Internal Versus External DTDs
May 3, 1999
As we have already alluded to several times throughout this
tutorial, a DTD can either be included as part of a
well-formed XML document, (standalone=yes),
or it can be referenced from an external source, (standalone=no).
The benefits of using external DTDs is that they
can more easily and efficiently be
shared by more than one XML document,
or in fact, many organizations with the
need to standardize communications and data.
You can write a DTD once
and have multiple documents reference it.
Not only does this save typing time, but
it assures that as the DTD manager makes changes to the
central DTD, all documents that rely on the DTD
are updated in one fell swoop. (Of course, DTD changes
will not necessarily be backwards compatible,
so watch out!).
In order to reference an external DTD, you must change both
the XML declaration and the DOCTYPE declaration.
The XML Declaration must be changed to reflect the fact that
the XML document will not work on its own. That is, it will
not be standalone.
<?xml version = "1.0"
encoding="UTF-8"
standalone = "no"?>
You will also need to change the DOCTYPE declaration to add
the SYSTEM attribute.
<!DOCTYPE ROOT_ELEMENT
SYSTEM "URL_OF_EXTERNAL_DTD">
such as....
<!DOCTYPE CONTACTS
SYSTEM "http://www.mydomain.com/dtds/contacts.dtd">
Note also, that the URL may be a relative or absolute file
location such as....
<!DOCTYPE CONTACTS
SYSTEM "contacts.dtd">
which specifies a dtd file in the same directory as the XML
document that references it. Or, similarly, you can reference
the same document up one directory and down one into the "dtds"
directory using something like the following.
<!DOCTYPE CONTACTS
SYSTEM "../dtds/contacts.dtd">
Using this method, you can simply cut out the DTD from your
XML document and paste it into the separate document called
contacts.dtd. Thus, you have one file with the DTD and one
file with the well-formed XML document.
Entity Declarations
Introduction to XML For Web Developers | Table of Contents
Public DTDs
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