There are several reasons synchronization on this or class expressions may be a bad idea: First, it makes synchronization part of the external interface of the class. This makes a future change to a different locking mechanism difficult. Second, it makes it hard to track just who is locking on a given object. And third , it makes a denial-of-service attack possible, either on purpose or it can happen easily by accident when subclassing.
As an alternative, consider synchronizing on a private final lock object, access to which can be completely controlled.