It is not that easy, man

Now it is DA who represent the People (your state) to sue, rather than you v. your roommate. He/she stated that you assaulted him/her. If he/she change his/her mind and does not want to testify against you at trial, DA still got physical evidence and the cop's testimoney. If DA want, he may compell your roommate to testify. If he/she said differently at trial, DA could even charge him/her with perjury. But probably what I said may not happen. Just warn you of being serious.