character data only]]> - 64 - Figure 3.8

- 64 -
Figure 3.8:
Parser output of CDATA snippet.
Document Type Declaration
A 
document type declaration 
is a statement embedded in an XML document whose 
purpose is to herald the existence and location of a document type definition (DTD). 
Read that statement twice. A document type 
declaration
is different from a document 
type 
definition
. The first is a statement that points to the second. A DTD is a set of rules 
that defines the structure of an XML document. A document type declaration is a 
statement that tells the parser which DTD to use for checking and validation.
Let’s say we have an XML document called myfile.xml that we want the parser to validate 
against a DTD called my_rules.dtd. The way to associate (that is, tell the parser to check) 
the content structure of myfile.xml against the rules specified in my_rules.dtd is to insert 
the following line after the XML declaration:
The above simply states that for this particular document, whose root element’s name is 
myfile (note: the root element name MUST match the root element declared in the 
declaration), a DTD is available, and its name and path is my_rules.dtd. Intuitive, isn’t it? 
Seriously though, now you have a glimpse as to why we detest DTDs so much.
Document Type Declarations can have external and internal components; that is, they 
can refer to Document Type 
Definitions 
that are either declared within the XML document 
itself (so the structural rules are contained within the XML document itself) or described in 
an external file/location (that is, it points to an external document that contains structural 
information about the document, just as a model number refers to specifications 
contained in an external manual).
If the DTDs are internal, the syntax to use is this:
]>
Even though we’re getting ahead of ourselves (internal subsets are discussed later on in 
this chapter), here’s an example of an internal subset declaration and its usage:
]>

- 65 -
Alex, Faraz
Similarly, for external subsets, the syntax found in the XML document is:

...

To exemplify this, we need to have two documents: the external DTD and the XML 
document. The XML document looks like this:
Alex, Faraz
while the DTD, good_guys.dtd looks like this:
A document can have 
both 
internal and external document type declarations so that you 
can further define your structure. In keeping with the same theme as above, our 
good_guys.dtd should look like this:
If we further wanted to define the structure of our good_guy element, our good_guys.xml 
XML file could look like this:
]>
Alex, Faraz
Hold on to your hats; it gets worse before it gets better. But rest assured—it does get 
better!

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