Friday, February 20, 2009

where is the lock used by Vector synchronization?

If you know a collection class (Vector for eg) is thread-safe, you can't assume[1] the synchronization lock(s) used is the Vector instance itself. There could be one or more locks internally employed.

A *real* challenge -- create a method that writes[2] to a vector object twice *atomically*. A novice might decide to synchronize on the vector object, with Assumption [1] above. Not wrong, but beware -- 2 threads synchronizing on different locks won't get serialized.

[2] atomic read-then-write is also a challenge, since the write could be based on the read. See Repeated Read in my post on "Transaction Isolation"

ON another note, a local Vector is locked before each of the add operations to ensure that it would not be modified by other threads, but because it is strictly local to the method this is an unnecessary performance hit, and u can't avoid it with Vectors.

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