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Document Type Definition (DTD)
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A formal, machine-readable specification thatdefines the structure of an XML document andprovides some information about the requiredcontent
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DTDs Provides syntax for declaring elements,attribute lists, entities, and notations
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DTD element declarations begin with the openingdelimiter sequence
, followed by one of the four
keywords
ELEMENT
,^
ATTLIST
,^
ENTITY
, or
NOTATION
then a case sensitive element name, a contentdescription, and end with
, e.g.,
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A DTD is a sequence of these declarations enclosedin a
DOCTYPE
declaration or stored separately and
referred to from a
DOCTYPE
DTD in XML Prolog (Internal Subset)
** ]>**
**BossTroops15 April 1951The buck stops here. ** •^
Parentheses are grouping operators and commas are andoperators
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#PCDATA
means parsed character data
Document Type Definition (DTD)
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DTDs can be declared as
PUBLIC
rather than
SYSTEM
where a unique name is specified as the URI and a URIis supplied following the DTD unique name (typicalpractice)
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Both external subset and internal subset DTDs may beused in the same document
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Operators and keywords available for declaring contentdescriptions: ,
(comma)ordered list (and operator) |
or operator
content grouping
preceding item may occur zero or one time
preceding item may occur one or more times
preceding item may occur zero or more times
#PCDATA
parsed character data
EMPTY
element may not contain content
ANY
element may contain any content
Document Type Definition (DTD)
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Attributes are declared with an
declaration, which contains the element name for whichthe attributes are being declared followed by a list ofattribute declarations. Each declaration includes theattribute name, a data type specification, and a defaultdefinition that tells whether the attribute is required andif not, what action the parser should take. Example:
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In this example, the
date
attribute is required
(#REQUIRED
) and is of type
CDATA
(character data).
The
priority
attribute is an enumerated type. The
values for the enumeration must be legal XML names,enclosed in parentheses, and separated by
operators. The quoted string
"medium"
in the example
makes the
priority
attribute optional with a default
value of
medium
XML Schema
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Created to solve DTD shortcomings.
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DTDs have a very weak typing system that can onlyrestrict XML elements to contain no data, other XMLelements, or text data
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DTDs do not support data types like integers,decimals, booleans, dates, or enumerations
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DTDs do not allow one to specify that the dataappear in a specific format.
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DTDs do not support namespaces
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DTDs use a different syntax than the XMLdocuments they describe
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An XML
s
chema is an XML document that conforms
to the XML
S
chema specification
XML Schema, cont.
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Binding an XML schema to an XML document is done via attributes inthe root XML element.
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xmlns:xsi
attribute declares the XML Schema namespace
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xsi:schemaLocation
attribute declares the location of the XML
schema document being used
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xmlns
attribute declares the default namespace being used, which is
defined in the schema document **BossTroops15 April 1951The buck stops here. **
XML Schema, cont.
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The root XML element for the XML schema definition is
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The
xmlns
attribute of the schema definition binds the
namespace prefix
xsd
to the version of XML Schema
being used, in this case http://www.w3.org/2001/XMLSchema
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The
targetNamespace
for the XML schema is the
namespace for the elements and attributes defined by theschema definition. When this schema is referenced byanother XML document, the
targetNamespace
will be
used to qualify the elements defined by this schema.
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The
elementFormDefault
attribute set to
"qualified"
indicates that nested elements in the XML
document instance must be namespace qualified; defaultis unqualified.
XML Schema, cont.
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XML elements are defined using the
tag
and XML element attributes are defined using the
tag.
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The
name
and
type
attributes are used to define the
element/attribute name and data type, respectively.
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Elements can be defined as either
complexType
or
simpleType
, attributes can only be
simpleType
. Simple
types can have neither attributes nor child elements.Complex types can have either.
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XML Schema defines many built-in atomic types includingstrings, numbers, dates, and times.
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The built-in atomic types can be further constrained by a derived
simple type specifying
facets
using the
element, e.g.,
minLength
and
maxLength
