extensible markup language (xml). definition xml is a cross-platform, software and hardware...
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eXtensible Markup Language (XML)
Definition
XML is a cross-platform, software and hardware independent tool for
transmitting information.
“XMl is going to be everywhere” – W3C
XML BASICS
Introduction
• XML is a portable, widely supported, open technology for data storage and exchange
• Developed from SGML (Standard Generalised Markup Language )
•Became a W3C Recommendations in 1998
•A meta-markup language
•Deficiencies of HTML and SGML–Many complex features that are rarely used
•HTML is a markup language, XML is used to define markup languages
Introduction
•Markup languages defined in XML are known as applications
•XML can be written by hand or generated by computer
–Useful for data exchange
•Foundation for several next-generation web technologies:
–RSS, AJAX, Web Services, etc.
Introduction
What is XML?
• A specification for creating markup languages to store data and exchange data
• Tags are not predefined (user generated)
Why XML?
• Sample Catalog Entry in HTML
<TITLE> Laptop Computer </TITLE> <BODY> <UL>
<LI> IBM Thinkpad 600E<LI>400 MHz<LI> 64 Mb<LI>8 Gb<LI> 4.1 pounds<LI> $3200
</UL> </BODY>
•How can I parse the content? E.g. price?•Need a more flexible mechanism than HTML to interpret content.
Why XML?
<COMPUTER TYPE=“Laptop”><MANUFACTURER>IBM</MANUFACTURER><LINE> ThinkPad</LINE><MODEL>600E</MODEL><SPECIFICATIONS>
<SPEED UNIT = “MHz”>400</SPEED><MEMORY UNIT=“MB”>64</MEMORY><DISK UNIT=“GB”>8</DISK><WEIGHT UNIT=“POUND”>4.1</WEIGHT><PRICE CURRENCY=“USD”>3200</PRICE>
</SPECIFICATIONS></COMPUTER>
Sample Catalog Entry using XMl
What does XML do?
• XML is used to structure and describe information.
• Intended to be used with the Internet
• Used to facilitate sharing data between different systems
Smart processing using XMl
• <COMPUTER> and <SPECIFICATIONS> provide logical containers for extracting and manipulating product information as a unit
• – Sort by <MANUFACTURER>, <SPEED>,<WEIGHT>, <PRICE>, etc.• Explicit identification of each part enablesits automated processing
– Convert <PRICE> from “USD” to Euro, Yen,etc.
Document exchange
• Use of XMl allows companies to exchange information that can be processed automatically without human intervention e.g.– Purchase orders– Invoices– Catalogues etc
Difference between HTML and XML?
• XML was designed to carry data.
• XML is not a replacement for HTML.
• XML and HTML were designed with different goals:•XML was designed to describe data and to focus on what data is.•HTML was designed to display data and to focus on how data looks.•HTML is about displaying information, while XML is about describing information.
How can XML be Used?
• XML can Separate Data from HTML• With XML, your data is stored outside your HTML• XML is used to Exchange Data• With XML, data can be exchanged between
incompatible systems• With XML, financial information can be exchanged over
the Internet• XML can be used to Share Data• XML can be used to Store Data• XML can make your Data more Useful• XML can be used to Create new Languages
XML Pros and Cons
•pro: –human-readable, self-documenting format
–strict syntax allows standardized tools
–international, platform-independent
–can represent almost any general kind of data (record, list, tree)
•con:
–bulky syntax/structure makes files large; can decrease performance
An example XML file
Components of an XML Document
Processing Instructions
Elements
Elements with Attributes
<?xml version="1.0" ?><?xml-stylesheet type="text/xsl" href="template.xsl"?><ROOT>
<ELEMENT1><SUBELEMENT1 /><SUBELEMENT2 /></ELEMENT1><ELEMENT2> </ELEMENT2><ELEMENT3 type='string'> </ELEMENT3><ELEMENT4 type='integer' value='9.3'> </ELEMENT4>
</ROOT>
XML Document Structure
•A header, then a single document tag that can contain other tags
–<?xml version="1.0" encoding="UTF-8"?>
•Tag syntax:
–<element attributes> text or tags </element>
•Attribute syntax:
–name="value"
•comments: <!--comment -->
XML Document Structure
•Well-formed XML documents–All XML documents begin with an XML declaration.–All begin tags have a matching end tag
•Empty tags–There is one root tag that contains all the other
tags in a document–Attributes must have a value assigned, the value
must be quoted–The characters <, >, & can only appear with their
special meaning–XML tags are case sensitive–XML elements must be properly nested
XML Document Structure
•Specified by either DTD or XML schemas. (specify the tags and their order)
•Valid documents are well-formed and also conform to a schema which defines details of the allowed content
•Validity is tested against a schema, discussed later
XML SCHEMA
XML Schema
•Schema: describes the structure of an XML document, by setting rules specifying which tags and attributes are valid, and how they can be used together
•Used to check XML files to make sure they follow the rules set in the schema; W3C validator uses it to validate doctypeat top of XHTML file specifies schema
•Two ways to define a schema: –Document Type Definition (DTD)–W3C XML Schema
Document Type Definitions
•A set of structural rules called declarations
•Define tags, attributes, entities
•Specify the order and nesting of tags
•Specify which attributes can be used with which tags.
•DTD can be:–Embedded inside XML (internal DTD).–Stored in a separate file (external DTD saved as .dtd).
Document Type Definitions
General syntax for declarations –<!keyword …. >–Note, not XML!
•Four possible keywords:–ELEMENT, for defining tags
–ATTLIST, for specifying attributes in your tags
–ENTITY, for identifying sources of data
–NOTATION, for defining data types for non-XML data
Declaring Elements
•General syntax-<!ELEMENT element-name(content-description)>- Content description specifies what tags may appear inside the named element and whether there may be any plain text in the content
EX: <!ELEMENT person (parent+, age, spouse?, sibling*)>•An element can be either an internal or a leaf node.•Multiplicity
–+–*–?
•Leaf elements can be:–#PCDATA –EMPTY–ANY
Declaring Attributes
•General syntax–<!ATTLIST element-name
(attribute-name attribute-type default-value?)+ >
•There are 10 attribute types, only CDATA will be used.•Default values
–A value–#FIXED value–#REQUIRED (no default value, each instance must specify value)–#IMPLIED (default, if not specified)
Declaring Attributes
Example:
•DTD<!ATTLIST airplane places CDATA “4”><!ATTLIST airplane engine_type CDATA #REQUIRED><!ATTLIST airplane price CDATA #IMPLIED><!ATTLIST airplane make CDATA #FIXED “Cessna”>
•XML Element
<airplane places=“10” engine_type=“jet”></airplane>
ENTITIES
XML document may be distributed among a number of files
–Each unit of information is called an entity–Each entity has a name to identify it–Defined using an entity declaration –Used by calling an entity reference
General Entities
•Declared in DTD
–<!DOCTYPE My_XML_Doc [<!ENTITY name "replacement"> ]>
•<!ENTITY xml "eXtensible Markup Language">
•The &xml; includes entities
•The eXtensible Markup Language includes entities
Attributesvs. Elements
•There are no rules about when to use attributes and when to use elements.
•Avoid XML Attributes?
•Some of the problems with using attributes are:
–attributes cannot contain multiple values (elements can) –attributes cannot contain tree structures (elements can) –attributes are not easily expandable (for future changes)
Internal and External DTDs
A document type declaration can either contain declarations directly or can refer to another file
•Internal
–<!DOCTYPE root-element[declarations]>
•External file
–<!DOCTYPE root-nameSYSTEM “file-name”>
DTD Example
<?xml version="1.0"?> <!DOCTYPE note SYSTEM "note.dtd"> <note> <to>Tove</to> <from>Jani</from> <heading>Reminder</heading> <body>Don't forget me this weekend!</body></note>
Syntax: <!Doctype root-element SYSTEM “filename”
You usually specify the DTD for your XML document by providing a reference to it near the top of the XMl document.
DTD Example
The DTD (note.DTD) for XMl document Note: is
.
<!DOCTYPE note [
<!ELEMENT note (to,from,heading,body)> <!ELEMENT to (#PCDATA)> <!ELEMENT from (#PCDATA)> <!ELEMENT heading (#PCDATA)> <!ELEMENT body (#PCDATA)> ]>
Lists the document type (note) and the valid elements, and the type of content they can accept
Why use a DTD?
With DTD, each of your XML files can carry a description of its own format with it.
With a DTD, independent groups of people can agree to use a common DTD for interchanging data.
Your application can use a standard DTD to verify that the data you receive from the outside world is valid.
NAMESPACES
NAMESPACES
•“XML namespaces provide a simple method for qualifying element and attribute names used in Extensible Markup Language documents by associating them with namespaces identified by URI references.”•Multiple namespaces can be used in a single document•Two types of Namespaces:
–Default namespace•<element xmlns=“URI”>
–Explicit•Use a prefix.•<element xmlns[:prefix]=“URI”>•Prefix is used for two reasons:
–URI is too long to be typed–URI includes illegal characters
NAMESPACES
•Example:<states xmlns= “http://www.states.org/states”xmlns:cap= “http://www.states.org/capitals”><state><name>South Dakota</name><capital><cap:name>South Dakota</cap:name></capital></state></states>
•DTDs do not support namespaces very well
NAMESPACES
NAMESPACES
W3C XML SCHEMA (XSD)
Remember “Valid” XML
•Adheres to specification in DTD or XSD
XML Schemas (XSD)
•XSD , can specify elements, attributes, nesting, ordering, #occurrences.
•However DTDs have several deficits
–They do not use XML syntax
–They do not support namespaces
–Data types cannot be strictly specified•Example date vs. string is like DTD
Schema Fundamentals
•Documents that conform to a schema’s rules are considered instances of that schema
•Schema purposes–Structure of instances.–Data types of elements and attributes.
•W3C XML Schemas support namespaces–The XML Schema language itself is a set of XML tags–The application being described is another set of tags
Defining a Schema
•The root of an XML Schema document is the <schema> tag
•Attributes–xmlns attributes for the schema namespace and for the namespace being defined–A targetNamespaceattribute declaring the namespace being defined
–An elementFormDefaultattribute with the value qualified to indicate that all elements defined in the target namespace must be namespace qualified (either with a prefix or default) when used
Defining a Schema
•Example<xsd:schemaxmlns:xsd= “http://www.w3.org/2001/XMLSchema” targetNamespace= “http://www.sustech.edu” xmlns= “http://www.sustech.edu” elementFormDefault= "qualified">...</xsd:schema>
An Overview of Data Types
•Data types are of two kinds–Simple data types with string content–Complex data types with elements, attributes and string content
•Predefined types–Primitive: string, Boolean, float .. –Derived: PositiveInteger, long, decimal
•Restrictions (user defined types)–Facets(to limit its content, or require the data to match a specific pattern)
Simple Types
•Named types can be used to give the type of –an attribute (which must be simple) or –an element (which may be simple or complex)
•Elements or attributes with simple type may have default values specified•Syntax
–<xsd:elementname="xxx" type="yyy"/>–where xxx is the name of the element and yyy is the data type of the element–E.g.: <xsd:elementname=“engine” type =“xsd:string” default=“fuel inj”/>
Simple Types
•New simple types can be defined by restriction of base types
•Example:<xsd:simpleTypename = “FirstName”><xsd:restrictionbase = “xsd:string”><xsd:maxLengthvalue=“10”></ xsd:restriction></ xsd:simpleType>
Complex Types
•A complex element is an XML element that contains other elements and/or attributes.
•There are four kinds of complex elements:–empty elements –elements that contain only other elements–elements that contain only text –elements that contain both other elements and text
–Check http://www.w3schools.com/schema/schema_complex.asp
Empty Elements
<xs:elementname="product"><xs:complexType><xs:attributename="prodid“ type="xs:positiveInteger"/></xs:complexType></xs:element>
Elements Only
•Element-only element must be contained in an ordered group, an unordered group, a choice or a named group.
•The sequenceelement is used to contain an ordered group.
•The allelement is used to contain an unordered group.
•An element definition can be associated with a type by
–Referring to a named type directly in the type attribute
Elements Only
<xs:elementname="person"> <xs:complexType> <xs:sequence> <xs:elementname="firstname" type="xs:string"/> <xs:elementname="lastname" type="xs:string"/> </xs:sequence> </xs:complexType></xs:element>
XSD (note.xsd)
>
Defining a Schema Instance
•The xmlnsattribute declares a namespace for an element and its descendants
–<element xmlns[:prefix+=“URI”>–The element itself may not be in the namespace
–Multiple elements may be defined
•The http://www.w3.org/2001/XMLSchema-instancenamespace includes one attribute, schemaLocation
Reference to XSD (notes.xml)
Validating Instances of Schemas
eVarious systems for validating instances against schemas
–Online http://www.w3.org/2001/03/webdata/xsv or http://validator.w3.org/
–Standalone automatic validation tools: AltovaXMLSpy, oXygenXML editor, XML Copy ditor, XMLLINT
Example
XPATH
What is Xpath?
•XPathis a syntax used for selecting parts of an XML document
•The way Xpath describes paths to elements is similar to the way an operating system describes paths to files
•Xpath is almost a small programming language; it has functions, tests, and expressions
•Xpath is a W3C standard
•Xpath is not itself written as XML, but is used heavily in XSLT
Xpath capabilities
•Xpath provides a syntax for:
–navigating around an XML document
–selecting nodes and values
–comparing node values
–performing arithmetic on node values
•Xpath= Path expressions + Conditions
•Xpath provides some functions (e.g., concat(), substring(), etc.) to facilitate the above.
Today XPath expressions can also be used in JavaScript, Java, XML Schema, PHP, Python, C and C++, and lots of other languages.
Xpath Basic Constructs
Example
Example
eXtensibleStyle Language Transformation
Displaying Raw XML Documents
•Plain XML documents are generally displayed literally by browsers•Firefox notes that there is no style information
Displaying Raw XML Documents
• Raw XML files can be viewed in IE 5.0 (and higher) and in Netscape 6– but to make it display like a web page,
you have to add some display information
• XML documents do not carry information about how to display the data
• Different solutions to the display problem, using CSS, XSL, JavaScript, and XML Data Islands
Displaying XML Documents with CSS
• An xml-stylesheet processing instruction can be used to associate a general XML document with a style sheet– <?xml-stylesheet type=“text/css” href=“planes.css”>
• The style sheet selectors will specify tags that appear in a particular document
XSLT Style Sheets
• A family of specifications for transforming XML documents– XSLT: specifies how to transform
documents– XPath: specifies how to select parts of a
document and compute values– XSL-FO: specifies a target XML language
describing the printed page• XSLT describes how to transform XML
documents into other XML documents such as XHTML– XSLT can be used to transform to non-XML
documents as well
Overview of XSLT
• A functional style programming language
• Basic syntax is XML
• An XSLT processor takes an XML document as input and produces output based on the specifications of an XSLT document
XSLT Processing
XSLTDocument
XMLDocument
XSLTProcessor
XSLDocument
Creating XSLT Stylesheets
• Uses the http://www.w3c.org/1999/XSL/Transformnamespace, and is typically assigned XSL prefix
• XSLT Stylesheets are defined using the <xsl:stylesheet> root tag
• Stylesheets typically contain one or more <xsl:template> tags that define each template– Templates have name or/and match attributes
• Templates contain other XSLT tags that control how the XML data is transformed
Common XSLT Elements
• •<xsl:stylesheet>• •<xsl:templatename=“name” match
=“xpath”>• •<xsl:value-of select=“xpath”>• •<xsl:attribute>• •<xsl:text>• •<xsl:for-each select=“xpath”>• •<xsl:iftest=“condition”>• •<xsl:choose>, <xsl:when>, <xsl:otherwise>• •<xsl:sortselect=“xpath”>
An Example XSLT Template
Example: the xml file<?xml version="1.0" encoding="ISO-8859-1"?><?xml-stylesheet type="text/xsl" href="simple.xsl" ?><breakfast_menu>
<food><name>Belgian Waffles</name><price>$5.95</price><description>two of our famous Belgian Waffles with
plenty of real maple syrup</description><calories>650</calories>
</food><food>
<name>Strawberry Belgian Waffles</name><price>$7.95</price><description>light Belgian waffles covered with
strawberries and whipped cream</description><calories>900</calories>
</food>…
</breakfast_menu>
Example: the xsl file<?xml version="1.0" encoding="ISO-8859-1"?><html xsl:version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xmlns="http://www.w3.org/TR/xhtml1/strict"> <body style="font-family:Arial,helvetica,sans-serif;font-size:12pt; background-color:#EEEEEE"> <xsl:for-each select="breakfast_menu/food"> <div style="background-color:teal;color:white;padding:4px"> <span style="font-weight:bold;color:white"> <xsl:value-of select="name"/></span> - <xsl:value-of select="price"/> </div> <div style="margin-left:20px;margin-bottom:1em;font-size:10pt"> <xsl:value-of select="description"/> <span style="font-style:italic"> (<xsl:value-of select="calories"/> calories per serving) </span> </div> </xsl:for-each> </body></html>
• Demo– XSLT EXAMPLE
XML PROCESSOR
Uses of XML
• To exchange data between incompatible systems(just send an XML document, with an agreed definition of the tags)
• For B2B e-commerce – exchange of business documents between businesses - XML is flexible enough to describe any logical text structure e.g. Purchase order, invoice
• To store data – as plain text files, or in databases
• To create new mark-up languages (I.e. that uses tags) – Can use XML to agree what the tags mean. Many mark-up languages already created that have been based on XML – e.g. JSTL, WML, VoiceXML, XHTML
Using an XMl document
Need an XML Parser to “use” or parse out the data held in the XMl document
XML Processors
• XML processors provide tools in programming languages to read in XML documents, manipulate them and to write them out
Purposes of XML Processors
• Four purposes– Check the basic syntax of the input document– Replace entities– Insert default values specified by schemas or DTD’s– If the parser is able and it is requested, validate the input
document against the specified schemas or DTD’s• The basic structure of XML is simple and repetitive, so
providing library support is reasonable• Examples
– Xerces-J from the Apache foundation provides library support for Java
– Command line utilities are provided for checking well-formedness and validity
• Two different standards/models for processing– Tree based – Document Object Model (DOM)– Event based – Simple API for XMl (SAX)
Parsing
• The process of reading in a document and analyzing its structure is called parsing
• The parser provides as output a structured view of the input document
XML Parsers
An XML parser does the following:
• Retrieves and read the an XML document – I.e. “parses” the document to figure out what’s in it,
• Ensures the document adheres to specific standards (e.g. well formed? Adheres to DTD?)
• Makes the document contents available to your application
XML Parsers: Tree based DOM interface
• Uses Document Object Model (DOM)
• Tree based interface (navigates through the document)
• Developed by W3C• XML parsers that use DOM exist for
java, javascript, perl, C++
Tree based DOM parser - example
Object/Tree Interface (DOM)
Definition: Parser reads the XML document, and creates an in-memory“tree” of data – an object module of the data
For example: Given a sample XML document on the next slide, what kind of tree would be produced?
Tree based DOM parser - example
Sample XML Document
<?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE WEATHER SYSTEM "Weather.dtd"> <WEATHER> <CITY NAME="Hong Kong"> <HI>87</HI> <LOW>78</LOW> </CITY></WEATHER>
Tree based DOM parser - example
XML Parsers: Event based SAX parser
• Simple API for XML• Event based• Developed by volunteers on the
XML-dev mailing list• http://www.megginson.com/SAX/
Event based SAX parserEvent Based Parser
Definition: Parser reads the XML document, and generates events for each parsing event.
They don’t create an in memory object model of the document – it’s up to the programmer to write the code to interpret the events
For example: Given the same XML document, what kind of events would be produced?
Event based SAX parser: example
<?xml version="1.0" encoding="UTF-8"?><!DOCTYPE WEATHER SYSTEM
"Weather.dtd"><WEATHER> <CITY NAME="Hong Kong"> <HI>87</HI> <LOW>78</LOW> </CITY></WEATHER>
Event based SAX parser: example
Events generated:
• 1. Start of <Weather> Element• 2. Start of <CITY> Element• 3. Start of <HI> Element• 4. Character Event: 87• 5. End of </HI> Element• 6. Start of <LOW> Element• 7. Character Event: 78• 8. End of </LOW> Element• 9. End of </CITY> Element• 10. End of </WEATHER> Element
Event based parsers
For each of these events, the your application implements “event handlers.”
Each time an event occurs, a different event handler is called.
Your application intercepts these events, and handles them in any way you want.
Comparing tree based DOM parser with event based SAX
parser
Questions: • Which parser is faster?
• Which parser is more efficient?
• Which parser is suitable for which type of XML documents?
Comparing tree based DOM parser with event based SAX
parserTree based:
slower takes up more memory
Simpler to use
More suitable for documentsthat are less structured, with
less repetition of tags.
More suitable where the program needs to move around the
document alot within the program need to keep easy access to
full document at all time.
Event based: Faster
Takes up much less memoryBut More complex to
implement
Good for large, machine generated, structured documents e.g. book
contents (because repetitive nature of tags allows for re-use of event handling code and therefore less
work for programmer
Good where only parts of the document needed at any one time within the document (event based parsers cannot “skip around” from
one part of the document to the other
Comparing tree based DOM parser with event based SAX
parser
Performance and Memory
Therefore, when high performance and low-memory are the most important criteria, use an event-based parser.
Examples:• Java applets• Palm Pilot Applications• Parsing Huge Data files
The SAX Approach
• In the SAX approach, an XML document is read in serially
• As certain conditions, called events, are recognized, event handlers are called
• The program using this approach only sees part of the document at a time
The DOM Approach
• In the DOM approach, the parser produces an in-memory representation of the input document– Because of the well-formedness rules of XML, the
structure is a tree• Advantages over SAX
– Parts of the document can be accessed more than once
– The document can be restructured– Access can be made to any part of the document at
any time– Processing is delayed until the entire document is
checked for proper structure and, perhaps, validity• One major disadvantage is that a very large document
may not fit in memory entirely
DOM Parser example
XML IN PRACTICE
•Ajax
•RSS
•KML (Keyhole Markup Language) Google Earth
•ODF (Open Document Format) Open Office
•eBooks and ePub
•Web Services: SOAP & WSDL
Web resources
•http://www.deitel.com/ResourceCenters/Programming/XML/tabid/279/Default.aspx •W3 Schools XML Tutorial
•http://www.w3schools.com/xml/default.asp•W3C XML page
•http://www.w3.org/XML/•XML Tutorials
•http://www.programmingtutorials.com/xml.aspx •Online resource for markup language technologies
•http://xml.coverpages.org/•Several Online Presentations