java - In what situations could an empty synchronized block achieve correct threading semantics?

Question

I was looking through a Findbugs report on my code base and one of the patterns that was triggered was for an empty synchronzied block (i.e. synchronized (var) {}). The documentation says:

Empty synchronized blocks are far more subtle and hard to use correctly than most people recognize, and empty synchronized blocks are almost never a better solution than less contrived solutions.

In my case it occurred because the contents of the block had been commented out, but the synchronized statement was still there. In what situations could an empty synchronized block achieve correct threading semantics?

Answer 1

An empty synchronized block will wait until nobody else is using that synchronizer. That may be what you want, but because you haven't protected the subsequent code in the synchronized block, nothing is stopping somebody else from modifying what ever it was you were waiting for while you run the subsequent code. That's almost never what you want.