TriberStack

看JDK源碼FutureTask的時候看到裏面有描述用到Triber stack，想到不知道這是什麼數據結構，於是來學習一下。

代碼如下：

public class FutureTask<V> implements RunnableFuture<V> {
    /** Treiber stack of waiting threads */
    private volatile WaitNode waiters;
}

等待節點用的是一個Treiber stack，而WaitNode的結構如下：

    /**
     * Simple linked list nodes to record waiting threads in a Treiber
     * stack.  See other classes such as Phaser and SynchronousQueue
     * for more detailed explanation.
     */
    static final class WaitNode {
        volatile Thread thread;
        volatile WaitNode next;
        WaitNode() { thread = Thread.currentThread(); }
    }

看不出來有什麼端倪，又google一番，參考wikipedia

The Treiber stack algorithm is a scalable lock-free stack utilizing the fine-grained concurrency primitive compare-and-swap.[1] It is believed that R. Kent Treiber was the first to publish it in his 1986 article "Systems Programming: Coping with Parallelism".[2]

即Treiber stack是一個使用細粒度的併發原語compare-and-swap(CAS)的可擴展的無鎖棧。R. Kent Treiber在他的文章Systems Programming: Coping with Parallelism中首次提出。

即通過CAS來保證入棧和出棧的線程安全，相對來說實現起來比較簡單。

如下所示：

public class TreiberStack<E> {

    AtomicReference<Node<E>> top = new AtomicReference<>();

    public void push(E element) {
        Node<E> newHead = new Node<>(element);
        Node<E> oldHead;
        do {
            oldHead = top.get();
            newHead.next = oldHead;
        } while (!top.compareAndSet(oldHead, newHead));
    }

    public E pop() {
        Node<E> newHead;
        Node<E> oldHead;
        do {
            oldHead = top.get();
            if (null == oldHead) {
                return null;
            }
            newHead = oldHead.next;
        } while (!top.compareAndSet(oldHead, newHead));
        return oldHead.emement;
    }

    final class Node<E> {
        private final E emement;
        private Node next;

        public Node(E element) {
            this.emement = element;
        }
    }
}