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It's possible that you have leaking resources.
I'd start with a thread dump of the "frozen" process to see if there's any hints there.
Stephen, thanks for the thread dump advice, it's valueable for me. I'm not so sure that so "frozen" process which do not respond to kill signal will respond to kill -QUIT signal, but I'll try to do this next time.
Wold-Dieter Fink, it's hard to catch for me whether it was ClassNoFoundEx or not because there are too much clients and possibly one of the hundred got the ClassNotFoundEx before I got server "frozen".