Creating a DTD

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Creating a DTD

• COIN 78
• Lecture 3

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What is a DTD?

• A DTD is a contract for an XML application
• The DTD is a text file for validating XML
  documents. It has a .dtd file extension
• DTDs specify what elements, attributes and
  entities may appear in an XML file
• DTDs also specify the order of elements
• and what each of those elements may contain
• DTDs also describe attributes and entities

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Internal vs. External DTDs

• It is possible to write internal DTDs for XML
  documents, but it is not really practical
• External DTDs can be leveraged among many XML
  files, analogous to CSS / HTML
• External DTDs are easier to write, modify, and
  apply to one or more XML documents
• DTDs can be generated from XML files, and used to
  generate XML schema (XML Spy).

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Tips for Writing DTDs

• First, print out your XML document
• Write down the order of elements, and include all
  attributes that may appear
• Try drawing a tree with nodes and leafs
• Text entries appear at leafs / attributes
• EditML Pro builds very good DTDS
• XML Spy builds DTDs alphabetically

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Using Internal / External DTDs

• Declaring an internal DTD
• Writing an external DTD
• Naming an external DTD
• Declaring a personal external DTD
• Declaring a public external DTD

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Declaring an Internal DTD

• At the top of the XML document, after the XML
  declaration, type
• lt!DOCTYPE root
• where root corresponds to the name of the root
  element in the document that the DTD will be
  applied to.
• Type gt to complete the DTD.

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Example Code Internal DTD

• lt?xml version1.0?gt
• lt!DOCTYPE endangered_species
• (you will put your declarations here)
• gt
• Leave room between and for document type
  declarations.

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Writing an External DTD

• Create a new text file with a text editor
• Define the rules for the DTD (document type
  definitions for defining elements and attributes,
  and entities and notations)
• Save the file as text only with a .dtd file
  extension (be careful not to save as .txt)

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Example Code External DTD

• lt!ELEMENT endangered_species (animal)gt
• lt!ELEMENT animal (name, threats, weight?,
  length, source, picture, subspecies)gt
• lt!ELEMENT name (PCDATA)gt
• lt!ATTLIST name language (English Latin)gt
• ...

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Naming an External DTD

• Type
• if DTD is approved by a standards body
• - if DTD is not a recognized standard
• Type
• // Owner//DTD where owner identifies who wrote
  or maintains the DTD
• Type a space followed by a label for the DTD,
  then //XX// where XX defines the language

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Naming an External DTD

• - //Liz Castro//DTD End_Species//EN//
• Liz Castro is the owner
• End_Species is the DTD description
• EN means the DTD is written in English

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Declaring a Personal External DTD

• In the XML declaration at the top of the
  document, add standaloneno
• Type lt!DOCTYPE root (name of root element)
• Type SYSTEM to indicate that the external DTD is
  a personal, non-standardized DTD
• Type file.dtd, where file.dtd is the DTD file
• Type gt to complete the document type declaration
• Personal (SYSTEM) is compared to Public DTDs

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Example Code

• lt?xml version1.0 standaloneno?gt
• lt!DOCTYPE endangered_species SYSTEM
  http//www.cookwood.com/xml/examples/
• dtd_creating/end_species.dtdgt
• lt!-- an additional internal DTD is shown below--gt
• lt!ELEMENT continent (Asia Europe Africa)gt

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Declaring a Public External DTD

• In the XML declaration at the top of the
  document, add standaloneno
• Type lt!DOCTYPE root (name of root element)
• Type PUBLIC to indicate that the external DTD is
  a standardized set of rules
• Type DTD_name where DTD_name is the official
  name of the DTD you are referencing
• Type file.dtd, where file.dtd is the DTD file
• Type gt to complete the document type declaration

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Example Code

• lt?xml version1.0 standaloneno?gt
• lt!DOCTYPE endangered_species PUBLIC
• -//Liz Castro//DTD End_Species//EN//
• http//www.cookwood.com/xml/examples/
• dtd_creating/end_species.dtdgt

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Defining Elements, Attributes, Entities and
Notations in a DTD
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Defining Elements and Attributes in a DTD

• Type lt!ELEMENT tag
• Type EMPTY if no contents
• Type (contents) to specify contents
• Type (ANY) to allow any combination of elements
  or text
• TYPE gt to complete the element declaration

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Defining an Element to Contain Only Text

• Type lt!ELEMENT tag
• Next type (PCDATA)
• Finally type gt

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Defining an Element to Contain One Child

• Type lt!ELEMENT tag
• Type (child)
• Type gt to complete the declaration

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Defining an Element to Contain a Sequence

• Type lt!ELEMENT tag
• Type (child1
• Type (child2
• Repeat for (child-n
• Type ) to complete the sequence
• Type gt to close the declaration
• lt!ELEMENT (child1,child2,child3)gt

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Defining Choices

• Type lt!ELEMENT tag
• Type (child1
• Type to indicate that if the first element
  appears that the second can not appear
• Type child2
• Repeat for additional choices
• Type ) to complete the list of choices
• Type gt to complete the element declaration

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Defining How Many Units

• To define how many units
• Type ? to indicate that the unit can appear at
  most once, if at all.
• Type to indicate that the unit must appear at
  least once, but is unbounded.
• Type to indicate that the unit can appear as
  many times as necessary, or not at all.
• lt!ELEMENT (child1?,child2,child3)gt

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About Attributes

• Attributes add information about an element and
  the content of that element
• Information contained in attributes tends to be
  about the content of the document
• Elements are perhaps a better choice for
  information you may want to display (css)
• attributes are information about information

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Defining Simple Attributes

• Type lt!ATTLIST tag
• Type attribute
• Type CDATA (with no parenthesis
• Or type (choice_1 choice_2)
• Next type default
• or type fixed default
• or type required
• or type implied
• Type gt to complete the attribute declaration

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Defining Attributes with Unique Values

• Type lt!ATTLIST tag
• Type attribute
• Type ID if you want the value to be unique
• Next type default
• or type fixed default
• or type required
• or type implied
• Type gt to complete the attribute declaration

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Referencing Attributes to Valid XML Names

• Type lt!ATTLIST tag
• Type attribute
• Type IDREF to define an attribute that will
  contain a value that matches an other attributes
  ID value
• Type IDREFS for attributes that contain several
  white-space-separated ID values
• Type default, fixed default, required,
  implied
• Type gt to complete the attribute declaration

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Restricting Attributes to Valid XML Names

• To ensure attribute values follow the rules for
  valid XML names
• Type lt!ATTLIST tag
• Type attribute
• Type NMTOKEN if you want the value to be an XML
  name
• Or TYPE NMTOKENS if you want the value to be a
  white space separated list of XML names

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Creating Shortcuts for Text

• In the DTD type lt!ENTITY
• Type abbreviation
• Type content
• Type gt to complete the declaration
• lt! ENTITY RDC Robert D. Cormiagt

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Using Shortcuts for Text

• In the XML document first type an
• Type abbreviation, where abbreviation is the
  identifying name of your entity
• (and matches the one used in the previous
  example)
• Type
• Ex. RDC would be Robert D. Cormia

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Shortcuts for Text in External Files

• Create the content for the entity in an external
  file
• In the XML document itself, add standaloneno
• In the DTD for the XML document type lt!ENTITY to
  begin the entity definition
• Type abbreviation
• Type SYSTEM entity.url
• Type gt

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Creating and UsingShortcuts for DTDs

• Create content for the entity
• add standalone no
• In the DTD for the XML document type lt!ENTITY
• Type
• Type abbreviation
• Type SYSTEM or PUBLIC
• Type SYSTEM entity.url
• Type gt

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Creating Entities for Unparsed Content

• In the DTD type lt!ENTITY
• Type abbreviation
• Type SYSTEM
• Type entity.url
• Type NDATA id
• Type gt

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Embedding Unparsed Content

• In the DTD define the element
• Type lt!ATTLIST element_name
• Type att_name
• Type ENTITY or ENTITIES
• Type the default value of the document
• Type gt to close the declaration

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Writing Filenames

• address_book_empty.dtd
• address_book_empty_dtd.xml
• address_book_mixed.dtd
• address_book_mixed_dtd.xml
• address_book_nested.dtd
• address_book_nested_dtd.xml

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Common DTD Mistakes

• Inconsistent data structure in your file
• Having an element appear more than once, or
  omitting an element in a DTD
• Not declaring an attribute for an element
• Changing an XML file but not the DTD
• Not checking your DTD with a validating parser
  (EditML Pro / XML Spy etc.)
• A Web browser provides weak validation

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Summary

• DTDs are contracts for XML documents
• They validate the data structure of a file
• DTDs are fairly straightforward to write
• Draw a picture of your XML application
• Clearly show the elements and attributes
• Declare all entities and entity references
• Use validating tools like EditML Pro and XML Spy
  to validate XML using your DTD!