Thursday 21 November 2013

XML

What is XML?


  • XML stands for EXtensible Markup Language
  • XML is a markup language much like HTML
  • XML was designed to carry data, not to display data
  • XML tags are not predefined. You must define your own tags
  • XML is designed to be self-descriptive
  • XML is a W3C Recommendation

Learn XML Tutorial 

The Difference Between XML and HTML

XML is not a replacement for HTML.
XML and HTML were designed with different goals:
  • XML was designed to transport and store data, with focus on what data is
  • HTML was designed to display data, with focus on how data looks
HTML is about displaying information, while XML is about carrying information.


HTMLXML
For displayfor data stucture
no knowledge of datapresentatation independent
standardopen language
case sensitivecase sensitive
 


With XML You Invent Your Own Tags

The tags in the example above (like <to> and <from>) are not defined in any XML standard. These tags are "invented" by the author of the XML document.
That is because the XML language has no predefined tags.
The tags used in HTML are predefined. HTML documents can only use tags defined in the HTML standard (like <p>, <h1>, etc.).
XML allows the author to define his/her own tags and his/her own document structure.

(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 />
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 />.

What is an XML Element?

An XML element is everything from (including) the element's start tag to (including) the element's end tag.
An element can contain:
  • other elements
  • text
  • attributes
  • or a mix of all of the above...
<bookstore>
  <book category="CHILDREN">
    <title>Harry Potter</title>
    <author>J K. Rowling</author>
    <year>2005</year>
    <price>29.99</price>
  </book>
  <book category="WEB">
    <title>Learning XML</title>
    <author>Erik T. Ray</author>
    <year>2003</year>
    <price>39.95</price>
  </book>
</bookstore>
In the example above, <bookstore> and <book> have element contents, because they contain other elements. <book> also has an attribute (category="CHILDREN"). <title>, <author>, <year>, and <price> have text content because they contain text.
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:
<img src="madonna.jpg" alt='Foligno Madonna, by Raphael'/>
Another example would be
<step number="3">Connect A to B.</step>
where the name of the attribute is "number" and the value is "3".
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 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,a space-delimited list can be used.
<div class="inner greeting-box" >Hello!</div>
where the attribute "class" has both the value "inner greeting-box", indicating the CSS class names "inner" and "greeting-box".
XML declaration
XML documents may begin by declaring some information about themselves, as in the following example:
<?xml version="1.0" encoding="UTF-8"?>

What is XML Used For?

XML is one of the most widely-used formats for sharing structured information today: between programs, between people, between computers and people, both locally and across networks.
A short example:
<part number="1976">
  <name>Windscreen Wiper</name>
  <description>The Windscreen wiper
    automatically removes rain
    from your windscreen, if it
    should happen to splash there.
    It has a rubber <ref part="1977">blade</ref>
    which can be ordered separately
    if you need to replace it.
  </description>
</part>

XML Does Not DO Anything

Maybe it is a little hard to understand, but XML does not DO anything. XML was created to structure, store, and transport information.
The following example is a note to Tove, from Jani, stored as XML:

XML Document Example

<?xml version="1.0"?>
<note>
    <to>Tove</to>
    <from>Jani</from>
    <heading>Reminder</heading>
    <body>Don't forget me this weekend!</body>
</note>

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