Does anybody know if you will fail automatically if your assignment is an executable jar?
I am not sure what exactly you mean here. Many of us chose to submit a jar that contained two other jars, -- server.jar and client.jar that can be started as follows:
java -jar client.jar
java -jar server.jar
That's a perfectly acceptable solution. Now, if you mean that you would ask the assessor to just double-click the submission jar or one of the jars inside it, that's another matter, and I would say is a risky one.
Eugene.