Offline database usage

I looking for some advice regarding using a disconnected java application
running on PG.

Scenario: A Java application read/writes a PG database in a transactional
environment. To be more specific, status information about clients is
looked up from some table(s), and a number of lines are then entered into an
order. The items that can be ordered are in another table, which must also
be available offline. Lastly the completed order must of course also be
available offline, for submission later when an internet connection is
available again.

Now this application should be able to work when disconnected from the
server. Of course the application can be written to create offline copies
of the data in a local file, but basically that would cause some problems,
since it would involve a lot of coding firstly, but also mean that different
code would be required for online and offline operation.

My question is, is there any technology that would allow this type of
funtionality with Postgres, like some type of caching enjine, or something
else, acting as a sort of mini "middleware" of sorts (to borrow that term)
to do this for me.

Thanks

Roland Giesler