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java.util.concurrent
Class SynchronousQueue<E>
java.lang.Object
java.util.AbstractCollection<E>
java.util.AbstractQueue<E>
java.util.concurrent.SynchronousQueue<E>
-
-
Type Parameters:
-
E - the type of elements held in this collection
-
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
in which each
peek
put
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
must wait for a
poll()
take
will return
, 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 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
|
SynchronousQueue
()
Creates a
SynchronousQueue
with nonfair access policy. |
SynchronousQueue
(boolean fair)
Creates a
SynchronousQueue
with
the
specified fairness policy. |
Method Summary
|
void |
clear
()
Does nothing. |
boolean |
contains
(
Object
o)
Always returns
false
. |
boolean |
containsAll
(
Collection
Returns
false
unless
the
given collection is empty. |
int |
drainTo
(
Collection
<? super
E
Removes all available elements from this queue and adds them
to
into
the given collection. |
int |
drainTo
(
Collection
<? super
E
Removes at most the given number of available elements from this queue and adds them
to
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
Inserts the specified element into this queue, if another thread is waiting to receive it. |
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. |
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
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. |
Object
[] |
toArray
()
Returns a zero-length array. |
|
toArray
(T[] a)
Sets the zeroeth element of the specified array to
null
(if the array has non-zero length) and returns it. |
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.
put
public void put(E o)
throws InterruptedException
-
Adds the specified element to this queue, waiting if necessary for another thread to receive it.
-
-
Specified by:
-
put
in interface
BlockingQueue
<
E
>
-
-
Parameters:
-
o - the element to add
-
Throws:
-
InterruptedException
- if interrupted while
waiting
waiting.
-
NullPointerException
- if the specified element is
null
.
offer
public boolean offer(E o,
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:
-
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
waiting.
-
NullPointerException
- if the specified element is
null
.
offer
take
public boolean
public Eoffer take ( E()
throws InterruptedException e)
-
Inserts
Retrieves and removes
the
specified element into
head of
this queue,
waiting
if
necessary for
another thread
is waiting
to
receive
insert
it.
-
-
Specified by:
-
offer
take
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
the head of this queue
-
Throws:
-
NullPointerException
InterruptedException
- if the specified element is null
- if interrupted while waiting.
take
poll
public Etake poll ()
throws InterruptedException(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:
-
take
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
the head of this queue, or
null
if the specified waiting time elapses before an element is present.
-
Throws:
-
InterruptedException
- if interrupted while
waiting
waiting.
poll
offer
public E
public boolean poll offer (long timeout,
TimeUnit( E unit)
throws InterruptedException o)
-
Retrieves and removes the head of this queue, waiting if necessary up to the specified wait time, for another thread to insert it.
Inserts the specified element into this queue, if another thread is waiting to receive it.
-
-
Specified by:
-
poll
offer
in interface
BlockingQueue
<
E
> -
Specified by:
-
offer
in interface
Queue
<
E
>
-
-
Parameters:
-
timeout - how long to wait before giving up, in units of
unit
-
unit - a
TimeUnit
determining how to interpret the
timeout
parameter
o - the element to add.
-
Returns:
-
the head of this queue, or
null
true
if the specified waiting time elapses before an element is present.
if it was possible to add the element to this queue, else
false
-
Throws:
-
InterruptedException
NullPointerException
- if interrupted while waiting
- if the specified element is
null
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.
isEmpty
public boolean isEmpty()
-
Always returns
true
. A
SynchronousQueue
has no internal capacity.
-
-
Specified by:
-
isEmpty
in interface
Collection
<
E
>
-
Overrides:
-
isEmpty
in class
AbstractCollection
<
E
>
-
-
Returns:
-
true
size
public int size()
-
Always returns zero. A
SynchronousQueue
has no internal capacity.
-
-
Specified by:
-
size
in interface
Collection
<
E
>
-
Specified by:
-
size
in class
AbstractCollection
<
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
>
-
contains
public boolean contains(Object o)
-
Always returns
false
. A
SynchronousQueue
has no internal capacity.
-
-
Specified by:
-
contains
in interface
Collection
<
E
>
-
Specified by:
-
contains
in interface
BlockingQueue
<
E
>
-
Overrides:
-
contains
in class
AbstractCollection
<
E
>
-
-
Parameters:
-
o - the element
-
Returns:
-
false
remove
public boolean remove(Object o)
-
Always returns
false
. A
SynchronousQueue
has no internal capacity.
-
-
Specified by:
-
remove
in interface
Collection
<
E
>
-
Specified by:
-
remove
in interface
BlockingQueue
<
E
>
-
Overrides:
-
remove
in class
AbstractCollection
<
E
>
-
-
Parameters:
-
o - the element to remove
-
Returns:
-
false
containsAll
public boolean containsAll(Collection<?> c)
-
Returns
false
unless
the
given collection is empty. A
SynchronousQueue
has no internal capacity.
-
-
Specified by:
-
containsAll
in interface
Collection
<
E
>
-
Overrides:
-
containsAll
in class
AbstractCollection
<
E
>
-
-
Parameters:
-
c - the collection
-
Returns:
-
false
unless given collection is empty
-
See Also:
-
AbstractCollection.contains(Object)
removeAll
public boolean removeAll(Collection<?> c)
-
Always returns
false
. A
SynchronousQueue
has no internal capacity.
-
-
Specified by:
-
removeAll
in interface
Collection
<
E
>
-
Overrides:
-
removeAll
in class
AbstractCollection
<
E
>
-
-
Parameters:
-
c - the collection
-
Returns:
-
false
-
See Also:
-
AbstractCollection.remove(Object)
,
AbstractCollection.contains(Object)
retainAll
public boolean retainAll(Collection<?> c)
-
Always returns
false
. A
SynchronousQueue
has no internal capacity.
-
-
Specified by:
-
retainAll
in interface
Collection
<
E
>
-
Overrides:
-
retainAll
in class
AbstractCollection
<
E
>
-
-
Parameters:
-
c - the collection
-
Returns:
-
false
-
See Also:
-
AbstractCollection.remove(Object)
,
AbstractCollection.contains(Object)
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
iterator
public Iterator<E> iterator()
-
Returns an empty iterator in which
hasNext
always returns
false
.
-
-
Specified by:
-
iterator
in interface
Iterable
<
E
>
-
Specified by:
-
iterator
in interface
Collection
<
E
>
-
Specified by:
-
iterator
in class
AbstractCollection
<
E
>
-
-
Returns:
-
an empty iterator
toArray
public Object[] toArray()
-
Returns a zero-length array.
-
-
Specified by:
-
toArray
in interface
Collection
<
E
>
-
Overrides:
-
toArray
in class
AbstractCollection
<
E
>
-
-
Returns:
-
a zero-length array
toArray
public <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.
-
-
Specified by:
-
toArray
in interface
Collection
<
E
>
-
Overrides:
-
toArray
in class
AbstractCollection
<
E
>
-
-
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
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
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
the number of elements transferred.
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
into
the given collection. A failure encountered while attempting to
add elements to collection
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
the number of elements transferred.