java.util.concurrent
Interface BlockingQueue<E>

Type Parameters:

E - the type of elements held in this collection

All Superinterfaces:

Collection <E>, Iterable <E>, Queue <E>

All Known Subinterfaces: Implementing Classes:

BlockingDeque ArrayBlockingQueue <E> , DelayQueue , LinkedBlockingQueue , PriorityBlockingQueue , SynchronousQueue

All Known Implementing Classes:

ArrayBlockingQueue , DelayQueue , LinkedBlockingDeque , LinkedBlockingQueue , PriorityBlockingQueue , SynchronousQueue

public interface BlockingQueue<E>
extends Queue<E>

A Queue that additionally supports operations that wait for the queue to become non-empty when retrieving an element, and wait for space to become available in the queue when storing an element.

BlockingQueue methods come in four forms, with different ways of handling operations that cannot be satisfied immediately, but may be satisfied at some point in the future: one throws an exception, the second returns a special value (either null or false , depending on the operation), the third blocks the current thread indefinitely until the operation can succeed, and the fourth blocks for only a given maximum time limit before giving up. These methods are summarized in the following table:

	Throws exception	Special value	Blocks	Times out
Insert	add(e)	offer(e)	put(e)	offer(e, time, unit)
Remove	remove()	poll()	take()	poll(time, unit)
Examine	element()	peek()	not applicable	not applicable

A BlockingQueue does not accept null elements. Implementations throw NullPointerException on attempts to add , put or offer a null . A null is used as a sentinel value to indicate failure of poll operations.

A BlockingQueue may be capacity bounded. At any given time it may have a remainingCapacity beyond which no additional elements can be put without blocking. A BlockingQueue without any intrinsic capacity constraints always reports a remaining capacity of Integer.MAX_VALUE .

BlockingQueue implementations are designed to be used primarily for producer-consumer queues, but additionally support the Collection interface. So, for example, it is possible to remove an arbitrary element from a queue using remove(x) . However, such operations are in general not performed very efficiently, and are intended for only occasional use, such as when a queued message is cancelled.

BlockingQueue implementations are thread-safe. All queuing methods achieve their effects atomically using internal locks or other forms of concurrency control. However, the bulk Collection operations addAll , containsAll , retainAll and removeAll are not necessarily performed atomically unless specified otherwise in an implementation. So it is possible, for example, for addAll(c) to fail (throwing an exception) after adding only some of the elements in c .

A BlockingQueue does not intrinsically support any kind of "close" or "shutdown" operation to indicate that no more items will be added. The needs and usage of such features tend to be implementation-dependent. For example, a common tactic is for producers to insert special end-of-stream or poison objects, that are interpreted accordingly when taken by consumers.

Usage example, based on a typical producer-consumer scenario. Note that a BlockingQueue can safely be used with multiple producers and multiple consumers.

 class Producer implements Runnable {
 private final BlockingQueue queue;
 Producer(BlockingQueue q) { queue = q; }
 public void run() {
 try {
 while (true) while(true) { queue.put(produce()); }
 } catch (InterruptedException ex) { ... handle ...}
   }
   Object produce() { ... }
 }

 class Consumer implements Runnable {
 private final BlockingQueue queue;
 Consumer(BlockingQueue q) { queue = q; }
 public void run() {
 try {
 while (true) while(true) { consume(queue.take()); }
 } catch (InterruptedException ex) { ... handle ...}
   }
   void consume(Object x) { ... }
 }

 class Setup {
   void main() {
     BlockingQueue q = new SomeQueueImplementation();
     Producer p = new Producer(q);
     Consumer c1 = new Consumer(q);
     Consumer c2 = new Consumer(q);
     new Thread(p).start();
     new Thread(c1).start();
     new Thread(c2).start();
   }
 }
 

Memory consistency effects: As with other concurrent collections, actions in a thread prior to placing an object into a BlockingQueue happen-before actions subsequent to the access or removal of that element from the BlockingQueue in another thread.

This interface is a member of the Java Collections Framework .

Since:

1.5

Method Summary
boolean	add ( E           Inserts           Adds the specified element into to this queue if it is possible to do so immediately without violating capacity restrictions, immediately, returning true upon success and success, else throwing an IllegalStateException if no space is currently available. IllegalStateException.
boolean	contains ( Object           Returns true if this queue contains the specified element.
int	drainTo ( Collection <? super E           Removes all available elements from this queue and adds them into the given collection.
int	drainTo drainTo ( Collection <? super E           Removes all at most the given number of 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 the given collection.
boolean	offer ( E           Inserts the specified element into this queue, if possible.
boolean	offer offer ( E  o, long timeout, TimeUnit           Inserts the specified element into this queue queue, waiting if it is possible necessary up to do so immediately without violating capacity restrictions, returning true upon success and false if no the specified wait time for space is currently to become available.
boolean   E	offer poll ( E  e, long timeout, (long timeout, TimeUnit           Inserts           Retrieves and removes the specified element into head of this queue, waiting if necessary up to the specified wait time if necessary for space to become available. no elements are present on this queue.
E  void	poll put (long timeout, TimeUnit ( E           Retrieves and removes           Adds the head of specified element to this queue, waiting up to the specified wait time if necessary for an element space to become available.
void	put ( E           Inserts the specified element into this queue, waiting if necessary for space to become available.
int	remainingCapacity ()           Returns the number of elements that this queue can ideally (in the absence of memory or resource constraints) accept without blocking, or Integer.MAX_VALUE if there is no intrinsic limit.
int	remainingCapacity ()           Returns the number of additional elements that this queue can ideally (in the absence of memory or resource constraints) accept without blocking, or Integer.MAX_VALUE if there is no intrinsic limit.
E	take ()           Retrieves and removes the head of this queue, waiting if no elements are present on this queue.
boolean	remove ( Object           Removes a single instance of the specified element from this queue, if it is present.
E	take ()           Retrieves and removes the head of this queue, waiting if necessary until an element becomes available.

 

Methods inherited from interface java.util. Queue
element , peek , poll , remove

 

Methods inherited from interface java.util. Collection
addAll , clear , contains , containsAll , equals , hashCode , isEmpty , iterator , remove , removeAll , retainAll , size , toArray , toArray

 

Method Detail

add

 
boolean add ( E e) 

Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions, returning true upon success and throwing an IllegalStateException if no space is currently available. When using a capacity-restricted queue, it is generally preferable to use offer .

Specified by:

add in interface Collection < E >

Specified by:

add in interface Queue < E >

Parameters:

e - the element to add

Returns:

true (as specified by Collection.add(E) )

Throws:

IllegalStateException - if the element cannot be added at this time due to capacity restrictions

ClassCastException - if the class of the specified element prevents it from being added to this queue

NullPointerException - if the specified element is null

IllegalArgumentException - if some property of the specified element prevents it from being added to this queue

offer

boolean offer(E e)  o) 

Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions, returning Inserts the specified element into this queue, if possible. When using queues that may impose insertion restrictions (for example capacity bounds), method true offer upon success and false if no space is currently available. When using a capacity-restricted queue, this method is generally preferable to add(E) is generally preferable to method Collection.add(E) , which can fail to insert an element only by throwing an exception.

Specified by:

offer in interface Queue < E >

Parameters:

e o - the element to add add.

Returns:

true if it was possible to add the element was added to this queue, else false

Throws:

ClassCastException - if the class of the specified element prevents it from being added to this queue

NullPointerException - if the specified element is null

IllegalArgumentException - if some property of the specified element prevents it from being added to this queue - if the specified element is null

put offer

void 
boolean put offer (E e)
 throws InterruptedException o,
 long timeout,
 TimeUnit unit)
 throws InterruptedException

Inserts the specified element into this queue, waiting if necessary up to the specified wait time for space to become available.

Parameters:

e - the element to add 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 space is available.

Throws:

InterruptedException - if interrupted while waiting waiting.

ClassCastException - if the class of the specified element prevents it from being added to this queue

NullPointerException - if the specified element is null

IllegalArgumentException - if some property of the specified element prevents it from being added to this queue - if the specified element is null .

offer poll

boolean 
 Eoffer poll ( E(long timeout,
 TimeUnit e,
 long timeout,
 TimeUnit unit)
 throws InterruptedException unit)
 throws InterruptedException

Inserts the specified element into this queue, waiting up to the specified wait time if necessary for space to become available. Retrieves and removes the head of this queue, waiting if necessary up to the specified wait time if no elements are present on this queue.

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:

the head of this queue, or true null if successful, or false if the specified waiting time elapses before space an element is available present.

Throws:

InterruptedException - if interrupted while waiting

ClassCastException - if the class of the specified element prevents it from being added to this queue

NullPointerException - if the specified element is null

IllegalArgumentException - if some property of the specified element prevents it from being added to this queue - if interrupted while waiting.

take

E take()
       throws InterruptedException

Retrieves and removes the head of this queue, waiting if necessary until an element becomes available. no elements are present on this queue.

Returns:

the head of this queue

Throws:

InterruptedException - if interrupted while waiting waiting.

poll put

 E
void poll put (long timeout,
 TimeUnit( E unit)
  o)
 throws InterruptedException

Retrieves and removes Adds the head of specified element to this queue, waiting up to the specified wait time if necessary for an element space to become available.

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: Throws:

the head of this queue, or InterruptedException - if interrupted while waiting.

NullPointerException - if the specified element is null if the specified waiting time elapses before an element is available

Throws:

InterruptedException - if interrupted while waiting .

remainingCapacity

int remainingCapacity()

Returns the number of additional elements that this queue can ideally (in the absence of memory or resource constraints) accept without blocking, or Integer.MAX_VALUE if there is no intrinsic limit.

Note that you cannot always tell if an attempt to insert an element will succeed by inspecting add an element will succeed by inspecting remainingCapacity because it may be the case that another thread is about to insert put or remove take an element.

Returns:

the remaining capacity

remove add

boolean remove add ( ObjectE o)

Removes a single instance of the specified element from this queue, if it is present. More formally, removes an element Adds the specified element to this queue if it is possible to do so immediately, returning e such that o.equals(e) , if this queue contains one or more such elements. Returns true if this queue contained the specified element (or equivalently, if this queue changed as a result of the call). upon success, else throwing an IllegalStateException.

Specified by:

remove add in interface Collection < E >

Parameters:

o - element to be removed from this queue, if present o - the element

Returns:

true if this queue changed as a result of the call (as per the general contract of Collection.add ).

Throws:

ClassCastException NullPointerException - if the class of the specified element is incompatible with this queue (optional) null

NullPointerException IllegalStateException - if the specified element is null (optional) - if element cannot be added

contains

 
boolean contains ( Object o) 

Returns true if this queue contains the specified element. More formally, returns true if and only if this queue contains at least one element e such that o.equals(e) .

Specified by:

contains in interface Collection < E >

Parameters:

o - object to be checked for containment in this queue

Returns:

true if this queue contains the specified element

Throws:

ClassCastException - if the class of the specified element is incompatible with this queue (optional)

NullPointerException - if the specified element is null (optional)

drainTo

int drainTo(Collection<? super E> c)

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.

Parameters:

c - the collection to transfer elements into

Returns:

the number of elements transferred 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 c 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 - if c is this queue

drainTo

int drainTo(Collection<? super E> c,
            int maxElements)

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.

Parameters:

c - the collection to transfer elements into

maxElements - the maximum number of elements to transfer

Returns:

the number of elements transferred 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 c 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 - if c is this queue