XML StudySession: Part II

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XML Study-Session Part II

• Validating XML Documents

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Objectives

• By completing this study-session, you should be
  able to
• Validate XML documents against a DTD.
• Understand basic DTD syntax.
• Create simple DTDs of your own.

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What is a DTD?

• Document Type Definition
• Standard originally developed for SGML.
• Provides a description of the XML documents
  structure, and serves as a grammar to specify
  what tags and attributes are valid in an XML
  document and in what context they are valid.
• E.g. The following is an example DTD statement

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Why use a DTD?

• DTDs are used to allow an application to
  construct valid XML that conforms to that
  specification. Also
• Self documentation
• Portability
• Provides defaults for attributes
• Entity declaration

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Using a DTD in an XML document

• An XML document may do any of the following
• Refer to a DTD, using its URI.
• Include a DTD inline as part of the XML document.
• Omit a DTD altogether. Without a DTD, an XML
  document can be checked for well-formedness, but
  not for validity.
• The DTD used by the XML document may be internal
  or external. An external DTD is stored as an
  ASCII text .dtd file.

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Example Using a DTD inline

• ISBN CDATA REQUIRED
• section (fictionnonfiction) fiction
• To Kill a Mockingbird
• Harper Lee
• Description

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Doctype declaration

• The Document Type (Doctype) declaration is used
  to indicate the DTD used for the document.
  Syntax may be in any of the following forms

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Example External DTD

• The following is an example of an XML document
  that uses an external DTD
• Moby Dick
• Herman Melville
• The external DTD must be located in the same
  directory as the XML document.

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Example Using DTDs with URLS

• The following is an example of an XML document
  that references an external DTD with an URL
• ooklist.dtd
• Moby Dick
• Herman Melville

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Specifying Elements

• In the DTD, this is done with the notation
• where elemName is the actual element name, and
  elemDefinitionOrType indicates whether the
  content of the content is pure data or a compound
  type of data and other elements.

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Some Element Types

• The element type keyword ANY allows the element
  to contain textual data, nested elements, or any
  legal XML combination of the two.
• The element type keyword PCDATA indicates
  textual data, and can be used to store regular
  character data we want the XML document to handle
  normally.
• The element type keyword EMPTY indicates that the
  element is always empty.

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Nesting elements

• To define the allowed nestings within a DTD, the
  following notation is used
• where the order of elements is enforced as a
  validity constraint within an XML document.
• By default, an element can appear exactly once
  when specified without any modifiers in the DTD.

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Recurrence Operators

• Recurrence operators can be used to indicate how
  many times an element must appear in an XML
  document

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Grouping elements

• Often, recurrence occurs for a block or group of
  elements rather than with a single element.
• To signify a group, enclose a set of elements
  within parantheses. Nested parentheses are
  acceptable.
• In this way, a recurrence operator can then be
  applied to the group.
• E.g.
• group1Elem2), (group2Elem1, group2Elem2)?)

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Either Or

• In the DTD, an OR operator is signified by
  using . This allows one thing or the other to
  occur, and can be used in conjunction with
  groupings.
• E.g.
• ent2)

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Defining Attributes

• Attribute definitions are in the following form
• attributeType attributeModifier
• The attributeType keyword CDATA allows an
  attribute to take on any value, and may represent
  a comment or additional information about an
  element.
• Another attribute type is an enumeration, where
  any of the specified values may be used, but any
  other value for the attribute results in an
  invalid document.
• E.g.
• (value1value2) attributeModifier

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Attribute Modifiers

• We can indicate in the attribute definition
  whether the attribute is required within an
  element.
• The three modifier keywords are IMPLIED,
  REQUIRED, and FIXED.
• An implied attribute may be given a value, or
  left unspecified.
• A required attribute must be given a value.
• A fixed attribute has a specified value that can
  never change. The notation for this is
• fixedValue

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Parameter Entities in DTDs

• Parameter entities are entities that can only be
  used in the DTD.
• A simple internal parameter entity has the
  format
• E.g.
• Note?)
• sum

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Parameter Entities in DTDs (contd.)

• External parameter entitites can be declared
  using the following
• or
• E.g. The following orders.dtd file could be
  created
• XHTML 1.0 Transitional//EN http//www.w3.org/TR/x
  html1/DTD/xhtml1-transitional.dtd
• XHTML1-t.dtd

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Using INCLUDE and IGNORE

• We can customize our DTDs using the INCLUDE and
  IGNORE statements, which have the following
  syntax
• E.g. In the orders.dtd file, add the following
  lines
• (same as before)

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Example Using the XHTML 1.1 DTD

• The XHTML 1.1 DTD is a DTD driver which includes
  various XHTML 1.1 modules (i.e. DTD sections)
  using parameter entities.
• E.g.
• -//W3C//ELEMENTS XHTML 1.1 Tables 1.0//EN
  xhtml11-table-1.mod
• xhtml-table.mod
• The above allows us to customize the XHTML 1.1
  DTD to include/exclude support for tables.

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Next session

• Parsing XML Documents
• Parsing techniques
• Writing your own XML applications