HELP NEEDED:  problem with rxtxcomm.jar

Hello, I am having a problem with running arduino and complie my java code

I using netbeans to to have access to the ttyUSB0

it works fine from the first time then if I stop and do it again it will give this following problem

Exception in thread "main" at at tester.Main.main( Java Result: 1 BUILD SUCCESSFUL (total time: 0 seconds)

the only way I can debug again by restarting my ubuntu 9.04 32bit and run arduino and make the upload then close it then run netbeans then run the java program I am sure why this ttyUSB port not found.

is it may the port is unlocked from the previous run

I also noticed that the the previous sketch was still there even I made another upload(it was successful ) using another computer.

its maybe the memory is not cleared and that why I cannot establish any communication I am using arduino NG or older with ATmega168