BlockingQueue (Java Platform SE 7 b138)

Type Parameters:: E - the type of elements held in this collection

All Superinterfaces:: Collection<E>, Iterable<E>, Queue<E>

All Known Subinterfaces:: BlockingDeque<E>, TransferQueue<E>

All Known Implementing Classes:: ArrayBlockingQueue, DelayQueue, LinkedBlockingDeque, LinkedBlockingQueue, LinkedTransferQueue, 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) { 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();
   }
 }

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

Methods
Modifier and Type	Method and Description
`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.
`boolean`	`contains(Object o)` Returns `true` if this queue contains the specified element.
`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`	`offer(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 `false` if no space is currently available.
`boolean`	`offer(E e, long timeout, TimeUnit unit)` Inserts the specified element into this queue, waiting up to the specified wait time if necessary for space to become available.
`E`	`poll(long timeout, TimeUnit unit)` Retrieves and removes the head of this queue, waiting up to the specified wait time if necessary for an element to become available.
`void`	`put(E e)` Inserts the specified element into this queue, waiting if necessary for space to become available.
`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.
`boolean`	`remove(Object o)` 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, containsAll, equals, hashCode, isEmpty, iterator, 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)
```
    Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions, returning true upon success and false if no space is currently available. When using a capacity-restricted queue, this method is generally preferable to add(E), which can fail to insert an element only by throwing an exception.
    
    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
    
    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
  - put
```
void put(E e)
         throws InterruptedException
```
    Inserts the specified element into this queue, waiting if necessary for space to become available.
    
    Parameters:
    e - the element to add
    
    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
  - offer
```
boolean offer(E e,
            long timeout,
            TimeUnit unit)
              throws InterruptedException
```
    Inserts the specified element into this queue, waiting up to the specified wait time if necessary for space to become available.
    
    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 space is available
    
    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
  - take
```
E take()
       throws InterruptedException
```
    Retrieves and removes the head of this queue, waiting if necessary until an element becomes available.
    
    Returns:
    the head of this queue
    
    Throws:
    
    InterruptedException - if interrupted while waiting
  - poll
```
E poll(long timeout,
     TimeUnit unit)
       throws InterruptedException
```
    Retrieves and removes the head of this queue, waiting up to the specified wait time if necessary for an element 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
    
    Returns:
    the head of this queue, or 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 remainingCapacity because it may be the case that another thread is about to insert or remove an element.
    
    Returns:
    the remaining capacity
  - remove
```
boolean remove(Object o)
```
    Removes a single instance of the specified element from this queue, if it is present. More formally, removes an element 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).
    
    Specified by:
    
    remove in interface Collection<E>
    
    Parameters:
    o - element to be removed from this queue, if present
    
    Returns:
    true if this queue changed as a result of the call
    
    Throws:
    
    ClassCastException - if the class of the specified element is incompatible with this queue (optional)
    
    NullPointerException - if the specified element is null (optional)
  - 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 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:
    
    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
```
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. 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:
    
    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

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DRAFT ea-b138

Interface BlockingQueue<E>

Method Summary

Methods inherited from interface java.util.Queue

Methods inherited from interface java.util.Collection

Method Detail

add

offer

put

offer

take

poll

remainingCapacity

remove

contains

drainTo

drainTo