Friday, March 9, 2007

synchronized protects against inconsistency?

Not always, unfortunately. After throwing exceptions, the synchronized method may leave the data in a /bizarre/, illogical, impossible state, but JVM always releases the lock automatically, and permits other threads to read/write this wierd data! For more on inconsistency, read other blog posts.

This exception issue is Arguably the Achilles' heel of "synchronized" -- perhaps the only weakness in its promised protection against inconsistency.

Luckily, Lock interface comes to the rescue. It doesn't release the lock after an exception. However, this can create deadlocks, since the lock held is never automatically released. You need finally{}.

Conclusion: Lock interface with finally{} protects against the twin evils of inconsistency && deadlock, in the event of exceptions.

In fact, exceptions, consistency and deadlock are right at the heart of a major design challenge in JVM thread implementation. See P 123 [[ java threads ]].

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