A block of code that synchronizes on a field and updates that field while the lock is held is unlikely to provide the desired thread safety. Such a synchronized block does not prevent multiple unsynchronized assignments to that field because it obtains a lock on the object stored in the field rather than the field itself.

Instead of synchronizing on the field itself, consider synchronizing on a separate lock object when you want to avoid simultaneous updates to the field. You can do this by declaring a synchronized method and using it for any field updates.

In the following example, in class A, synchronization takes place on the field that is updated in the body of the setField method.

In class B, the recommended approach is shown, where synchronization takes place on a separate lock object.

Java Language Specification: The synchronized Statement, synchronized Methods.

The Java Tutorials: Lock Objects.