Why Does ArrayBlockingQueue Use Local Copies of its Lock and Item Array?

ArrayBlockingQueue Lock Optimization

ArrayBlockingQueue, a bounded concurrent queue, employs an optimization technique in its methods that require the lock. Rather than referencing the final lock field directly, these methods copy it to a local final variable (lock) before calling lock().

Reason for Local Copy of Lock

This optimization is driven by machine-oriented code optimization. Copying the lock field into a local variable results in smaller bytecode, which improves efficiency for low-level code. As explained by Doug Lea, the class's author, this approach "produces the smallest bytecode."

Additional Local Copy in extract()

In the extract() method, another local copy (items) is created for the items array before it is acted upon. This optimization also serves the purpose of reducing bytecode size and improving code efficiency.

Conclusion

While final fields are typically accessed directly, ArrayBlockingQueue's optimization demonstrates the benefits of copying them into local variables for performance improvements. This technique, employed by Doug Lea, optimizes the implementation by minimizing bytecode and enhancing efficiency, particularly in low-level code scenarios.

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