Why Does ArrayBlockingQueue Copy Final Member Fields to Local Variables?

Why the ArrayBlockingQueue Class Copies a Final Member Field into a Local Final Variable

Background:

The ArrayBlockingQueue class in Java provides a bounded buffer to store and retrieve elements. All methods that require the lock copy a final instance variable, this.lock, to a local final variable, lock, before acquiring the lock.

Question:

Why is this copying operation necessary when the this.lock field is already a final variable?

Answer:

It's an optimization technique known as "extra local variables" or "field widening," used by Doug Lea, the original author of the ArrayBlockingQueue class. According to a discussion on the core-libs-dev mailing list, this optimization aims to reduce bytecode size and improve performance for low-level code.

Optimization Benefits:

• Smaller Bytecode: Copying the final field into a local variable allows the compiler to optimize the bytecode. The local variable lookup is often faster than accessing the instance field, which may involve pointer dereferencing.
• Improved Performance: For low-level code that manipulates memory directly, accessing a local variable has less overhead than accessing a field.

Additional Copying:

The ArrayBlockingQueue class also copies a final array field, this.items, into a local final array, items. This optimization serves the same purpose of reducing bytecode size and improving performance, especially in memory-intensive operations.

Conclusion:

The copying of final member fields into local final variables in the ArrayBlockingQueue class is an optimization technique that enhances performance and bytecode efficiency for demanding code scenarios.

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