/*
File: WaitFreeQueue.java
Originally written by Doug Lea and released into the public domain.
This may be used for any purposes whatsoever without acknowledgment.
Thanks for the assistance and support of Sun Microsystems Labs,
and everyone contributing, testing, and using this code.
History:
Date Who What
16Jun1998 dl Create public version
5Aug1998 dl replaced int counters with longs
17nov2001 dl Simplify given Bill Pugh's observation
that counted pointers are unnecessary.
*/
/**
* A wait-free linked list based queue implementation.
* <p>
*
* While this class conforms to the full Channel interface, only the
* <code>put</code> and <code>poll</code> methods are useful in most
* applications. Because the queue does not support blocking
* operations, <code>take</code> relies on spin-loops, which can be
* extremely wasteful. <p>
*
* This class is adapted from the algorithm described in <a
* href="http://www.cs.rochester.edu/u/michael/PODC96.html"> Simple,
* Fast, and Practical Non-Blocking and Blocking Concurrent Queue
* Algorithms</a> by Maged M. Michael and Michael L. Scott. This
* implementation is not strictly wait-free since it relies on locking
* for basic atomicity and visibility requirements. Locks can impose
* unbounded waits, although this should not be a major practical
* concern here since each lock is held for the duration of only a few
* statements. (However, the overhead of using so many locks can make
* it less attractive than other Channel implementations on JVMs where
* locking operations are very slow.) <p>
*
* @see BoundedLinkedQueue
* @see LinkedQueue
*
* <p>[<a href="http://gee.cs.oswego.edu/dl/classes/EDU/oswego/cs/dl/util/concurrent/intro.html"> Introduction to this package. </a>]
**/
package rttm.WaitFreeQueue;
public class WaitFreeQueueLOCK extends WaitFreeQueue {
/** List nodes for Queue **/
protected final static class Node {
protected final Object value;
protected volatile Node next = null;
protected volatile Node previous = null;
/** Make a new node with indicated item, and null link **/
protected Node(Object x) {
value = x;
}
}
/** Head of list is always a dummy node **/
protected volatile Node head = new Node(null);
/** Pointer to last node on list **/
protected volatile Node tail = new Node(null);
protected volatile int size = 0;
public WaitFreeQueueLOCK() {
head.next = tail;
head.previous = null;
tail.previous = head;
tail.next = null;
}
// /** Lock for simulating CAS for tail field **/
protected final Object lock = new Object();
public void put(Object x) {
Node n = new Node(x);
synchronized (lock) {
n.previous = tail.previous;
n.next = tail;
tail.previous.next = n;
tail.previous = n;
}
}
public Object get() {
for (;;) {
if (head.next == tail) {
continue;
}
Node n = null;
Node h = head;
synchronized (lock) {
if (h.next != tail) {
n = head.next;
head.next = n.next;
n.next.previous = head;
}
}
if (n != null)
return n.value;
}
}
public boolean offer(Object x, long msecs) {
// TODO Auto-generated method stub
return false;
}
public Object peek() {
// TODO Auto-generated method stub
return null;
}
public Object poll(long msecs) {
// TODO Auto-generated method stub
return null;
}
public Object take() {
// TODO Auto-generated method stub
return null;
}
}