Have a look at org/postgresql/test/jdbc2/NotifyTest.java for exactly what you proposed. As for when it gets notified, I do not know how you would tell as the spec does not provide this functionality (AFAIK)
I've seen that :) May be I'm missing something, but that's not asynchronous. I need to wait until statement finishes before calling those getWarnings() methods. May be I can call them from another thread (at least on Connection/Statement object) as the SQL is being processed, and they will return current messages, but I'm not sure when to - i.e. how do I know when a new message has been made available.
Is it possible to get output of raise notice statements as produced by a PL function, as the function is being executed? And if yes, what version was that introduced in?