BlockingQueue (Java SE 25 & JDK 25)
Description: declaration: module: java.base, package: java.util.concurrent, interface: BlockingQueue
Keywords: java.util.concurrent.BlockingQueue interface

# Interface BlockingQueue<E>

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

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:

Summary of BlockingQueue methods
| 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

  Modifier and Type
  Method
  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 declared in interface Collection

  `addAll, clear, containsAll, equals, hashCode, isEmpty, iterator, parallelStream, removeAll, removeIf, retainAll, size, spliterator, stream, toArray, toArray, toArray`

  ### Methods declared in interface Iterable

  `forEach`

  ### Methods declared in interface Queue

  `element, peek, poll, remove`

- ## Method Details

  - ### 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