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public class: WaitFreeQueue [javadoc | source]

All Implemented Interfaces:

A wait-free linked list based queue implementation.

While this class conforms to the full Channel interface, only the put and poll methods are useful in most applications. Because the queue does not support blocking operations, take relies on spin-loops, which can be extremely wasteful.

This class is adapted from the algorithm described in Simple, Fast, and Practical Non-Blocking and Blocking Concurrent Queue Algorithms 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.)

Nested Class Summary:
protected static final class  WaitFreeQueue.Node  List nodes for Queue 
Field Summary
protected volatile  Node head    Head of list is always a dummy node * 
protected volatile  Node tail    Pointer to last node on list * 
protected final  Object tailLock    Lock for simulating CAS for tail field * 
Method from EDU.oswego.cs.dl.util.concurrent.WaitFreeQueue Summary:
CASHead,   CASTail,   extract,   offer,   peek,   poll,   put,   take
Methods from java.lang.Object:
clone,   equals,   finalize,   getClass,   hashCode,   notify,   notifyAll,   toString,   wait,   wait,   wait
Method from EDU.oswego.cs.dl.util.concurrent.WaitFreeQueue Detail:
 protected synchronized boolean CASHead(Node oldHead,
    Node newHead) 
    Simulate CAS for head field, using 'this' lock *
 protected boolean CASTail(Node oldTail,
    Node newTail) 
    Simulate CAS for tail field *
 protected Object extract() throws InterruptedException 
    Main dequeue algorithm, called by poll, take. *
 public boolean offer(Object x,
    long msecs) throws InterruptedException 
 public Object peek() 
 public Object poll(long msecs) throws InterruptedException 
    Spin until poll returns a non-null value or time elapses. if msecs is positive, a Thread.sleep(0) is performed on each iteration as a heuristic to reduce contention.
 public  void put(Object x) throws InterruptedException 
 public Object take() throws InterruptedException 
    Spin until poll returns a non-null value. You probably don't want to call this method. A Thread.sleep(0) is performed on each iteration as a heuristic to reduce contention. If you would rather use, for example, an exponential backoff, you could manually set this up using poll.