are element-tags (with their attributes).

XML is used to define (other) markup languages (tag sets), like XHTML, or iTunes libraries¹?. While HTML is designed to specifically to display web pages, XML is used to structure and manage many sorts of information.

• Here's a simple XML document: lect11b-ch01-xml-example.xml (use “show-source”).

  • What information is being stored?
    
  • What is the structure of the information?
    
  • What tags are used to describe the information?
    

  At first glance, XML doesn't look so different from HTML: it is populated with tags, attributes, and values. Notice, however, that the tags are different than HTML, and in particular how the tags describe the contents they enclose. XML is also written much more strictly, the rules will be described in much more detail.

  The XML describes information only; no formatting/presentation is implied by the tag names. We can use CSS to specify formatting details:
  XML document referencing a CSS
  the referenced CSS

• XML is not a language that is intended to format information so that it can be displayed. It is a language that is intended to structure information so that it can be stored, transferred, and searched much, much more easily than information formatted using XHTML.
• XML is also easily extended and adapted. XML is used to design custom markup languages, those languages are then used to structure information. These defined markup languages (tag sets) can be shared and used by others to describe similar information. Many specific XML tag sets are being widely used.
• XML is a non-proprietary specification and is free to anyone who wishes to use it. It was created by the W3C (www.w3.org), an international consortium primarily responsible for the development of platform-independent Web standards and specifications.
• At one point, it was thought XML was going to replace HTML. For this to occur, world-wide adoption of new browsers supporting additional XML technologies and webmasters everywhere would need to rewrite all of their HTML documents. This will obviously not happen anytime soon. However, XML is used to manage and organize information, XSL is used to convert XML to XHTML. With this strategy, XML helps to store, structure, and transfer data and the universality of HTML is used to format and display.

Every custom markup language created using the XML specification must adhere to XML's underlying grammar. Custom markup languages created with XML are called “XML applications”. In other words, these custom markup languages are applications of XML, such as XSLT, RSS, SOAP, etc.. XML, like HTML, can be written using any text editor or word processor. There are many XML editors that are available (e.g. EditiX Lite), or plugins such as nxml mode for emacs.

XML documents, like HTML documents, are comprised of tags and data. One big difference between the two documents is that the tags used by an XML document are invented by the author. They are used to describe and structure the data. There are no implied presentation details for XML tag sets.

XML document rules

• The first line of the XML document must be the XML declaration.
  <?xml version="1.0" ?>
  
  There may be some DOCTYPE directives after the XML declaration.
• Following the opening declarations, the data part of the document is specified. The first line is an element and is the root element or tag of the document. There can only be one root element in an XML document.
  lect11b-ch01-wonders.xml

• Notice that tags must be properly nested (a key rule in XML): You must close the nearest unclosed tag. Attributes can be specified within the open-tag.

  Although browers will handle improperly nested tags (like “<em>blah <strong>blah</em> blah</strong>”), it's strictly dis-allowed in XML.

• Every element must have a closing tag. Empty elements can use a separate closing tag, or an all-in-one opening and closing tag with a slash before the final >.
• Case Matters - XML is case sensitive. Elements named wonder and WONDER are considered entirely separate and unrelated to each other.
• An attribute must be enclosed in either matching single or double quotation marks.

XML uses the same building blocks as HTML, tags that define elements, values of those elements, and attributes. An XML element is the most basic unit of an XML document. It can contain text, attributes, and other elements. Although whitespace between adjacent tags is ignored by the parser, but is important for human readability. (Whitespace within strings, or between tags and strings, is technically preserved, but may or may not actually be important to the information being represented.)

Tags that begin with <? and end with ?> are called processing instructions. The <?xml?> processing instruction tells the computer what parser to use (it commonly contains attributes for version, encoding). We have also seen the <?xml-stylesheet?> processing instruction, as well as (duh) the <?php?> processing instruction.

Comments in XML are just like XHTML (as in all XML tag sets).
<!-- here's the comment -->
Comments do not nest (arrrgh!), though a comment can contain multiple lines.

Entities

• Entities are a kind of auto text; a way of entering text into an XML document without typing it all out. There are many letters and symbols that can be inserted into HTML documents by using entities. In XML, there are only five predefined entitites.
• &amp; -- ampersand(&)
• &lt; -- less than (<)
• &gt; -- greater than (>)
• &quot; -- double quotation mark (")
• &apos; -- apostrophe {single quote} ( ' )
• You can create additional entities for a given document. (For example, html speficies many other character entities). You can even create entities which refer to more than individual characters. We'll see examples later.

• If you want to include a long string which you don't want processed as XML, you can you use a special CDATA declaration to tell an XML parser “The followimg is raw text — special characters and all”. CDATA sections occur between “<![CDATA[” and a matching “]]>”
  Example: lect11b-ch01-cdata.xml CDATA sections do not nest (arrrgh!),

  (CDATA sections are very reminiscent of PHP's “nowdoc” multi-line literals, which spanned from an opening “<<<'someID'” and a closing “someID”.)

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©2012, Ian Barland, Radford University Last modified 2012.Apr.04 (Wed)

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