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Does Java still have a word in every object allocation for locking it? Adding a 64-bit pointer sounds terribly inefficient.

Did you know Objective-C does locks and retain counting without allocating any extra fields in objects?

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On hotspot: There are two bits in the header of every object. This is enough for an object that's never been used as a contended lock, CAS operations on the header can be used to handle the locking and that's that. As soon as you actually block on it, a 'real' lock is created (you can't get around the need for a list of threads to wake up as the lock is released) and the header grows to accomodate it. The process is called 'monitor inflation'. At a later date this might be cleaned up by a 'monitor deflation'.

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