Akka/Scala example source code file (AbstractNodeQueue.java)

This example Akka source code file (AbstractNodeQueue.java) is included in my "Source Code Warehouse" project. The intent of this project is to help you more easily find Akka and Scala source code examples by using tags.

All credit for the original source code belongs to akka.io; I'm just trying to make examples easier to find. (For my Scala work, see my Scala examples and tutorials.)

Akka tags/keywords

abstractnodequeue, akka, dispatch, exceptionininitializererror, node, suppresswarnings, t, util

The AbstractNodeQueue.java Akka example source code

/**
 * Copyright (C) 2009-2014 Typesafe Inc. <http://www.typesafe.com>
 */

package akka.dispatch;

import akka.util.Unsafe;

import java.util.concurrent.atomic.AtomicReference;

/**
 * Lock-free MPSC linked queue implementation based on Dmitriy Vyukov's non-intrusive MPSC queue:
 * http://www.1024cores.net/home/lock-free-algorithms/queues/non-intrusive-mpsc-node-based-queue
 */
@SuppressWarnings("serial")
public abstract class AbstractNodeQueue<T> extends AtomicReference<AbstractNodeQueue.Node<T>> {
    // Extends AtomicReference for the "head" slot (which is the one that is appended to) since Unsafe does not expose XCHG operation intrinsically
    @SuppressWarnings("unused")
    private volatile Node<T> _tailDoNotCallMeDirectly;

    protected AbstractNodeQueue() {
       final Node<T> n = new Node<T>();
       _tailDoNotCallMeDirectly = n;
       set(n);
    }

    /*
     * !!! There is a copy of this code in pollNode() !!!
     */
    @SuppressWarnings("unchecked")
    protected final Node<T> peekNode() {
        for(;;) {
          final Node<T> tail = ((Node<T>)Unsafe.instance.getObjectVolatile(this, tailOffset));
          final Node<T> next = tail.next();
          if (next != null || get() == tail)
            return next;
        }
    }

    public final T peek() {
        final Node<T> n = peekNode();
        return (n != null) ? n.value : null;
    }

    public final void add(final T value) {
        final Node<T> n = new Node<T>(value);
        getAndSet(n).setNext(n);
    }
    
    public final void addNode(final Node<T> n) {
        n.setNext(null);
        getAndSet(n).setNext(n);
    }

    public final boolean isEmpty() {
        return peek() == null;
    }

    public final int count() {
        int count = 0;
        for(Node<T> n = peekNode();n != null; n = n.next())
          ++count;
        return count;
    }

    /*
     * !!! There is a copy of this code in pollNode() !!!
     */
    public final T poll() {
        final Node<T> next = peekNode();
        if (next == null) return null;
        else {
            final T ret = next.value;
            next.value = null;
            Unsafe.instance.putOrderedObject(this, tailOffset, next);
            return ret;
        }
    }
    
    @SuppressWarnings("unchecked")
    public final Node<T> pollNode() {
      Node<T> tail;
      Node<T> next;
      for(;;) {
        tail = ((Node<T>)Unsafe.instance.getObjectVolatile(this, tailOffset));
        next = tail.next();
        if (next != null || get() == tail)
          break;
      }
      if (next == null) return null;
      else {
        tail.value = next.value;
        next.value = null;
        Unsafe.instance.putOrderedObject(this, tailOffset, next);
        return tail;
      }
    }

    private final static long tailOffset;

    static {
        try {
          tailOffset = Unsafe.instance.objectFieldOffset(AbstractNodeQueue.class.getDeclaredField("_tailDoNotCallMeDirectly"));
        } catch(Throwable t){
            throw new ExceptionInInitializerError(t);
        }
    }

    public static class Node<T> {
        public T value;
        @SuppressWarnings("unused")
        private volatile Node<T> _nextDoNotCallMeDirectly;

        public Node() {
            this(null);
        }

        public Node(final T value) {
            this.value = value;
        }

        @SuppressWarnings("unchecked")
        public final Node<T> next() {
            return (Node<T>)Unsafe.instance.getObjectVolatile(this, nextOffset);
        }

        protected final void setNext(final Node<T> newNext) {
          Unsafe.instance.putOrderedObject(this, nextOffset, newNext);
        }
        
        private final static long nextOffset;
        
        static {
            try {
                nextOffset = Unsafe.instance.objectFieldOffset(Node.class.getDeclaredField("_nextDoNotCallMeDirectly"));
            } catch(Throwable t){
                throw new ExceptionInInitializerError(t);
            } 
        }
    } 
}