在 ArrayBlockingQueue 中，为什么要将 final 成员字段复制到本地 final 变量中？

Question

In ArrayBlockingQueue , all the methods that require the lock copy it to a local final variable before calling lock() .

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

Is there any reason to copy this.lock to a local variable lock when the field this.lock is final ?

Additionally, it also uses a local copy of E[] before acting on it:

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

Is there any reason for copying a final field to a local final variable?

Answer 1

It's an extreme optimization Doug Lea, the author of the class, likes to use. Here's a post on a recent thread on the core-libs-dev mailing list about this exact subject which answers your question pretty well.

from the post:

...copying to locals produces the smallest bytecode, and for low-level code it's nice to write code that's a little closer to the machine

Answer 2

This thread gives some answers. In substance:

• the compiler can't easily prove that a final field does not change within a method (due to reflection / serialization etc.)
• most current compilers actually don't try and would therefore have to reload the final field everytime it is used which could lead to a cache miss or a page fault
• storing it in a local variable forces the JVM to perform only one load