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Date: Sat, 03 May 2003 17:25:40 +0200
From: "J.Pietschmann" <j3322ptm@yahoo.de>
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To: Ant Developers List <dev@ant.apache.org>
Subject: Re: Namespaces in Ant
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Nicola Ken Barozzi wrote:
> This seems interestig, and brings up what XML namespaces can be used for.

XML namespaces are indented to disambiguate short local element
and attribute names. Any sematic associated to XML namespaces
beside this has to be weighted carefully.

Lets take an example. There are two projects, Foo and Bar,
each providing a taske, lets call them <foo> and <bar>
respectively. Both tasks take a <part> child, by coincidence.
Of course, because the projects act uncoordinated, the <part>
child element has a different semantic. In order to make this
clearer, let's say the Foo <part> takes an optional <mumble>
child while the Bar <part> takes three mandatory <xonx>
children.
Someone finds both the <foo> and the <bar> task exciting and
wants to use both in an Ant build file. No problem so far:
because ot the way Ant elements get their child elements and
create associated Java objects, this should work.
Now said someone got a super-duper schema directed XML editor
and wants to use it for editing the build.xml file. He asks
all projects for a schema (DTD, XSD, RNG, whatever) for this
purpose and merges them in order to get a schema for his build
file. At this point the two <part> elements are likely to clash
(at least for DTDs, where element names are global). While
it is possible to merge the content models so that <part> now
takes either an optional <mumble> or three <xonx> children, this
would allow the user to put <xonx> childre into the <part> of
the <foo> task. This is only a minor inconvenience for most
people, but an unthinkable horror for true purists.
Introduce namespaces: the Foo projects names its namespace
"http://www.fooproject.org/anttask" while the Bar project uses
"URI:bar" or whatever. For the XML parser it is only really
important that two different strings are used. You see, the
longer the strings the less tha chance they will clash, and
they probably won't clash if they start with the URLs of the
project's homepages (the intent behind the recommendation to
use URLs, because it's the closest thing to a global registry
you can get short of actually creating a global registry).
Anyway, because the expanded names of the <part> elements are
now "{http://www.fooproject.org/anttask}part" and "{URI:bar}part"
respectively they obviously no longer clash.
BTW you can write this as
  <target name="foo">
    <foo xmlns="http://www.fooproject.org/anttask">
      <part>
        <mumble>
      </part>
    </foo>
    <bar xmlns="URI:bar">
      <part><xonx/><xonx/><xonx/></part>
    </bar>
  <target>
or as
  <target name="foo"
     xmlns:foo="http://www.fooproject.org/anttask"
     xmlns:bar="URI:bar">
    <foo:foo>
      <foo:part>
        <foo:mumble>
      </foo:part>
    </foo:foo>
    <bar:bar>
      <bar:part><bar:xonx/><bar:xonx/><bar:xonx/></bar:part>
    </bar:bar>
  <target>
take your pick (if you think the "foo" and "bar" prefixes are too
long, use "a" and "b" instead, it doesn't matter).
So far, the namespace names should only be different for different
projects, so why is it dangerous to associate some semantic with it,
like letting them point to a jar file? The problem is again that
general purpose XML tools, like the above mentioned super-duper XML
editor may associate their own semantics with the namespace, like
how to auto-format certain elements. This information will be stored
in some config files, and it requires that the namespace name is
the same until the semantics of the elements in it have changed
enough that it warrants assigning a new namespace name.

Summary:
1. XML namespaces are there to facilitate aggregation of XML adhering
  to schemas (content models) of different, uncoordinated origin.
2. XML Namespaces should be used in a way that no end user action
  can result in two namespace names becoming unintentionally the
  same.
3. XML Namespace names should preferably be assigned by the people
  or project which specifies the semantics of the XML elemnets and
  attributes therein.
4. XML Namespace names should be kept unchanged until a change of
  the semantic of the elements warrants a change.
5. Good tools should not monopolize XML namespace syntax for its
  own semantics.

The schema directed editor should provide an example hoe tools
can take advantage of XML namespaces: use them as a key into a
DB/config to get it's own associated semantic.
In particular for Ant/Antlib I can imagine that each library
provides a factory object associated to the XML namespace for
the library.

The FOP parser uses such a two stage lookup: first the namespace
is used to get a factory object from a hash table, then the factory
is used with the local XML element name to create a Java object
which is inserted into the FO tree. The hash table with the factories
is initialized at startup, the associations between namespace name
and factory class name is read from a Services file. Want to add
a FOP extension? Get the default Services file, add a line with
your namespace-to-factoryclassname mapping put it into the jar with
all the classes and drop the jar as first into the classpath. If the
user wants to use multiple extensions, well, edit the main Services
instead, dead easy.

HTH
J.Pietschmann