jdk concurrent concurrentlinkedqueue

ConcurrentLinkedQueue

初始化

public class ConcurrentLinkedQueue<E> extends AbstractQueue<E>
        implements Queue<E>, java.io.Serializable {
    transient volatile Node<E> head;
    private transient volatile Node<E> tail;

    public ConcurrentLinkedQueue() {
        head = tail = new Node<E>();
    }
}

offer

public class ConcurrentLinkedQueue<E> extends AbstractQueue<E>
        implements Queue<E>, java.io.Serializable {
    static final VarHandle NEXT;

    public boolean offer(E e) {
        final Node<E> newNode = new Node<E>(Objects.requireNonNull(e));
        // p = tail
        for (Node<E> t = tail, p = t; ; ) {
            Node<E> q = p.next;
            if (q == null) {
                // 说明无竞争
                // cas 将 tail.next = newNode
                if (NEXT.compareAndSet(p, null, newNode)) {
                    // 至少第一次循环不会出现这个情况
                    if (p != t)
                        TAIL.weakCompareAndSet(this, t, newNode);
                    return true;
                }
            } else if (p == q) {
                p = (t != (t = tail)) ? t : head;
            } else {
                p = (p != t && t != (t = tail)) ? t : q;
            }
        }
    }
}

第一个元素 offer 后的结果

• tail.next = nodeA

第二个元素 offer 后的结果

1. 第一次循环
   • q = nodeA
   • p = tail
   • t = tail
   • p = q = nodeA
2. 第二次循环
   • p = nodeA
   • q = p.next = nodeA.next
   • nodeA.next = nodeB
   • t = tail
   • p != t == true
   • tail = nodeB