edu.emory.mathcs.backport.java.util.concurrent

Class SynchronousQueue

Implemented Interfaces:
BlockingQueue, Collection, Queue, java.io.Serializable

public class SynchronousQueue
extends AbstractQueue
implements BlockingQueue, java.io.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. Fairness generally decreases throughput but reduces variability and avoids starvation.

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

This class is a member of the ../../../../../../../../guide/collections/index.html"> Java Collections Framework.

Author:
Doug Lea
Since:
1.5
See Also:
Serialized Form

Constructor Summary

SynchronousQueue()
Creates a SynchronousQueue with nonfair access policy.
SynchronousQueue(boolean fair)
Creates a SynchronousQueue with specified fairness policy.

Method Summary

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 c)
int
drainTo(Collection c, int maxElements)
boolean
isEmpty()
Always returns true.
Iterator
iterator()
Returns an empty iterator in which hasNext always returns false.
boolean
offer(Object e)
Inserts the specified element into this queue, if another thread is waiting to receive it.
boolean
offer(Object 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.
Object
peek()
Always returns null.
Object
poll()
Retrieves and removes the head of this queue, if another thread is currently making an element available.
Object
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(Object 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.
Object
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.
Object[]
toArray(Object[] a)
Sets the zeroeth element of the specified array to null (if the array has non-zero length) and returns it.

Methods inherited from class edu.emory.mathcs.backport.java.util.AbstractQueue

add, addAll, clear, element, remove

Methods inherited from class edu.emory.mathcs.backport.java.util.AbstractCollection

toArray, toArray

Constructor Details

SynchronousQueue

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

SynchronousQueue

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.

Method Details

clear

public void clear()
Does nothing. A SynchronousQueue has no internal capacity.
Overrides:
clear in interface AbstractQueue

contains

public boolean contains(Object o)
Always returns false. A SynchronousQueue has no internal capacity.
Specified by:
contains in interface BlockingQueue
Parameters:
o - object to be checked for containment in this queue
Returns:
false

containsAll

public boolean containsAll(Collection c)
Returns false unless the given collection is empty. A SynchronousQueue has no internal capacity.
Parameters:
c - the collection
Returns:
false unless the given collection is empty

drainTo

public int drainTo(Collection c)
Specified by:
drainTo in interface BlockingQueue

drainTo

public int drainTo(Collection c,
                   int maxElements)
Specified by:
drainTo in interface BlockingQueue

isEmpty

public boolean isEmpty()
Always returns true. A SynchronousQueue has no internal capacity.
Returns:
true

iterator

public Iterator iterator()
Returns an empty iterator in which hasNext always returns false.
Returns:
an empty iterator

offer

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

offer

public boolean offer(Object 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
Returns:
true if successful, or false if the specified waiting time elapses before a consumer appears.

peek

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

poll

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

poll

public Object 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
Returns:
the head of this queue, or null if the specified waiting time elapses before an element is present.

put

public void put(Object e)
            throws InterruptedException
Adds the specified element to this queue, waiting if necessary for another thread to receive it.
Specified by:
put in interface BlockingQueue

remainingCapacity

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

remove

public boolean remove(Object o)
Always returns false. A SynchronousQueue has no internal capacity.
Specified by:
remove in interface BlockingQueue
Parameters:
o - the element to remove
Returns:
false

removeAll

public boolean removeAll(Collection c)
Always returns false. A SynchronousQueue has no internal capacity.
Parameters:
c - the collection
Returns:
false

retainAll

public boolean retainAll(Collection c)
Always returns false. A SynchronousQueue has no internal capacity.
Parameters:
c - the collection
Returns:
false

size

public int size()
Always returns zero. A SynchronousQueue has no internal capacity.
Returns:
zero

take

public Object 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
Returns:
the head of this queue

toArray

public Object[] toArray()
Returns a zero-length array.
Overrides:
toArray in interface AbstractCollection
Returns:
a zero-length array

toArray

public Object[] toArray(Object[] a)
Sets the zeroeth element of the specified array to null (if the array has non-zero length) and returns it.
Overrides:
toArray in interface AbstractCollection
Parameters:
a - the array
Returns:
the specified array