An example of classic Michael-Scott queue implementation in java

MSQueue.java

import java.util.concurrent.atomic.AtomicReference;

/**
 * An example of classic Michael-Scott queue
 *
 * @param <T>
 */
public class MSQueue<T> {

    private final class Node<T> {

        // TODO it could be replaced by AtomicFieldUpdater in order to reduce memory footprint,
        //  but I especially avoided to do this because intent of MSQueue is just to be a learning eaxample
        private final AtomicReference<Node<T>> nextRef;

        private T item;

        private Node(T item, Node<T> next) {
            this.nextRef = new AtomicReference<>(next);
            this.item = item;
        }

    }

    private final AtomicReference<Node<T>> tailRef = new AtomicReference<>(new Node<>(null, null));
    private final AtomicReference<Node<T>> headRef = new AtomicReference<>(tailRef.get());

    public final void put(T item) {
        Node<T> newNode = new Node<T>(item, null);
        while (true) {
            Node<T> currentTail = tailRef.get();
            Node<T> nextTail = currentTail.nextRef.get();
            if (currentTail == tailRef.get()) {
                if (nextTail != null) {
                    tailRef.compareAndSet(currentTail, nextTail);
                } else {
                    if (currentTail.nextRef.compareAndSet(null, newNode)) {
                        tailRef.compareAndSet(currentTail, newNode);
                        return;
                    }
                }
            }
        }
    }

    public final T poll() {
        while (true) {
            Node<T> currentHead = headRef.get();
            Node<T> currentTail = tailRef.get();
            Node<T> next = currentHead.nextRef.get();
            if (currentHead == headRef.get()) {
                if (currentHead == currentTail) {
                    if (next == null) {
                        return null;
                    } else {
                        // help to producer
                        tailRef.compareAndSet(currentTail, next);
                    }
                } else {
                    if (headRef.compareAndSet(currentHead, next)) {
                        T item = next.item;
                        next.item = null; // help to gc
                        return item;
                    }
                }
            }
        }
    }

}