This question has been asked and answered many times:
to reference a few.
Personally I recommend SerialPort from http://serialio.com – it’s not free, but it’s well worth the developer (no royalties) licensing fee for any commercial project. Sadly, it is no longer royalty free to deploy, and SerialIO.com seems to have remade themselves as a hardware seller; I had to search for information on SerialPort.
From personal experience, I strongly recommend against the Sun, IBM and RxTx implementations, all of which were unstable in 24/7 use. Refer to my answers on some of the aforementioned questions for details. To be perfectly fair, RxTx may have come a long way since I tried it, though the Sun and IBM implementations were essentially abandoned, even back then.
A newer free option that looks promising and may be worth trying is jSSC (Java Simple Serial Connector), as suggested by @Jodes comment.