The Name Game - XML

Setting up a namespace is simple - here's the syntax:

<elementName xmlns: prefix="namespaceURL">

In this case, the prefix is the unique string used to identify the namespace; this is linked to a specific namespace URL.

A namsepace is usually declared at the root element level, although authors are free to declare it at a lower level of the tree structure too.

Once the namespace has been declared within the document, it can be used by prefixing each element within that namespace with the unique namespace identifier. Take a look at my revised stock portfolio, which now uses a "mytrades" namespace to avoid name clashes.

<mytrades:portfolio
xmlns:mytrades="http://www.somedomain.com/namespaces/mytrades/">
<mytrades:stock>Cisco 
Systems</mytrades:stock>
<mytrades:stock>Nortel Networks</mytrades:stock>
<mytrades:stock>eToys</mytrades:stock>
<mytrades:stock>IBM</mytrades:stock>
</mytrades:portfolio>

And once Tom gets over his hangover, I guess he'd find it pretty easy to fix his document too.

<?xml version="1.0"?>
<toms_store:inventory xmlns:toms_store="http://www.toms_store.com/">
<toms_store:category>Mice</toms_store:category>
<toms_store:item>Mouse 
C106</toms_store:item>
<toms_store:vendor>Logitech</toms_store:vendor>
<toms_store:stock>100</toms_store:stock>
<toms_store:category>Handhelds</toms_store:category>
<toms_store:item>Visor 
Deluxe</toms_store:item>
<toms_store:vendor>HandSpring</toms_store:vendor>
<toms_store:stock>23</toms_store:stock>
<toms_store:category>MP3 
players</toms_store:category>
<toms_store:item>Nomad</toms_store:item>
<toms_store:vendor>Creative</toms_store:vendor>
<toms_store:stock>2</toms_store:stock>
</toms_store:inventory>

In case you're wondering, the namespace URL is simply a pointer to a Web address, and is meaningless in practical terms; the XML specification doesn't really care where the URL points, or even if it's a valid link.

In case a single document contains two or more namespaces, adding namespace declarations and prefixes to every element can get kind of messy - as the following example demonstrates.

<?xml version="1.0"?>
<me:person xmlns:me="http://www.mywebsite.com/">
My 
name is <me:name>Huey</me:name>. I'm <me:age>seven</me:age> years 
old,
and I live in <me:address>Ducktown</me:address> with <rel:relationships
xmlns:rel="http://www.mywebsite.com/relationships/">my 
brothers
<rel:name>Dewey</rel:name> and
<rel:name>Louie</rel:name></rel:relationships>.
</me:person>

In such a situation, XML allows you to specify any one namespace as the default namespace, by omitting the prefix from the namespace declaration. Modifying the document above to make "me" the default namespace, we have

<?xml version="1.0"?>
<person xmlns="http://www.mywebsite.com/">
My name 
is <name>Huey</name>. I'm <age>seven</age> years old, and I live in
<address>Ducktown</address> 
with <rel:relationships
xmlns:rel="http://www.mywebsite.com/relationships/">my 
brothers
<rel:name>Dewey</rel:name> and
<rel:name>Louie</rel:name></rel:relationships>.
</person>

Namespaces need not be restricted to elements alone - attributes can use namespaces too, as the following example demonstrates.

<?xml version="1.0"?>
<mystuff:collection xmlns:mystuff="http://www.somedomain.com/mystuff"
xmlns:music="http://www.somedomain.com/music_genres">
<mystuff:artist>Spears, 
Britney</mystuff:artist>
<mystuff:title music:genre="Pop">Oops, I Did It 
Again!</mystuff:title>
</mystuff:collection>

This article copyright Melonfire 2001. All rights reserved.

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