Introduction to XML

Introduction to XML Kostas Kontogiannis Evan Mamas

Outline • Introduce XML, HTML and SGML • Compare and Contrast • XML vs. HTML • XML vs. SGML • XML • Components, Applications, Industry • Thoughts on XML

What is XML? • eXtensible Markup Language • Proper subset of SGML for web use • Meta-language • Allows you to create your own markup languages • Compromise between HTML and SGML

What is HTML ? • HyperText Markup Language • Language to describe information for transmission over the web. • Uses tags to markup the information • Tags are just a formatting tool • Example • <H1> Hello, World </H1> • Hello, World

Why isn’t HTML enough? • Good enough for presenting text on the web • Not accepted as an authoring or archival form • Extensibility • HTML standard changes continually • Uses tags for formatting • Structures • Has no defined or definable structural rules

What is SGML ? • Standard Generalized Markup Language • International Standard for over 10 years • Language for specifying markup languages • Describes only the formal properties and inter-relations of the components of a document • Document, Entities, Elements, Attributes

Uses of SGML • Formally structured documents • Technical Manuals • Exchange documents • Product documentation • Data encoding • Interchange specification • Provide long-term storage of information which was independent of suppliers and changes in h/w and s/w

SGML Example • Memo • DTD (Document Type Definition) <to>All staff <from>Martin Bryan <date>5th November <subject>Cats and Dogs <text>Please remember to keep all cats and dogs indoors tonight. <!DOCTYPE memo [ <!ELEMENT memo O O ((to & from & date & subject?), text) > <!ELEMENT text - O (para+) > <!ELEMENT para O O (#PCDATA) > <!ELEMENT (to, from, date, subject) - O (#PCDATA) > ]>

Why isn’t SGML enough? • Specification is very long • Contains many options not needed for Web applications • Time consuming and high cost • Expensive tools • Too much for small applications • Bad reputation

XML vs. HTML • New tags and attributes definitions allowed • Document structures can be nested to any level of complexity • Structural validation is possible by describing the grammar

XML vs. SGML • XML is the minimum required subset of SGML for web use • Easier to implement and to create tools for • A new attempt at structured markup languages with a new “face”

XML Components • XML Style Language (XSL) • Cascading Style Sheets, level 2 CCS2 • XML Document Object Model (DOM) • XML Linking Language (XLL) • XML Pointer Language (XPL) • XML Name Spaces • Synchronized Multimedia Integration Language (SMIL) • Resource Description Framework (RDF) • Mathematical Markup Language (MathML)

XML Components (cont.) • XML Style Language (XSL) • Defines a way to present the documents • Separates formatting from content • Has two steps: • Generate a result tree (associate patterns with templates) • Use XML Namespace (formatting vocabulary) to generate formatted output. • Similar to DSSSL for SGML

XML Components (cont.) • Cascading Style Sheets, level 2 CCS2 • Defines a way to present documents • Similar to XSL (Not as strong) • Supported by most browsers <HTML> <TITLE>Bach's home page</TITLE> <STYLE type="text/css"> H1 { color: blue } </STYLE> <BODY> <H1>Bach's home page</H1> <P>Johann Sebastian Bach was a prolific composer. </BODY> </HTML>

XML Components (cont.) • XML Document Object Model (DOM) • In-memory model for representing parsed XML documents • Designed to provide common structures in XML browsers • Intended to enable interoperable XML processing across browsers • Implemented by Internet Explorer and Netscape

XML Components (cont.) • XML Linking Language (XLL) • Links by reference rather than exact location • Provides hyperlinking elements • Simple links like HTML links • Extended • Multi-directional links • Links with multiple destinations • Placing content inline from a linked document • Requires use of XML Pointer Language

XML Components (cont.) • XML Name Spaces • Vocabulary of all elements and attribute types • Namespace prefix (mapped to Uniform Redource Identifier) • Local Part • Allows use of names defined in other documents • Modularity and reuse of a markup • Mechanisms to establish name scope

XML Components (cont.) • Synchronized Multimedia Integration Language (SMIL) • Language for describing interactive synchronized multimedia distributed on the Web • Several components (images, video, audio) can be linked together to create a presentation on the web • Resource Description Framework (RDF) • Abstract mechanism for defining simple relationships among web resources • Mathematical Markup Language (MathML) • Language to describe mathematical expressions

XML DTD • Defines the hierarchy of all user-defined elements (tags) in the XML document • Declares the attributes and behaviour of each XML element • Each XML document calls a specific DTD file to validate its elements

XML DTD • <?xml version="1.0" encoding="UTF-8"?> •  •  • <!ELEMENT Language (FileTag*,Declaration*,Function_Call*)> • <!ELEMENT FileTag (IncludeTag*,SourceTag*)> • <!ELEMENT IncludeTag (#PCDATA)*> • <!ELEMENT SourceTag (#PCDATA)*> • <!ELEMENT Declaration (Type_Name|Identifier)*> • <!ELEMENT Type_Name (#PCDATA)*> • <!ELEMENT Identifier (#PCDATA)*> • <!ELEMENT Function_Call (Return_Type*,Function_Name*,Argument*)> • <!ELEMENT Return_Type (Return_Var*)> • <!ELEMENT Return_Var (#PCDATA)> • <!ELEMENT Function_Name (#PCDATA)> • <!ELEMENT Argument (parameterName*)> • <!ELEMENT parameterName (#PCDATA)> •  • <!ELEMENT External_Call EMPTY> • <!ELEMENT Graphics EMPTY> •  Defines what other tags are within the <Language> tag Defines data types for contents within the <IncludeTag> tag

XML Document (page 1 of 2) • <?xml version="1.0"?> • <?xml:stylesheet type="text/xsl" href="studentXSL1.xsl" ?> • <!DOCTYPE Language SYSTEM "Student.dtd"> • <Language> • <FileTag> • <IncludeTag>include stdio.h:</IncludeTag> • </FileTag> • <FileTag> • <IncludeTag>include math.h</IncludeTag> • </FileTag> • <FileTag> • <SourceTag>code statement3:</SourceTag> • </FileTag> • <FileTag> • <SourceTag>code statement2:</SourceTag> • </FileTag> • <Declaration> • <Type_Name>char*</Type_Name> • <Identifier>UW</Identifier> • </Declaration> Calls a XSL style sheet Calls a DTD document

XML Document (page 2 of 2) • <Declaration> • <Type_Name>int</Type_Name> • <Identifier>numOfstudents</Identifier> • </Declaration> • <Declaration> • <Type_Name>char*</Type_Name> • <Identifier>facultyName</Identifier> • </Declaration> • <Function_Call> • <Return_Type> • <Return_Var>student_profile</Return_Var> • </Return_Type> • <Function_Name>elec_eng</Function_Name> • <Argument> • <parameterName>name</parameterName> • </Argument> • </Function_Call> • </Language>

XML Namespaces • Latest milestone for W3C's XML technology (14-January-1999 ) • W3C’s definition of XML 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.” • Why use it? • Maintain tag meaningfulness and uniqueness • How does it solve the problem? • Add context to XML tags by using prefix and URL

XSL Document (Page 1 of 3) • <?xml version="1.0"?> • <DIV xmlns:xsl="http://www.w3.org/TR/WD-xsl"> • <html:html xmlns:html="http://www.w3.org/TR/REC-html40"> • <i>This page consists of XML, XSL, Namespace, HTML, and Java Applet</i> • <html:head><html:title><H1>Sample C Code (hidden XML tag)</H1></html:title></html:head> • <xsl:for-each select="Language"> • <TD STYLE="padding-left:1em"> • <DIV><xsl:value-of select="/"/></DIV> • <html:font color="red">The above command prints out all contents within tags without any formmating, ordering, linebreaks, etc.</html:font> • </TD> • </xsl:for-each> • <xsl:for-each order-by="+ IncludeTag" select="Language/FileTag"> • <TD STYLE="padding-left:1em"> • <html:BR></html:BR> • <DIV><html:BR><xsl:value-of select="IncludeTag"/></html:BR></DIV> • </TD> • </xsl:for-each> • <html:font color="red">End of IncludeTag, ascending sort on Include Tag Content</html:font> Namespace for XSL Namespace for HTML

XSL Document (Page 2 of 3) • <xsl:for-each order-by="+ SourceTag" select="Language/FileTag"> • <TD STYLE="padding-left:1em"> • <html:BR></html:BR> • <DIV><xsl:value-of page-break-after="SourceTag" select="SourceTag"/></DIV> • </TD> • </xsl:for-each> • <html:font color="red">End of SourceTag, ascending sort on SourceTag Content</html:font> • <html:BR></html:BR> • <xsl:for-each order-by="+ Type_Name" select="Language/Declaration"> • <TD STYLE="padding-left:1em"> • <html:BR></html:BR> • <DIV><html:BR><xsl:value-of select="Type_Name"/></html:BR></DIV> • <DIV><html:BR><xsl:value-of select="Identifier"/></html:BR></DIV> • </TD> • </xsl:for-each> • <html:font color="red">End of Declaration, ascending sort on Type_Name</html:font> • <DIV></DIV>

XSL Document (Page 3 of 3) • <xsl:for-each select="Language/Function_Call"> • <TD STYLE="padding-left:1em"> • <html:BR><DIV><xsl:value-of select="Return_Type"/></DIV></html:BR> • <html:font color="red">End of Return_Type</html:font> • <html:BR><DIV><xsl:value-of select="Function_Name"/></DIV></html:BR> • <html:font color="red">End of Function_Name</html:font> • <html:BR></html:BR> • <html:BR><DIV><xsl:value-of select="Argument"/></DIV></html:BR> • <html:font color="red">End of Argument</html:font> • <html:BR></html:BR> • </TD> • </xsl:for-each> • <html:BR></html:BR> • <html:APPLET code="AgentAction.class" width="400" height="200"></html:APPLET> • <html:BR></html:BR> • </html:html> • </DIV>

Applications that require XML • Information exchange between heterogeneous databases • Health care example • Distributed processing • Semiconductor industry example • Multiple views of the same data • “Intelligent” information agents

Using XML • XML for Storage • Compact syntax • Generalized and standarized • Product independent • XML for Searching • Use of content specific markup enables robust searching • Search engines need to be XML aware • Can use current SGML search engines

What is DOM? • A programming API for XML • logical structure of document • Access and Manipulation of documents

What is DOM? • As an object model, DOM identifies • Interface and Objects used for the doc. • Behaviours and Attributes • Relationships and Collaborations of Interfaces and Objects

What is DOM? • 2 Major Components for DOM Level 1 • DOM Core = Basic functionalities for XML • DOM HTML = Objects and Methods specific to HTML • Level 2 • DOM CSS, DOM Event, DOM Filters and Iterators, DOM Range

Advantages of using DOM • Easy to create, navigate, add, modify documents • DOM abstraction avoids implementation dependencies • DOM applications may use additional language bindings

A Typical DOM Structure <condition_statement> <if_statement> <if_tag> if </if_tag> <expression_tag> (b == c) </expression_tag> <statement_tag> {a += c} </statement_tag> </if_statement> </condition_statement>

<condition_statements> <if_statements> <if_tag>> <expression_tag> <statement_tag> if (b==c) {a+=c} A Typical DOM Structure (2)

A Typical DOM Structure (3) • DOM abstraction is a Tree or Forest Structure • Users have full flexibility to specify the structure • Structural Isomorphism

Some Key Objects • Node • Tree node of the document • root node, parents and children • Element (is a Node object) • Elements of a document • Represents contents between the start tag and end tag • Attributes: defined by DTD

Some Key Objects (2) • Document • root node of a document • NodeIterator • iterates over a set of nodes specified by a filter • AttributeList • collection of Attribute objects, indexed by attribute name

Some Key Objects (3) • Attribute • attribute of an Element Object • DocumentContext • respository for metadata about a document • DOM • provides instance-independent document operations

Memory Management for DOM • DOM APIs operate across a variety of memory implementation methods: • Language platforms that do not expose memory management to user • Language (Java) that provides constructors with Garbage collection capability • Language (C/C++) that requires explicit memory allocations

Resources/Quirks • IE 5 and Navigator 5.0 implement different features: • IE 5.0 - XML/XSL Navigator - XML/CSS • Navigator to support RDF • XML Resources: • http://www.swen.uwaterloo.ca/~group1

Using XML (cont.) • XML for Presentation • Convert to HTML at server • Use Java applications to render in browser • Slow • Use XSL or CSS to render in browser • Fast

XML in the industry • Explosive growth of XML tools and specifications • Tools: JADE, MSXML, JUMBO,... • Specifications: CDF, CFML,EDI • Browsers: IE, Netscape

Thoughts on XML • Seems like a transition stage between HTML and SGML • Will we eventually end up using SGML? • XML follows basic principles of SE • Higher abstraction layer • Reuse • Modularity

References • XML.COM - A guide to XML • http://www.xml.com/xml/pub/w3j/s3.walsh.html • XML.COM - The Road to XML: Adapting SGML to the Web • http://www.xml.com/xml/pub/w3j/s1.discussion.html • The Computer Bulletin - The XML Files • http://www.bcs.org.uk/publicat/ebull/may98/xml.htm • XML, Java, and the future of the Web • http://sunsite.unc.edu/pub/sun-info/standards/xml/why/xmlapps.htm • XML: What is it • http://iai.sgml.com/980106-01.asp • Why do we need XML? • http://info.admin.kth.se/SGML/Konferenser/xml98sve/seminar.html • An Introduction to the Standard Generalized Markup Language • http://www.personal.u-net.com/~sgml/sgml.htm • SGML101 • http://www.uslynx.com/sgml101.htm

Introduction to XML