XML
Extensible Markup Language (XML) is markup language that defines a set of rules for encoding documents in aformat that is both human-readable and machine-readable. It is defined in the XML 1.0 Specification produced by theW3C, and several other related specifications,all free open standards.
The design goals of XML emphasize simplicity, generality, and usability over the Internet. It is a textual data format with strong support via Unicode for different human languages. Although the design of XML focuses on documents, it is widely used for the representation of arbitrary data structures, for example in web services.
Many application programming interfaces (APIs) have been developed to aid software developers with processing XML data, and several schema systems exist to aid in the definition of XML-based languages.
Key terminology
The material in this section is based on the XML Specification. This is not an exhaustive list of all the constructs that appear in XML; it provides an introduction to the key constructs most often encountered in day-to-day use.
- (Unicode) character
- By definition, an XML document is a string of characters. Almost every legal Unicode character may appear in an XML document.
- Processor and application
- The processor analyzes the markup and passes structured information to an application. The specification places requirements on what an XML processor must do and not do, but the application is outside its scope. The processor (as the specification calls it) is often referred to colloquially as an XML parser.
- Markup and content
- The characters making up an XML document are divided into markup and content, which may be distinguished by the application of simple syntactic rules. Generally, strings that constitute markup either begin with the character
<and end with a>, or they begin with the character&and end with a;. Strings of characters that are not markup are content. However, in a CDATA section, the delimiters<![CDATA[and]]>are classified as markup, while the text between them is classified as content. In addition, whitespace before and after the outermost element is classified as markup. - Tag
- A markup construct that begins with
<and ends with>. Tags come in three flavors:
- start-tags; for example:
<section> - end-tags; for example:
</section> - empty-element tags; for example:
<line-break />
- start-tags; for example:
- Element
- A logical document component which either begins with a start-tag and ends with a matching end-tag or consists only of an empty-element tag. The characters between the start- and end-tags, if any, are the element's content, and may contain markup, including other elements, which are called child elements. An example of an element is
<Greeting>Hello, world.</Greeting>(see hello world). Another is<line-break />. - Attribute
- A markup construct consisting of a name/value pair that exists within a start-tag or empty-element tag. In the example (below) the element img has two attributes,src and alt:
- Another example would be
<img src="madonna.jpg" alt='Foligno Madonna, by Raphael' /> - where the name of the attribute is "number" and the value is "3".
<step number="3">Connect A to B.</step>
- where the attribute "class" has both the value "inner greeting-box" and also indicates the two CSS class names "inner" and "greeting-box".
<div class="inner greeting-box" >Hello!</div>
- XML declaration
- XML documents may begin by declaring some information about themselves, as in the following example:
<?xml version="1.0" encoding="UTF-8"?>Full summary
XML Filename extension .xmlInternet media type application/xml,text/xml[1]Uniform Type Identifier (UTI) public.xml UTI conformation public.text Developed by World Wide Web Consortium Type of format Markup language Extended from SGML Extended to Numerous, including:
XHTML, RSS, Atom, KMLStandard 1.0 (Fifth Edition)November 26, 2008
1.1 (Second Edition)August 16, 2006Open format? Yes Extensible Markup Language (XML) Current status Published Year started 1996 Editors Tim Bray, Jean Paoli, C. M. Sperberg-McQueen, Eve Maler, François Yergeau, John Cowan Related standards XML Schema Domain Data Serialization Abbreviation XML Website XML 1.0
An XML attribute can only have a single value and each attribute can appear at most once on each element. In the common situation where a list of multiple values is desired, this must be done by encoding the list into a well-formed XML attribute[note 1] with some format beyond what XML defines itself. Usually this is either a comma or semi-colon delimited list or, if the individual values are known not to contain spaces,[note 2] a space-delimited list can be used.
0 comments:
Post a Comment