gnu.io.PortInUseException: Unknown Application?

Check that /var/lock folder exist on your machine.

mkdir /var/lock
chmod go+rwx /var/lock

Reboot the system / disable the port.
Actual problem is when the program runs port is opened and it didn't close after the program terminates.
it works.

I ran into this problem because the port was actually in use. A previous instance of javaw.exe appeared in the Windows task manager, it hogged the port.

The reason why that previous java process hung was a hardware issue: When plugging the USB-2-serial converter that I happened to use into a USB-2 port, all worked fine. When plugged into a USB-3 port, RXTX CommPortIdentifier code would hang, and then subsequent instances of Java received the PortInUseException.