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    <p><strong>Synchronization is done upon <em>objects</em>, not <em>variables</em>.</strong></p> <p>Variables/members [sometimes] contain <em>objects</em> and it is the resulting <em>object</em> contained in [variable] <code>x</code> that is actually synchronized upon in <code>synchronized(x)</code>.</p> <p>There are a few other issues with thread-visibility of variables (e.g. might read a "stale" object from a variable), but that does not apply here: there is no re-assignment of <code>_lock</code> and the visibility of the initial ("final") assignment is guaranteed. Because of this it is guaranteed that, in this case, the method parameter will always contain the correct (same) <em>object</em> used for the synchronization.</p> <p>If the lock object used (where presumably <code>_lock</code> is not final) changes, however, then that would require re-evaluation of the appropriate values/thread-visibility but otherwise does not differ from any cross-thread access.</p> <p>Happy coding.</p>
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    1. POCan you safely synchronize on a Java method parameter?
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    1. POCan you safely synchronize on a Java method parameter?
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    1. COActually, my example wasn't really correct. I didn't mean to mark _mutableObject as final. I don't think this locking will work *IF* _mutableObject in the parent object is non-final. If you pass that value as a variable into the helper method, and then you change _mutableObject to point to something else in the parent class, when the helper method runs it is no longer holding onto the correct reference for _mutableObject. The lock will still lock, but you will be "protecting" a stale value of _mutableObject. That is really what my question was supposed to point out.
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    2. COI will accept your answer assuming that this will only work if _mutableObject is 'final' in the parent class. The _lock object always needs to be declared final, no matter which version of my example you use. In fact, I shouldn't have created my example to include a "mutable" object. I should have just described that I wanted to protected a non-final variable, call it "private Object _object". I want the lock to protect that value changing. If the example is changed to that, I don't think you can pass the locking off to a helper method, because helper method gets stale _object reference.
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    3. CO@Android Dev An object used for a lock *object* should be stable and unique for each mutually exclusive synchronization desired. The variable (or object) being protected by the synchronization can change (the synchronization context will ensure the memory barriers and happens-before guarantees as long as all such synchronizations operate upon the same lock *object*). It's generally a bad idea to swap an object used for a lock (the lock *object* can be mutated and this will not affect it's usage with `synchronized` -- the importance is that the *same object* is used for synchronization.)
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