Module java.base
Package java.util.concurrent

Class SynchronousQueue<E>

  • Type Parameters:
    E - the type of elements held in this queue
    All Implemented Interfaces:
    Serializable, Iterable<E>, Collection<E>, BlockingQueue<E>, Queue<E>
    public class SynchronousQueue<E>
extends AbstractQueue<E>
implements BlockingQueue<E>, Serializable
    A blocking queue in which each insert operation must wait for a corresponding remove operation by another thread, and vice versa. A synchronous queue does not have any internal capacity, not even a capacity of one. You cannot peek at a synchronous queue because an element is only present when you try to remove it; you cannot insert an element (using any method) unless another thread is trying to remove it; you cannot iterate as there is nothing to iterate. The head of the queue is the element that the first queued inserting thread is trying to add to the queue; if there is no such queued thread then no element is available for removal and poll() will return null. For purposes of other Collection methods (for example contains), a SynchronousQueue acts as an empty collection. This queue does not permit null elements.

    Synchronous queues are similar to rendezvous channels used in CSP and Ada. They are well suited for handoff designs, in which an object running in one thread must sync up with an object running in another thread in order to hand it some information, event, or task.

    This class supports an optional fairness policy for ordering waiting producer and consumer threads. By default, this ordering is not guaranteed. However, a queue constructed with fairness set to true grants threads access in FIFO order.

    This class and its iterator implement all of the optional methods of the Collection and Iterator interfaces.

    This class is a member of the Java Collections Framework.

    Since:
    1.5
    See Also:
    Serialized Form

    • Constructor Summary

      Constructors 
      Constructor Description
      SynchronousQueue()
      Creates a SynchronousQueue with nonfair access policy.
      SynchronousQueue​(boolean fair)
      Creates a SynchronousQueue with the specified fairness policy.

    • Method Summary

      Modifier and Type Method Description
      void clear()
      Does nothing.
      boolean contains​(Object o)
      Always returns false.
      boolean containsAll​(Collection<?> c)
      Returns false unless the given collection is empty.
      int drainTo​(Collection<? super E> c)
      Removes all available elements from this queue and adds them to the given collection.
      int drainTo​(Collection<? super E> c, int maxElements)
      Removes at most the given number of available elements from this queue and adds them to the given collection.
      boolean isEmpty()
      Always returns true.
      Iterator<E> iterator()
      Returns an empty iterator in which hasNext always returns false.
      boolean offer​(E e)
      Inserts the specified element into this queue, if another thread is waiting to receive it.
      boolean offer​(E e, long timeout, TimeUnit unit)
      Inserts the specified element into this queue, waiting if necessary up to the specified wait time for another thread to receive it.
      E peek()
      Always returns null.
      E poll()
      Retrieves and removes the head of this queue, if another thread is currently making an element available.
      E poll​(long timeout, TimeUnit unit)
      Retrieves and removes the head of this queue, waiting if necessary up to the specified wait time, for another thread to insert it.
      void put​(E e)
      Adds the specified element to this queue, waiting if necessary for another thread to receive it.
      int remainingCapacity()
      Always returns zero.
      boolean remove​(Object o)
      Always returns false.
      boolean removeAll​(Collection<?> c)
      Always returns false.
      boolean retainAll​(Collection<?> c)
      Always returns false.
      int size()
      Always returns zero.
      Spliterator<E> spliterator()
      Returns an empty spliterator in which calls to trySplit always return null.
      E take()
      Retrieves and removes the head of this queue, waiting if necessary for another thread to insert it.
      Object[] toArray()
      Returns a zero-length array.
      <T> T[] toArray​(T[] a)
      Sets the zeroth element of the specified array to null (if the array has non-zero length) and returns it.
      String toString()
      Always returns "[]".

    • Constructor Detail

      • SynchronousQueue

        public SynchronousQueue()
        Creates a SynchronousQueue with nonfair access policy.

      • SynchronousQueue

        public SynchronousQueue​(boolean fair)
        Creates a SynchronousQueue with the specified fairness policy.
        Parameters:
        fair - if true, waiting threads contend in FIFO order for access; otherwise the order is unspecified.

    • Method Detail

      • offer

        public boolean offer​(E e,
                     long timeout,
                     TimeUnit unit)
              throws InterruptedException
        Inserts the specified element into this queue, waiting if necessary up to the specified wait time for another thread to receive it.
        Specified by:
        offer in interface BlockingQueue<E>
        Parameters:
        e - the element to add
        timeout - how long to wait before giving up, in units of unit
        unit - a TimeUnit determining how to interpret the timeout parameter
        Returns:
        true if successful, or false if the specified waiting time elapses before a consumer appears
        Throws:
        InterruptedException - if interrupted while waiting
        NullPointerException - if the specified element is null

      • offer

        public boolean offer​(E e)
        Inserts the specified element into this queue, if another thread is waiting to receive it.
        Specified by:
        offer in interface BlockingQueue<E>
        Specified by:
        offer in interface Queue<E>
        Parameters:
        e - the element to add
        Returns:
        true if the element was added to this queue, else false
        Throws:
        NullPointerException - if the specified element is null

      • take

        public E take()
       throws InterruptedException
        Retrieves and removes the head of this queue, waiting if necessary for another thread to insert it.
        Specified by:
        take in interface BlockingQueue<E>
        Returns:
        the head of this queue
        Throws:
        InterruptedException - if interrupted while waiting

      • poll

        public E poll​(long timeout,
              TimeUnit unit)
       throws InterruptedException
        Retrieves and removes the head of this queue, waiting if necessary up to the specified wait time, for another thread to insert it.
        Specified by:
        poll in interface BlockingQueue<E>
        Parameters:
        timeout - how long to wait before giving up, in units of unit
        unit - a TimeUnit determining how to interpret the timeout parameter
        Returns:
        the head of this queue, or null if the specified waiting time elapses before an element is present
        Throws:
        InterruptedException - if interrupted while waiting

      • poll

        public E poll()
        Retrieves and removes the head of this queue, if another thread is currently making an element available.
        Specified by:
        poll in interface Queue<E>
        Returns:
        the head of this queue, or null if no element is available

      • size

        public int size()
        Always returns zero. A SynchronousQueue has no internal capacity.
        Specified by:
        size in interface Collection<E>
        Returns:
        zero

      • remainingCapacity

        public int remainingCapacity()
        Always returns zero. A SynchronousQueue has no internal capacity.
        Specified by:
        remainingCapacity in interface BlockingQueue<E>
        Returns:
        zero

      • clear

        public void clear()
        Does nothing. A SynchronousQueue has no internal capacity.
        Specified by:
        clear in interface Collection<E>
        Overrides:
        clear in class AbstractQueue<E>

      • peek

        public E peek()
        Always returns null. A SynchronousQueue does not return elements unless actively waited on.
        Specified by:
        peek in interface Queue<E>
        Returns:
        null

      • toArray

        public <T> T[] toArray​(T[] a)
        Sets the zeroth element of the specified array to null (if the array has non-zero length) and returns it.
        Specified by:
        toArray in interface Collection<E>
        Overrides:
        toArray in class AbstractCollection<E>
        Type Parameters:
        T - the component type of the array to contain the collection
        Parameters:
        a - the array
        Returns:
        the specified array
        Throws:
        NullPointerException - if the specified array is null

      • drainTo

        public int drainTo​(Collection<? super E> c)
        Description copied from interface: BlockingQueue
        Removes all available elements from this queue and adds them to the given collection. This operation may be more efficient than repeatedly polling this queue. A failure encountered while attempting to add elements to collection c may result in elements being in neither, either or both collections when the associated exception is thrown. Attempts to drain a queue to itself result in IllegalArgumentException. Further, the behavior of this operation is undefined if the specified collection is modified while the operation is in progress.
        Specified by:
        drainTo in interface BlockingQueue<E>
        Parameters:
        c - the collection to transfer elements into
        Returns:
        the number of elements transferred
        Throws:
        UnsupportedOperationException - if addition of elements is not supported by the specified collection
        ClassCastException - if the class of an element of this queue prevents it from being added to the specified collection
        NullPointerException - if the specified collection is null
        IllegalArgumentException - if the specified collection is this queue, or some property of an element of this queue prevents it from being added to the specified collection

      • drainTo

        public int drainTo​(Collection<? super E> c,
                   int maxElements)
        Description copied from interface: BlockingQueue
        Removes at most the given number of available elements from this queue and adds them to the given collection. A failure encountered while attempting to add elements to collection c may result in elements being in neither, either or both collections when the associated exception is thrown. Attempts to drain a queue to itself result in IllegalArgumentException. Further, the behavior of this operation is undefined if the specified collection is modified while the operation is in progress.
        Specified by:
        drainTo in interface BlockingQueue<E>
        Parameters:
        c - the collection to transfer elements into
        maxElements - the maximum number of elements to transfer
        Returns:
        the number of elements transferred
        Throws:
        UnsupportedOperationException - if addition of elements is not supported by the specified collection
        ClassCastException - if the class of an element of this queue prevents it from being added to the specified collection
        NullPointerException - if the specified collection is null
        IllegalArgumentException - if the specified collection is this queue, or some property of an element of this queue prevents it from being added to the specified collection