API java : BlockingQueue

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 Implementing Classes:

ArrayBlockingQueue, DelayQueue, 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.

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) { 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) { 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();
   }
 }

This interface is a member of the Java Collections Framework.

Since:

1.5

offer

boolean offer(E o)

Inserts the specified element into this queue, if possible. When using queues that may impose insertion restrictions (for example capacity bounds), method offer 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:

o - the element to add.

Returns:

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

Throws:

NullPointerException - if the specified element is null

offer

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 space to become available.

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 space is available.

Throws:

InterruptedException - if interrupted while waiting.

NullPointerException - if the specified element is null.

poll

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 if no elements are present on this queue.

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:

InterruptedException - if interrupted while waiting.

take

E take()
       throws InterruptedException

Retrieves and removes the head of this queue, waiting if no elements are present on this queue.

Returns:

the head of this queue

Throws:

InterruptedException - if interrupted while waiting.

put

void put(E o)
         throws InterruptedException

Adds the specified element to this queue, waiting if necessary for space to become available.

Parameters:

o - the element to add

Throws:

InterruptedException - if interrupted while waiting.

NullPointerException - if the specified element is null.

remainingCapacity

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.

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

Returns:

the remaining capacity

add

boolean add(E o)

Adds the specified element to this queue if it is possible to do so immediately, returning true upon success, else throwing an IllegalStateException.

Specified by:

add in interface Collection<E>

Parameters:

o - the element

Returns:

true (as per the general contract of Collection.add).

Throws:

NullPointerException - if the specified element is null

IllegalStateException - if element cannot be added

drainTo

int drainTo(Collection<? super E> c)

Removes all available elements from this queue and adds them 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 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.

Throws:

NullPointerException - if c is null

IllegalArgumentException - 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 into 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.

Parameters:

c - the collection to transfer elements into

maxElements - the maximum number of elements to transfer

Returns:

the number of elements transferred.

Throws:

NullPointerException - if c is null

IllegalArgumentException - if c is this queue