Chapter 6 – Document Type Definition (DTD)

2002 Prentice Hall, Inc. All rights reserved.

Chapter 6 – Document Type Definition (DTD)

Outline6.1 Introduction6.2 Parsers, Well-formed and Valid XML Documents6.3 Document Type Declaration6.4 Element Type Declarations 6.4.1 Sequences, Pipe Characters and Occurrence Indicators 6.4.2 EMPTY, Mixed Content and ANY6.5 Attribute Declarations 6.5.1 Attribute Defaults (#REQUIRED, #IMPLIED, #FIXED)6.6 Attribute Types: strings (CDATA), tokenized, enumerated 6.6.1 Tokenized Attribute Type (ID, IDREF, ENTITY, NMTOKEN) 6.6.2 Enumerated Attribute Types6.7 Conditional Sections6.8 Whitespace Characters6.9 Case Study: Writing a DTD for the Day Planner Application


6.1 Introduction

• Document Type Definitions (DTDs)– Define structure of XML document

• i.e., what elements, attributes, etc. are permitted in document

– XML document not required to have DTD• Usually recommended for document conformity

– Use Extended Backus-Naur Form (EBNF) grammar• Wikipedia Definition: The Backus–Naur form (also known as

BNF, the Backus–Naur formalism, Backus normal form, or Panini–Backus Form) is a metasyntax used to express context-free grammars: that is, a formal way to describe formal languages.


Simple DTD (not in the Deitel Text)

<?xml version=”1.0” encoding=”UTF-8”?>

<!ELEMENT rootelement (firstelement, secondelement)>

<!ELEMENT firstelement (level1)>

<!ATTLIST firstelement position CDATA #REQUIRED>

<!ELEMENT level1 (#PCDATA | level2)*>

<!ATTLIST level1 children (0 | 1) #REQUIRED>

<!ATTLIST secondelement position CDATA #REQUIRED>

<!ELEMENT level2 (#PCDATA)>

<!ELEMENT secondelement (level1)>


XML Document using a simple DTD

<?xml version=”1.0” encoding=”UTF-8”?><!DOCTYPE rootelement SYSTEM “verysimplexml.dtd”><rootelement>

<firstelement position=”1”><level1 children=”0”>This is level 1 of the nested

elements</level1></firstelement><secondelement position=”2”>

<level1 children=”1”><level2>This is level 2 of the nested

elements</level2></level1>

</secondelement></rootelement>


6.2 Parsers, Well-formed and Valid XML Documents

• Parsers– Validating

• Able to read DTD

• Determine whether XML document conforms to DTD

– Valid document conforms to DTD

» Document is then well formed, by definition

» Documents can be well formed, but not valid

– Nonvalidating• Able to read DTD

• Cannot check document against DTD for conformity


6.3 Document Type Declaration

• Document Type Declaration– Introduce DTDs into XML documents– Placed in XML document’s prolog (markup preceding the

root element)– Begins with <!DOCTYPE– Ends with >– Can point to

• External subsets– Declarations outside document– Exist in different file

» typically ending with .dtd extension• Internal subsets

– Declarations inside document– Visible only within document in which it resides


6.3 Document Type DeclarationExternal

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

<!DOCTYPE root-element SYSTEM "filename">

– Example (from intro.xml Fig 6.1)

<!DOCTYPE myMessage SYSTEM "intro.dtd">


6.3 Document Type DeclarationInternal

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

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

– Example (from intro2.xml in Fig 6.7)

<!DOCTYPE myMessage [

<!ELEMENT myMessage ( message )>

<!ELEMENT message ( #PCDATA )>

<!ATTLIST message id CDATA #REQUIRED>

]>


6.3 Document Type Declarationcombined

Example (from p. 136 of Deitel)

<!DOCTYPE myMessage SYSTEM “myDTD.dtd” [

<!ELEMENT myElement ( #PCDATA )>

]>


Outline

u

Fig. 6.1 XML document declaring its associated DTD.

DOCTYPE starts document type declaration

Document type declaration is named myMessage

Keyword SYSTEM specifies external subset

intro.dtd is DTD

1 <?xml version = "1.0"?>

2

3 

4 

5

6 <!DOCTYPE myMessage SYSTEM "intro.dtd">

7

8 <myMessage>

9 <message>Welcome to XML!</message>

10 </myMessage>

Document type declaration is named myMessage

Keyword SYSTEM specifies external subset

DOCTYPE starts document type declaration

intro.dtd is DTD


6.4 Element Type Declarations

• Element type declarations– Declare elements in XML documents– Begin with <!ELEMENT, End with >– Syntax:

<!ELEMENT element-name category> or

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

<!ELEMENT myElement ( #PCDATA )>• myElement is generic identifier• Parentheses specify element’s content (content specification)• Keyword PCDATA

– Element must contain parsable character data» All text treated as markup


Outline

Fig. 6.2 Validation with using an external DTD.

Declare element myMessage

Element myMessage contains child element message

Declare element message

Element message contains parsable character data

1 

2 

3

4 <!ELEMENT myMessage ( message )>

5 <!ELEMENT message ( #PCDATA )>

Declare element myMessage

Element myMessage contains child element message

Declare element message

Element message contains parsable character data


Fig. 6.2 Validation with using an external DTD.


Fig. 6.2 Validation with using an external DTD (cont.)


Outline

Fig. 6.3 Non-valid XML document.

Element myMessage’s structure does not adhere to that specified in intro.dtd


2

3 

4 

5

6 <!DOCTYPE myMessage SYSTEM "intro.dtd">

7

8 

9 <myMessage>

10 </myMessage>

Element myMessage’s structure does not adhere to

that specified in intro.dtd


Fig. 6.3 Non-valid XML document.


6.4.1 Sequences, Pipe Characters and Occurrence Indicators

• Sequences– Specify order in which elements occur

– Comma (,) used as delimiter<!ELEMENT classroom ( teacher, student )>


6.4.1 Sequences, Pipe Characters and Occurrence Indicators (cont.)

• Pipe characters (|)– Specify choices<!ELEMENT dessert ( iceCream | pastry )>


6.4.1 Sequences, Pipe Characters and Occurrence Indicators (cont.)

• Occurrence indicators– Specify element’s frequency

– Plus sign (+) indicates one or more occurrences<!ELEMENT album ( song+ )>

– Asterisk (*) indicates optional element<!ELEMENT library ( book* )>

– Question mark (?) indicates element can occur only once<!ELEMENT seat ( person? )>


Fig. 6.4 Occurrence indicators.

Occurrence Indicator Description

Plus sign ( + ) An element can appear any number of times, but must be appear at least once (i.e., the element appears one or more times).

Asterisk ( * ) An element is optional and if used, the element can appear any number of times (i.e., the element appears zero or more times).

Question mark ( ? ) An element is optional, and if used, the element can appear only once (i.e., the element appears zero or one times).


6.4.2 EMPTY, Mixed Content and ANY

• Content specification types– EMPTY

• Elements do not contain character data

• Elements do not contain child elements<!ELEMENT oven EMPTY>

• Markup for oven element<oven/>



• Content specification types– Mixed content

• Combination of elements and PCDATA<!ELEMENT myMessage ( #PCDATA | message )*>

• Markup for myMessage <myMessage>Here is some text, some <message>other text</message>and <message>even more text</message> </myMessage>


Outline

Fig. 6.5 Example of a mixed-content element.

Specify DTD as internal subset

Declare format as mixed content element

Elements bold and italic have PCDATA only for content specification

Element format adheres to structure in DTD

1 <?xml version = "1.0" standalone = "yes"?>

2

3 

4 

5

6 <!DOCTYPE format [

7 <!ELEMENT format ( #PCDATA | bold | italic )*>

8 <!ELEMENT bold ( #PCDATA )>

9 <!ELEMENT italic ( #PCDATA )>

10 ]>

11

12 <format>

13 This is a simple formatted sentence.

14 <bold>I have tried bold.</bold>

15 <italic>I have tried italic.</italic>

16 Now what?

17 </format>


Declare format as mixed content element

Elements bold and italic have PCDATA only for content specification

Element format adheres to structure in DTD


Fig. 6.5 Example of a mixed-content element.


Fig. 6.6 Illegal mixed-content element syntax.



• Content specification types– ANY

• Can contain any content– PCDATA, elements or combination

– Can also be empty elements

• Commonly used in early DTD-development stages

– Replace with specific content as DTD evolves


6.5 Attribute Declarations

• Attribute declaration– Specifies element’s attribute list– Uses ATTLIST attribute list declaration

An attribute declaration has the following syntax:<!ATTLIST element-name attribute-name attribute-type

default-value>

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

XML example:<payment type="check" /><payment type=“cash" />


Outline

Fig. 6.7 Declaring attributes.


Declare element myMessagewith child element message

Declare that attribute idcontain required CDATA


2

3 

4 

5

6 <!DOCTYPE myMessage [

7 <!ELEMENT myMessage ( message )>

8 <!ELEMENT message ( #PCDATA )>

9 <!ATTLIST message id CDATA #REQUIRED>

10 ]>

11

12 <myMessage>

13

14 <message id = "445">

15 Welcome to XML!

16 </message>

17

18 </myMessage>


Declare element myMessagewith child element message

Declare that attribute idcontain required CDATA


Fig. 6.7 Declaring attributes.


6.5.1 Attribute Defaults (#REQUIRED, #IMPLIED, #FIXED)

• Attribute defaults– Specify attribute’s default value– #IMPLIED

• Use (application’s) default value if attribute value not specified

[from w3schools: Use the #IMPLIED keyword if you don't want to force the author to include an attribute, and you don't have an option for a default value.]

– #REQUIRED• Attribute must appear in element

• Document is not valid if attribute is missing

– #FIXED• Attribute value is constant

• Attribute value cannot differ in XML document


6.6 Attribute Types

• Attribute types– Strings (CDATA)

• No constraints on attribute values

– Except for disallowing <, >, &, ’and ” characters

– Tokenized attributes• Constraints on permissible characters for attribute values

– Enumerated attributes• Most restrictive

• Take only one value listed in attribute declaration


6.6 Attribute Types

Type Value Explanation

String CDATA The value is character data

Enumerated (en1|en2|…) The value must be one from an enumerated list

Tokenized ID The value is a 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 a predefined xml value


6.6.1 Tokenized Attribute Type (ID, IDREF, ENTITY, NMTOKEN)

• Tokenized attribute types– Restrict attribute values– ID

• Uniquely identifies an element

– IDREF• Points to elements with ID attribute


Outline

Fig. 6.8 XML document with ID and IDREF attribute types.

Each shipping element has a unique identifier (shipID)

Attribute shippedBy points to shipping element by matching shipID attribute


2

3 

4 

5

6 <!DOCTYPE bookstore [

7 <!ELEMENT bookstore ( shipping+, book+ )>

8 <!ELEMENT shipping ( duration )>

9 <!ATTLIST shipping shipID ID #REQUIRED>

10 <!ELEMENT book ( #PCDATA )>

11 <!ATTLIST book shippedBy IDREF #IMPLIED>

12 <!ELEMENT duration ( #PCDATA )>

13 ]>

14

15 <bookstore>

16 <shipping shipID = "s1">

17 <duration>2 to 4 days</duration>

18 </shipping>

19

Each shipping element has a unique identifier (shipID)

Attribute shippedBy points to shipping element by

matching shipID attribute


Outline

Fig. 6.8 XML document with ID and IDREF attribute types . (Part 2)

Declare book elements with attribute shippedBy

20 <shipping shipID = "s2">

21 <duration>1 day</duration>

22 </shipping>

23

24 <book shippedBy = "s2">

25 Java How to Program 3rd edition.

26 </book>

27


29 C How to Program 3rd edition.

30 </book>

31


33 C++ How to Program 3rd edition.

34 </book>

35 </bookstore>

Declare book elements with attribute shippedBy


Fig. 6.7 XML document with ID and IDREF attribute types.


Fig. 6.9 Error displayed by XML Validator when an invalid ID is

referenced.

Assign shippedBy (line 28) value “s3”

Outline

Assign shippedBy (line 28) value “s3”


6.6.1 Tokenized Attribute Type (ID, IDREF, ENTITY, NMTOKEN) (cont.)

• ENTITY tokenized attribute type– Indicate that attribute has entity for its value

– Entity declaration

<!ENTITY digits “0123456789”>

– Entity may be used as follows:<useAnEntity>&digits;</useAnEntity>

– Entity reference &digits; replaced by its value<useAnEntity>0123456789</useAnEntity>


Outline

Declare entity city that refers to external document elements tour.html

Fig. 6.10XML document that contains an ENTITY attribute type.

Declare entity city that refers to external document elements tour.html

NDATA indicates that external-entity content is not XML

Attribute tour for element company requires ENTITY attribute type


2

3 

4 

5

6 <!DOCTYPE database [

7 <!NOTATION html SYSTEM "iexplorer">

8 <!ENTITY city SYSTEM "tour.html" NDATA html>

9 <!ELEMENT database ( company+ )>

10 <!ELEMENT company ( name )>

11 <!ATTLIST company tour ENTITY #REQUIRED>

12 <!ELEMENT name ( #PCDATA )>

13 ]>

14

15 <database>

16 <company tour = "city">

17 <name>Deitel & Associates, Inc.</name>

18 </company>

19 </database>

Declare entity city that refers to external document

elements tour.html

NDATA indicates that external-entity content is not XML

Attribute tour for element company requires

ENTITY attribute type


Fig. 6.10 XML document that contains an ENTITY attribute type.


Fig. 6.11 Error generated by XML Validator when a DTD contains a reference to an undefined entity.

Replace line 16<company tour = "city">

with<company tour = "country">

Outline

Replace line 16


6.6.1 Tokenized Attribute Type (ID, IDREF, ENTITY, NMTOKEN) (cont.)

• NMTOKEN tokenized attribute type– “Name token”

– Value consists of letters, digits, periods, underscores, hyphens and colon characters (i.e. cannot contain spaces)


6.6.2 Enumerated Attribute Types

• Enumerated attribute types– Declare list of possible values for attribute

<!ATTLIST person gender ( M | F ) “F”>

• Attribute gender can have either value M or F• F is default value


6.7 Conditional Sections

• Conditional sections– Include declarations

• Keyword INCLUDE

– Exclude declarations• Keyword IGNORE

– Often used with entities• Parameter entities

– Preceded by percent character (%)

– Creates entities specific to DTD

– Can be used only inside DTD in which they are declared


Outline

Entities accept and reject represent strings INCLUDE and IGNORE, respectively

Fig. 6.12Conditional sections in a DTD.


Include this element message declaration

Exclude this element message declaration

1 

2 

3

4 <!ENTITY % reject "IGNORE">

5 <!ENTITY % accept "INCLUDE">

6

7 <![ %accept; [

8 <!ELEMENT message ( approved, signature )>

9 ]]>

10

11 <![ %reject; [

12 <!ELEMENT message ( approved, reason, signature )>

13 ]]>

14

15 <!ELEMENT approved EMPTY>

16 <!ATTLIST approved flag ( true | false ) "false">

17

18 <!ELEMENT reason ( #PCDATA )>

19 <!ELEMENT signature ( #PCDATA )>


Include this element message declaration

Exclude this element message declaration


Outline

Fig. 6.13XML document that conforms to conditional.dtd.

1 <?xml version = "1.0" standalone = "no"?>

2

3 

4 

5

6 <!DOCTYPE message SYSTEM "conditional.dtd">

7

8 <message>

9 <approved flag = "true"/>

10 <signature>Chairman</signature>

11 </message>


Fig. 6.13 XML document that conforms to conditional.dtd.


Whitespace Characters

• Whitespace– Either preserved or normalized

• Depending on context in which it is used


Outline

Fig. 6.14Processing whitespace in an XML document.

Attribute hasCDATA requires CDATA, which preserves whitespace

Other attributes normalize (do not preserve) whitespace


2

3 

4 

5

6 <!DOCTYPE whitespace [

7 <!ELEMENT whitespace ( hasCDATA,

8 hasID, hasNMTOKEN, hasEnumeration, hasMixed )>

9

10 <!ELEMENT hasCDATA EMPTY>

11 <!ATTLIST hasCDATA cdata CDATA #REQUIRED>

12

13 <!ELEMENT hasID EMPTY>

14 <!ATTLIST hasID id ID #REQUIRED>

15

16 <!ELEMENT hasNMTOKEN EMPTY>

17 <!ATTLIST hasNMTOKEN nmtoken NMTOKEN #REQUIRED>

18

19 <!ELEMENT hasEnumeration EMPTY>

20 <!ATTLIST hasEnumeration enumeration ( true | false )

21 #REQUIRED>

22

23 <!ELEMENT hasMixed ( #PCDATA | hasCDATA )*>

24 ]>

25

Attribute cdata requires CDATA, which preserves whitespace

Other attributes normalize (do not preserve) whitespace


Outline

Fig. 6.14Processing whitespace in an XML document. (Part 2)

Whitespace preserved

Whitespace normalized

26 <whitespace>

27

28 <hasCDATA cdata = " simple cdata "/>

29

30 <hasID id = " i20"/>

31

32 <hasNMTOKEN nmtoken = " hello"/>

33

34 <hasEnumeration enumeration = " true"/>

35

36 <hasMixed>

37 This is text.

38 <hasCDATA cdata = " simple cdata"/>

39 This is some additional text.

40 </hasMixed>

41

42 </whitespace>




Outline

Output from Fig. 6.14



>java Tree yeswhitespace.xmlURL: file:C:/Examplesps/Files/deleted/ch09/Tree/whitespace.xml[ document root ]+-[ element : whitespace ] +-[ ignorable ] +-[ ignorable ] +-[ ignorable ] +-[ element : hasCDATA ] +-[ attribute : cdata ]" simple cdata “ +-[ ignorable ] +-[ ignorable ] +-[ ignorable ]+-[ element : hasID ] +-[ attribute : id ] "i20“ +-[ ignorable ] +-[ ignorable ] +-[ ignorable ] +-[ element : hasNMTOKEN ] +-[ attribute : nmtoken ] "hello“ +-[ ignorable ] +-[ ignorable ] +-[ ignorable ] +-[ element : hasEnumeration ] +-[ attribute : enumeration ] "true“ +-[ ignorable ] +-[ ignorable ] +-[ ignorable ] +-[ element : hasMixed ] +-[ text ] ““ +-[ text ] " This is text.“ +-[ text ] “




Outline

Output from Fig. 6.14

“ +-[ text ] " “ +-[ element : hasCDATA ] +-[ attribute : cdata ] " simple cdata“ +-[ text ] ““ +-[ text ] " This is some additional text.“ +-[ text ] ““ +-[ text ] " “ +-[ ignorable ] +-[ ignorable ][ document end ]


6.9 Case Study: Writing a DTD for the Day Planner Application

• Continue case study from Chapter 5– External subset of DTD for day planner


Outline

Fig. 6.15DTD for planner.xml.

Root element planner

Element year contains one or more date elements

Element year contains attribute value that has character data

Element date contains one or more note elements

Element date contains attributes month and day

Element note contains parsed character data and optional attribute time

1 

2 

3

4 <!ELEMENT planner ( year* )>

5

6 <!ELEMENT year ( date+ )>

7 <!ATTLIST year value CDATA #REQUIRED>

8

9 <!ELEMENT date ( note+ )>

10 <!ATTLIST date month CDATA #REQUIRED>

11 <!ATTLIST date day CDATA #REQUIRED>

12

13 <!ELEMENT note ( #PCDATA )>

14 <!ATTLIST note time CDATA #IMPLIED>

Root element planner contains any number of (optional) year elements

Element year contains one or more date elements

Element year contains attribute value that has character data

Element date contains one or more note elements

Element date contains attributes month and day, which

contain has character data

Element note contains parsed character data and optional attribute time


Fig. 6.15 DTD for planner.xml.