Document
Type Definition
The purpose of a DTD
(Document Type Definition) is to define the legal building blocks of an XML
document. A DTD defines the document structure with a list of legal elements
and attributes. A DTD can be declared inline inside an XML document, or as an
external reference.
Internal
DTD Declaration:
If the DTD is declared inside the XML file, it should be wrapped in a
DOCTYPE definition with the following syntax:
<! DOCTYPE root-element
[element-declarations]>
XML document with an internal DTD:
<?xml
version="1.0" encoding="UTF-8"?>
<!DOCTYPE
weather[
<!ELEMENT
weather (day1)>
<!ELEMENT
day1 (location,date,temp)>
<!ELEMENT
location (#PCDATA)>
<!ELEMENT
date (#PCDATA)>
<!ELEMENT
temp (#PCDATA)>
<!ENTITY
GM "Good Morning">
<!ENTITY
GN "Good Night">
]>
<weather>
<day1
>
<location>&GM;
Sanfransico </location>
<date>20/04/2013</date>
<temp>20'c</temp>
</day1>
</weather>
The DTD above is interpreted like this:
!DOCTYPE note defines that the root
element of this document is note
!ELEMENT note defines that the note
element contains four elements: "to, from, heading, body"
!ELEMENT to defines the to
element to be of type "#PCDATA"
!ELEMENT from defines the from
element to be of type "#PCDATA"
!ELEMENT heading defines
the heading element to be of type "#PCDATA"
!ELEMENT body defines the body
element to be of type "#PCDATA"
External DTD Declaration
If the DTD is declared in an external
file, it should be wrapped in a DOCTYPE definition with the syntax:
<! DOCTYPE root-element SYSTEM
"filename">
whether.xml
<?xml version="1.0"
encoding="UTF-8"?>
<!DOCTYPE weather SYSTEM
"weather.dtd">
<weather>
<day1>
<location>&GM;Vita</location>
<date>20/04/2013</date>
<temp>20'c</temp>
</day1>
</weather>
weather.dtd
<!ELEMENT weather (day1)>
<!ELEMENT day1 (location,date,temp)>
<!ELEMENT location (#PCDATA)>
<!ELEMENT date (#PCDATA)>
<!ELEMENT temp (#PCDATA)>
<!ENTITY GM "Good Morning">
<!ENTITY GN "Good Night">
Why to Use a DTD?
With a
DTD, each of your XML files can carry a description of its own format. With a
DTD, independent groups of people can agree to use a standard DTD for
interchanging data. Your application can use a standard DTD to verify that the
data you receive from the outside world is valid. You can also use a DTD to
verify your own data.
The Building Blocks of XML Documents
All XML documents are made up by the following building blocks:
Elements, Attributes, Entities, PCDATA, and CDATA .
Elements
Elements
are the main
building blocks of both XML and HTML documents.
Attributes
Attributes
provide extra information about elements. Attributes are always placed inside
the opening tag of an element. Attributes always come in name/value pairs.
Entities .Some characters have a
special meaning in XML, like the less than sign (<) that defines the start
of an XML tag.
PCDATA
PCDATA means parsed character data.
CDATA
CDATA means character data.
