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

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

在ArrayBlockingQueue中，所有需要锁的方法都在调用lock()之前将其复制到本地最终变量。

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?

有什么理由复制这个吗?当字段锁定时，锁定一个局部变量。锁是最后?

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

此外，它还使用E[]的本地副本进行操作:

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?

是否有任何理由将最终字段复制到本地最终变量?

2 个解决方案

#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.

这个类的作者Doug Lea喜欢使用这种极端的优化。这是最近在核心-libs-dev邮件列表中发布的一篇关于这个主题的帖子，它很好地回答了你的问题。

from the post:

职务:

...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

…复制到本地代码可以生成最小的字节码，对于低级代码来说，编写更接近机器的代码是很好的

#2

This thread gives some answers. In substance:

这条线给出了一些答案。在物质:

the compiler can't easily prove that a final field does not change within a method (due to reflection / serialization etc.)
编译器不能很容易地证明最终字段在方法中不会改变(由于反射/序列化等原因)。
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
大多数当前的编译器实际上不会尝试，因此每次使用最终字段时都必须重新加载，这可能导致缓存丢失或页面错误
storing it in a local variable forces the JVM to perform only one load
将其存储在本地变量中会迫使JVM只执行一个负载

#1