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public class

SynchronousQueue

extends AbstractQueue<E>
implements Serializable BlockingQueue<E>
java.lang.Object
   ↳ java.util.AbstractCollection<E>
     ↳ java.util.AbstractQueue<E>
       ↳ java.util.concurrent.SynchronousQueue<E>

Class Overview

A blocking queue in which each put must wait for a take, 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 take it; you cannot add 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 thread is trying to add to the queue; if there are no queued threads then no element is being added and the head is 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. Fairness generally decreases throughput but reduces variability and avoids starvation.

This class implements all of the optional methods of the Collection and Iterator interfaces.

Summary

Public Constructors
SynchronousQueue()
Creates a SynchronousQueue with nonfair access policy.
SynchronousQueue(boolean fair)
Creates a SynchronousQueue with specified fairness policy.
Public Methods
void clear()
Does nothing.
boolean contains(Object o)
Always returns false.
boolean containsAll(Collection<?> c)
Returns false unless given collection is empty.
int drainTo(Collection<? super E> c, int maxElements)
Removes at most the given number of available elements from this queue and adds them into the given collection.
int drainTo(Collection<? super E> c)
Removes all available elements from this queue and adds them into the given collection.
boolean isEmpty()
Always returns true.
Iterator<E> iterator()
Returns an empty iterator in which hasNext always returns false.
boolean offer(E o, 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.
boolean offer(E o)
Inserts the specified element into this queue, if another thread is waiting to receive it.
E peek()
Always returns null.
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.
E poll()
Retrieves and removes the head of this queue, if another thread is currently making an element available.
void put(E o)
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.
E take()
Retrieves and removes the head of this queue, waiting if necessary for another thread to insert it.
<T> T[] toArray(T[] a)
Sets the zeroeth element of the specified array to null (if the array has non-zero length) and returns it.
Object[] toArray()
Returns a zero-length array.
[Expand]
Inherited Methods
From class java.util.AbstractQueue
From class java.util.AbstractCollection
From class java.lang.Object
From interface java.lang.Iterable
From interface java.util.Collection
From interface java.util.Queue
From interface java.util.concurrent.BlockingQueue

Public Constructors

public SynchronousQueue ()

Creates a SynchronousQueue with nonfair access policy.

public SynchronousQueue (boolean fair)

Creates a SynchronousQueue with specified fairness policy.

Parameters
fair if true, threads contend in FIFO order for access; otherwise the order is unspecified.

Public Methods

public void clear ()

Does nothing. A SynchronousQueue has no internal capacity.

public boolean contains (Object o)

Always returns false. A SynchronousQueue has no internal capacity.

Parameters
o the element
Returns
  • false

public boolean containsAll (Collection<?> c)

Returns false unless given collection is empty. A SynchronousQueue has no internal capacity.

Parameters
c the collection
Returns
  • false unless given collection is empty

public int drainTo (Collection<? super E> c, int maxElements)

Removes at most the given number of available elements from this queue and adds them into 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.

Parameters
c the collection to transfer elements into
maxElements the maximum number of elements to transfer
Returns
  • the number of elements transferred.

public int drainTo (Collection<? super E> c)

Removes all available elements from this queue and adds them into 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.

Parameters
c the collection to transfer elements into
Returns
  • the number of elements transferred.

public boolean isEmpty ()

Always returns true. A SynchronousQueue has no internal capacity.

Returns
  • true

public Iterator<E> iterator ()

Returns an empty iterator in which hasNext always returns false.

Returns
  • an empty iterator

public boolean offer (E o, 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.

Parameters
o 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.

public boolean offer (E o)

Inserts the specified element into this queue, if another thread is waiting to receive it.

Parameters
o the element to add.
Returns
  • true if it was possible to add the element to this queue, else false
Throws
NullPointerException if the specified element is null

public E peek ()

Always returns null. A SynchronousQueue does not return elements unless actively waited on.

Returns
  • null

public 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.

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.

public E poll ()

Retrieves and removes the head of this queue, if another thread is currently making an element available.

Returns
  • the head of this queue, or null if no element is available.

public void put (E o)

Adds the specified element to this queue, waiting if necessary for another thread to receive it.

Parameters
o the element to add
Throws
InterruptedException if interrupted while waiting.
NullPointerException if the specified element is null.

public int remainingCapacity ()

Always returns zero. A SynchronousQueue has no internal capacity.

Returns
  • zero.

public boolean remove (Object o)

Always returns false. A SynchronousQueue has no internal capacity.

Parameters
o the element to remove
Returns
  • false

public boolean removeAll (Collection<?> c)

Always returns false. A SynchronousQueue has no internal capacity.

Parameters
c the collection
Returns
  • false

public boolean retainAll (Collection<?> c)

Always returns false. A SynchronousQueue has no internal capacity.

Parameters
c the collection
Returns
  • false

public int size ()

Always returns zero. A SynchronousQueue has no internal capacity.

Returns
  • zero.

public E take ()

Retrieves and removes the head of this queue, waiting if necessary for another thread to insert it.

Returns
  • the head of this queue
Throws
InterruptedException if interrupted while waiting.

public T[] toArray (T[] a)

Sets the zeroeth element of the specified array to null (if the array has non-zero length) and returns it.

Parameters
a the array
Returns
  • the specified array

public Object[] toArray ()

Returns a zero-length array.

Returns
  • a zero-length array