Why Does ArrayBlockingQueue Use Local Final Variables for Performance Optimization?

ArrayBlockingQueue: Local Final Variables for Performance Optimization

In the ArrayBlockingQueue class, it's noticeable that certain methods copy member fields to local final variables before invoking critical operations.

For instance, in the offer() method:

public boolean offer(E e) {
    ...
    final ReentrantLock lock = this.lock;
    lock.lock();
    ...
}

Here, this.lock is copied to the local variable lock. This practice extends to other member fields as well, such as E[] items in the extract() method.

One might question the necessity of these local copies, given that the member fields are already declared as final. However, there's a subtle performance optimization at play.

According to Doug Lea, the author of the class, copying to local variables produces smaller bytecode. For low-level code, this optimization can marginally reduce the overhead associated with retrieving and using the member fields, particularly for frequently invoked methods.

As stated in a core-libs-dev mailing list thread, this optimization "produces the smallest bytecode, and it's nice to write code that's a little closer to the machine."

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