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.
This is an XML document with a Document Type Definition: (Open
it in IE5, and select view source)
<?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…..
This is the same XML document with an external DTD: (Open
it in IE5, and select view source)
<?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)>
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.
A lot of forums are emerging to define standard DTDs for almost everything in the areas of data exchange. Take a look at: CommerceNet’s XML exchange and http://www.schema.net.
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 are declared with the keyword EMPTY inside the parentheses:
<!ELEMENT element-name (EMPTY)> example: <!ELEMENT img (EMPTY)>
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 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)>
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>
<!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.
<!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.
<!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.
<!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.
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!
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 |
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.
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.
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.
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.
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.
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.
Syntax: <!ENTITY entity-name "entity-value"> DTD Example: <!ENTITY writer "Jan Egil Refsnes."> <!ENTITY copyright "Copyright XML101."> XML example: <author>&writer;©right;</author>
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;©right;</author>
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)
or or just look at the XML file
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)
You can read more about the parseError object in the Dom section of this Web.
By David Moisan. Copied from his Web: http://www.davidmoisan.org/
<!DOCTYPE TVSCHEDULE [ <!ELEMENT TVSCHEDULE (CHANNEL+)>
<!ATTLIST TVSCHEDULE NAME CDATA #REQUIRED> ]> |
By Richard Erlander. Copied from his Web: http://pdbeam.uwaterloo.ca/~rlander/
<!ATTLIST REPORT security (high | medium | low ) “low”>
|
Copied from http://www.vervet.com/
<!ENTITY NEWSPAPER “Vervet Logic Times”>
|
Copied from http://www.vervet.com/
<!ATTLIST PRICE MSRP CDATA #IMPLIED>]> |