XML and DTD

ThePlace	[ Home ] [ Search ] [ Resources ] [ Site Map ] [ Contact Me ] Dave's Information Technology Resource
[ Up ]
[ What is XML ] [ Viewing XML ] [ XML Syntax ] [ Creating XML ] [ XML and CSS ] [ XML and DTD ] [ XML, XSL, and HTML ] [ The XML DOM ]
--- XML and DTD ---

XML and DTD's

What is a DTD?

	DTD = Document Type Definition
	The purpose of a DTD is to define the legal building blocks of an XML document.
	It defines the document structure with a list of legal elements.
	A DTD can be declared inline in your XML document, or as an external reference.

Why do we have DTD's?

Some XML parsers such as Internet Explorer merely check XML for "well-formedness".

DTD's are used to ensure that the XML data meets criteria for specific structure and layout.

DTD's are very useful when data is to be exchanged between different data systems.

Tools that act as validating parsers include:

	XML Parser for Java
	Chilkat XML Parser

Internal DTD's

This is an XML document with a internal Document Type Definition.

<?xml version="1.0"?>
<!DOCTYPE letter [
  <!ELEMENT letter    (to,from,subject,body)>
  <!ELEMENT to      (#PCDATA)>
  <!ELEMENT from    (#PCDATA)>
  <!ELEMENT subject (#PCDATA)>
  <!ELEMENT body    (#PCDATA)>
]>
<letter>
<to>John</to>
<from>Bob</from>
<subject>Something important</subject>
<body>See me in the morning for the next assignment...</body>
</letter>

The DTD is interpreted like this:

!ELEMENT note (in line 2) defines the element "letter" as having four elements: "to,from,subject,body".
!ELEMENT to (in line 3) defines the "to" element to be of the type "CDATA".
!ELEMENT from (in line 4) defines the "from" element to be of the type "CDATA"
and so on.....

External DTD

This is an XML document with an external DTD:

<?xml version="1.0"?>
<!DOCTYPE letter SYSTEM "note.dtd">
<letter>
<to>John</to>
<from>Bob</from>
<subject>Something important</subject>
<body>See me in the morning for the next assignment...</body>
</letter>

This is what you find in "note.dtd" containing the Document Type Definition:

<?xml version="1.0"?>
<!ELEMENT letter    (to,from,subject,body)>
<!ELEMENT to      (#PCDATA)>
<!ELEMENT from    (#PCDATA)>
<!ELEMENT subject (#PCDATA)>
<!ELEMENT body    (#PCDATA)>

Why use a DTD?

	XML provides an application independent way of sharing data.
	With a DTD, different groups of people can agree on a common DTD for interchanging data.
	Your application can use a standard DTD to verify that data that you receive from the outside world is valid.
	The DTD can be used to verify your data.

DTD - XML building blocks

XML documents (and HTML documents) are made up by the following building blocks:

	Elements
	Tags
	Attributes
	Entities
	PCDATA
	CDATA

Elements

	Elements are the main building blocks of both XML and HTML documents.
	Examples of HTML elements are "body" and "table".
	Examples of XML elements could be "note" and "message".
	Elements can contain text, other elements, or be empty.
	Examples of empty HTML elements are "hr", "br" and "img".

Attributes

	Attributes provide extra information about elements.
	Attributes are placed inside the start tag of an element.
	Attributes come in name/value pairs.
	The following "img" element has an additional information about a source file:

<img src="computer.gif" />

	The name of the element is "img".
	The name of the attribute is "src".
	The value of the attribute is "computer.gif".
	Since the element itself is empty it is closed by a " /".

Entities

	Entities as variables used to define common text.
	Entity references are references to entities.
	Consider the HTML entity reference: " " that is used to insert an extra space in an HTML document.
	Entities are expanded when a document is parsed by an XML parser.
	The following entities are predefined in XML:

Entity References	Character
<	<
>	>
&	&
"	"
'	'

PCDATA

	PCDATA means parsed character data.
	Think of character data as the text found between the start tag and the end tag of an XML element.
	PCDATA is text that will be parsed by a parser.
	Tags inside the text will be treated as markup and entities will be expanded.

CDATA

	CDATA also means character data.
	CDATA is text that will NOT be parsed by a parser.
	Tags inside the text will NOT be treated as markup and entities will not be expanded.

DTD - Elements

Declaring an Element

	In the DTD, XML elements are declared with an element declaration.
	An element declaration has the following syntax:

<!ELEMENT element-name (element-content)>

Empty elements

Empty elements are declared with the keyword EMPTY inside the parentheses:

<!ELEMENT element-name (EMPTY)>

example:

<!ELEMENT img (EMPTY)>

Elements with data

Elements with data are declared with the data type inside parentheses:

<!ELEMENT element-name (#CDATA)>
or
<!ELEMENT element-name (#PCDATA)>
or
<!ELEMENT element-name (ANY)>

example:

<!ELEMENT letter (#PCDATA)>

	#CDATA means the element contains character data that is not supposed to be parsed by a parser.
	#PCDATA means that the element contains data that IS going to be parsed by a parser.
	The keyword ANY declares an element with any content.
	If a #PCDATA section contains elements, these elements must also be declared.

Elements with children (sequences)

Elements with one or more children are defined with the name of the children elements inside the parentheses:

<!ELEMENT element-name (child-element-name)>
or
<!ELEMENT element-name (child-element-name,child-element-name,.....)>

example:

<!ELEMENT letter (to,from,heading,body)>

	When children are declared in a sequence separated by commas, the children must appear in the same sequence in the document.
	In a full declaration, the children must also be declared, and the children can also have children.
	The full declaration of the letter document will be:

<!ELEMENT letter    (to,from,subject,body)>
<!ELEMENT to      (#PCDATA)>
<!ELEMENT from    (#PCDATA)>
<!ELEMENT subject (#PCDATA)>
<!ELEMENT body    (#PCDATA)>

Wrapping

If the DTD is to be included in your XML source file, it should be wrapped in a DOCTYPE definition with the following syntax:

<!DOCTYPE root-element [element-declarations]>

example: 
<?xml version="1.0"?>
<!DOCTYPE letter [
  <!ELEMENT letter    (to,from,subject,body)>
  <!ELEMENT to      (#PCDATA)>
  <!ELEMENT from    (#PCDATA)>
  <!ELEMENT subject (#PCDATA)>
  <!ELEMENT body    (#PCDATA)>
]>
<letter>
<to>John</to>
<from>Bob</from>
<subject>Something important</subject>
<body>See me in the morning for the next assignment...</body>
</letter>

Declaring only one occurrence of the same element

<!ELEMENT element-name (child-name)>

example

<!ELEMENT letter (message)>

The example declaration above declares that the child element message can only occur one time inside the letter element.

Declaring minimum one occurrence of the same element

<!ELEMENT element-name (child-name+)>

example

<!ELEMENT letter (message+)>

The + sign in the example above declares that the child element message must occur one or more times inside the letter element.

Declaring zero or more occurrences of the same element

<!ELEMENT element-name (child-name*)>

example

<!ELEMENT letter (message*)>

The * sign in the example above declares that the child element message can occur zero or more times inside the letter element.

Declaring zero or one occurrences of the same element

<!ELEMENT element-name (child-name?)>

example

<!ELEMENT letter (message?)>

The ? sign in the example above declares that the child element message can occur zero or one times inside the letter element.

Declaring mixed content

example

<!ELEMENT letter (to+,from,subject,message*,#PCDATA)>

The example above declares that the element letter must contain at least one to child element, exactly one from child element, exactly one header, zero or more message, and some other parsed character data as well.

DTD - Attributes

Declaring Attributes

In the DTD, XML element attributes are declared with an ATTLIST declaration. An attribute declaration has the following syntax:

<!ATTLIST element-name attribute-name attribute-type default-value>

	As you can see from the syntax above, the ATTLIST declaration defines the element which can have the attribute, the name of the attribute, the type of the attribute, and the default attribute value.
	The attribute-type can have the following values:

Value	Explanation
CDATA	The value is character data
(eval\|eval\|..)	The value must be an enumerated value
ID	The value is an unique id
IDREF	The value is the id of another element
IDREFS	The value is a list of other ids
NMTOKEN	The value is a valid XML name
NMTOKENS	The value is a list of valid XML names
ENTITY	The value is an entity
ENTITIES	The value is a list of entities
NOTATION	The value is a name of a notation
xml:	The value is predefined

The attribute-default-value can have the following values:

Value	Explanation
#DEFAULT value	The attribute has a default value
#REQUIRED	The attribute value must be included in the element
#IMPLIED	The attribute does not have to be included
#FIXED value	The attribute value is fixed

Attribute declaration example

DTD example:
<!ELEMENT square EMPTY>
  <!ATTLIST square width CDATA "0">

XML example:
<square width="100"></square>

In the above example the element square is defined to be an empty element with the attributes width of type CDATA. The width attribute has a default value of 0.

Default attribute value

Syntax:
<!ATTLIST element-name attribute-name CDATA "default-value">

DTD example:
<!ATTLIST payment type CDATA "check">

XML example:
<payment type="check">

Specifying a default value for an attribute, assures that the attribute will get a value even if the author of the XML document didn't include it.

Implied attribute

Syntax:
<!ATTLIST element-name attribute-name attribute-type #IMPLIED>

DTD example:
<!ATTLIST contact fax CDATA #IMPLIED>

XML example:
<contact fax="555-667788">

Use an implied attribute if you don't want to force the author to include an attribute and you don't have an option for a default value either.

Required attribute

Syntax:
<!ATTLIST element-name attribute_name attribute-type #REQUIRED>

DTD example:
<!ATTLIST person number CDATA #REQUIRED>

XML example:
<person number="5677">

Use a required attribute if you don't have an option for a default value, but still want to force the attribute to be present.

Fixed attribute value

Syntax:
<!ATTLIST element-name attribute-name attribute-type #FIXED "value">

DTD example:
<!ATTLIST sender company CDATA #FIXED "Microsoft">


XML example:
<sender company="Microsoft">

Use a fixed attribute value when you want an attribute to have a fixed value without allowing the author to change it. If an author includes another value, the XML parser will return an error.

Enumerated attribute values

Syntax:
<!ATTLIST element-name attribute-name (eval|eval|..) default-value>

DTD example:
<!ATTLIST payment type (check|cash) "cash">

XML example:
<payment type="check">
or
<payment type="cash">

Use enumerated attribute values when you want the attribute values to be one of a fixed set of legal values.

DTD - Entities

Entities

	Entities as variables used to define shortcuts to common text.
	Entity references are references to entities.
	Entities can be declared internal.
	Entities can be declared external

Internal Entity Declaration

Syntax:

<!ENTITY entity-name "entity-value">

DTD Example:

<!ENTITY writer "Jan Egil Refsnes.">
<!ENTITY copyright "Copyright XML101.">

XML example:

<author>&writer;&copyright;</author>

External Entity Declaration

Syntax:

<!ENTITY entity-name SYSTEM "URI/URL">

DTD Example:

<!ENTITY writer    SYSTEM "http://www.xml101.com/entities/entities.xml">
<!ENTITY copyright SYSTEM "http://www.xml101.com/entities/entities.dtd">

XML example:

<author>&writer;&copyright;</author>

DTD Validation

Validating with the XML Parser

	If you try to open an XML document, the XML Parser might generate an error.
	By accessing the parseError object, the exact error code, the error text, and even the line that caused the error can be retrieved:

var xmlDoc = new ActiveXObject("Microsoft.XMLDOM")
xmlDoc.async="false"
xmlDoc.validateOnParse="true"
xmlDoc.load("note_dtd_error.xml")

document.write("<br>Error Code: ")
document.write(xmlDoc.parseError.errorCode)
document.write("<br>Error Reason: ")
document.write(xmlDoc.parseError.reason)
document.write("<br>Error Line: ")
document.write(xmlDoc.parseError.line)

Turning Validation off

Validation can be turned off by setting the XML parser's validateOnParse="false".

var xmlDoc = new ActiveXObject("Microsoft.XMLDOM")
xmlDoc.async="false"
xmlDoc.validateOnParse="false"
xmlDoc.load("note_dtd_error.xml")

document.write("<br>Error Code: ")
document.write(xmlDoc.parseError.errorCode)
document.write("<br>Error Reason: ")
document.write(xmlDoc.parseError.reason)
document.write("<br>Error Line: ")
document.write(xmlDoc.parseError.line)

DTD - Examples

TV Schedule DTD

By David Moisan. Copied from: http://www1.shore.net/~dmoisan/

<!DOCTYPE TVSCHEDULE [

<!ELEMENT TVSCHEDULE (CHANNEL+)> <!ELEMENT CHANNEL (BANNER, DAY+)> <!ELEMENT BANNER (#PCDATA)> <!ELEMENT DAY ((DATE, HOLIDAY) | (DATE, PROGRAMSLOT+))+> <!ELEMENT HOLIDAY (#PCDATA)> <!ELEMENT DATE (#PCDATA)> <!ELEMENT PROGRAMSLOT (TIME, TITLE, DESCRIPTION?)> <!ELEMENT TIME (#PCDATA)> <!ELEMENT TITLE (#PCDATA)> <!ELEMENT DESCRIPTION (#PCDATA)>


        <!ATTLIST TVSCHEDULE NAME CDATA #REQUIRED>

        <!ATTLIST CHANNEL CHAN CDATA #REQUIRED>

        <!ATTLIST PROGRAMSLOT VTR CDATA #IMPLIED>

        <!ATTLIST TITLE RATING CDATA #IMPLIED>

        <!ATTLIST TITLE LANGUAGE CDATA #IMPLIED>

]>

A Report DTD

By Richard Erlander. Copied from: http://pdbeam.uwaterloo.ca/~rlander/


        <!DOCTYPE REPORT [ 
        
<!ELEMENT REPORT (TITLE,(SECTION|SHORTSECT)+)>

        <!ELEMENT SECTION (TITLE,%BODY;,SUBSECTION*)>

        <!ELEMENT SUBSECTION (TITLE,%BODY;,SUBSECTION*)>

        <!ELEMENT SHORTSECT (TITLE,%BODY;)>

        <!ELEMENT TITLE %TEXT;>

        <!ELEMENT PARA %TEXT;>

        <!ELEMENT LIST (ITEM)+>

        <!ELEMENT ITEM (%BLOCK;)>

        <!ELEMENT CODE (#PCDATA)>

        <!ELEMENT KEYWORD (#PCDATA)>

        <!ELEMENT EXAMPLE (TITLE?,%BLOCK;)>

        <!ELEMENT GRAPHIC EMPTY>

        

        <!ATTLIST REPORT security (high | medium | low ) "low">

        <!ATTLIST CODE type CDATA #IMPLIED>

        <!ATTLIST GRAPHIC file ENTITY #REQUIRED>
        
<!ENTITY xml "Extensible Markup Language">

        <!ENTITY sgml "Standard Generalized Markup Language">

        <!ENTITY pxa "Professional XML Authoring">

        <!ENTITY % TEXT "(#PCDATA|CODE|KEYWORD|QUOTATION)*">

        <!ENTITY % BLOCK "(PARA|LIST)+">

        <!ENTITY % BODY "(%BLOCK;|EXAMPLE|NOTE)+">
        
<!NOTATION GIF SYSTEM "">

        <!NOTATION JPG SYSTEM "">

        <!NOTATION BMP SYSTEM "">

]>

Newspaper Article DTD

Copied from http://www.vervet.com/


        <!DOCTYPE NEWSPAPER [ 
        
<!ELEMENT NEWSPAPER (ARTICLE+)>

        <!ELEMENT ARTICLE (HEADLINE, BYLINE, LEAD, BODY, NOTES)>

        <!ELEMENT HEADLINE (#PCDATA)>

        <!ELEMENT BYLINE (#PCDATA)>

        <!ELEMENT LEAD (#PCDATA)>

        <!ELEMENT BODY (#PCDATA)>

        <!ELEMENT NOTES (#PCDATA)> 
        
<!ATTLIST ARTICLE AUTHOR CDATA #REQUIRED>

        <!ATTLIST ARTICLE EDITOR CDATA #IMPLIED>

        <!ATTLIST ARTICLE DATE CDATA #IMPLIED>

        <!ATTLIST ARTICLE EDITION CDATA #IMPLIED>

        

        <!ENTITY NEWSPAPER "Vervet Logic Times">

        <!ENTITY PUBLISHER "Vervet Logic Press">

        <!ENTITY COPYRIGHT "Copyright 1998 Vervet Logic Press">

]>

Product Catalog DTD

Copied from http://www.vervet.com/

<!DOCTYPE CATALOG [

<!ELEMENT CATALOG (PRODUCT+)> <!ELEMENT PRODUCT (SPECIFICATIONS+, OPTIONS?, PRICE+, NOTES?)> <!ELEMENT SPECIFICATIONS (#PCDATA)> <!ELEMENT OPTIONS (#PCDATA)> <!ELEMENT PRICE (#PCDATA)> <!ELEMENT NOTES (#PCDATA)> <!ATTLIST PRODUCT NAME CDATA #IMPLIED> <!ATTLIST CATEGORY (HandTool | Table | Shop-Professional) "HandTool"> <!ATTLIST PARTNUM CDATA #IMPLIED> <!ATTLIST PLANT (Pittsburgh | Milwaukee | Chicago) "Chicago"> <!ATTLIST INVENTORY (InStock | Backordered | Discontinued) "InStock"> <!ATTLIST SPECIFICATIONS WEIGHT CDATA #IMPLIED> <!ATTLIST POWER CDATA #IMPLIED> <!ATTLIST OPTIONS FINISH (Metal | Polished | Matte) "Matte"> <!ATTLIST OPTIONS ADAPTER (Included | Optional | NotApplicable) "Included"> <!ATTLIST OPTIONS CASE (HardShell | Soft | NotApplicable) "HardShell"> <!ATTLIST PRICE MSRP CDATA #IMPLIED> <!ATTLIST PRICE WHOLESALE CDATA #IMPLIED> <!ATTLIST PRICE STREET CDATA #IMPLIED> <!ATTLIST PRICE SHIPPING CDATA #IMPLIED> <!ENTITY AUTHOR "John Doe"> <!ENTITY COMPANY "JD Power Tools, Inc."> <!ENTITY EMAIL "jd@jd-tools.com">

[ Home ] [ Up ] [ Computer Architecture ] [ Programming Bootcamp ] [ Database Bootcamp ] [ Visual BasicS ] [ Web Basics ] [ Web Multimedia ] [ Web Programming ] [ Advanced Web Topics ] [ Developing Web Sites ] [ Web Glossary ]

XML and DTD's

Internal DTD's

External DTD

DTD - XML building blocks

Elements

Tags

Attributes

Entities

PCDATA

CDATA

DTD - Elements

Declaring an Element

Empty elements

Elements with data

Elements with children (sequences)

Wrapping

DTD - Attributes

DTD - Entities

DTD Validation

DTD - Examples

TV Schedule DTD

A Report DTD

Newspaper Article DTD

Product Catalog DTD