Bob Smith wrote:
hi all,
I pretty new to xml, what does xmlns mean?
like in:
<rdf:RDF
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns="http://purl.org/rss/1.0/"
xmlns:perljobs="http://jobs.perl.org/rss/"
xmlns:taxo="http://purl.org/rss/1.0/modules/taxonomy/"
xmlns:dc="http://purl.org/dc/elements/1.1/"
xmlns:syn="http://purl.org/rss/1.0/modules/syndication/"
xmlns:admin="http://webns.net/mvcb/"
tia
xmlns is used to bound prefixes to namespace URIs
it means that the *universal name* of the <rdf:RDF> element is
[
http://www.w3.org/1999/02/22-rdf-syntax-ns# , RDF ]
this notation [ URI , local-name ] denotes that the *universal name* is
in 2 parts (in most XML papers, it is displayed with another notation :
{http://www.w3.org/1999/02/22-rdf-syntax-ns#}RDF but I prefer the
former) ; notice that these notations are only for explanation and NEVER
appear in real XML documents
such names are *universal names* because URIs are unique ; notice that
an URI is not necessary an URL :
xmlns:foo="urn:guid:88d969c9-f192-11d4-a65f-0040963251e6"
xmlns:book="urn:isbn:2-84177-230-6"
xmlns:office="urn:schemas-microsoft-com:office:office"
namespace URIs are just use to qualify XML names
the prefix is just an alias for the URI, but its mean is local to the
xmlns declaration (that is to say, applies on its host element and its
descendents)
<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#">
<foo:RDF xmlns:foo="http://www.w3.org/1999/02/22-rdf-syntax-ns#">
<RDF xmlns="http://www.w3.org/1999/02/22-rdf-syntax-ns#">
are the same ; the last form doesn't use a prefix, it uses the default
namespace
attributes are told "owned" by their host elements, so they don't need
additional information ; that's why the default namespace doesn't apply
to attributes ; sometimes, it is necessary to "add" to an element
special purpose attributes, that are often called "foreign attributes" :
<image xmlns:xlink="http://www.w3.org/1999/xlink">
xlink:type="simple"
xlink:actuate="onLoad"
xlink:show="embed"
xlink:href="http://www.acme.com/foo/bar.jpg"
width="123"
height="234" />
some application may recognize a particular namespace URI :
-for example, an application might expose an hyperlink in conformance to
the XLink specification if it encounters attributes like those above
-an XSLT processor will do something special when it encounters
<xsl:template> not because the prefix is "xsl" but because this element
is in the scope of the xmlns declaration :
xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
-a graphic application that recognize SVG could draw a rectangle :
<svg:svg xmlns:svg "http://www.w3.org/2000/svg"
width="18cm" height="6cm" viewBox="0 0 200 120">
<svg:rect x="1" y="1" width="198" height="118"
fill="#888888" stroke="blue" />
</svg:svg>
if you have to design XML datas, I encourage you to use your own
namespace URI with your elements : thus, your application will be able
to process these elements without ambiguity ; XML namespaces have been
design for that purpose : distinguish elements that could have the same
local name, and grouping a set of names "related to the same feature"
--
Cordialement,
///
(. .)
-----ooO--(_)--Ooo-----
| Philippe Poulard |
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