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

java.lang.Object
  extended byjava.util.AbstractCollection
      extended byedu.emory.mathcs.backport.java.util.AbstractCollection
          extended byedu.emory.mathcs.backport.java.util.AbstractQueue
              extended byedu.emory.mathcs.backport.java.util.concurrent.SynchronousQueue
All Implemented Interfaces:
BlockingQueue, java.util.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 Java Collections Framework.

Since:
1.5
Author:
Doug Lea
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(java.lang.Object o)
          Always returns false.
 boolean containsAll(java.util.Collection c)
          Returns false unless the given collection is empty.
 int drainTo(java.util.Collection c)
          Removes all available elements from this queue and adds them to the given collection.
 int drainTo(java.util.Collection 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.
 java.util.Iterator iterator()
          Returns an empty iterator in which hasNext always returns false.
 boolean offer(java.lang.Object e)
          Inserts the specified element into this queue, if another thread is waiting to receive it.
 boolean offer(java.lang.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.
 java.lang.Object peek()
          Always returns null.
 java.lang.Object poll()
          Retrieves and removes the head of this queue, if another thread is currently making an element available.
 java.lang.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(java.lang.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(java.lang.Object o)
          Always returns false.
 boolean removeAll(java.util.Collection c)
          Always returns false.
 boolean retainAll(java.util.Collection c)
          Always returns false.
 int size()
          Always returns zero.
 java.lang.Object take()
          Retrieves and removes the head of this queue, waiting if necessary for another thread to insert it.
 java.lang.Object[] toArray()
          Returns a zero-length array.
 java.lang.Object[] toArray(java.lang.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, element, remove
 
Methods inherited from class java.util.AbstractCollection
toString
 
Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait
 
Methods inherited from interface edu.emory.mathcs.backport.java.util.concurrent.BlockingQueue
add
 
Methods inherited from interface edu.emory.mathcs.backport.java.util.Queue
element, remove
 
Methods inherited from interface java.util.Collection
addAll, equals, hashCode
 

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

put

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

Specified by:
put in interface BlockingQueue
Parameters:
e - the element to add
Throws:
java.lang.InterruptedException - if interrupted while waiting
java.lang.NullPointerException - if the specified element is null

offer

public boolean offer(java.lang.Object e,
                     long timeout,
                     TimeUnit unit)
              throws java.lang.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
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:
java.lang.InterruptedException - if interrupted while waiting
java.lang.NullPointerException - if the specified element is null

take

public java.lang.Object take()
                      throws java.lang.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
Throws:
java.lang.InterruptedException - if interrupted while waiting

poll

public java.lang.Object poll(long timeout,
                             TimeUnit unit)
                      throws java.lang.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
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:
java.lang.InterruptedException - if interrupted while waiting

offer

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

Specified by:
offer in interface BlockingQueue
Parameters:
e - the element to add
Returns:
true if the element was added to this queue, else false
Throws:
java.lang.NullPointerException - if the specified element is null

poll

public java.lang.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.

isEmpty

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

Specified by:
isEmpty in interface java.util.Collection
Returns:
true

size

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

Specified by:
size in interface java.util.Collection
Returns:
zero

remainingCapacity

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

Specified by:
remainingCapacity in interface BlockingQueue
Returns:
zero

clear

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

Specified by:
clear in interface java.util.Collection
Overrides:
clear in class AbstractQueue

contains

public boolean contains(java.lang.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

remove

public boolean remove(java.lang.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

containsAll

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

Specified by:
containsAll in interface java.util.Collection
Parameters:
c - the collection
Returns:
false unless the given collection is empty
Throws:
java.lang.NullPointerException - if the specified collection is null

removeAll

public boolean removeAll(java.util.Collection c)
Always returns false. A SynchronousQueue has no internal capacity.

Specified by:
removeAll in interface java.util.Collection
Parameters:
c - the collection
Returns:
false

retainAll

public boolean retainAll(java.util.Collection c)
Always returns false. A SynchronousQueue has no internal capacity.

Specified by:
retainAll in interface java.util.Collection
Parameters:
c - the collection
Returns:
false

peek

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

Specified by:
peek in interface Queue
Returns:
null

iterator

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

Specified by:
iterator in interface java.util.Collection
Returns:
an empty iterator

toArray

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

Specified by:
toArray in interface java.util.Collection
Overrides:
toArray in class AbstractCollection
Returns:
a zero-length array

toArray

public java.lang.Object[] toArray(java.lang.Object[] a)
Sets the zeroeth element of the specified array to null (if the array has non-zero length) and returns it.

Specified by:
toArray in interface java.util.Collection
Overrides:
toArray in class AbstractCollection
Parameters:
a - the array
Returns:
the specified array
Throws:
java.lang.NullPointerException - if the specified array is null

drainTo

public int drainTo(java.util.Collection 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
Parameters:
c - the collection to transfer elements into
Returns:
the number of elements transferred
Throws:
java.lang.UnsupportedOperationException - if addition of elements is not supported by the specified collection
java.lang.ClassCastException - if the class of an element of this queue prevents it from being added to the specified collection
java.lang.NullPointerException - if the specified collection is null
java.lang.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(java.util.Collection 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
Parameters:
c - the collection to transfer elements into
maxElements - the maximum number of elements to transfer
Returns:
the number of elements transferred
Throws:
java.lang.UnsupportedOperationException - if addition of elements is not supported by the specified collection
java.lang.ClassCastException - if the class of an element of this queue prevents it from being added to the specified collection
java.lang.NullPointerException - if the specified collection is null
java.lang.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