1. X-Informatics: I-400 and I-590 An Introduction to XML
Spring Semester MW 6:00 pm – 7:15 pm Indiana Time
Geoffrey Fox and Bryan Carpenter
PTLIU Laboratory for Community Grids
Informatics, (Computer Science , Physics)
Indiana University
Bloomington IN 47404
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2. Outline of Introduction to XML
The two drivers for XML
Original: A better way of specifying documents that is more powerful than HTML and easier to understand than SGML
Current: XML as a object structure for totally general entities
Basic XML: well formed and valid XML
Examples of use of XML
XML Syntax – this presentation
Further Presentations on
XML Schema
XML based Document Object Model and style sheets
Transforming XML documents XSLT
Searching XML documents
Applications of XML – Dublin Core, RDF, SVG, SOAP
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3. An Information Nugget
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4. Nugget In XML
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5. Essential Issues We have a world of objects
Objects are instances of “classes of object”
This SONY Viao is an instance of the general SONY Viao laptop class
This SONY Viao laptop class is a subclass of the laptop class
Object classes are made up of smaller things – which have “types”
Color is a special type of variable with a 24 bit (or some other) specification
String (characters) is a type (simple type); Integers are a type etc.
Date is also a simple type in XML as so common
XML has Schema to specify classes and these are made up of simple types and complex (complicated) types
Further one Schema can be built from one or more other Schemas
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6. An Example of RDF and Dublin Core
<rdf:RDF xmlns:rdf=" xmlns:dc="
<rdf:Description about="
<dc:Title>D-Lib Program - Research in Digital Libraries</dc:Title> <dc:Description>The D-Lib program supports the community of people with research interests in digital libraries and electronic publishing. </dc:Description> <dc:Publisher>Corporation For National Research Initiatives</dc:Publisher> <dc:Date> </dc:Date>
<dc:Subject>
<rdf:Bag> <rdf:li>Research; statistical methods</rdf:li> <rdf:li>Education, research, related topics</rdf:li> <rdf:li>Library use Studies</rdf:li> </rdf:Bag>
</dc:Subject> <dc:Type>World Wide Web Home Page</dc:Type>
<dc:Format>text/html</dc:Format>
<dc:Language>en</dc:Language>
</rdf:Description> </rdf:RDF>
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7. XML Example for SOAP
This is way to use XML to send command ls (list files) from one machine to another
First argument
HTTP Header
SOAP Envelope
With body
Specify ls as
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8. XML Example II HTTP Header
SOAP Envelope and body
XML Example II
And this is the result of ls sent back to client in SOAP over HTTP
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9. Overview of HTML HTML = Hypertext Markup Language
the lingua franca of the World Wide Web
HTML is a simple language well suited for hypertext, multimedia and the display of small and reasonably simple documents
HTML 2.0 spec completed in Nov 95
HTML+ and HTML 3.0 never released
HTML 3.2 (Jan 97) added tables, applets, and other capabilities (approximately 70 tags)
this is what most people are familiar with today
HTML 4.0 spec released in Dec 97
XHTML (XML Version of HTML 4.0) released January 2000 as a W3C recommendation
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10. W3C Process
The Web Consortium has a highly effective process for initiating and refining standards for the web
The agreed standards for protocols and API’s are as critical to success of the web as are technologies
The process to define standards involve moving from Working Draft to Last Call Working Draft to Candidate Recommendation to Proposed Recommendation and finally to Recommendation.
The standards discussed here are quite recent
XML Schema became a recommendation May
SVG (2D Vector Graphics done in XML – relevant for animated web pages) became a recommendation September
XQUERY (a proposed way of searching XML datastructures/documents) is currently a working draft dated December
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11. XML in the HTML world
XML = eXtensible Markup Language (name suggests documents not objects)
XML was originally designed as a subset of SGML -- Standard Generalized Markup Language, but unlike the latter, XML was specifically designed for the web and for comparative simplicity
Specification of W3C: and lots of links like
XML 1.0 in February 98, with continuing refinements
How XML fits into the Browser world:
XML with Application Specific Schema describes the logical structure of the document.
CSS (Cascading Style Sheets) or other style language describes the visual presentation of the document.
The DOM (Document Object Model) allows scripting languages, such as JavaScript to access document objects.
DHTML (Dynamic HTML) allows a dynamic presentation of the document.
XHTML is XML Syntax for specifying Text Display – so HTML just does DISPLAY; XML does “Knowledge” and DISPLAY
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12. Informatics View of Architecture
Raw Data
Resource
XML for Data
(Virtual) XML Interface
Note Server Tier uses lots of subsystems that are themselves separated by XML Interfaces
Processing
Server
Information/Knowledge
(Virtual) XML Interface
XML for Knowledge
Rendering to XML syntax Display Format
XHTML and SVG
are examples
Clients
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13. The original Motivation for XML as an enhancement of HTML separating Display and Knowledge
Limitations of HTML:
Extensibility: HTML does not allow users to specify their own tags or attributes in order to parameterize or otherwise semantically qualify their data.
Structure: HTML does not support the specification of deep structures needed to represent database schema or object-oriented hierarchies.
Validation: HTML does not support the kind of language specification that allows applications to check data for structural validity when it is imported.
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14. Logical vs. Visual Design
This is XML used as Interface between Knowledge and Rendering
The logical design of a document (content) should be separate from its visual design (presentation)
Separation of logical and visual (rendering) design
promotes sound typography
encourages better writing
is more flexible
Allows the same “knowledge/information” (defined in XML) to be displaced on PC’s, PDA’s, Braille devices etc.
XML can be used to define the logical design, while the XSL (Extensible Style Language) is used to define the visual design (usually by mapping XML into HTML but better XML for Knowledge into XHTML or SVG or ….).
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15. What is SGML?
SGML = Standard Generalized Markup Language defined as an ISO (not W3C) standard (ISO8879) in 1986
A SGML document carries with it a grammar called a Document Type Definition (DTD). The DTD defines the tags and the meaning of those tags
DTD syntax is not very nice
Presentation is governed by a style sheet written in the Document Style Semantics and Specification Language (DSSSL)
Note that HTML is a fixed SGML application, a hard-wired set of about 70 tags and 50 attributes, and does not need to have a DTD for each HTML instance.
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16. SGML Example
A simple SGML document with embedded DTD: <!DOCTYPE DOCUMENT [ <!ELEMENT DOCUMENT O O (p*,BIGP*)> <!ELEMENT p - O (#PCDATA)> <!ELEMENT BIGP - O (#PCDATA)> ]> <DOCUMENT> <p>Welcome to <BIGP>XML Style! </DOCUMENT>
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17. SGML Example (cont’d)
A corresponding DSSSL style sheet: <!DOCTYPE style-sheet PUBLIC "-//James Clark//DTD DSSSL Style Sheet//EN"> (root (make simple-page-sequence)) (element p (make paragraph)) (element BIGP (make paragraph font-size: 24pt space-before: 12pt))
DSSSL is simplified as XSL just as XML simplifies SGML
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18. XML as a simple SGML
XML is also an SGML application, but since XML is extensible (XML can be considered a metalanguage), every XML document must be accompanied by its DTD
XML is a compromise between the non-extensible, limited capabilities of HTML and the full power and complexity of SGML
XML offers “80% of the benefits of SGML for 20% of its complexity”
XML designers tried to leave out all the SGML that would be rarely used on the web
Note that XML specification is 30 pages and the SGML specification is 500 pages.
XML allows you to define your own tags and to describe nested hierarchies of information.
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19. Some Global Concepts
We are defining objects – possible just for documents as in SGML
We need to define “object templates” or their structure
This is class for Java
This is DTD for SGML and XML or Schema for XML
We have instances of objects
XML files or Java Objects with some way (optional for XML) of specifying
We need to transform objects
We can do this with “real software” i.e. read object into program, interpret and spit out in a different form
We can use specialized transformation language with some control data –this is DSSSL plus stylesheet for SGML; XSLT plus stylesheet for XML; browser plus CSS stylesheet for HTML
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20. XML Design Goals 1) XML shall be usable over the Internet
2) XML shall support a variety of applications
3) XML shall be compatible with SGML
4) It shall be easy to write programs that process XML documents
5) Optional features in XML shall be kept to the absolute minimum, ideally zero
6) XML documents should be human-legible and reasonably clear
7) Design of XML should be prepared quickly
8) Design of XML shall be formal and concise
9) XML documents shall be easy to create
10) Terseness in XML markup is of minimal importance
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21. Features of XML I
The documents are stored in plain text and thus can be transferred and processed anywhere.
Inline-reusability - documents can be composed of many pieces
Unifying principles make it easily acceptable
“everything is a tree”
UNICODE for different languages
XML documents enable several types of uses
traditional data processing - XML documents can be the data interchange medium
document-driven programming
archiving
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22. A Tree
<root> <onedown val=“abc”> <twodown> <threedown anotherval=“123”>stuff </threedown> </twodown> </onedown> <nextone> </nextone> </root>
root
onedown
twodown
threedown
anotherval
val
Content(stuff)
nextone
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23. Features of XML II
It is important to remember that XML is a markup language, not a programming language. XSL can be viewed as a way of programming data whose structure is defined in XML
Except this isn’t really correct – you can build a programming language with XML Syntax
<myshell program=“cat” args=“grades” /> <myshell program=“ls” args=“-l” />
M in XML is Markup reflecting its origin in the publication” community with markup specifying layout of document, fonts to use etc.
XML’s most important use is not this original specifying abstract data structures -- equivalent to structures in C++ or classes in Java or Entity relationship in database world
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24. Origins of XML
First draft of XML spec released by W3C in Nov 96 (four other drafts published in 1997)
The first XML parser (written in Java) released by Microsoft in July 97
Microsoft released version 1.8 of its XML parser (which supports XML 1.0) in Jan 98
W3C finalized the XML 1.0 spec in Feb 98
First XML-aware beta versions of Netscape and IE5.0 released in June 98
Sun announced Java Standard Extension for XML (XML API) in March 99
W3C ongoing effort as discussed
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25. HTML becomes XHTML
This shows how near HTML is to XML but also differences!
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26. XHTML II
You must have a body and a head section You canNOT use capital letters in element or attribute names
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27. XHTML III no
yes no or
yes
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28. XHTML IV no
yes
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29. XHTML V
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30. XHTML VI
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31. XHTML VII
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32. XHMTL VIII
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33. XHTML IX Converting HTML to XHTML requires … 11/22/2018
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34. Homework 2 Go to http://www.w3schools.com/xhtml/default.asp
Take XHTML Course
Take quiz returning Either screen dump or Saved HTML of results page
Build a valid XHTML file as your course home page
It should NOT have frames
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35. Homework 2 Continued
Validate your XHTML File and send to
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36. “Hello World!” in XML
An XML document with external DTD: <?xml version="1.0"?> <!DOCTYPE greeting SYSTEM "hello.dtd"> <greeting>Hello World!</greeting>
An XML document with embedded DTD: <?xml version="1.0"? standalone =“yes” ?> <!DOCTYPE greeting [ <!ELEMENT greeting (#PCDATA)> ]> <greeting>Hello World!</greeting>
Current XHTML has a DTD but not a Schema
Next version of XHTML with modules is Schema based
Don’t need to understand DTD to use XHTML
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37. XML and Related Acronyms
Document Type Definition (DTD), which defines the tags and their relationships – to be replaced (IMHO) by XML Schema
Extensible Style Language (XSL) style sheets, which specify the presentation of the document
Cascading Style Sheets(CSS) less powerful presentation technology without tag mapping capability
XPATH which specifies location in document
XLINK and XPOINTER which defines link-handling details
Resource Description Framework (RDF), document metadata
Document Object Model (DOM), API for converting the document to a tree object in your program for processing and updating
Simple API for XML (SAX), “serial access” protocol, fast-to-execute protocol for processing document on the fly
XML Namespaces, for an environment of multiple sets of XML tags
XHTML, a definition of HTML tags for XML documents (which are then just HTML documents)
XML Schema, offers a better alternative to DTD
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38. Document Type Definition
The DTD specifies the logical structure of the document; it is a formal grammar describing document syntax and semantics
The DTD does not describe the physical layout of the document; this is left to the style sheets and the scripts
It is no mean task to write a DTD, so most users will adopt predefined DTDs (or can write an XML document without a DTD).
DTDs can be written in separate files to facilitate re-use.
Content-providers, industries and other groups can collaborate to define sets of tags: the essence of “any” field (physics, music …) is captured in a domain specific DTD/Schema
XML documents are valid if they are consistent with a specified DTD or Schema
We will NOT discuss DTD significantly in this presentation
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39. XML Editors
There are several XML editors at various prices and capabilities
One list of available editors is at
We have good experience with XML Spy which costs money but renewable 30 day licenses are available
The capabilities of editors depends on how well they support Schemas
As XML gets more complicated, expect a new generation of “processing tools” that accept XML as input with multiple Schema and produce some sort of output for people and/or computers
Microsoft XML Notepad is simple free and dated
has a set of good XML Schema links which inter alia discuss XML
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40. XML must be “well-formed”
For the data contained in an XML document to be parsed correctly, its markup must be well-formed, meaning in part that properly nested and non-abbreviated starting and ending tags are used.
This well-formed-ness provides a well defined encapsulation mechanism allowing designated sections of the data to be accessed programmatically.
Current HTML browsers allow rule violations but XML is strict which is essential for many (robust) applications
If XML was just used to render, then sloppiness allowable but as XML aimed at capturing object structure or information, we cannot have errors interpreted unpredictably by parsers
Well-formed is less restrictive than valid
XML documents must be well-formed – user can decide if need to be valid
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41. Character Data in XML CDATA and PCDATA
XML documents are made up of markup and CDATA (character data)
PCDATA is text gotten from parsing document and processing markup as necessary
“markup” includes
Tags and attributes (ALL that is important), Entity references, Character references, Comments, CDATA Section delimiters, DTD declarations and Processing Instructions
XML allows you to specify chunks of text which may contain “reserved characters/strings” with an ugly syntax
<![CDATA <ignored>Anything </ignored> ]]>
Maybe (hopefully) this will be replaced by alternatives based on ideas like mail attachments – see
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42. Characters in XML
We can choose the character set such as UTF-8 (8 bit ASCII codes for characters) or the official default Unicode (16 bit character codes as used by Java) or even UCS which offers 32 bits for each character. This is specified in the xml processing instruction in the document prolog.
You can use character reference markup
π is Unicode for  wrapped in &# .. ; syntax for a 16 bit (4 hexadecimal symbols) character reference in Unicode (ISO/IEC 10646)
π is also  using decimal form of Unicode
One can use the five built-in entity references
& for &
&apos; for ‘
> for >
< for <
" for “
In the DTD approach (which we are ignoring), one can define arbitrary entity references
&#x----; Hexadecimal (base ABCDEF
&#----; Decimal (base
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43. White Space in XML
XML as default treats spaces, tabs, line feeds and carriage return “just” as white space. Thus <greeting>Hello World!</greeting> and <greeting>Hello World!</greeting> are identical
This is similar to HTML. One can overrule this using attribute xml:space with syntax
<greeting xml:space=“preserve” >Hello World!</greeting>
This attribute must be defined in DTD with
<!ATTLIST greeting xml:space (default|preserve) ‘preserve’ >
defines element greeting to allow an attribute xml:space which can take values default or preserve with latter as default
If you specify xml:space, then it holds not only for given element but all those contained within it.
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44. XML Example
Another example which could be used for URL exchanges between network capable applications: <LINK> <TITLE>XML Recommendation</TITLE> <URL> </URL> <DESCRIPTION> The official XML spec from W3C </DESCRIPTION> </LINK>
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45. XML Example (cont’d) A document may have many such links:
<?xml version="1.0" encoding=”UTF-8” standalone="yes"?> <?xml-stylesheet type=“text/css” href=“fred.css” ?> <DOCUMENT> <LINKS> <LINK>…</LINK> <LINK>…</LINK> … </LINKS> </DOCUMENT>
Here we have also added prolog processing instructions.
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46. XML Prolog and Processing Instructions
Every XML file starts with the prolog, giving information about the document. The minimal prolog identifies it as an xml document <?xml version=“1.0”?>
The prolog may also include the encoding and whether it is a standalone document: <?xml version="1.0" encoding="ISO ” standalone="yes” ?>
If it is not standalone, it may specifiy external “entities” which may be named in the document or an external DTD
An XML file may also contain more general processing instructions for the application processing the document: <?target instructions ?> where target is the name of the application.
Only <?xml … ?> is understood by all XML processors
Specification of a stylesheet by <?xml-stylesheet .. ?> is common
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47. XML Prolog and Comments
The Prolog can contain:
Processing Instructions
DTD Specifications -- we have illustrated these but will not discuss further
Comments
Comments have same form anywhere in the XML document and are just like comments in HTML
<!--This is the Prolog and <tag> Lousy Course</tag> is not treated as a tag-->
You cannot have -- inside comments but <tag> </tag> is not treated as markup
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48. Processing XML
So in the beginning (1999), it was not clear how XML would be used
One (major?) of original goals was specifying content of web pages and this implied processing of XML with “style-sheets” that specified mapping of XML into HTML
Obviously this is some sort of “processing”
XML was so popular that lots of other applications with lots of totally different processing were invented
<?target instructions ?> was insufficient in ability to specify the way processing to be done and not very useful as better always to be modular and NOT associate details of processing with data
So best to ignore XML processing tags unless used in very conventional way such as style sheets.
Modern web page technology tends not to use this way but rather has a separate “configuration file” matching XML and style-sheets
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49. Comments in XML
<!-- --> syntax represents a comment on “file” as ignored by XML parser
This is sometimes useful but more valuable is a comment that is preserved by parser as this can be either thrown away or preserved as you please
Do this with some sort of tag like <yourcomment> This is a comment</yourcomment>
Parsers read XML – check if well-formed/valid and return some sort of answer – in simplest model – this is a modified file
OLD MODEL
XML File
XML Parser
Output
Use processing instructions to control parsing <!-- --> ignored
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50. Role of Parsers New model disassociates data and action on the data
XML Parsers are critical technology – Editors built on top of them but parsers are basis of all use of XML in web services
Output
XML Parser
XML File
“Business
Logic”
Web Service
Specify in XML Web Service (WSDL does this)
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51. XML tag structure
In XML terminology, a pair of start and end tags is an element.
XML documents must have a strict hierarchical structure.
All start tags must have an end tag.
Any element must be properly nested within another.
<LI> XML requires <B><I>proper nesting</I></B>.</LI> is well formed
<LI> XML requires <B><I>proper nesting</I></LI>.</B> would be rejected by an XML Parser
Empty tags (no content except perhaps attributes) are allowed as elements in XML documents.
An empty tag is a start and end tag together and is identified by a trailing / after the tag name. So in XHTML one uses <br/> for the empty break tag. (So empty tags with no attributes are “flags”)
A start tag and end tag with nothing in-between can also be considered an empty tag. <IMG SRC=“face.gif”></IMG>
XML tags are case-sensitive. (<H1> is not the same as <h1>.
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52. Document is a Single Tree
XML documents allow only one root element.
So it must be
<?xml version=“1.0” ?> <rootoftree> ……… </rootoftree>
And not
<?xml version=“1.0” ?> <rootoftree> ……… </rootoftree> <rootoftree> ……… </rootoftree>
So there is only one tree in each document
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53. XML Attributes I
Tags can have any number of attributes (which must be declared inside the DTD or by the schema)
All attribute values must be within single or double quotes. <FONT COLOR=“#FF00CC”> quoted attribute </FONT>
If you have a double quote inside an attribute value, then either
Use " for inside quote as in quote=“"”
Enclose attribute value in single quotes as in quote=‘”’
Each attribute can only appear once in a given element definition
One can choose (matter of taste) between <person name=“Fox” role=“teacher” ></person> and
<person><name>Fox</name><role>teacher</role></person>
Note you can repeat elements but you cannot repeat attributes to represent multiple occurrences
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54. XML Attributes II
Note that with DTD (this changes with Schema), all element and attribute values are text not numbers and so must be “converted” by application to intended form
So <item> weekdays<quantity>5</quantity><item> or <item quantity=“5” >weekdays</item>
Returns string “5” not the number 5 for quantity
xml:lang is a useful attribute (in xml Namespace) which can be used (as always if declared in DTD or Schema as allowed attribute) to specify language
<text xml:lang=“en”>Good English</text> <text xml:lang=“x-youth” >Coolio,Wax On, Wax Off, Dude</text>
xml:lang can take values from an official vocabulary (such as en above which is ISO 639) or your private code starting with x-
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55. XML Names and NMTOKEN
Name Characters are letters, digits, hyphens, underscores, colons or full stops.
An NMTOKEN is any collection of Name Characters
NMTOKENS is any list of NMTOKEN’s separated by white space (space, tab, newline etc.)
Case is significant: PERSON and person are distinct names
Attribute and Element names must be (a subset of) NMTOKEN with restriction
Names cannot begin with a digit
Names cannot begin with xml (or any variant gotten by case changes) – system will use this prefix
Colons are ONLY to be used in Namespaces – currently an informal rule only
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56. CDATA Sections
CDATA sections allow you to include unparsed characters in a document <![CDATA <ignored>Anything </ignored> ]]>
In this example the ignored tag is not processed by XML parser
Unfortunately you must guarantee that there is no ]]> string in the text between <![CDATA and ]]>
<script language=“JavaScript”> <![CDATA var fred = 0; if( fred < 10) { document.writeln(“> and < here are NOT parsed”); } ]]> </script>
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57. XML Namespaces I
This is an extension to XML adopted January 1999 at
Namespaces address problem in DTD that labels (element and attribute names) cannot be repeated; more fundamentally (and for Schemas especially) it provides subroutine or library capability to XML
Suppose you had a XML file with <student> and <faculty> and you wanted to write <student><name>you</name><student> <teacher><name>me<special>Prof</special></name></teacher>
This is invalid unless <name> is identical in structure for both teacher and student, as each element in tree must have unique structure.
We can get round it by using <studentname> and <teachername> but this is not so satisfactory especially if you get this conflict by joining two different sets of tags together
This is seen in XHTML when you could add MathML SMIL SVG tags ….
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58. XML Namespaces II
So we use new syntax xmlns=“ to define an XML Namespace
The value of xmlns is hopefully a useful URL/URI telling you about tags. However this is not required.
Microsoft in its cunning way uses in Office web export:
<xml xmlns:v="urn:schemas-microsoft-com:vml“ xmlns:o="urn:schemas-microsoft-com:office:office“ xmlns:p="urn:schemas-microsoft-com:office:powerpoint">
And teaches Internet Explorer to understand these obscure “universal resource names” for VML Office and PowerPoint Namespaces respectively.
xmlns is an attribute which can be used in any element (depending on parser you may need to declare this as allowed attribute in DTD/Schema)
<student xmlns=“studentschema”><name> ….
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59. XML Namespaces III
And when we come to teacher use <bigboss:teacher xmlns:bigboss=“teacherschema”><bigboss:name> ….
In the above, we made student elements as default
We can more symmetrically write <university xmlns:bigboss=“teacherschema” xmlns:downtrodden=“studentschema” >
<downtrodden:student><downtrodden:name>you </downtrodden:name></downtrodden:student> …….. <bigboss:teacher><bigboss:name>me </bigboss:name></bigboss:student>
</university>
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