XML I

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XML I
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• Learning Objectives
• What is XML
• Features of XML
• Uses of XML
• Structure of an XML document
• Document Type Declaration
• Document Type Definitions (DTDs)

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• What is XML?
• XML means Extensible markup language.
• It is NOT a version of HTML
• Derived from SGML (Standard Generalized Mark-up
  language, which was established in 1986 as a
  standard for generalized electronic document
  exchange.
• Has 3 main features structure, extensibility and
  validation.
• XML defines a framework for transmitting
  structured data, hence an XML document is
  essentially a structured document for storing
  information.
• Allows creation of custom mark-up tags for
  describing virtually anything.
• XML documents are processed by an XML processor.

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• Uses of XML
• Applied use of its capability of storage, and
  exchange of structural data between applications,
  that constitute the core of systems).
• Examples of XML applications are Chemical Markup
  Language (CML), Extensible Financial Reporting
  Markup Language (XFRML), and Mathematical Markup
  Language.
• Used in e-commerce to store, and transmit
  product, and other data, including financial
  information.
• Used in Open Financial eXchange.
• Used in search engines to store, and search data.
• Applied use in virtually every sector.

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• By including, or referencing a Document type
  definition (DTD), XML documents can be validated.
• XML Syntax Fundamentals
• XML syntax describes the constructs used to
  define the structure and layout of an XML
  document, as well as the constraints involved.
• An XML processor is a software module that reads
  an XML document, and provides access to its
  content and structure.
• XML processors typically process documents on
  behalf of applications, and are readily available
  as software plug-ins.
• IE 5.0 is an e.g. of an XML application that
  processes and displays XML documents.

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• Entity The basic building block of an XML
  document. Contains either parsed or unparsed
  data.
• Parsed data consists of characters that are
  considered as character data or mark-up, and are
  processed by an XML processor.
• Unparsed character is handled as raw text and is
  not processed.
• E.g. ltnamegtJohnlt/namegt, ltnamegt and lt/namegt are
  mark-up, while John is character data.
• Markup Used to provide a description of a
  documents storage structure (entities) and
  logical structures (elements).
• Elements Describe the logical structure. They
  have start tags e.g. ltnamegt and end tags (
  lt/namegt ), or a single empty tag (ltname/gt).

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• XML mark-up components include
• Tags Most obvious component in XML syntax, used
  to describe elements.
• Processing instructions Passed by the parser to
  the application. Begin with lt? and end with ?gt.
  E.g lt?xml version1.0?gt indicates that the
  document is based on xml version 1.0
• Document type declarations Used to specify
  information about the document, including the
  documents root element, and the Document Type
  Definition (DTD). Must appear after the XML
  declaration, but before the root element e.g
  
  
  
• lt?xml version1.0gt
  lt!DOCTYPE addressbook
  SYSTEM Addressbook.dtdgt ltaddressbookgt
  
  ltcontactgt
• addressbook declared in line 2 must
  correspond to ltaddressbookgt in line 3, the root
  element of the document.

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• Entity references Used to assign aliases to
  pieces of data. They are made within an ampersand
  () and a colon (). E.g. apos corresponds to
  an apostrophe () while amp corresponds to .
  
• Comments Used to present information that is
  technically not part of the documents content.
  Begin with lt! and end with -- gt
• Marked (CDATA) Sections Used to block off text
  that is to be sidestepped by the parser. Defined
  by enclosing it in within lt!CDATA and gt. E.g.
  lt!CDATAltnamegtJohnlt/namegt. In this example,
  the name element is not recognized as mark-up and
  John is not recognized as parsed character data.
• It is common to use CDATA sections to
  quote a piece of XML code, e.g. in a tutorial.

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• Styling XML for display
• Accomplished in 2 ways
• With the use of CSS.
• With XSL. More complex and advanced than CSS
• Parsing XML
• Can be validating or non-validating.
• Validating parsers validate XML documents against
  a DTD or XML Schema.
• E.g.s of XML parsers are The Lark and Larval XML
  parsers for Java, Suns Project X Parser for
  Java, IBMs XML Parser for Java, Oracle XML
  parser for Java, IBMs XML Parser for C.

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• Example of an XML Document
• lt?xml version1.0?gt
• lt!DOCTYPE addressbook SYSTEM Addressbook.dtdgt
• ltaddressbookgt
• ltcontactgt
• ltnamegtTony Bennlt/namegt
• ltaddressgt210 Temple roadlt/addressgt
• ltcitygtLondonlt/citygt
• ltpostcodegtNW9 0RTlt/postcodegt
• ltphonegt02082049565lt/phonegt
• lt/contactgt
• ltcontactgt
• ltnamegtPeter Bloggslt/namegt

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• ltaddressgt230 The Valelt/addressgt
• ltcitygtLondonlt/citygt
• ltpostcodegtNW6 2BTlt/postcodegt
• ltphonegt02082029517lt/phonegt
• lt/contactgt
• lt/addressbookgt
• The above example is a well-formed XML document
  used to store contact information. However, it is
  not valid yet!
• Note that the root element (ltaddressbookgt) has
  nested child elements that are defined with
  opening and closing tags respectively.

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• XML Data Modelling
• Involves describing the structure of XML
  documents, for the purpose of validation.
• After defining a data model, you can create
  structured XML documents that must adhere to that
  model, to be valid.
• Valid vs Well-formed XML It is perfectly legal
  to create an XML document without a data model,
  in which case the document could be considered
  well-formed, but is not valid.
• There are 2 approaches to creating data models
• DTDs (Document Type Definitions) and
• XML Schemas
• The data model (DTD or XML Schema) defines the
  arrangement of mark-up and character data within
  a valid XML document, i.e. the order of nesting
  of the elements.

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• Modelling Data with DTDs
• DTDs (Document Type Definitions) rely on
  specialized syntax for describing the structure
  of XML vocabulary (class of document).
• DTDs can be broken down into 2 subsets
• Internal or Local DTD Mark-up declarations are
  contained in the prolog (section of document
  preceding the root element) of the same document.
• External DTD External mark-up declarations that
  can be referenced by one or more documents.
• The 2 subsets may be combined, with Internal
  having higher precedence.
• The DTD declares every element, attribute and
  entity used in the XML document.
• It must be declared, or referenced in the
  document type declaration.

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• Example Addressbook.dtd
• lt!ELEMENT addressbook (contact)gt
• lt!ELEMENT contact (name, address, city, postcode,
  phone)gt
• lt!ELEMENT name (PCDATA)gt
• lt!ELEMENT address (PCDATA)gt
• lt!ELEMENT city (PCDATA)gt
• lt!ELEMENT postcode (PCDATA)gt
• lt!ELEMENT phone (PCDATA)gt
• ltaddressbookgt
• ltcontactgt
• ltnamegtTony Bennlt/namegt
• ltaddressgt210 Temple roadlt/addressgt
• ltcitygtLondonlt/citygt
• ltpostcodegtNW9 0RTlt/postcodegt
• ltphonegt02082049565lt/phonegt
• lt/contactgt

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• Document type declaration syntax
• lt!DOCTYPE rootElem SYSTEM ExtDTDRef
  InternalDTDDeclgt
• where rootElem is the root element, ExtDTDRef is
  the External DTD reference, and InternalDTDDecl
  is the Internal DTD declaration.
• Illustration
• lt!DOCTYPE movies SYSTEM Movies.dtd
• lt!ELEMENT actor (PCDATA)gt gt
• ltmoviesgt
• lttitlegtLord of the ringslt/titlegt
• lt! the other child elements go here -- gt
• External DTDs are more commonly used, and are
  especially useful when you are creating multiple
  documents of the same class when you would like
  to use an existing DTD or to make your document
  as concise as possible.

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• Internal DTDs are preferable in situations where
  youre creating only one document, or to reduce
  the overhead associated with your documents.
• Elements and Attributes
• The primary contents described in a DTD are
  elements and attributes.
• Think of an element as a logical unit of
  information, and an
• Attribute as a characteristic of that
  information.
• By looking at a document as a group of
  information objects, it is usually possible to
  associate each object with an element. Any
  leftover information would usually be represented
  as attributes.
• Another approach is to consider the type of
  information and how it will be used.

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• Attributes provide tighter constraints on
  information, while elements on the other hand,
  are very loosely constrained and are better
  suited for long strings of text.
• Attributes can be constrained against a
  predefined list of values, and can have default
  values.
• Attributes are very concise, and are easier to
  parse.
• They however can not contain nested information.
• Elements
• Declared with element declarations in the DTD.
• Syntax lt!ELEMENT ElementName Typegt
• ElementName corresponds to the tag used to mark
  up that element in the XML document.
• Type specifies the content. 4 types are supported
  in XML

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• Empty types The element doesnt contain any
  content, but may contain attributes. In the DTD,
  they are declared in the form lt!ELEMENT
  ElementName EMPTYgt
• E.g lt!ELEMENT img EMPTYgt
• Empty elements are defined in the XML document in
  2 ways
• ltstart taggtltend taggt with no space in between e.g
  ltimg srcpic.gifgtlt/imggt.
• with an empty tag e.g ltimg/gt or ltimg
  srcpic.gif/gt
• Element only type The element only type contains
  child elements. Denoted by lt!ELEMENT ElementName
  contentModelgt
• The content model is specified using a
  combination of special element declaration
  symbols and child element names.
• The symbols represent the relationship of the
  child, to the container element.

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• Table of Special Symbols

Symbol Usage
Parentheses (()) Enclose a sequence or choice group of child elements
Comma (,) Separates the items in a sequence and establishes the order in which they must appear.
Pipe () Separates items in a choice group of elements.
No symbol Implies that the child element must appear exactly once
Question mark (?) Child element must appear only once or not at all
Asterisk () Child element can appear any number of times
Plus sign () Must appear at least once
Example lt!ELEMENT resume (intro, (education
experience),hobbies?,references)gt

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• Mixed Elements
• Contain both character and child elements. The
  simplest mixed element is that declared to
  contain only character data.
• Take the following form
• lt!ELEMENT ElementName (PCDATA)gt.
• E.g. lt!ELEMENT city (PCDATA)gt
• ANY Elements
• The ANY element, so named because it is declared
  with the symbol ANY, can contain any type of
  element, or a combination of elements.
• Due to its lack of structure, you should avoid
  using it.
• Typically used during development of a DTD, but
  should not appear in a production DTD.
• Form lt!ELEMENT ElementName ANYgt

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• Attributes
• Used to specify additional information about
  elements.
• Within an element, attributes are used to form
  name/value pairs that describe a particular
  property of the element.
• Declared in a DTD with attribute list declaration
  which take the form
• lt! ATTLIST ElementName AttrName AttrType Defaultgt
• There are 4 types of default types that can be
  specified
• REQUIRED The attribute is required
• IMPLIED The attribute is optional
• FIXED value The attribute has a fixed value
• default The default value of the attribute
• REQUIRED implies that the attribute is required,
  and you must define that attribute if you use the
  element.

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• Attribute Type
• Must be specified, in addition to the attribute
  default value.
• XML supports 10 attribute types
• CDATA- Unparsed character data
• Enumerated Series of string values
• NOTATION A notation declared somewhere else in
  the DTD
• ENTITY An external binary entity
• ENTITIES Multiple external binary entities
  separated by whitespace.
• ID A unique identifier
• IDREF Reference to an ID declared somewhere else
  in the DTD
• IDREFS Multiple references to IDs declared
  somewhere else in the DTD
• NMTOKEN A name consisting of XML token
  characters (letters, numbers, periods, dashes,
  colons and underscores).
• NMTOKENS Multiple names consisting of XML token
  characters.

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• String Attributes
• Most commonly used attribute
• Example
• lt!ATTLIST player team CDATA REQUIREDgt
• In the above example, the team to which a player
  belongs is a required character data attribute
  that must be defined in the player element.
• lt!ATTLIST player team CDATA IMPLIEDgt would have
  made the team optional.
• Another example
• lt!ELEMENT movie (Producer, Director, Actor,
  Writer, Duration)
• lt!ATTLIST movie type (comedy thriller)
  REQUIREDgt
• In this example, the movie element contains the
  child elements defined, but it also has a
  mandatory attribute called Type which has 2
  possible values.