DTD Tutorial

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 use a DTD?

XML provides an application independent way of sharing data. With a DTD, independent groups of people can agree to use 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. You can also use a DTD to verify your own data.


There are two type of DTD

  1. Intenal DTD
  2. External DTD

Internal DTD

This is an XML document with a Document Type Definition:

<?xml version="1.0"?>
<!DOCTYPE note [
  <!ELEMENT note    (to,from,heading,body)>
  <!ELEMENT to      (#PCDATA)>
  <!ELEMENT from    (#PCDATA)>
  <!ELEMENT heading (#PCDATA)>
  <!ELEMENT body    (#PCDATA)>
]>
<note>
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note> 

The DTD is interpreted like this:
!ELEMENT note (in line 2) defines the element “note” as having four elements: “to,from,heading,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 the same XML document with an external DTD:

<?xml version="1.0"?>
<!DOCTYPE note SYSTEM "note.dtd">
<note>
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note> 

This is a copy of the file “note.dtd” containing the Document Type Definition:

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


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


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 note (#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 note (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 note document will be:

<!ELEMENT note (to,from,heading,body)>
<!ELEMENT to      (#CDATA)>
<!ELEMENT from    (#CDATA)>
<!ELEMENT heading (#CDATA)>
<!ELEMENT body    (#CDATA)>


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 note [
  <!ELEMENT note (to,from,heading,body)>
  <!ELEMENT to      (#CDATA)>
  <!ELEMENT from    (#CDATA)>
  <!ELEMENT heading (#CDATA)>
  <!ELEMENT body    (#CDATA)>
]>
<note>
  <to>Tove</to>
  <from>Jani</from>
  <heading>Reminder</heading>
  <body>Don't forget me this weekend</body>
</note>


Declaring only one occurrence of the same element

<!ELEMENT element-name (child-name)>
example
<!ELEMENT note (message)>

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


Declaring minimum one occurrence of the same element

<!ELEMENT element-name (child-name+)>
example
<!ELEMENT note (message+)>

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


Declaring zero or more occurrences of the same element

<!ELEMENT element-name (child-name*)>
example
<!ELEMENT note (message*)>

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


Declaring zero or one occurrences of the same element

<!ELEMENT element-name (child-name?)>
example
<!ELEMENT note (message?)>

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


Declaring mixed content

example
<!ELEMENT note (to+,from,header,message*,#PCDATA)>

The example above declares that the element note 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. Puh!

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.

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)

Try it Yourself or or just look at the XML file


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)

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s