XML Demystified

CHAPTER 3 Document Type Defi nitions
NJ
07665
555-555-1212
555-555-5432
555-555-7678
555-555-9989
bsmith@mycompany.com
6970654
External DTD are commonly used when XML documents are exchanged among
business partners. Business partners agree to the structure of the DTD and then
share the same external DTD for multiple XML documents.
Typically one organization will be the “owner” of the DTD and is responsible for
updating it and making it accessible to any partner that’s using it. The most common
way to share a DTD is to make the source reference a URL. The DTD can be posted
on a public web server so anybody using it can reference it. Even though one
company or organization controls the DTD, they still need to agree on the content
of the XML with their partners.
In one sense, it’s good practice to use a DTD so that all parties using it agree and
understand the exact structure of the document. Any change to the DTD needs to be
clearly communicated. Another good reason to use a DTD is that the XML parsers
can validate the structure of the document. There’s no need to write a lot of custom
code for validation. The parser already has this built in to it. For example, if the
parser comes across an element tag in the above document that has ,
this tag isn’t defined in the DTD and the parser throws an error. There is no need to
check for this in the application code; the parser checks it for you.
Another feature in the parser is its ability to ignore certain types of white space.
Oftentimes an XML document may be structured like this to make it more human
readable:
Joe
Brown
Technically there’s a text node between the tag and tag.
The value (on a Windows platform) is three characters, which consist of a carriage
return, line feed, and tab. These characters aren’t part of the content of the
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