Xml basics

XML Document
• Physical View
• Logical View

XML Document: Physical View
• Either Markup or Character Data
• To store contents in one or more les
• Collectively they represent an XML document

XML Document: Logical View
• Structure of document
• XML document is viewed as tree
• Elements divide the document into its
constituent parts
• Elements are organized into a hierarchy

Document Structure
• Root element: Top element of the hierarchy
• Also called Document Element
• Root element defines boundary
• It encloses all the other elements
• Only one root element in a document
• XML documents are commonly stored as text files
• Any text editor may be used to create XML document
• Use extension .xml for clarity
• XML document does not contain formatting
information

Parsing XML document
• The XML parser reads the XML document,
checks its syntax, reports any errors and
allows programmatic access to the
document’s contents.
• If the document is well formed, the parser
makes the document’s data available to the
application (i.e., IE5), using the XML
document.

An Example XML Document

<?xml version=“1.0"?>
<!DOCTYPE Greeting SYSTEM “wel.dtd">


<Greeting>
<from>Instructor</from>
<to>CSIB 313 Class</to>
<myGreetings>
<greetings>Welcome to XML</greetings>
</myGreetings>
</Greeting>

Comments
• Tags: Enclosed between <>
• Tags demarcate and label the parts of
documents
• Comment start tag: 
• May extend several lines

Document Prolog
• XML document: Prolog and Body
• It may hold additional information
• Text encoding, Processing Instructions, and
Document Type Definition being used
• Ordering between prolog and body significant

XML declaration
• Syntax: <?xml name1 = “value1" name2 =
“value2" ...
• Three properties can be set
– version, encoding, and standalone
• <?xml version=“1.0" encoding=“US-ASCII“
standalone=“yes“ ?>
• Note: single and double quotes enclosing
values

Contd.
• All properties are optional
• Property names must be in lowercase
• Values must be quoted: single or double
• Desirable to include version

XML Character Set
• A character set consists of the characters that
may be represented in a document.
• ASCII: 7 bit coding
• ISO: 8 bit coding
• ISO-8859-1,-2,-3,..
• XML document may contain: carriage return,
line feed, and Unicode characters

Contd.
• The Unicode consortium: www.unicode.org
• Unicode: 16 bit code
• Encodes major scripts of world
• Universal Character System(UCS): 32 bits
• ISO-10646: Lower 2 bytes are Unicode

Character Encoding
• Each subset for a script
• This subset for a script is mapped to 8 bit code
• The characters are in same order, but starts at
lower number
• The mapped subset is known as character
encoding
• Most common encoding scheme: UTF-8
• UTF-8: Efficient to encode documents having
ASCII characters

Contd..
• UTF-8 is default XML character encoding
• Some common character encodings:
– US-ASCII, ISO-8859-1, ISO-8859-n
– ISO-8859-1-Windows-3.1-Latin-1
– UTF-7, UTF-8, UTF-16
– ISO-10646-UCS-2, ISO-10646-UCS-4
– UCS-2 is same as Unicode

Elements
• Elements are building blocks
• Elements are organized in an hierarchy
• Hierarchy defines the logical structure
• Elements acts as containers and labels
• Types of the elements are differentiated by tags
• Start-tag, End-tag, Empty-tag
• Empty-element tag: <name/>
• Start-tag must have a matching end tag
• XML IS CASE SENSITIVE
• <message> ....</Message> : Wrong

Attributes
• Attributes describe elements
• An element may have zero or more attributes
• Attributes are placed within element's start
tag
• Only one occurrence of each attribute
• Example: <car doors = “4"/>
• Attribute doors has value “4"

Elements and Attributes Names
• Can be of any length
• Must begin with a letter or an underscore
• May contain letters, digits, underscores,
hyphens, and periods
• Names are case sensitive
• Example:
<instructor Designation=“Professor"> PKD
</instructor>

Reserved Attributes
• xml:lang: Classies an element by language
• xml:lang=“en"
• xml:space: Species whether whitespace
should be preserved
• xml:space=“default"
• xml:link and xml:attribute

White Space Characters
• Spaces, tabs, line feeds and carriage return
• An XML parser is required to pass all
characters in a document
• Application need to decide the significance
• Insignificant white spaces may be collapsed
into single white space character or removed
• This process is called normalization

Entity References
• Reserved characters: <, >, & , `, “, etc.
• To use these characters in content: Entity
References
• Entity references begin with & and ends with ;
• Unicode may be used in document
• د أ ..

Built-in Entities
• XML provides built-in entities
• &, <, >, ', and &quote;
• Example:<message><>&quote;</messag
e>

CDATA Section
• CDATA sections are helpful for XML authors
• may contain text, reserved characters (e.g., <) and
whitespace characters
• It is set off by <![CDATA[ and ]]>
• Everything between <![CDATA[ and the ]]> is treated as raw
data
• Character data in a CDATA section is not processed by the
• XML parser
• In CDATA < is not start tag
• An acronym for “character data"
• An example:
<para> Then you may say <![CDATA[ if(&x < &y) ]]> and
be done with it.</para>

Entities
• An entity is a placeholder for content
• Declared once and may be used several times
• Does not add anything semantically
• Convenient to read, write and maintain XML
document
• It represents physical containers such as files
or URLs

Contd..
• Can be used for different reasons, but always
eliminate an inconvenience
• Standing in for impossible-to-type characters to
marking the place where file should be imported.
• Entities can hold a single character, a string of text,
or even a chunk of XML markup
• Without entities, XML would be much less useful.
• For e.g. define an entity w3url to represent the
W3C's URL. Whenever you enter the entity in a
document, it will be replaced with the text
http://www.w3.org/.

Different types of entities
• Entities are classified as: Parameter and General
• Parameter entities are used in only DTDs
• General entities: Character, Unparsed, and mixed-
content (General entities are placeholders for any
content that occurs at the level of or inside the
root element of an XML document)
• Mixed-content: Internal and External
• Character: Predefined, Numbered, and Named

Contd..
• An entity consists of a name and a value
• The value may be anything (a single character to a
file of XML markup)
• As the parser scans the XML document, it encounters
entity references(ER), which are special markers
derived from entity names.
• For each ER, the parser consults a table in memory
for something with which to replace the marker. It
replaces the entity reference with the appropriate
replacement text or markup,
• Two syntax for entity reference:
– General entities: &entity_name;
– Parameter entities: %entity_name;

Example
<!DOCTYPE message SYSTEM “xmldtds/message.dtd" [
<!ENTITY client “Mr. John">
<!ENTITY agent “Ms. Sally">
<!ENTITY phone “<number>8755944998 </number>“ >
]>
<message>
<opening>Dear &client;</opening>
<body>We have an exciting opportunity for you! A set of ocean-
front cliff dwellings in Piñata, Mexico have been renovated
as time-share vacation homes. They're going fast! To reserve a
place for your holiday, call &agent; at &phone;.
Hurry, &client;. Time is running out!</body>
</message>

Contd..
• The entities &client;, &agent;, and &phone; are
declared in the internal subset of this document
and referenced in the <message> element.
• A fourth entity, ñ, is a numbered character
entity that represents the character ñ.
• This entity is referenced but not declared;
• no declaration is necessary because numbered
character entities are implicitly defined in XML as
references to characters in the current character
set.

• All entities (besides predefined ones) must be
declared before they are used in a document
• Two acceptable places to declare them are
– in the internal subset, which is ideal for local
entities, and
– in an external DTD, which is more suitable for
entities shared between documents.

Character Entities
• Character entities: Contain single character
• Classified into several groups
• Predefined character entities: amp, apos, lt,....
• Numbered character entities: &#231, &#xe7
• Hexadecimal version is distinguished with x

Contd..
• Named character entities
• They have to be declared
• Easy to remember mnemonic
• Large numbers have been defined in DTDs
• ISO-8879: Standardized set of named
character entities

Mixed Content Entities
• Unlimited length
• May include markup as well as text
• Categories: Internal and External
• Internal: Replacement text is defined in the
declaration
• &client; &agent; &phone;

Contd..
• Predefined character entities must not be
used in markup if they are used in entity
definition
• Exceptions: quote and apos
• Entities can contain entity reference if it has
been declared
• Recursive declaration not permitted

• External: Replacement text in another file
• Useful for sharing the contents
• Breaks a document into multiple physical
parts
• A linking mechanism

An example:
<!DOCTYPE doc SYSTEM “http://www.dtds.com/generic.dtd" [
<!ENTITY part1 SYSTEM “p1.xml">
]>
<longdoc>
&part1;
&part2;
&part3;
</longdoc>

• Replacement text is inserted at the time of parsing
• &part1;, &part2;, and &part3; are external entities
• Keyword SYSTEM is used.
• File names are under single or double quotes.
• System identifier: To identify resource location.
• Quoted string may be URL
• Alternative is to use keyword PUBLIC
• XML processor will locate the resource
• Public identifier may be accompanied with a system
identifier

Unparsed Entities
• Kind of entity that holds replacement
text/content that should not be parsed
• Because it contains something other than text and
would likely confuse the parser
• Used to import graphic, sound files and other
non-character data.
• Declaration looks similar to that of an external
entity, with additional information

Example
An Example:
<!DOCTYPE doc [
<!ENTITY mypic SYSTEM “photos/erick.gif“
NDATA GIF>
]>
< doc >
&mypic;
</doc>

Contd..
• Keyword NDATA used after system identifier
• NDATA: Notation for data
• tells the parser that the entity's content is in a
special format, or notation, other than the
usual parsed mixed content.
• It is followed by notation identifier that
specifies the data format.

Processing Instructions
• Presentational information should be kept out of
document
• For example: if we want to store page numbers
in the document
• The prescription for this kind of information is a
processing instruction
• It is container for data that is targeted toward a
specific XML processor

Contd..
• PI contain two pieces of information: a target
keyword and target data
• The parser passes processing instructions up to the
next level of processing.
• If the processing instruction handler recognizes the
target keyword, it may choose to use the data;
otherwise, the data is discarded.
• A PI starts with a two-character delimiter
– consisting of an open angle bracket and a question mark
(<?),
– followed by a target and an optional string of characters
that is the data portion of the PI
– a closing delimiter, consisting of a question mark and
closing angle bracket (?>).

Contd..
• The target is a keyword that an XML processor uses
to determine whether the data is meant for it or not
• More than one program can use a PI, and a single
program can accept multiple PIs.
• The PI can contain any data except the combination
?>
• Examples:
– <? flubber pg = 9 recto ?>
– <? things ?>
– <title> The Introduction to the XML <? lb ?> portability <?
lb ?> and integratin</title>, where <? lb ?> forces line
break

Well-Formed Documents
• An XML parser or processor
• Parses XML documents
• Produce parse tree
• Document is not well-formed if parser is not
able to generate tree
• Every XML document must be well formed

Well-Formed Documents (Contd...)
• Some of the syntactic rules checked
– Case sensitive: message and Message are different
tags
– Single root element
– A start and end tag for each element
– Properly nested tags
– Quoted attribute values
– Comments and processing instructions may not
appear inside tags

Document Validation
• A valid document includes a Document Type
Declaration
• The DTD defines rules for the documents
• These are additional constraints
• Validating parser compares documents to
their DTDs
• The DTD lists all elements, attributes, and
entities the document use and their contexts.

Document Modeling
• Define a Markup Language for documents
• Define a grammar for documents
• It is also called as XML Application Modeling
• Markups may appear in the documents
• It describes the restrictions which each
document instance has to honor

Document Type Definitions
• Document Type Definition is written in formal syntax
• Describes elements, attributes, entities, and contents
which may appear in the documents
• Validating Parser compares documents to their DTDs
• The DTD does not say:
– What is the document's root element?
– How many instances of each element?
– What character data are inside elements look like?
– What is the meaning of an element?

A Simple DTD Example
<!ELEMENT person (name, profession*) >
<!ELEMENT name (first name, last name) >
<!ELEMENT first name (#PCDATA) >
<!ELEMENT last name (#PCDATA) >
<!ELEMENT profession (#PCDATA) >

Contd..
• The DTD describes person element
• The person element has two children elements /
sub-elements
• Sub-elements are: name and profession
• A person may have “zero or more" profession
• The name has two sub-elements

Document Type Declaration
• A valid document includes a reference to its
DTD
• The DTD declaration is included in prolog
• <!DOCTYPE person SYSTEM
“http://www.upes.ac.in/xml/dtds/person.dtd"
>
• The DTD is generally stored in separate file
• Optionally, it may have extension .dtd

Contd..
• Root element is person
• The DTD can be found at the URL
• Relative URL may be used if DTD is on same
site
• File name may be used if the document and
the DTD are in the same directory
• DTD may be stored at several URLs

Xml basics

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Xml basics