Lecture Introduction to XML Lecture Introduction to XML What is XML.pdfWhat is XML

7/25/2019 Lecture Introduction to XML Lecture Introduction to XML What is XML.pdfWhat is XML

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Copyright IBM Corporation 2004

Course materials may not be reproduced in whole or in part without the prior written permission of IBM.

Welcome to:

3.1

What Is XMLWhat Is XML??


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Unit Objectives

After completing this unit, you should be able to:

Describe the basic rules of XML

Describe what it means for an XML document to be well-formed

List the components that make up an XML document

Differentiate between XML and HTML

Describe the internationalization support in XML

Define some best practices for XML


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What Is XML?

At its core XML is text formatted to follow a well-defined set of rules.

XML documents consist primarily of tags and text.

If you've ever seen the source to an HTML document, then theXML structureshould look familiar

This text may be stored/represented in:A normal file stored on disk

A message being sent over HTTP

A character string in a programming language

A CLOB (character large object) in a database

Any other way textual data can be used

XML documents do notneed to exist as documents --they may be:

Byte streams sent between applications

Fields in a database record

Collections of XML Infoset information items

For simplicity they will be referred to as though they aredocuments and files.


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XML documents should be thought of as a hierarchical tree

structure.

Example Tree Representation of XML

"Tom

Wolfe"

"$6.00""TheRight

Stuff"

ROOT

=

Tom Wolfe

The Right Stuff

$6.00



"Optional" first line; only required ifencoding IS NOT UTF-8 or UTF-16*

Root element start tag

Alphabet from A to Z

First child element with data

Empty element (no data)

Begin element tag

Boreng Riter Nested child elements

End element tag

The letter A is the first in the alphabet. It is also the first of five vowels.

Element containing an attribute andparsed character data (PCDATA) [TBD]

Comment

The letter Z is the last letter in the alphabet.

Last element in document

Root element end tag

A Simple XML Document - Basic Structure



A Simple XML Document -Basic Nomenclature

The XML instance on the previous page consists of:

One main elementbook

Subelementstitle, isbn, author, chapter, and comment

Authorcontains other subelements firstNameand lastNameISBNand chaptercontain attributesnumberand title, respectively

Title, firstName, and lastNamecontain only strings:

Elements that contain numbers, strings, dates, and so forth (TBD) but no

subelements (or attributes) are said to have simple types

ISBN and chapter carry attributes; author has subelements:

Elements that contain subelements or carry attributes are said to havecomplex types

Attributes always have simple types (that is, they are numbers, strings,dates, and so forth.

TBD -- In a later chapter we describe XML Schemas which have access to

a collection of built-in simple types



Basics of Well-formedXML (1 of 2)

XML documents are considered to be well-formedwhen they

adhere to a set of five rules that define basic XML syntax andstructure + a sixth for worldwide conformity.

1. There must be a single root element:

All other elements are nested inside the root element

2. Elements must be properly terminated:For every opening tag"" there must be a matching closing tag

""The exception is an empty (no content or body) tag ""

3. Elements must be properly nested underneath a parent tag

(except for the single, root element):A nested tag-pair may not overlap another tagThere is no limit to the nesting level of children elements



Basics of Well-formedXML (2 of 2)

4. Tag names are case sensitive:

All tag and attribute names, attribute values, and data must complywith XML naming rules.

5.Attributes, extra information that can be provided for elements,

must be properly quoted:That is, all attributevalues must be in quotes.

6. The first line should/must contain the special tag that identifies

the version of the XML specification to apply:XML 1.0 is currently the most common.



Element Rules - Rule 1. Single Root Element

All XML documents must have a single root element.

Legal: Not legal:

red green

red

green

Colors is the root element forthis XML.

Color represents multiple rootelements.



Element Rules - Rule 2. Element Tag Rules

Elements consist of start and end tags.

End tag is identified by the /.Example: red

Elements may contain attributes within the start tag.

Example: Note: The attribute is isbn.

Empty elements contain no child elements or data.

These elements can be represented with a special shorthandnotation.

Example:

Can be shortened to:(preferred)

Or, if the element has no data as:



Element Rules - Rule 3. Element Nesting

Elements must be properly nested.

The end tags of inner elements must occur before the end tags ofouter elements.

Any number of child elements or data may be nested within the startand end tags of an element.

El t N ti E l



Element Nesting Example

Legal: Not legal: Polo

red large

large red Polo

All elements are properly nested.

The element tags are mixed up

and not ordered.

Best Practice:Use indentation to represent the document's hierarchy.

Important if your document will likely be read by humans.Computers and programs don't usually care.

El t R l R l 4 XML N i R l


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Element Rules - Rule 4. XML Naming Rules

XML name construction:

The first character must be A-Z, a-z, or _ (underscore)Any number of subsequent letters, numbers, hyphens,periods, colons, and underscore characters.

XML names are case sensitive.Names cannot contain spaces.

Names must not have a prefix of xml in any case combination

(such names are reserved).Best Practice:Brevity in tag names is not necessary.

Use descriptive names for elements and attributes.

oris far better than.Best Practice:Maintain standard naming conventions andquoting.

Camelback, dot and underscore notation are all common(For example, camelBackNotation, dot.notation, andunderscore_notation).

R l 4 T N i S l


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Rule 4. Tag Naming - Samples

Legal Not Legal Commentstitle, book.isdn,lastName, _street,addrLine1, name:first

1name, -street,&name

Examples of legal andillegal element names.

red

small

red

small

Element names arecase sensitive andstart and end tagsmust match.

John

John

Element names must

not contain spaces.

John

John

Elements must notcontain any W3Creserved words.

R l 4 El t C t t (1 f 2) G l


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Rule 4. Element Content (1 of 2): General

An XML instance is composed of elementsexpressed in tag pairs

(except for empty tags) plus optional attributesthat always havequoted values and optional datathat appears between the elementstart tag and the element end tag.

Mixed content - element content that contains data (PCDATA is

shown) and other elements.

Example (snippet):XMLExample

Chapter informationWhat is XML

What is HTMLMore chapter information

Rule 4 Element Content (2 of 2): Data


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Rule 4. Element Content (2 of 2): Data

Element data content is handled in one of two ways:

1. Parsed Character Data (PCDATA): is examined by the XMLparser to discover XML content embedded within it.

2. Character Data (CDATA): is delimited by the special syntax and is not processed by the parser.

Rule 4 PCDATA Parsed Character Data


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Rule 4. PCDATA - Parsed Character Data

Predefined entities exist to address ambiguous syntax situations,

situations where the literal would be interpreted as part of theXML document syntax rather than its content.

Examples:> 6& < 20

Entity Description Character

< "less than" "greater than" >

& "ampersand" &' "apostrophe" '" "quote" "

Rule 4 CDATA Character Data


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Rule 4. CDATA - Character Data

Syntax:

Note: Anything except the literal string "]]>";to embed "]]>" use "]]>"

CDATA is not parsed and is treated as-is.Useful for embedding other languages within the XML.

HTML documents.

XML documents.

JavaScript source.

Or any other text with a lot of special characters.

Generally speaking the escaping rules inside a CDATA section are

those of the embedded languageFor example, to escape an ampersand in Javascript use &.

Rule 4 CDATA Examples


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Rule 4. CDATA Examples

These script elements contain JavaScript:

This nameXML element stores actual XML to be treated as text:

{ return 1 } else { return 0 }}

]]>

{ return 1 } else { return 0 }}]]>

Sir Frederick of Ledyard's End

]]>

Element Rules - Rule 5 Element Attributes


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Element Rules - Rule 5. Element Attributes

Attributes are used to attach information to elements.

Attributes consist of a name="value" pair, where the name is a legalXML name. This is often referred to as a "key-value" pair.

Attributes are placed in the start tag of the element to which they

apply.An element may have several attributes, each uniquely named.

Examples:XML overview

Yacht

Notice the different usage of the attribute "type" in the two elements;semantically they are not the same.

Attributes must have a value.

Values must be quoted with either double or single quotes.

Convention is to stick with one or the other.

Element Rules - Rule 6


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Element Rules - Rule 6.XML Declaration (1 of 2)

The XML Declaration is an optional first line in all XML documents:

If this declaration is used, the version attribute is mandatory.The encodingattribute indicates the character encoding used in thedocument; if UTF-8 or UTF-16 is used it may be omitted.

ASCII is a subset of UTF-8 and need not be declared.

Comments are notallowed before this statement.The XML Declaration follows the syntax of a Processing Instructionor PI,which is described on a subsequent chart, but it is considered to beunique and is treated separately in the 1.0 XML specification.

GENERAL NOTE OF CAUTION: You can not always rely on a browser ortool to completely/correctly enforce the specifications. Nor are thespecifications alwayswritten in language that, to a particular reader, is

unambiguous. Still, the best advice is when in doubt, refer to thespecification, which for XML is www.w3.org/XML.

Element Rules - Rule 6


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The stand-aloneattribute is included here for completeness: it is used toindicate if this XML document depends on information declared externally tothis document (in a DTD or XSL file (TBD), for examples); value may be yesor no.

A value of "yes" indicates there are no external markup declarations; ifthere are no external markup declarations, the declaration has nomeaning.

A value of "no"indicates there are or may be such external markupdeclarations; if there are such declarations but there is no standalone

declaration, "no" is assumed.. . . so it is typically not used.

In any event, the inclusion in the XML instance of references to externalentities, such as those in an embedded DTD, does not change its

standalonestatus.

A bigger issue associated with the stand-aloneattribute is that of defining orsetting values in anyentity that may be external to the XML instance.

Arguably, the principal reason for using XML is that it explicitly defines theelements it includes. If attribute values are overridden then the XMLinstance before us is no longer declarative.

Element Rules - Rule 6.XML Declaration (2 of 2)

Comments


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Comments

Defines a comment.A space after the beginning and before the trailing hyphens isrecommended but not required.

A is the first letterZ is the last letter

Improper usage:


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Internationalization and Encoding (1 of 2)

Support for different character encodings is provided through theencoding attribute of the XML Declaration.

The encoding attribute indicates the set of characters that are

permitted in the document.In the absence of an encoding declaration, Unicode UTF-8 orUTF-16 characters may be used.

Documents exchanged via network may be presented to theprocessor in an encoding format other than the specified encodingas long as the transport protocol (for example, HTTP) indicates theencoding used.



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It is very important that the editor and operating system used to

write and save an XML document support the encoding specified inthe XML Declaration.

Sample encoding declarations:

ASCII (subset of UTF-8)

16 bit UNICODE...

Japanese

...

Note: Encoding names are case-insensitive

Processing Instruction


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Processing Instruction

Syntax

Processing Instruction is often abbreviated as PI indocumentation.

A feature inherited from SGML.

Used to embed application-specific instructions in documents.

The target name immediately follows "


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e o ed e sus a d

A well-formed XML document:

Consists of XML elements that are nested within another.Has a unique root element.

Follows the XML naming conventions.

Follows the XML rules for quoting attributes.

Has tags that are properly terminated.All XML parsers check for well-formedness.

A valid XML document has an associated vocabularyand obeys the

structural rules specified by that vocabulary.Associated vocabulary is typically defined by either a DTD or anXML Schema.

XML parsers may be validating or non-validating depending upon

whether or not they can apply an associated grammar.Studio is an example of a tool whose XML capabilities includevalidation.

HTML versus XML (1 of 2)


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XML is about structured informationinterchange

HTML is about presentation andbrowsing

( )

Java ProgrammingEECS

Paul Thompson

Ron Jones

Uma Abingdon

Lindsay Garmon

HTML versus XML (2 of 2)


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( )

HTML XMLCourse Roster Course Roster

XML Programming Department: EECS

Teacher Paul Thompson Student
List Ron Jones
Uma Abingdon

Lindsay Garmon

Java Programming EECS

Paul Thompson Ron Jones Uma Abingdon Lindsay Garmon


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Checkpoint Questions (1 of 3)


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p ( )

1. Basic XML can be described as:

A. A hierarchical structure of tagged elements, attributes and text.B. All the HTML tags plus a set of new XML only tags.

C. Object-oriented structure of rows and columns.

D. Processing instructions (PIs) for text data.

E. Textual data with tags for visual presentation.

2. Which of these XML fragments is not well-formed?

A. XMLB. XML

C.

D. XMLXML

E. XML



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3. XML Comments are allowed (Select all that apply):

A. Before the XML DeclarationB. Anywhere

C. Between element tags

D. Before the root element

E. All of the Above

4. Which of these XML elements with attributes is not well-formed?

A. B.

C.

D.E.

F. All of the Above



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5. Which of these comments regarding HTML and XML is not true?

A. HTML markup is focused on presentation.B. XML markup is based on defining the data.

C. XML is based on HTML.

D. HTML tags are not case sensitive.

E. XML tags are case sensitive.F. Both XML and HTML support attributes.

Unit Summary


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Having completed this unit, you should be able to:

Describe the basic rules of XML

Describe what it means for an XML document to be well-formed

List the components that make up an XML document

Describe the differences between XML and HTML

Describe the internationalization support in XML

Describe some best practices in XML