Brett Morgan asks what happened to JavaSpaces? My two cents is that Javaspaces is a really neat solution in search of a problem. It stores data, but it's not a database. It sends messages but it's not a message-queue. It's networked shared memory, but there's always some more convenient, more familiar solution. One day, I'll find a problem that JavaSpaces is more suited to than anything else, but I'm yet to find it.

Oh, and on top of that, the overhead of working through Jini is decidedly non-trivial.