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

Class SynchronousQueue

public class SynchronousQueue extends AbstractQueue implements BlockingQueue, 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 Java Collections Framework.

Since: 1.5

Author: Doug Lea

Constructor Summary
SynchronousQueue()
Creates a SynchronousQueue with nonfair access policy.
SynchronousQueue(boolean fair)
Creates a SynchronousQueue with specified fairness policy.
Method Summary
voidclear()
Does nothing.
booleancontains(Object o)
Always returns false.
booleancontainsAll(Collection c)
Returns false unless the given collection is empty.
intdrainTo(Collection c)
intdrainTo(Collection c, int maxElements)
booleanisEmpty()
Always returns true.
Iteratoriterator()
Returns an empty iterator in which hasNext always returns false.
booleanoffer(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.
booleanoffer(Object e)
Inserts the specified element into this queue, if another thread is waiting to receive it.
Objectpeek()
Always returns null.
Objectpoll(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.
Objectpoll()
Retrieves and removes the head of this queue, if another thread is currently making an element available.
voidput(Object e)
Adds the specified element to this queue, waiting if necessary for another thread to receive it.
intremainingCapacity()
Always returns zero.
booleanremove(Object o)
Always returns false.
booleanremoveAll(Collection c)
Always returns false.
booleanretainAll(Collection c)
Always returns false.
intsize()
Always returns zero.
Objecttake()
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.

Constructor Detail

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 Detail

clear

public void clear()
Does nothing. A SynchronousQueue has no internal capacity.

contains

public boolean contains(Object o)
Always returns false. A SynchronousQueue has no internal capacity.

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

Throws: NullPointerException if the specified collection is null

drainTo

public int drainTo(Collection c)

Throws: UnsupportedOperationException {@inheritDoc } ClassCastException {@inheritDoc } NullPointerException {@inheritDoc } IllegalArgumentException {@inheritDoc }

drainTo

public int drainTo(Collection c, int maxElements)

Throws: UnsupportedOperationException {@inheritDoc } ClassCastException {@inheritDoc } NullPointerException {@inheritDoc } IllegalArgumentException {@inheritDoc }

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, 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.

Returns: true if successful, or false if the specified waiting time elapses before a consumer appears.

Throws: InterruptedException {@inheritDoc } NullPointerException {@inheritDoc }

offer

public boolean offer(Object e)
Inserts the specified element into this queue, if another thread is waiting to receive it.

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

peek

public Object peek()
Always returns null. A SynchronousQueue does not return elements unless actively waited on.

Returns: null

poll

public 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.

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

Throws: InterruptedException {@inheritDoc }

poll

public Object 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.

put

public void put(Object e)
Adds the specified element to this queue, waiting if necessary for another thread to receive it.

Throws: InterruptedException {@inheritDoc } NullPointerException {@inheritDoc }

remainingCapacity

public int remainingCapacity()
Always returns zero. A SynchronousQueue has no internal capacity.

Returns: zero

remove

public boolean remove(Object o)
Always returns false. A SynchronousQueue has no internal capacity.

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()
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 {@inheritDoc }

toArray

public Object[] toArray()
Returns a zero-length array.

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.

Parameters: a the array

Returns: the specified array

Throws: NullPointerException if the specified array is null