java中的无锁队列：ConcurrentLinkedQueue

java 1.5提供了一种无锁队列（wait-free/lock-free）ConcurrentLinkedQueue，可支持多个生产者多个消费者线程的环境：

http://docs.oracle.com/javase/1.5.0/docs/api/java/util/concurrent/ConcurrentLinkedQueue.html

下面这个是网上别人自己实现的一种无锁算法队列，原理和jdk官方的ConcurrentLinkedQueue相似：通过volatile关键字来保证数据唯一性（注：java的volatile和c++的volatile关键字是两码事！），但是里面又用到了atomic，感觉有点boost::lockfree::queue的风格，估计作者是参考了boost的代码来编写这个java无锁队列。

实现代码：

import java.util.concurrent.atomic.AtomicReference;

public class LockFreeQueue<T> {
    private static class Node<E> {
        E value;
        volatile Node<E> next;

        Node(E value) {
            this.value = value;
        }
    }

    private AtomicReference<Node<T>> head, tail;

    public LockFreeQueue() {
        // have both head and tail point to a dummy node
        Node<T> dummyNode = new Node<T>(null);
        head = new AtomicReference<Node<T>>(dummyNode);
        tail = new AtomicReference<Node<T>>(dummyNode);
    }

    /**
     * Puts an object at the end of the queue.
     */
    public void putObject(T value) {
        Node<T> newNode = new Node<T>(value);
        Node<T> prevTailNode = tail.getAndSet(newNode);
        prevTailNode.next = newNode;
    }

    /**
     * Gets an object from the beginning of the queue. The object is removed
     * from the queue. If there are no objects in the queue, returns null.
     */
    public T getObject() {
        Node<T> headNode, valueNode;

        // move head node to the next node using atomic semantics
        // as long as next node is not null
        do {
            headNode = head.get();
            valueNode = headNode.next;
            // try until the whole loop executes pseudo-atomically
            // (i.e. unaffected by modifications done by other threads)
        } while (valueNode != null && !head.compareAndSet(headNode, valueNode));

        T value = (valueNode != null ? valueNode.value : null);

        // release the value pointed to by head, keeping the head node dummy
        if (valueNode != null)
            valueNode.value = null;

        return value;
}

代码出处：

Is this (Lock-Free) Queue Implementation Thread-Safe?

http://stackoverflow.com/questions/1634368/is-this-lock-free-queue-implementation-thread-safe