Hi, I'm working on the B&S. As the Data class, I wanna maintain a collection(ArrayList) of the Contractors(synchronized with the record in the db file) on whose element synchronize() and notify() are performed in lock() and unlock(), and the cookie-record pairs are in a map in the DBLock class(with static methods which deal with the locking/unlocking for the data class). I still haven't implement it, can anyone tell me is it necessary to do this, and is there a better approach? U know I don't wanna manipulate the records directly on the db file, but I am not quiet sure is it reasonable. Thanks a lot!