Imposing Structure on XML Documents Using a DTD
An Introduction to Document Type Definitions
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Prof. David Bernstein
James Madison University
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Computer Science Department
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bernstdh@jmu.edu
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Motivation
- To be well-formed an XML document only need satisfy
a few simple requirements.
- These requirements say nothing about the structure of the
document (e.g., the attributes for each element, the
hierarchical structure of the elements).
Document Type Defintions (DTDs)
- Purpose:
- Impose structure on XML documents
- Associating a DTD with an XML Document:
- Include a
DOCTYPE declaration in
the XML document
- Syntax of the
DOCTYPE Declaration:
-
<!DOCTYPE
root
SYSTEM
"
dtd
"
>
-
where root is the root element of the XML document and
dtd is the URL of the DTD
DOCTYPE Declaration (in the XML Document)
An Example
xmlexamples/timetable/dtd/amtrak.xml
(Fragment: 0)
Declarations in a DTD
DOCTYPE
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The root declaration.
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ELEMENT
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Declares an element type.
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ATTLIST
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Declares an attribute list for an element type.
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Unary Operators in a DTD
+
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The element can appear one or more times.
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*
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The element can appear zero or more times.
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?
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The element can appear zero or one times.
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Binary Operators in a DTD
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Essentially the delimiter in a list of possible elements.
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Types in a DTD
#PCDATA
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The element can contain any text (i.e., parsed character data).
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EMPTY
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The element has no content.
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ANY
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The element can have any type of content.
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The DOCTYPE Declaration (in a DTD)
- Purpose:
- Syntax:
-
<!DOCTYPE
root
[
declarations
]>
- where root is the root element of the XML document and
declarations are the ELEMENT and ATTLIST declarations
that provide the structure
The ELEMENT Declaration (in a DTD)
- Purpose:
- Syntax:
-
<!ELEMENT
tag
(
contents [, ...])
>
- where tag is the tag that will be used for this element
and content is a description of the allowed contents
The ELEMENT Declaration (cont.)
An Example
xmlexamples/timetable/dtd/timetable.dtd
(Fragment: 0)
The ATTLIST Declaration (in a DTD)
- Purpose:
- Declares an attribute list for an element type
- Syntax:
-
<!ATTLIST
tag
attribute
values
#modifier ["
default
"]
>
- where tag is the relevant tag; attribute
is the name of the attribute being declared;
values is a description of the allowed values (or
CDATA for any text), modifier is either
REQUIRED, IMPLIED (i.e., optional with no default) or FIXED; and
default is the default value of this attribute.
The ATTLIST Declaration (cont.)
An Example
xmlexamples/timetable/dtd/timetable.dtd
(Fragment: 1)
A Complete Example
The DTD
xmlexamples/timetable/dtd/timetable.dtd
The XML
xmlexamples/timetable/dtd/amtrak.xml
An Important Note
There's Always More to Learn