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Partial documents with JAXB?

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I'm working to create some services with JAX-RS, and am relatively new to JAXB (actually XML in general) so please don't assume I know the pre-requisites that I probably should know! Here's the questions: I want to send and receive "partial" objects in XML. That is, imagine one has an object (Java form, obviously) with:

class Thing { int x, String y, Customer z }

I want to be able to send an XML output that contains (dynamically chosen, so I can't use XmlTransient) just x, or just z, or x and y, but not z, or any other combination that suits my client. The point, obviously, is that sometimes the client doesn't need everything, so I can save some bandwidth (particularly with lists of deep, complex objects, which this example clearly doesn't illustrate!).

Also, for input, the same bandwidth argument applies; I would like to be able to have the client send just the particular fields that should be updated in, say, a PUT operation, and ignore the rest, then have the server "merge" those new values onto existing objects and leave the un-mentioned fields unchanged.

This seems to be supported in the Jackson JSON libraries (though I'm still working on it), but I'm having trouble finding it in JAXB. Any ideas?

One thought that I was pondering is whether one can do this in some way via Maps. If I created a Map (potentially nested Maps, for nested coplex objects) of what I want to send, could JAXB send that with a plausible structure? And if it could create such a map on input, I guess I could work through it to make the updates. Not perfect, but maybe?

And yes, I know that the "documents" that will be flying around will probably fail to comply with schemas, having missing fields and all that, but I'm ok with that, provided the infrastructure can be made to work.

Oh, and I know I could do this "manually" with SAX, StAX, or DOM parsing, but I'm hoping there's a rather more automatic way, particularly since JAXB handles the whole objects so effortlessly.

Cheers, Toby
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