在ArrayBlockingQueue 中，为什么要将final 成员字段复制到本地final 变量中? [英] In ArrayBlockingQueue, why copy final member field into local final variable?

问题描述

在ArrayBlockingQueue中，所有需要锁的方法在调用lock()之前将其复制到本地final变量中.

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();
    }
}

当 this.lock 字段为 final<时，是否有任何理由将 this.lock 复制到局部变量 lock/代码>?

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

此外，它还使用了 E[] 的本地副本，然后再对其进行操作:

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;
}

是否有任何理由将 final 字段复制到局部 final 变量?

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

推荐答案

这是该类的作者 Doug Lea 喜欢使用的极端优化.这是关于最近的话题的帖子在关于这个确切主题的 core-libs-dev 邮件列表中，它很好地回答了您的问题.

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.

来自帖子:

...复制到本地产生最小的字节码，对于低级代码，编写代码很好离机器近一点

...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

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