Hello Dennis,
Glad to have you as an Axis committer and guess I should give you some
explanations to why there are toOM(...) and fromOM(...) methods. Seems
I'm a little late on answering but guess I should provide you an
explanation anyway :)
The Axis2 client API depends upon OM and expects and OMElement as the
input to a message being sent. Hence we must have the objects converted
to OM represntations and vice-versa. However building the OM does not
mean that the whole XML stream is parsed and converted into the Object
tree. It's OM and OM builds the Object model when absolutely necessary
:). Inside the databind supporteres what happens is this conversion,
where the pull stream is taken from the object and fed into the
builder(toOM) or the pullstream is taken from the OMElement and fed into
the XMLBeans factory to create the Object (fromOM). The streaming API
gives us the chance to get the events as late as possible. (There are
some problems we need to fix to have this running smoothely with maximum
performance. For starters attemting to get the events without caching
throws out an error with the XBeans pull stream. Perhaps the object
reference needs to be explicitly maintained through out the process but
seems I'll have to dig deep to solve it. Until then the generated stubs
have a 'element.build()' statement). What I wanted to stress is that OM
is virtual by it's nature :)
You are absolutely right about the pull support in XMLBeans. AFAIK it is
the only known framework that has the capability of interefacing through
StAX. The best solution for doing it with other framworks I guess is
what Eran explained, a 'Push' builder. Push builders create havoc in
serialization (:)) but I guess we can make it work. To keep the perf
figures intact we actually made the builder hang on to the object and
push the events directly to the stream (through a content handler) if
needed. I'm not sure whether this code is still in the code base but I
remember well that we did something similar.
However the generated code is architectured in such a way that the
databind mechanism (may it be push,pull,XMLBeans, JiBX or JAXB) has
nothing to do with the stubs. The seperate databind supporter is the one
that handles the whole thing. Theoreticaly one can modify the databind
supporter and customize how objects are serialized. So If we figure out
how to do the serialization, putting it in shouldn't be a problem.
Dennis Sosnoski wrote:
I took a look at the XMLBeans data binding implementation for Axis2,
and have got some questions about this. For one thing, it looks like
the toOM() methods actually build the OM representation of the
databound objects, while I thought the idea behind Axis2 was to keep
the OM virtual unless absolutely necessary (for something like signing).
Also, the way in which Axis gets the data out of the databound objects
is by using an XMLStreamReader. AFAIK, XMLBeans is the only framework
which directly supports using a reader to get the XML representation
of the objects. Most frameworks (including JiBX and JAXB 2.0) only
support push-style output, where you tell the framework to serialize
the XML representation to some form of output interface and it does so
before returning. We could write an adapter to basically store the
output from one of these frameworks and make it readable using an
XMLStreamReader, but it would probably have a substantial impact on
performance.
For other data binding frameworks it's probably best to define a
special type of OMElement to hold on to the object and the
corresponding serializer until the serialize() method of the OMElement
is called, then serialize directly to the supplied target. If we need
to instead create a full OM representation, we can use a builder that
implements the XMLStreamWriter interface and supply this to the
binding framework as the target for the marshalling output. This does
make for an all-or-nothing expansion, but this is probably the best we
can do with most data binding frameworks.
There's also the issue of how to link data binding to the MTOM
support, so that a data binding framework can pass off components to
be marshalled as attachments (and likewise access attachments for
unmarshalling). I don't know if XMLBeans has any support for this yet,
but JAXB 2.0 has it and I'll be adding it into JiBX.
- Dennis
- Re: [Axis2] Databinding API issues Ajith Ranabahu
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