The namespace repairing performed by the serializer should be configurable
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Key: AXIOM-402
URL: https://issues.apache.org/jira/browse/AXIOM-402
Project: Axiom
Issue Type: Improvement
Components: API
Reporter: Andreas Veithen
Priority: Minor
When serializing (with or without caching) an object model tree, Axiom performs
namespace repairing. This comprises two things:
1. Axiom generates namespace declarations for namespace bindings that are
visibly used (in element or attribute names) but for which no corresponding
declaration is in scope. This is important for two reasons:
* It ensures that Axiom always produces XML that is well-formed with respect to
namespaces, even when serializing a subtree.
* Some operations (such as moving elements from one location to another) may
result in an object model that is not well-formed with respect to namespaces.
The serializer therefore needs to add the missing namespace declarations
on-the-fly.
2. Redundant namespace declarations are removed. A namespace declaration is
considered redundant if it corresponds to a namespace binding for which another
namespace declaration is already in scope. The primary reason for this is that
when creating or modifying a tree programmatically, it will in general contain
such redundant namespace declarations. This occurs when creating an orphaned
element and adding it to a location in the tree where an appropriate namespace
declaration is already in scope (this contrasts with the case where the element
is created directly as the child of an existing element). It is also likely
that OMSourcedElements backed by some types of OMDataSources would generate
lots of redundant namespace declarations.
It should be noted that these operations don't alter the namespace context of
any of the elements being serialized. They don't alter the XPath data model in
any way either. This is a consequence of section 5.4 (as well as the last
sentence of section 5.3) of the XPath 1.0 specification. Therefore other
standards based on the XPath data model (such as XML C14N and XML Signature)
are not affected by the removal of redundant namespace declarations. Finally in
the XML infoset (as defined by the XML Information Set specification) the
removal of a namespace declaration only modifies the [namespace attributes]
property (which contains attribute information items, and not namespace
information items) of the element information item, but not the [in-scope
namespaces] property.
There will nevertheless be a problem in some very particular situations, e.g.
when a document containing a redundant namespace is parsed into an Axiom tree
and later serialized and processed by an application or service that fails to
implement the aforementioned standards correctly. For these situations, Axiom
should allow to disable the removal of redundant namespace declarations.
One can also see that if one parses an XML document and later serializes it
again without doing any structural modifications, then it will automatically be
well formed with respect to namespaces. In that case, namespace repairing is
just unnecessary overhead. Therefore Axiom should also allow to disable
namespace repairing altogether.
To summarize, Axiom should have a configuration option (e.g. in OMOutputFormat)
to choose the namespace repairing mode, with three supported modes:
* Default: complete namespace repairing with removal of redundant namespace
declarations.
* Namespace repairing without removal of redundant namespace declarations.
* No namespace repairing at all.
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