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260 lines
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<title>Apache Ant - Antlib Namespaces</title>
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<meta name="author" content="Antoine Levy-Lambert">
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<div class="lightbluebar"> </div>
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<div class="main">
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<div class="content">
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<h1 class="title">Antlib Namespaces</h1>
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<h3 class="section">
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<a name="J.Pietschmann 03.05.2003 17:25"></a>
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J.Pietschmann 03.05.2003 17:25
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</h3>
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<p>
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Nicola Ken Barozzi wrote:
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< This seems interesting, and brings up what XML namespaces can be used for.
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</p>
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<p>
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XML namespaces are indented to disambiguate short local element
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and attribute names. Any sematic associated to XML namespaces
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beside this has to be weighted carefully.
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</p>
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<p>
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Lets take an example. There are two projects, Foo and Bar,
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each providing a task, lets call them <foo> and <bar>
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respectively. Both tasks take a <part> child, by coincidence.
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Of course, because the projects act uncoordinated, the <part>
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child element has a different semantic. In order to make this
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clearer, let's say the Foo <part> takes an optional <mumble>
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child while the Bar <part> takes three mandatory <xonx>
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children.
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</p>
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<p>
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Someone finds both the <foo> and the <bar> task exciting and
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wants to use both in an Ant build file. No problem so far:
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because ot the way Ant elements get their child elements and
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create associated Java objects, this should work.
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Now said someone got a super-duper schema directed XML editor
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and wants to use it for editing the build.xml file. He asks
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all projects for a schema (DTD, XSD, RNG, whatever) for this
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purpose and merges them in order to get a schema for his build
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file. At this point the two <part> elements are likely to clash
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(at least for DTDs, where element names are global). While
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it is possible to merge the content models so that <part> now
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takes either an optional <mumble> or three <xonx> children, this
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would allow the user to put <xonx> children into the <part> of
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the <foo> task. This is only a minor inconvenience for most
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people, but an unthinkable horror for true purists.
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</p>
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<p>
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Introduce namespaces: the Foo projects names its namespace
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"http://www.fooproject.org/anttask" while the Bar project uses
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"URI:bar" or whatever. For the XML parser it is only really
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important that two different strings are used. You see, the
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longer the strings the less tha chance they will clash, and
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they probably won't clash if they start with the URLs of the
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project's homepages (the intent behind the recommendation to
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use URLs, because it's the closest thing to a global registry
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you can get short of actually creating a global registry).
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Anyway, because the expanded names of the <part> elements are
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now "{http://www.fooproject.org/anttask}part" and "{URI:bar}part"
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respectively they obviously no longer clash.
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BTW you can write this as
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</p>
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<pre class="code">
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<target name="foo">
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<foo xmlns="http://www.fooproject.org/anttask">
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<part>
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<mumble>
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</part>
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</foo>
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<bar xmlns="URI:bar">
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<part><xonx/><xonx/><xonx/></part>
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</bar>
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<target>
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</pre>
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<p>
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or as
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</p>
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<pre class="code">
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<target name="foo"
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xmlns:foo="http://www.fooproject.org/anttask"
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xmlns:bar="URI:bar">
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<foo:foo>
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<foo:part>
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<foo:mumble>
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</foo:part>
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</foo:foo>
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<bar:bar>
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<bar:part><bar:xonx/><bar:xonx/><bar:xonx/></bar:part>
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</bar:bar>
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<target>
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</pre>
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<p>
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take your pick (if you think the "foo" and "bar" prefixes are too
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long, use "a" and "b" instead, it doesn't matter).
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</p>
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<p>
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So far, the namespace names should only be different for different
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projects, so why is it dangerous to associate some semantic with it,
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like letting them point to a jar file? The problem is again that
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general purpose XML tools, like the above mentioned super-duper XML
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editor may associate their own semantics with the namespace, like
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how to auto-format certain elements. This information will be stored
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in some config files, and it requires that the namespace name is
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the same until the semantics of the elements in it have changed
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enough that it warrants assigning a new namespace name.
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</p>
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<p>
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Summary:
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</p>
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<ol>
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<li>
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XML namespaces are there to facilitate aggregation of XML adhering
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to schemas (content models) of different, uncoordinated origin.
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</li><li>
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XML Namespaces should be used in a way that no end user action
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can result in two namespace names becoming unintentionally the
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same.
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</li><li>
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XML Namespace names should preferably be assigned by the people
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or project which specifies the semantics of the XML elemnets and
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attributes therein.
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</li><li>
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XML Namespace names should be kept unchanged until a change of
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the semantic of the elements warrants a change.
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</li><li>
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Good tools should not monopolize XML namespace syntax for its
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own semantics.
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</li>
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</ol>
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<p>
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The schema directed editor should provide an example hoe tools
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can take advantage of XML namespaces: use them as a key into a
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DB/config to get it's own associated semantic.
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In particular for Ant/Antlib I can imagine that each library
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provides a factory object associated to the XML namespace for
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the library.
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</p>
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<p>
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The FOP parser uses such a two stage lookup: first the namespace
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is used to get a factory object from a hash table, then the factory
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is used with the local XML element name to create a Java object
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which is inserted into the FO tree. The hash table with the factories
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is initialized at startup, the associations between namespace name
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and factory class name is read from a Services file. Want to add
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a FOP extension? Get the default Services file, add a line with
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your namespace-to-factoryclassname mapping put it into the jar with
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all the classes and drop the jar as first into the classpath. If the
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user wants to use multiple extensions, well, edit the main Services
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instead, dead easy.
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</p>
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<p>
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HTH
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J.Pietschmann
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</p>
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</div>
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</div>
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<p class="copyright">
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Copyright © 2000-2003 The Apache Software Foundation. All rights reserved.
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