Java 数据结构之BlockingDeque

Deque还支持阻塞操作，即等待直到队列非空则检索一个元素，等待dqueue中有剩余空间再存储元素。

当请求不能立即被满足，但在未来某个时间点可以被满足时，BlockingDeque接口存在4中形式，分别使用不同的处理方式。分别为：

1）抛出异常

2）返回一个特殊值（null或false）

3）无限期阻塞当前线程直到该操作可以成功执行

4）以一个给定的最大时间限制阻塞当前线程，最大时间期限到，还未成功执行则放弃

总结如下表：

Summary of BlockingDeque methodsFirst Element (Head) Throws exceptionSpecial valueBlocksTimes outInsert{@link #addFirst addFirst(e)}{@link #offerFirst(Object) offerFirst(e)}{@link #putFirst putFirst(e)}{@link #offerFirst(Object, long, TimeUnit) offerFirst(e, time, unit)}Remove{@link #removeFirst removeFirst()}{@link #pollFirst pollFirst()}{@link #takeFirst takeFirst()}{@link #pollFirst(long, TimeUnit) pollFirst(time, unit)}Examine{@link #getFirst getFirst()}{@link #peekFirst peekFirst()}not applicablenot applicableLast Element (Tail) Throws exceptionSpecial valueBlocksTimes outInsert{@link #addLast addLast(e)}{@link #offerLast(Object) offerLast(e)}{@link #putLast putLast(e)}{@link #offerLast(Object, long, TimeUnit) offerLast(e, time, unit)}Remove{@link #removeLast() removeLast()}{@link #pollLast() pollLast()}{@link #takeLast takeLast()}{@link #pollLast(long, TimeUnit) pollLast(time, unit)}Examine{@link #getLast getLast()}{@link #peekLast peekLast()}not applicablenot applicable

像任何BlockingQueue，BlockingDeque是线程安全的，不允许元素null，且容量可限制也可不限制。BlockingDeque的实现可以直接用作FIFOBlockingQueue，继承自BlockingQueue接口的方法与BlockingDeque中的一些方法相同的。如下表所示：

Comparison of BlockingQueue and BlockingDeque methods{@code BlockingQueue} MethodEquivalent {@code BlockingDeque} MethodInsert{@link #add(Object) add(e)}{@link #addLast(Object) addLast(e)}{@link #offer(Object) offer(e)}{@link #offerLast(Object) offerLast(e)}{@link #put(Object) put(e)}{@link #putLast(Object) putLast(e)}{@link #offer(Object, long, TimeUnit) offer(e, time, unit)}{@link #offerLast(Object, long, TimeUnit) offerLast(e, time, unit)}Remove{@link #remove() remove()}{@link #removeFirst() removeFirst()}{@link #poll() poll()}{@link #pollFirst() pollFirst()}{@link #take() take()}{@link #takeFirst() takeFirst()}{@link #poll(long, TimeUnit) poll(time, unit)}{@link #pollFirst(long, TimeUnit) pollFirst(time, unit)}Examine{@link #element() element()}{@link #getFirst() getFirst()}{@link #peek() peek()}{@link #peekFirst() peekFirst()}

内存一致性效果(Memory consistency effects)：与其他并发集合一样，线程的向BlockingDeque中增加一个元素的优先级高于访问或移除一个元素的操作。

源码实现：

public interface BlockingDeque<E> extends BlockingQueue<E>, Deque<E> {    /*     * We have "diamond" multiple interface inheritance here, and that     * introduces ambiguities.  Methods might end up with different     * specs depending on the branch chosen by javadoc.  Thus a lot of     * methods specs here are copied from superinterfaces.     */    /**     * Inserts the specified element at the front of this deque if it is     * possible to do so immediately without violating capacity restrictions,     * throwing an {@code IllegalStateException} if no space is currently     * available.  When using a capacity-restricted deque, it is generally     * preferable to use {@link #offerFirst(Object) offerFirst}.     *     * @param e the element to add     * @throws IllegalStateException {@inheritDoc}     * @throws ClassCastException {@inheritDoc}     * @throws NullPointerException if the specified element is null     * @throws IllegalArgumentException {@inheritDoc}     */    void addFirst(E e);    /**     * Inserts the specified element at the end of this deque if it is     * possible to do so immediately without violating capacity restrictions,     * throwing an {@code IllegalStateException} if no space is currently     * available.  When using a capacity-restricted deque, it is generally     * preferable to use {@link #offerLast(Object) offerLast}.     *     * @param e the element to add     * @throws IllegalStateException {@inheritDoc}     * @throws ClassCastException {@inheritDoc}     * @throws NullPointerException if the specified element is null     * @throws IllegalArgumentException {@inheritDoc}     */    void addLast(E e);    /**     * Inserts the specified element at the front of this deque if it is     * possible to do so immediately without violating capacity restrictions,     * returning {@code true} upon success and {@code false} if no space is     * currently available.     * When using a capacity-restricted deque, this method is generally     * preferable to the {@link #addFirst(Object) addFirst} method, which can     * fail to insert an element only by throwing an exception.     *     * @param e the element to add     * @throws ClassCastException {@inheritDoc}     * @throws NullPointerException if the specified element is null     * @throws IllegalArgumentException {@inheritDoc}     */    boolean offerFirst(E e);    /**     * Inserts the specified element at the end of this deque if it is     * possible to do so immediately without violating capacity restrictions,     * returning {@code true} upon success and {@code false} if no space is     * currently available.     * When using a capacity-restricted deque, this method is generally     * preferable to the {@link #addLast(Object) addLast} method, which can     * fail to insert an element only by throwing an exception.     *     * @param e the element to add     * @throws ClassCastException {@inheritDoc}     * @throws NullPointerException if the specified element is null     * @throws IllegalArgumentException {@inheritDoc}     */    boolean offerLast(E e);    /**     * Inserts the specified element at the front of this deque,     * waiting if necessary for space to become available.     *     * @param e the element to add     * @throws InterruptedException if interrupted while waiting     * @throws ClassCastException if the class of the specified element     *         prevents it from being added to this deque     * @throws NullPointerException if the specified element is null     * @throws IllegalArgumentException if some property of the specified     *         element prevents it from being added to this deque     */    void putFirst(E e) throws InterruptedException;    /**     * Inserts the specified element at the end of this deque,     * waiting if necessary for space to become available.     *     * @param e the element to add     * @throws InterruptedException if interrupted while waiting     * @throws ClassCastException if the class of the specified element     *         prevents it from being added to this deque     * @throws NullPointerException if the specified element is null     * @throws IllegalArgumentException if some property of the specified     *         element prevents it from being added to this deque     */    void putLast(E e) throws InterruptedException;    /**     * Inserts the specified element at the front of this deque,     * waiting up to the specified wait time if necessary for space to     * become available.     *     * @param e the element to add     * @param timeout how long to wait before giving up, in units of     *        {@code unit}     * @param unit a {@code TimeUnit} determining how to interpret the     *        {@code timeout} parameter     * @return {@code true} if successful, or {@code false} if     *         the specified waiting time elapses before space is available     * @throws InterruptedException if interrupted while waiting     * @throws ClassCastException if the class of the specified element     *         prevents it from being added to this deque     * @throws NullPointerException if the specified element is null     * @throws IllegalArgumentException if some property of the specified     *         element prevents it from being added to this deque     */    boolean offerFirst(E e, long timeout, TimeUnit unit)        throws InterruptedException;    /**     * Inserts the specified element at the end of this deque,     * waiting up to the specified wait time if necessary for space to     * become available.     *     * @param e the element to add     * @param timeout how long to wait before giving up, in units of     *        {@code unit}     * @param unit a {@code TimeUnit} determining how to interpret the     *        {@code timeout} parameter     * @return {@code true} if successful, or {@code false} if     *         the specified waiting time elapses before space is available     * @throws InterruptedException if interrupted while waiting     * @throws ClassCastException if the class of the specified element     *         prevents it from being added to this deque     * @throws NullPointerException if the specified element is null     * @throws IllegalArgumentException if some property of the specified     *         element prevents it from being added to this deque     */    boolean offerLast(E e, long timeout, TimeUnit unit)        throws InterruptedException;    /**     * Retrieves and removes the first element of this deque, waiting     * if necessary until an element becomes available.     *     * @return the head of this deque     * @throws InterruptedException if interrupted while waiting     */    E takeFirst() throws InterruptedException;    /**     * Retrieves and removes the last element of this deque, waiting     * if necessary until an element becomes available.     *     * @return the tail of this deque     * @throws InterruptedException if interrupted while waiting     */    E takeLast() throws InterruptedException;    /**     * Retrieves and removes the first element of this deque, waiting     * up to the specified wait time if necessary for an element to     * become available.     *     * @param timeout how long to wait before giving up, in units of     *        {@code unit}     * @param unit a {@code TimeUnit} determining how to interpret the     *        {@code timeout} parameter     * @return the head of this deque, or {@code null} if the specified     *         waiting time elapses before an element is available     * @throws InterruptedException if interrupted while waiting     */    E pollFirst(long timeout, TimeUnit unit)        throws InterruptedException;    /**     * Retrieves and removes the last element of this deque, waiting     * up to the specified wait time if necessary for an element to     * become available.     *     * @param timeout how long to wait before giving up, in units of     *        {@code unit}     * @param unit a {@code TimeUnit} determining how to interpret the     *        {@code timeout} parameter     * @return the tail of this deque, or {@code null} if the specified     *         waiting time elapses before an element is available     * @throws InterruptedException if interrupted while waiting     */    E pollLast(long timeout, TimeUnit unit)        throws InterruptedException;    /**     * Removes the first occurrence of the specified element from this deque.     * If the deque does not contain the element, it is unchanged.     * More formally, removes the first element {@code e} such that     * {@code o.equals(e)} (if such an element exists).     * Returns {@code true} if this deque contained the specified element     * (or equivalently, if this deque changed as a result of the call).     *     * @param o element to be removed from this deque, if present     * @return {@code true} if an element was removed as a result of this call     * @throws ClassCastException if the class of the specified element     *         is incompatible with this deque     *         (<a href="../Collection.html#optional-restrictions">optional</a>)     * @throws NullPointerException if the specified element is null     *         (<a href="../Collection.html#optional-restrictions">optional</a>)     */    boolean removeFirstOccurrence(Object o);    /**     * Removes the last occurrence of the specified element from this deque.     * If the deque does not contain the element, it is unchanged.     * More formally, removes the last element {@code e} such that     * {@code o.equals(e)} (if such an element exists).     * Returns {@code true} if this deque contained the specified element     * (or equivalently, if this deque changed as a result of the call).     *     * @param o element to be removed from this deque, if present     * @return {@code true} if an element was removed as a result of this call     * @throws ClassCastException if the class of the specified element     *         is incompatible with this deque     *         (<a href="../Collection.html#optional-restrictions">optional</a>)     * @throws NullPointerException if the specified element is null     *         (<a href="../Collection.html#optional-restrictions">optional</a>)     */    boolean removeLastOccurrence(Object o);    // *** BlockingQueue methods ***    /**     * Inserts the specified element into the queue represented by this deque     * (in other words, at the tail of this deque) if it is possible to do so     * immediately without violating capacity restrictions, returning     * {@code true} upon success and throwing an     * {@code IllegalStateException} if no space is currently available.     * When using a capacity-restricted deque, it is generally preferable to     * use {@link #offer(Object) offer}.     *     * <p>This method is equivalent to {@link #addLast(Object) addLast}.     *     * @param e the element to add     * @throws IllegalStateException {@inheritDoc}     * @throws ClassCastException if the class of the specified element     *         prevents it from being added to this deque     * @throws NullPointerException if the specified element is null     * @throws IllegalArgumentException if some property of the specified     *         element prevents it from being added to this deque     */    boolean add(E e);    /**     * Inserts the specified element into the queue represented by this deque     * (in other words, at the tail of this deque) if it is possible to do so     * immediately without violating capacity restrictions, returning     * {@code true} upon success and {@code false} if no space is currently     * available.  When using a capacity-restricted deque, this method is     * generally preferable to the {@link #add} method, which can fail to     * insert an element only by throwing an exception.     *     * <p>This method is equivalent to {@link #offerLast(Object) offerLast}.     *     * @param e the element to add     * @throws ClassCastException if the class of the specified element     *         prevents it from being added to this deque     * @throws NullPointerException if the specified element is null     * @throws IllegalArgumentException if some property of the specified     *         element prevents it from being added to this deque     */    boolean offer(E e);    /**     * Inserts the specified element into the queue represented by this deque     * (in other words, at the tail of this deque), waiting if necessary for     * space to become available.     *     * <p>This method is equivalent to {@link #putLast(Object) putLast}.     *     * @param e the element to add     * @throws InterruptedException {@inheritDoc}     * @throws ClassCastException if the class of the specified element     *         prevents it from being added to this deque     * @throws NullPointerException if the specified element is null     * @throws IllegalArgumentException if some property of the specified     *         element prevents it from being added to this deque     */    void put(E e) throws InterruptedException;    /**     * Inserts the specified element into the queue represented by this deque     * (in other words, at the tail of this deque), waiting up to the     * specified wait time if necessary for space to become available.     *     * <p>This method is equivalent to     * {@link #offerLast(Object,long,TimeUnit) offerLast}.     *     * @param e the element to add     * @return {@code true} if the element was added to this deque, else     *         {@code false}     * @throws InterruptedException {@inheritDoc}     * @throws ClassCastException if the class of the specified element     *         prevents it from being added to this deque     * @throws NullPointerException if the specified element is null     * @throws IllegalArgumentException if some property of the specified     *         element prevents it from being added to this deque     */    boolean offer(E e, long timeout, TimeUnit unit)        throws InterruptedException;    /**     * Retrieves and removes the head of the queue represented by this deque     * (in other words, the first element of this deque).     * This method differs from {@link #poll poll} only in that it     * throws an exception if this deque is empty.     *     * <p>This method is equivalent to {@link #removeFirst() removeFirst}.     *     * @return the head of the queue represented by this deque     * @throws NoSuchElementException if this deque is empty     */    E remove();    /**     * Retrieves and removes the head of the queue represented by this deque     * (in other words, the first element of this deque), or returns     * {@code null} if this deque is empty.     *     * <p>This method is equivalent to {@link #pollFirst()}.     *     * @return the head of this deque, or {@code null} if this deque is empty     */    E poll();    /**     * Retrieves and removes the head of the queue represented by this deque     * (in other words, the first element of this deque), waiting if     * necessary until an element becomes available.     *     * <p>This method is equivalent to {@link #takeFirst() takeFirst}.     *     * @return the head of this deque     * @throws InterruptedException if interrupted while waiting     */    E take() throws InterruptedException;    /**     * Retrieves and removes the head of the queue represented by this deque     * (in other words, the first element of this deque), waiting up to the     * specified wait time if necessary for an element to become available.     *     * <p>This method is equivalent to     * {@link #pollFirst(long,TimeUnit) pollFirst}.     *     * @return the head of this deque, or {@code null} if the     *         specified waiting time elapses before an element is available     * @throws InterruptedException if interrupted while waiting     */    E poll(long timeout, TimeUnit unit)        throws InterruptedException;    /**     * Retrieves, but does not remove, the head of the queue represented by     * this deque (in other words, the first element of this deque).     * This method differs from {@link #peek peek} only in that it throws an     * exception if this deque is empty.     *     * <p>This method is equivalent to {@link #getFirst() getFirst}.     *     * @return the head of this deque     * @throws NoSuchElementException if this deque is empty     */    E element();    /**     * Retrieves, but does not remove, the head of the queue represented by     * this deque (in other words, the first element of this deque), or     * returns {@code null} if this deque is empty.     *     * <p>This method is equivalent to {@link #peekFirst() peekFirst}.     *     * @return the head of this deque, or {@code null} if this deque is empty     */    E peek();    /**     * Removes the first occurrence of the specified element from this deque.     * If the deque does not contain the element, it is unchanged.     * More formally, removes the first element {@code e} such that     * {@code o.equals(e)} (if such an element exists).     * Returns {@code true} if this deque contained the specified element     * (or equivalently, if this deque changed as a result of the call).     *     * <p>This method is equivalent to     * {@link #removeFirstOccurrence(Object) removeFirstOccurrence}.     *     * @param o element to be removed from this deque, if present     * @return {@code true} if this deque changed as a result of the call     * @throws ClassCastException if the class of the specified element     *         is incompatible with this deque     *         (<a href="../Collection.html#optional-restrictions">optional</a>)     * @throws NullPointerException if the specified element is null     *         (<a href="../Collection.html#optional-restrictions">optional</a>)     */    boolean remove(Object o);    /**     * Returns {@code true} if this deque contains the specified element.     * More formally, returns {@code true} if and only if this deque contains     * at least one element {@code e} such that {@code o.equals(e)}.     *     * @param o object to be checked for containment in this deque     * @return {@code true} if this deque contains the specified element     * @throws ClassCastException if the class of the specified element     *         is incompatible with this deque     *         (<a href="../Collection.html#optional-restrictions">optional</a>)     * @throws NullPointerException if the specified element is null     *         (<a href="../Collection.html#optional-restrictions">optional</a>)     */    public boolean contains(Object o);    /**     * Returns the number of elements in this deque.     *     * @return the number of elements in this deque     */    public int size();    /**     * Returns an iterator over the elements in this deque in proper sequence.     * The elements will be returned in order from first (head) to last (tail).     *     * @return an iterator over the elements in this deque in proper sequence     */    Iterator<E> iterator();    // *** Stack methods ***    /**     * Pushes an element onto the stack represented by this deque (in other     * words, at the head of this deque) if it is possible to do so     * immediately without violating capacity restrictions, throwing an     * {@code IllegalStateException} if no space is currently available.     *     * <p>This method is equivalent to {@link #addFirst(Object) addFirst}.     *     * @throws IllegalStateException {@inheritDoc}     * @throws ClassCastException {@inheritDoc}     * @throws NullPointerException if the specified element is null     * @throws IllegalArgumentException {@inheritDoc}     */    void push(E e);}