Package org.jctools.queues

Class BaseLinkedQueue<E>

  • All Implemented Interfaces:
    java.lang.Iterable<E>, java.util.Collection<E>, java.util.Queue<E>, MessagePassingQueue<E>
    Direct Known Subclasses:
    MpscLinkedQueue, SpscLinkedQueue
    abstract class BaseLinkedQueue<E>
extends BaseLinkedQueuePad2<E>
    A base data structure for concurrent linked queues. For convenience also pulled in common single consumer methods since at this time there's no plan to implement MC.

    • Constructor Detail

      • BaseLinkedQueue

        BaseLinkedQueue()

    • Method Detail

      • iterator

        public final java.util.Iterator<E> iterator()
        Specified by:
        iterator in interface java.util.Collection<E>
        Specified by:
        iterator in interface java.lang.Iterable<E>
        Specified by:
        iterator in class java.util.AbstractCollection<E>

      • toString

        public java.lang.String toString()
        Overrides:
        toString in class java.util.AbstractCollection<E>

      • size

        public final int size()
        This method's accuracy is subject to concurrent modifications happening as the size is estimated and as such is a best effort rather than absolute value. For some implementations this method may be O(n) rather than O(1).

        IMPLEMENTATION NOTES:
        This is an O(n) operation as we run through all the nodes and count them.
        The accuracy of the value returned by this method is subject to races with producer/consumer threads. In particular when racing with the consumer thread this method may under estimate the size.

        Specified by:
        size in interface java.util.Collection<E>
        Specified by:
        size in interface MessagePassingQueue<E>
        Specified by:
        size in class java.util.AbstractCollection<E>
        Returns:
        number of messages in the queue, between 0 and Integer.MAX_VALUE but less or equals to capacity (if bounded).
        See Also:
        Collection.size()

      • isEmpty

        public boolean isEmpty()
        This method's accuracy is subject to concurrent modifications happening as the observation is carried out.

        IMPLEMENTATION NOTES:
        Queue is empty when producerNode is the same as consumerNode. An alternative implementation would be to observe the producerNode.value is null, which also means an empty queue because only the consumerNode.value is allowed to be null.

        Specified by:
        isEmpty in interface java.util.Collection<E>
        Specified by:
        isEmpty in interface MessagePassingQueue<E>
        Overrides:
        isEmpty in class java.util.AbstractCollection<E>
        Returns:
        true if empty, false otherwise
        See Also:
        MessagePassingQueue.isEmpty()

      • poll

        public E poll()
        Called from the consumer thread subject to the restrictions appropriate to the implementation and according to the Queue.poll() interface.

        IMPLEMENTATION NOTES:
        Poll is allowed from a SINGLE thread.
        Poll is potentially blocking here as the Queue.poll() does not allow returning null if the queue is not empty. This is very different from the original Vyukov guarantees. See relaxedPoll() for the original semantics.
        Poll reads consumerNode.next and:

        1. If it is null AND the queue is empty return null, if queue is not empty spin wait for value to become visible.
        2. If it is not null set it as the consumer node and return it's now evacuated value.
        This means the consumerNode.value is always null, which is also the starting point for the queue. Because null values are not allowed to be offered this is the only node with it's value set to null at any one time.
        Returns:
        a message from the queue if one is available, null iff empty
        See Also:
        MessagePassingQueue.poll(), Queue.poll()

      • peek

        public E peek()
        Called from the consumer thread subject to the restrictions appropriate to the implementation and according to the Queue.peek() interface.

        IMPLEMENTATION NOTES:
        Peek is allowed from a SINGLE thread.
        Peek is potentially blocking here as the Queue.peek() does not allow returning null if the queue is not empty. This is very different from the original Vyukov guarantees. See relaxedPeek() for the original semantics.
        Poll reads the next node from the consumerNode and:

        1. If it is null AND the queue is empty return null, if queue is not empty spin wait for value to become visible.
        2. If it is not null return it's value.
        Returns:
        a message from the queue if one is available, null iff empty
        See Also:
        MessagePassingQueue.peek(), Queue.peek()

      • relaxedPoll

        public E relaxedPoll()
        Description copied from interface: MessagePassingQueue
        Called from the consumer thread subject to the restrictions appropriate to the implementation. As opposed to Queue.poll() this method may return null without the queue being empty.
        Returns:
        a message from the queue if one is available, null if unable to poll

      • relaxedPeek

        public E relaxedPeek()
        Description copied from interface: MessagePassingQueue
        Called from the consumer thread subject to the restrictions appropriate to the implementation. As opposed to Queue.peek() this method may return null without the queue being empty.
        Returns:
        a message from the queue if one is available, null if unable to peek

      • relaxedOffer

        public boolean relaxedOffer​(E e)
        Description copied from interface: MessagePassingQueue
        Called from a producer thread subject to the restrictions appropriate to the implementation. As opposed to Queue.offer(Object) this method may return false without the queue being full.
        Parameters:
        e - not null, will throw NPE if it is
        Returns:
        true if element was inserted into the queue, false if unable to offer

      • drain

        public int drain​(MessagePassingQueue.Consumer<E> c,
                 int limit)
        Description copied from interface: MessagePassingQueue
        Remove up to limit elements from the queue and hand to consume. This should be semantically similar to: 

        
   M m;
   int i = 0;
   for(;i < limit && (m = relaxedPoll()) != null; i++){
     c.accept(m);
   }
   return i;

        There's no strong commitment to the queue being empty at the end of a drain. Called from a consumer thread subject to the restrictions appropriate to the implementation.

        WARNING: Explicit assumptions are made with regards to MessagePassingQueue.Consumer.accept(T) make sure you have read and understood these before using this method.

        Returns:
        the number of polled elements

      • drain

        public int drain​(MessagePassingQueue.Consumer<E> c)
        Description copied from interface: MessagePassingQueue
        Remove all available item from the queue and hand to consume. This should be semantically similar to: 
         M m;
 while((m = relaxedPoll()) != null){
 c.accept(m);
 }
        There's no strong commitment to the queue being empty at the end of a drain. Called from a consumer thread subject to the restrictions appropriate to the implementation.

        WARNING: Explicit assumptions are made with regards to MessagePassingQueue.Consumer.accept(T) make sure you have read and understood these before using this method.

        Returns:
        the number of polled elements