If it works in Eclipse testing the .class file, but does not work from a jar file, you probably have a standard class/library not found issue.
( not directly related to RxTx or Processing or Arduino )
What makes it perhaps harder than usual here, is the required native library, which has to be found and must be for the desired environment ( 32 / 64 bit )
won’t even return a exception message:
Native libraries can crash the JVM ! That produces an error message (and file) on a different level
Before testing the your jar file:
Do you get the class file to execute outside Eclipse ? This shows if you have control over the required environment.
.class to .jar adds another level of possible problems.
A bit more information about your environment, what happens, and what you expect, might help us give you more hints
getPortIdentifiers() does not need many lines of code to be in a suitable environment, can you post them here ?