在ConcurrentLinkedQueue源代码中无法获得此条件 [英] Can't get this condition in ConcurrentLinkedQueue source code

问题描述

在ConcurrentLinkedQueue的源代码中，使用 offer 方法：

In the source code of ConcurrentLinkedQueue, in the offer method:

public boolean offer(E e) {
checkNotNull(e);
final Node<E> newNode = new Node<E>(e);

for (Node<E> t = tail, p = t;;) {
    Node<E> q = p.next;
    if (q == null) {
        // p is last node
        if (p.casNext(null, newNode)) {
                // Successful CAS is the linearization point
                // for e to become an element of this queue,
                // and for newNode to become "live".
                if (p != t) // hop two nodes at a time
                    casTail(t, newNode);  // Failure is OK.
                    return true;
            }
            // Lost CAS race to another thread; re-read next
        }
        else if (p == q)
            // We have fallen off list.  If tail is unchanged, it
            // will also be off-list, in which case we need to
            // jump to head, from which all live nodes are always
            // reachable.  Else the new tail is a better bet.
            p = (t != (t = tail)) ? t : head;
        else
            // Check for tail updates after two hops.
            p = (p != t && t != (t = tail)) ? t : q;
    }
}

在第352行，存在以下情况：

at line 352, there is this condition:

p = (p != t && t != (t = tail)) ? t : q;

我知道代码会将p放在最后，但是为什么要使用如此复杂的代码呢？ （p！= t&& t！=（t = tail））是什么意思？ t！=（t = tail））和 t！= t 有什么区别？

I know that code is to put p to the tail, but why use so complex code? and what does (p != t && t != (t = tail))mean? what the different between t!=(t=tail)) and t!=t? should it always be false ?

是否有任何材料可以清楚地解释ConcurrentLinkedQueue？

Is there any materials can explain ConcurrentLinkedQueue clearly?

推荐答案

这是一个有趣的问题，我更广泛地提出了这个问题，并得到了一些答案。根据我在该主题上可以读到的内容：

It is an interesting question, I asked it more broadly there and got a few answers. From what I can read on this topic:

t！=（t =尾）只是一种怪异的书写方式：

t != (t = tail) is just a weird way to write:

if (t != tail)
    t = tail;

简而言之，您正在将 t 的值与右边分配给 t 的值，这里是 tail 。

In short, you are comparing the value of t with the value of what is assigned to t on the right, here tail.

（所有功劳归因于 Eran ，以使他对这个主题和答案有所了解）

( All the credits go to Eran for his understanding of the topic and his answer )

所以要完全回答您的问题：

So to answer your question entirely:

• 我不知道他们为什么使用如此复杂的代码。


• （p！= t&&t！=（t =尾巴））表示如果p！= t且如果t！=尾巴t取尾值


• 差异解释


• （显然）它不应该总是错误的

• I don't know why they used so complex code.
• (p != t && t != (t = tail)) means if p != t and if t != tail t takes the tail value
• Difference explained
• It should not be always false (obviously)

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