eXtensible Markup Language, is a specification for creating custom markup languages W3C Recommendation Primary purpose is to help computers to share data XML is meta-language. This means that you use it for creating languages. XML is an extensive concept.
Every XML-document is text-based => sharing data between different computers! => sharing data in Internet! => platform independence!
Problems with Binary format › Platform depence › Firewalls › Hard to debug › Inspecting the file can be hard Since XML is text-based, it does not have the problems mentioned above. What are the disadvantages in text format?
XML is meta language, which you can use to create your own markup languages. There are several XML Markup Languages made for different purposes All the languages have common xml-rules Languages: XHTML, OOXML, Open Document, RSS, SVG, SOAP, SMIL, MathML... List: › http://en.wikipedia.org/wiki/List_of_XML_markup_languag es
<?xml version="1.0"?><!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"><html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> <head> <title>Minimal XHTML 1.0 Document</title> </head> <body> <p>This is a minimal <a href="http://www.w3.org/TR/xhtml1/">XHTML 1.0</a> document.</p> </body></html>
There are two levels of correctness of an XML document: 1. Well-formed. A well-formed document conforms to all of XMLs syntax rules. 2. Valid. A valid document additionally conforms to some semantic rules. Lets first look at the XMLs syntax rules (1).
<?xml version="1.0" encoding="utf-8" standalone="yes"?><presentation> <slide number="1"> <name>Introduction to XML</name> <contents>XML is ...</contents> </slide></presentation>
XML-declaration is optional in XML 1.0, mandatory in 1.1. › Recommendation: use it. Version: 1.0 or 1.1 Encoding: character encoding, default utf-8 Standalone: › is the xml-document linked to external markup declaration › yes: no external markup declarations › no: can have external markup declaration (open issue..) › default: "no"
<?xml version="1.0" encoding="utf-8" standalone="no"?><presentation> <slide> <name>Introduction to XML</name> <contents>XML is ...</contents> </slide></presentation> Same Declaration<?xml version="1.0"?><presentation> <slide> <name>Introduction to XML</name> <contents>XML is ...</contents> </slide></presentation>
Element consists of start tag, optional content and an end tag: › <name>Introduction to XML</name> Start tag › <name> Content › Introduction to XML End tag › </name> Start tag may have attribute › <slide number="1">
Only one root - element Every element contains starting tag and an ending tag Content is optional: Empty element › <x></x> <!-- same as --> › <x/> Tag – names are case-sensitive: › <X></x> <!-- Error --> Elements must be ended with the end tag in correct order: › <p><i>problem here</p></i> <!– Error
XML elements can have attributes in the start tag. Attributes must be quoted: › <person sex="female"> › <person sex=female> › <gangster name=George "Shotgun" Ziegler> › <gangster name="George "Shotgun" Ziegler">
Names can contain letters, numbers, and other characters Names must not start with a number or punctuation character Names must not start with the letters xml (or XML, or Xml, etc) Names cannot contain spaces
XML document is well-formed if it follows the syntax rules. XML document must be well-formed! › its not an xml-document, if it does not follow the rules..
XML document is valid if › 1) It is well formed AND › 2) It follows some semantic rules XML document is usually linked to an external file, that has semantic rules for the document. › The file can be dtd (.dtd) or schema (.xsd) Semantic rules? › Name of tags, order of elements
Because of HTML heritage, browsers try to understand invalid XHTML-pages This is not the case in other XML- languages. In general, if XML-document is invalid, the processing of the document is cancelled.
XML has strict rules for WF and Valid If application tries to manipulate xml- document it does not have to try to understand the possible errors in the document This means that handling xml-files via programming language is much easier › If the document is correctly formed, manipulate it › If it isnt display error
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