Why Does `ArrayBlockingQueue` Copy Final Member Fields to Local Final Variables?

Why Copy Final Member Fields into Local Final Variables in ArrayBlockingQueue?

In ArrayBlockingQueue, methods requiring the lock copy it to a local final variable before invoking lock(). For instance, in the offer method:

public boolean offer(E e) {
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
        if (count == items.length)
            return false;
        else {
            insert(e);
            return true;
        }
    } finally {
        lock.unlock();
    }
}

Additionally, the extract method employs a local copy of the items array:

private E extract() {
    final E[] items = this.items;
    E x = items[takeIndex];
    items[takeIndex] = null;
    takeIndex = inc(takeIndex);
    --count;
    notFull.signal();
    return x;
}

Why are these final fields being copied to local variables?

Optimization for Concurrency

According to Doug Lea, the author of ArrayBlockingQueue, copying final fields to locals is an optimization to reduce bytecode size. In low-level code, it's beneficial to write code that's closer to the machine.

By copying the final fields, the compiler generates smaller bytecode, which improves performance in concurrent environments. Accessing final fields through local variables can be faster than accessing them directly from the instance, particularly in highly concurrent scenarios where multiple threads may be accessing the same shared memory.

Performance Benefits

By copying final member fields into local final variables, ArrayBlockingQueue optimizes for performance and concurrency. This optimization may be subtle, but it can make a noticeable difference in situations where high levels of concurrency are involved.

Copying final fields to locals may not be a universal best practice for all situations, but in the context of ArrayBlockingQueue, it is a deliberate design choice made to enhance performance in concurrent environments specifically.

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