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 queue
- 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 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.
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
Constructor | Description |
---|---|
SynchronousQueue() | Creates a |
SynchronousQueue(boolean fair) | Creates a |
Method Summary
Modifier and Type | Method | Description |
---|---|---|
void | clear() | Does nothing. |
boolean | contains(Object o) | Always returns |
boolean | containsAll(Collection<?> c) | Returns |
int | drainTo(Collection<? super E> c) | Removes all available elements from this queue and adds them to the given collection. |
int | drainTo(Collection<? super E> 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 |
Iterator<E> | iterator() | Returns an empty iterator in which |
boolean | offer(E e) | Inserts the specified element into this queue, if another thread is waiting to receive it. |
boolean | offer(E 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. |
E | peek() | Always returns |
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 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 |
boolean | removeAll(Collection<?> c) | Always returns |
boolean | retainAll(Collection<?> c) | Always returns |
int | size() | Always returns zero. |
Spliterator<E> | spliterator() | Returns an empty spliterator in which calls to |
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. |
<T> T[] | toArray(T[] a) | Sets the zeroth element of the specified array to |
String | toString() | Always returns |
Methods declared in class java.util.AbstractQueue
add, addAll, element, remove
Methods declared in class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait
Methods declared in interface java.util.concurrent.BlockingQueue
add
Methods declared in interface java.util.Collection
addAll, equals, hashCode, parallelStream, removeIf, stream, toArray
Methods declared in interface java.lang.Iterable
forEach
Methods declared in interface java.util.Queue
element, remove
Constructor Detail
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.
Method Detail
put
public void put(E e) throws InterruptedException
Adds the specified element to this queue, waiting if necessary for another thread to receive it.
- Specified by:
-
put
in interfaceBlockingQueue<E>
- Parameters:
-
e
- the element to add - Throws:
-
InterruptedException
- if interrupted while waiting -
NullPointerException
- if the specified element is null
offer
public boolean offer(E 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 interfaceBlockingQueue<E>
- Parameters:
-
e
- the element to add -
timeout
- how long to wait before giving up, in units ofunit
-
unit
- aTimeUnit
determining how to interpret thetimeout
parameter - Returns:
-
true
if successful, orfalse
if the specified waiting time elapses before a consumer appears - Throws:
-
InterruptedException
- if interrupted while waiting -
NullPointerException
- if the specified element is null
offer
public boolean offer(E e)
Inserts the specified element into this queue, if another thread is waiting to receive it.
- Specified by:
-
offer
in interfaceBlockingQueue<E>
- Specified by:
-
offer
in interfaceQueue<E>
- Parameters:
-
e
- the element to add - Returns:
-
true
if the element was added to this queue, elsefalse
- Throws:
-
NullPointerException
- if the specified element is null
take
public E take() throws InterruptedException
Retrieves and removes the head of this queue, waiting if necessary for another thread to insert it.
- Specified by:
-
take
in interfaceBlockingQueue<E>
- Returns:
- the head of this queue
- Throws:
-
InterruptedException
- if interrupted while waiting
poll
public E 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 interfaceBlockingQueue<E>
- Parameters:
-
timeout
- how long to wait before giving up, in units ofunit
-
unit
- aTimeUnit
determining how to interpret thetimeout
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
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 interfaceQueue<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 interfaceCollection<E>
- Overrides:
-
isEmpty
in classAbstractCollection<E>
- Returns:
true
size
public int size()
Always returns zero. A SynchronousQueue
has no internal capacity.
- Specified by:
-
size
in interfaceCollection<E>
- Returns:
- zero
remainingCapacity
public int remainingCapacity()
Always returns zero. A SynchronousQueue
has no internal capacity.
- Specified by:
-
remainingCapacity
in interfaceBlockingQueue<E>
- Returns:
- zero
clear
public void clear()
Does nothing. A SynchronousQueue
has no internal capacity.
- Specified by:
-
clear
in interfaceCollection<E>
- Overrides:
-
clear
in classAbstractQueue<E>
contains
public boolean contains(Object o)
Always returns false
. A SynchronousQueue
has no internal capacity.
- Specified by:
-
contains
in interfaceBlockingQueue<E>
- Specified by:
-
contains
in interfaceCollection<E>
- Overrides:
-
contains
in classAbstractCollection<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 interfaceBlockingQueue<E>
- Specified by:
-
remove
in interfaceCollection<E>
- Overrides:
-
remove
in classAbstractCollection<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 interfaceCollection<E>
- Overrides:
-
containsAll
in classAbstractCollection<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 interfaceCollection<E>
- Overrides:
-
removeAll
in classAbstractCollection<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 interfaceCollection<E>
- Overrides:
-
retainAll
in classAbstractCollection<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.
iterator
public Iterator<E> iterator()
Returns an empty iterator in which hasNext
always returns false
.
- Specified by:
-
iterator
in interfaceCollection<E>
- Specified by:
-
iterator
in interfaceIterable<E>
- Specified by:
-
iterator
in classAbstractCollection<E>
- Returns:
- an empty iterator
spliterator
public Spliterator<E> spliterator()
Returns an empty spliterator in which calls to trySplit
always return null
.
- Specified by:
-
spliterator
in interfaceCollection<E>
- Specified by:
-
spliterator
in interfaceIterable<E>
- Returns:
- an empty spliterator
- Since:
- 1.8
toArray
public Object[] toArray()
Returns a zero-length array.
- Specified by:
-
toArray
in interfaceCollection<E>
- Overrides:
-
toArray
in classAbstractCollection<E>
- Returns:
- a zero-length array
toArray
public <T> T[] toArray(T[] a)
Sets the zeroth element of the specified array to null
(if the array has non-zero length) and returns it.
- Specified by:
-
toArray
in interfaceCollection<E>
- Overrides:
-
toArray
in classAbstractCollection<E>
- Type Parameters:
-
T
- the component type of the array to contain the collection - Parameters:
-
a
- the array - Returns:
- the specified array
- Throws:
-
NullPointerException
- if the specified array is null
toString
public String toString()
Always returns "[]"
.
- Overrides:
-
toString
in classAbstractCollection<E>
- Returns:
"[]"
drainTo
public int drainTo(Collection<? super E> 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 interfaceBlockingQueue<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
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 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 interfaceBlockingQueue<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