Java多线程系列--“JUC集合”09之 LinkedBlockingDeque

概要

本章介绍JUC包中的LinkedBlockingDeque。内容包括：
LinkedBlockingDeque介绍
LinkedBlockingDeque原理和数据结构
LinkedBlockingDeque函数列表
LinkedBlockingDeque源码分析(JDK1.7.0_40版本)
LinkedBlockingDeque示例

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LinkedBlockingDeque介绍

LinkedBlockingDeque是双向链表实现的双向并发阻塞队列。该阻塞队列同时支持FIFO和FILO两种操作方式，即可以从队列的头和尾同时操作(插入/删除)；并且，该阻塞队列是支持线程安全。

此外，LinkedBlockingDeque还是可选容量的(防止过度膨胀)，即可以指定队列的容量。如果不指定，默认容量大小等于Integer.MAX_VALUE。

LinkedBlockingDeque原理和数据结构

LinkedBlockingDeque的数据结构，如下图所示：

/*
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*
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/*
*
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*
* Written by Doug Lea with assistance from members of JCP JSR-166
* Expert Group and released to the public domain, as explained at
* http://creativecommons.org/publicdomain/zero/1.0/ */

package java.util.concurrent;

import java.util.AbstractQueue;
import java.util.Collection;
import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;

/**
* An optionally-bounded {@linkplain BlockingDeque blocking deque} based on
* linked nodes.
*
* <p> The optional capacity bound constructor argument serves as a
* way to prevent excessive expansion. The capacity, if unspecified,
* is equal to {@link Integer#MAX_VALUE}.  Linked nodes are
* dynamically created upon each insertion unless this would bring the
* deque above capacity.
*
* <p>Most operations run in constant time (ignoring time spent
* blocking).  Exceptions include {@link #remove(Object) remove},
* {@link #removeFirstOccurrence removeFirstOccurrence}, {@link
* #removeLastOccurrence removeLastOccurrence}, {@link #contains
* contains}, {@link #iterator iterator.remove()}, and the bulk
* operations, all of which run in linear time.
*
* <p>This class and its iterator implement all of the
* <em>optional</em> methods of the {@link Collection} and {@link
* Iterator} interfaces.
*
* <p>This class is a member of the
* <a href="{@docRoot}/../technotes/guides/collections/index.html">
* Java Collections Framework</a>.
*
* @since 1.6
* @author  Doug Lea
* @param <E> the type of elements held in this collection
*/
public class LinkedBlockingDeque<E>
extends AbstractQueue<E>
implements BlockingDeque<E>,  java.io.Serializable {

/*
* Implemented as a simple doubly-linked list protected by a
* single lock and using conditions to manage blocking.
*
* To implement weakly consistent iterators, it appears we need to
* keep all Nodes GC-reachable from a predecessor dequeued Node.
* That would cause two problems:
* - allow a rogue Iterator to cause unbounded memory retention
* - cause cross-generational linking of old Nodes to new Nodes if
*   a Node was tenured while live, which generational GCs have a
*   hard time dealing with, causing repeated major collections.
* However, only non-deleted Nodes need to be reachable from
* dequeued Nodes, and reachability does not necessarily have to
* be of the kind understood by the GC.  We use the trick of
* linking a Node that has just been dequeued to itself.  Such a
* self-link implicitly means to jump to "first" (for next links)
* or "last" (for prev links).
*/

/*
* We have "diamond" multiple interface/abstract class inheritance
* here, and that introduces ambiguities. Often we want the
* BlockingDeque javadoc combined with the AbstractQueue
* implementation, so a lot of method specs are duplicated here.
*/

private static final long serialVersionUID = -387911632671998426L;

/** Doubly-linked list node class */
static final class Node<E> {
/**
* The item, or null if this node has been removed.
*/
E item;

/**
* One of:
* - the real predecessor Node
* - this Node, meaning the predecessor is tail
* - null, meaning there is no predecessor
*/
Node<E> prev;

/**
* One of:
* - the real successor Node
* - this Node, meaning the successor is head
* - null, meaning there is no successor
*/
Node<E> next;

Node(E x) {
item = x;
}
}

/**
* Pointer to first node.
* Invariant: (first == null && last == null) ||
*            (first.prev == null && first.item != null)
*/
transient Node<E> first;

/**
* Pointer to last node.
* Invariant: (first == null && last == null) ||
*            (last.next == null && last.item != null)
*/
transient Node<E> last;

/** Number of items in the deque */
private transient int count;

/** Maximum number of items in the deque */
private final int capacity;

/** Main lock guarding all access */
final ReentrantLock lock = new ReentrantLock();

/** Condition for waiting takes */
private final Condition notEmpty = lock.newCondition();

/** Condition for waiting puts */
private final Condition notFull = lock.newCondition();

/**
* Creates a {@code LinkedBlockingDeque} with a capacity of
* {@link Integer#MAX_VALUE}.
*/
public LinkedBlockingDeque() {
this(Integer.MAX_VALUE);
}

/**
* Creates a {@code LinkedBlockingDeque} with the given (fixed) capacity.
*
* @param capacity the capacity of this deque
* @throws IllegalArgumentException if {@code capacity} is less than 1
*/
public LinkedBlockingDeque(int capacity) {
if (capacity <= 0) throw new IllegalArgumentException();
this.capacity = capacity;
}

/**
* Creates a {@code LinkedBlockingDeque} with a capacity of
* {@link Integer#MAX_VALUE}, initially containing the elements of
* the given collection, added in traversal order of the
* collection's iterator.
*
* @param c the collection of elements to initially contain
* @throws NullPointerException if the specified collection or any
*         of its elements are null
*/
public LinkedBlockingDeque(Collection<? extends E> c) {
this(Integer.MAX_VALUE);
final ReentrantLock lock = this.lock;
lock.lock(); // Never contended, but necessary for visibility
try {
for (E e : c) {
if (e == null)
throw new NullPointerException();
if (!linkLast(new Node<E>(e)))
throw new IllegalStateException("Deque full");
}
} finally {
lock.unlock();
}
}

// Basic linking and unlinking operations, called only while holding lock

/**
* Links node as first element, or returns false if full.
*/
private boolean linkFirst(Node<E> node) {
// assert lock.isHeldByCurrentThread();
if (count >= capacity)
return false;
Node<E> f = first;
node.next = f;
first = node;
if (last == null)
last = node;
else
f.prev = node;
++count;
notEmpty.signal();
return true;
}

/**
* Links node as last element, or returns false if full.
*/
private boolean linkLast(Node<E> node) {
// assert lock.isHeldByCurrentThread();
if (count >= capacity)
return false;
Node<E> l = last;
node.prev = l;
last = node;
if (first == null)
first = node;
else
l.next = node;
++count;
notEmpty.signal();
return true;
}

/**
* Removes and returns first element, or null if empty.
*/
private E unlinkFirst() {
// assert lock.isHeldByCurrentThread();
Node<E> f = first;
if (f == null)
return null;
Node<E> n = f.next;
E item = f.item;
f.item = null;
f.next = f; // help GC
first = n;
if (n == null)
last = null;
else
n.prev = null;
--count;
notFull.signal();
return item;
}

/**
* Removes and returns last element, or null if empty.
*/
private E unlinkLast() {
// assert lock.isHeldByCurrentThread();
Node<E> l = last;
if (l == null)
return null;
Node<E> p = l.prev;
E item = l.item;
l.item = null;
l.prev = l; // help GC
last = p;
if (p == null)
first = null;
else
p.next = null;
--count;
notFull.signal();
return item;
}

/**
* Unlinks x.
*/
void unlink(Node<E> x) {
// assert lock.isHeldByCurrentThread();
Node<E> p = x.prev;
Node<E> n = x.next;
if (p == null) {
unlinkFirst();
} else if (n == null) {
unlinkLast();
} else {
p.next = n;
n.prev = p;
x.item = null;
// Don't mess with x's links.  They may still be in use by
// an iterator.
--count;
notFull.signal();
}
}

// BlockingDeque methods

/**
* @throws IllegalStateException {@inheritDoc}
* @throws NullPointerException  {@inheritDoc}
*/
public void addFirst(E e) {
if (!offerFirst(e))
throw new IllegalStateException("Deque full");
}

/**
* @throws IllegalStateException {@inheritDoc}
* @throws NullPointerException  {@inheritDoc}
*/
public void addLast(E e) {
if (!offerLast(e))
throw new IllegalStateException("Deque full");
}

/**
* @throws NullPointerException {@inheritDoc}
*/
public boolean offerFirst(E e) {
if (e == null) throw new NullPointerException();
Node<E> node = new Node<E>(e);
final ReentrantLock lock = this.lock;
lock.lock();
try {
return linkFirst(node);
} finally {
lock.unlock();
}
}

/**
* @throws NullPointerException {@inheritDoc}
*/
public boolean offerLast(E e) {
if (e == null) throw new NullPointerException();
Node<E> node = new Node<E>(e);
final ReentrantLock lock = this.lock;
lock.lock();
try {
return linkLast(node);
} finally {
lock.unlock();
}
}

/**
* @throws NullPointerException {@inheritDoc}
* @throws InterruptedException {@inheritDoc}
*/
public void putFirst(E e) throws InterruptedException {
if (e == null) throw new NullPointerException();
Node<E> node = new Node<E>(e);
final ReentrantLock lock = this.lock;
lock.lock();
try {
while (!linkFirst(node))
notFull.await();
} finally {
lock.unlock();
}
}

/**
* @throws NullPointerException {@inheritDoc}
* @throws InterruptedException {@inheritDoc}
*/
public void putLast(E e) throws InterruptedException {
if (e == null) throw new NullPointerException();
Node<E> node = new Node<E>(e);
final ReentrantLock lock = this.lock;
lock.lock();
try {
while (!linkLast(node))
notFull.await();
} finally {
lock.unlock();
}
}

/**
* @throws NullPointerException {@inheritDoc}
* @throws InterruptedException {@inheritDoc}
*/
public boolean offerFirst(E e, long timeout, TimeUnit unit)
throws InterruptedException {
if (e == null) throw new NullPointerException();
Node<E> node = new Node<E>(e);
long nanos = unit.toNanos(timeout);
final ReentrantLock lock = this.lock;
lock.lockInterruptibly();
try {
while (!linkFirst(node)) {
if (nanos <= 0)
return false;
nanos = notFull.awaitNanos(nanos);
}
return true;
} finally {
lock.unlock();
}
}

/**
* @throws NullPointerException {@inheritDoc}
* @throws InterruptedException {@inheritDoc}
*/
public boolean offerLast(E e, long timeout, TimeUnit unit)
throws InterruptedException {
if (e == null) throw new NullPointerException();
Node<E> node = new Node<E>(e);
long nanos = unit.toNanos(timeout);
final ReentrantLock lock = this.lock;
lock.lockInterruptibly();
try {
while (!linkLast(node)) {
if (nanos <= 0)
return false;
nanos = notFull.awaitNanos(nanos);
}
return true;
} finally {
lock.unlock();
}
}

/**
* @throws NoSuchElementException {@inheritDoc}
*/
public E removeFirst() {
E x = pollFirst();
if (x == null) throw new NoSuchElementException();
return x;
}

/**
* @throws NoSuchElementException {@inheritDoc}
*/
public E removeLast() {
E x = pollLast();
if (x == null) throw new NoSuchElementException();
return x;
}

public E pollFirst() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
return unlinkFirst();
} finally {
lock.unlock();
}
}

public E pollLast() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
return unlinkLast();
} finally {
lock.unlock();
}
}

public E takeFirst() throws InterruptedException {
final ReentrantLock lock = this.lock;
lock.lock();
try {
E x;
while ( (x = unlinkFirst()) == null)
notEmpty.await();
return x;
} finally {
lock.unlock();
}
}

public E takeLast() throws InterruptedException {
final ReentrantLock lock = this.lock;
lock.lock();
try {
E x;
while ( (x = unlinkLast()) == null)
notEmpty.await();
return x;
} finally {
lock.unlock();
}
}

public E pollFirst(long timeout, TimeUnit unit)
throws InterruptedException {
long nanos = unit.toNanos(timeout);
final ReentrantLock lock = this.lock;
lock.lockInterruptibly();
try {
E x;
while ( (x = unlinkFirst()) == null) {
if (nanos <= 0)
return null;
nanos = notEmpty.awaitNanos(nanos);
}
return x;
} finally {
lock.unlock();
}
}

public E pollLast(long timeout, TimeUnit unit)
throws InterruptedException {
long nanos = unit.toNanos(timeout);
final ReentrantLock lock = this.lock;
lock.lockInterruptibly();
try {
E x;
while ( (x = unlinkLast()) == null) {
if (nanos <= 0)
return null;
nanos = notEmpty.awaitNanos(nanos);
}
return x;
} finally {
lock.unlock();
}
}

/**
* @throws NoSuchElementException {@inheritDoc}
*/
public E getFirst() {
E x = peekFirst();
if (x == null) throw new NoSuchElementException();
return x;
}

/**
* @throws NoSuchElementException {@inheritDoc}
*/
public E getLast() {
E x = peekLast();
if (x == null) throw new NoSuchElementException();
return x;
}

public E peekFirst() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
return (first == null) ? null : first.item;
} finally {
lock.unlock();
}
}

public E peekLast() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
return (last == null) ? null : last.item;
} finally {
lock.unlock();
}
}

public boolean removeFirstOccurrence(Object o) {
if (o == null) return false;
final ReentrantLock lock = this.lock;
lock.lock();
try {
for (Node<E> p = first; p != null; p = p.next) {
if (o.equals(p.item)) {
unlink(p);
return true;
}
}
return false;
} finally {
lock.unlock();
}
}

public boolean removeLastOccurrence(Object o) {
if (o == null) return false;
final ReentrantLock lock = this.lock;
lock.lock();
try {
for (Node<E> p = last; p != null; p = p.prev) {
if (o.equals(p.item)) {
unlink(p);
return true;
}
}
return false;
} finally {
lock.unlock();
}
}

// BlockingQueue methods

/**
* Inserts the specified element at the end of this deque unless it would
* violate capacity restrictions.  When using a capacity-restricted deque,
* it is generally preferable to use method {@link #offer(Object) offer}.
*
* <p>This method is equivalent to {@link #addLast}.
*
* @throws IllegalStateException if the element cannot be added at this
*         time due to capacity restrictions
* @throws NullPointerException if the specified element is null
*/
public boolean add(E e) {
addLast(e);
return true;
}

/**
* @throws NullPointerException if the specified element is null
*/
public boolean offer(E e) {
return offerLast(e);
}

/**
* @throws NullPointerException {@inheritDoc}
* @throws InterruptedException {@inheritDoc}
*/
public void put(E e) throws InterruptedException {
putLast(e);
}

/**
* @throws NullPointerException {@inheritDoc}
* @throws InterruptedException {@inheritDoc}
*/
public boolean offer(E e, long timeout, TimeUnit unit)
throws InterruptedException {
return offerLast(e, timeout, unit);
}

/**
* Retrieves and removes the head of the queue represented by 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
*/
public E remove() {
return removeFirst();
}

public E poll() {
return pollFirst();
}

public E take() throws InterruptedException {
return takeFirst();
}

public E poll(long timeout, TimeUnit unit) throws InterruptedException {
return pollFirst(timeout, unit);
}

/**
* Retrieves, but does not remove, the head of the queue represented by
* 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 the queue represented by this deque
* @throws NoSuchElementException if this deque is empty
*/
public E element() {
return getFirst();
}

public E peek() {
return peekFirst();
}

/**
* Returns the number of additional elements that this deque can ideally
* (in the absence of memory or resource constraints) accept without
* blocking. This is always equal to the initial capacity of this deque
* less the current {@code size} of this deque.
*
* <p>Note that you <em>cannot</em> always tell if an attempt to insert
* an element will succeed by inspecting {@code remainingCapacity}
* because it may be the case that another thread is about to
* insert or remove an element.
*/
public int remainingCapacity() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
return capacity - count;
} finally {
lock.unlock();
}
}

/**
* @throws UnsupportedOperationException {@inheritDoc}
* @throws ClassCastException            {@inheritDoc}
* @throws NullPointerException          {@inheritDoc}
* @throws IllegalArgumentException      {@inheritDoc}
*/
public int drainTo(Collection<? super E> c) {
return drainTo(c, Integer.MAX_VALUE);
}

/**
* @throws UnsupportedOperationException {@inheritDoc}
* @throws ClassCastException            {@inheritDoc}
* @throws NullPointerException          {@inheritDoc}
* @throws IllegalArgumentException      {@inheritDoc}
*/
public int drainTo(Collection<? super E> c, int maxElements) {
if (c == null)
throw new NullPointerException();
if (c == this)
throw new IllegalArgumentException();
final ReentrantLock lock = this.lock;
lock.lock();
try {
int n = Math.min(maxElements, count);
for (int i = 0; i < n; i++) {
c.add(first.item);   // In this order, in case add() throws.
unlinkFirst();
}
return n;
} finally {
lock.unlock();
}
}

// Stack methods

/**
* @throws IllegalStateException {@inheritDoc}
* @throws NullPointerException  {@inheritDoc}
*/
public void push(E e) {
addFirst(e);
}

/**
* @throws NoSuchElementException {@inheritDoc}
*/
public E pop() {
return removeFirst();
}

// Collection methods

/**
* 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
*/
public boolean remove(Object o) {
return removeFirstOccurrence(o);
}

/**
* Returns the number of elements in this deque.
*
* @return the number of elements in this deque
*/
public int size() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
return count;
} finally {
lock.unlock();
}
}

/**
* 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
*/
public boolean contains(Object o) {
if (o == null) return false;
final ReentrantLock lock = this.lock;
lock.lock();
try {
for (Node<E> p = first; p != null; p = p.next)
if (o.equals(p.item))
return true;
return false;
} finally {
lock.unlock();
}
}

/*
* TODO: Add support for more efficient bulk operations.
*
* We don't want to acquire the lock for every iteration, but we
* also want other threads a chance to interact with the
* collection, especially when count is close to capacity.
*/

//     /**
//      * Adds all of the elements in the specified collection to this
//      * queue.  Attempts to addAll of a queue to itself result in
//      * {@code IllegalArgumentException}. Further, the behavior of
//      * this operation is undefined if the specified collection is
//      * modified while the operation is in progress.
//      *
//      * @param c collection containing elements to be added to this queue
//      * @return {@code true} if this queue changed as a result of the call
//      * @throws ClassCastException            {@inheritDoc}
//      * @throws NullPointerException          {@inheritDoc}
//      * @throws IllegalArgumentException      {@inheritDoc}
//      * @throws IllegalStateException         {@inheritDoc}
//      * @see #add(Object)
//      */
//     public boolean addAll(Collection<? extends E> c) {
//         if (c == null)
//             throw new NullPointerException();
//         if (c == this)
//             throw new IllegalArgumentException();
//         final ReentrantLock lock = this.lock;
//         lock.lock();
//         try {
//             boolean modified = false;
//             for (E e : c)
//                 if (linkLast(e))
//                     modified = true;
//             return modified;
//         } finally {
//             lock.unlock();
//         }
//     }

/**
* Returns an array containing all of the elements in this deque, in
* proper sequence (from first to last element).
*
* <p>The returned array will be "safe" in that no references to it are
* maintained by this deque.  (In other words, this method must allocate
* a new array).  The caller is thus free to modify the returned array.
*
* <p>This method acts as bridge between array-based and collection-based
* APIs.
*
* @return an array containing all of the elements in this deque
*/
@SuppressWarnings("unchecked")
public Object[] toArray() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
Object[] a = new Object[count];
int k = 0;
for (Node<E> p = first; p != null; p = p.next)
a[k++] = p.item;
return a;
} finally {
lock.unlock();
}
}

/**
* Returns an array containing all of the elements in this deque, in
* proper sequence; the runtime type of the returned array is that of
* the specified array.  If the deque fits in the specified array, it
* is returned therein.  Otherwise, a new array is allocated with the
* runtime type of the specified array and the size of this deque.
*
* <p>If this deque fits in the specified array with room to spare
* (i.e., the array has more elements than this deque), the element in
* the array immediately following the end of the deque is set to
* {@code null}.
*
* <p>Like the {@link #toArray()} method, this method acts as bridge between
* array-based and collection-based APIs.  Further, this method allows
* precise control over the runtime type of the output array, and may,
* under certain circumstances, be used to save allocation costs.
*
* <p>Suppose {@code x} is a deque known to contain only strings.
* The following code can be used to dump the deque into a newly
* allocated array of {@code String}:
*
* <pre>
*     String[] y = x.toArray(new String[0]);</pre>
*
* Note that {@code toArray(new Object[0])} is identical in function to
* {@code toArray()}.
*
* @param a the array into which the elements of the deque are to
*          be stored, if it is big enough; otherwise, a new array of the
*          same runtime type is allocated for this purpose
* @return an array containing all of the elements in this deque
* @throws ArrayStoreException if the runtime type of the specified array
*         is not a supertype of the runtime type of every element in
*         this deque
* @throws NullPointerException if the specified array is null
*/
@SuppressWarnings("unchecked")
public <T> T[] toArray(T[] a) {
final ReentrantLock lock = this.lock;
lock.lock();
try {
if (a.length < count)
a = (T[])java.lang.reflect.Array.newInstance
(a.getClass().getComponentType(), count);

int k = 0;
for (Node<E> p = first; p != null; p = p.next)
a[k++] = (T)p.item;
if (a.length > k)
a[k] = null;
return a;
} finally {
lock.unlock();
}
}

public String toString() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
Node<E> p = first;
if (p == null)
return "[]";

StringBuilder sb = new StringBuilder();
sb.append('[');
for (;;) {
E e = p.item;
sb.append(e == this ? "(this Collection)" : e);
p = p.next;
if (p == null)
return sb.append(']').toString();
sb.append(',').append(' ');
}
} finally {
lock.unlock();
}
}

/**
* Atomically removes all of the elements from this deque.
* The deque will be empty after this call returns.
*/
public void clear() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
for (Node<E> f = first; f != null; ) {
f.item = null;
Node<E> n = f.next;
f.prev = null;
f.next = null;
f = n;
}
first = last = null;
count = 0;
notFull.signalAll();
} finally {
lock.unlock();
}
}

/**
* 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).
*
* <p>The returned iterator is a "weakly consistent" iterator that
* will never throw {@link java.util.ConcurrentModificationException
* ConcurrentModificationException}, and guarantees to traverse
* elements as they existed upon construction of the iterator, and
* may (but is not guaranteed to) reflect any modifications
* subsequent to construction.
*
* @return an iterator over the elements in this deque in proper sequence
*/
public Iterator<E> iterator() {
return new Itr();
}

/**
* Returns an iterator over the elements in this deque in reverse
* sequential order.  The elements will be returned in order from
* last (tail) to first (head).
*
* <p>The returned iterator is a "weakly consistent" iterator that
* will never throw {@link java.util.ConcurrentModificationException
* ConcurrentModificationException}, and guarantees to traverse
* elements as they existed upon construction of the iterator, and
* may (but is not guaranteed to) reflect any modifications
* subsequent to construction.
*
* @return an iterator over the elements in this deque in reverse order
*/
public Iterator<E> descendingIterator() {
return new DescendingItr();
}

/**
* Base class for Iterators for LinkedBlockingDeque
*/
private abstract class AbstractItr implements Iterator<E> {
/**
* The next node to return in next()
*/
Node<E> next;

/**
* nextItem holds on to item fields because once we claim that
* an element exists in hasNext(), we must return item read
* under lock (in advance()) even if it was in the process of
* being removed when hasNext() was called.
*/
E nextItem;

/**
* Node returned by most recent call to next. Needed by remove.
* Reset to null if this element is deleted by a call to remove.
*/
private Node<E> lastRet;

abstract Node<E> firstNode();
abstract Node<E> nextNode(Node<E> n);

AbstractItr() {
// set to initial position
final ReentrantLock lock = LinkedBlockingDeque.this.lock;
lock.lock();
try {
next = firstNode();
nextItem = (next == null) ? null : next.item;
} finally {
lock.unlock();
}
}

/**
* Returns the successor node of the given non-null, but
* possibly previously deleted, node.
*/
private Node<E> succ(Node<E> n) {
// Chains of deleted nodes ending in null or self-links
// are possible if multiple interior nodes are removed.
for (;;) {
Node<E> s = nextNode(n);
if (s == null)
return null;
else if (s.item != null)
return s;
else if (s == n)
return firstNode();
else
n = s;
}
}

/**
* Advances next.
*/
void advance() {
final ReentrantLock lock = LinkedBlockingDeque.this.lock;
lock.lock();
try {
// assert next != null;
next = succ(next);
nextItem = (next == null) ? null : next.item;
} finally {
lock.unlock();
}
}

public boolean hasNext() {
return next != null;
}

public E next() {
if (next == null)
throw new NoSuchElementException();
lastRet = next;
E x = nextItem;
advance();
return x;
}

public void remove() {
Node<E> n = lastRet;
if (n == null)
throw new IllegalStateException();
lastRet = null;
final ReentrantLock lock = LinkedBlockingDeque.this.lock;
lock.lock();
try {
if (n.item != null)
unlink(n);
} finally {
lock.unlock();
}
}
}

/** Forward iterator */
private class Itr extends AbstractItr {
Node<E> firstNode() { return first; }
Node<E> nextNode(Node<E> n) { return n.next; }
}

/** Descending iterator */
private class DescendingItr extends AbstractItr {
Node<E> firstNode() { return last; }
Node<E> nextNode(Node<E> n) { return n.prev; }
}

/**
* Save the state of this deque to a stream (that is, serialize it).
*
* @serialData The capacity (int), followed by elements (each an
* {@code Object}) in the proper order, followed by a null
* @param s the stream
*/
private void writeObject(java.io.ObjectOutputStream s)
throws java.io.IOException {
final ReentrantLock lock = this.lock;
lock.lock();
try {
// Write out capacity and any hidden stuff
s.defaultWriteObject();
// Write out all elements in the proper order.
for (Node<E> p = first; p != null; p = p.next)
s.writeObject(p.item);
// Use trailing null as sentinel
s.writeObject(null);
} finally {
lock.unlock();
}
}

/**
* Reconstitute this deque from a stream (that is,
* deserialize it).
* @param s the stream
*/
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
s.defaultReadObject();
count = 0;
first = null;
last = null;
// Read in all elements and place in queue
for (;;) {
@SuppressWarnings("unchecked")
E item = (E)s.readObject();
if (item == null)
break;
add(item);
}
}

}

View Code

下面从ArrayBlockingQueue的创建，添加，取出，遍历这几个方面对LinkedBlockingDeque进行分析

1. 创建

下面以LinkedBlockingDeque(int capacity)来进行说明。

public LinkedBlockingDeque(int capacity) {
if (capacity <= 0) throw new IllegalArgumentException();
this.capacity = capacity;
}

说明：capacity是“链式阻塞队列”的容量。

LinkedBlockingDeque中相关的数据结果定义如下：

// “双向队列”的表头
transient Node<E> first;
// “双向队列”的表尾
transient Node<E> last;
// 节点数量
private transient int count;
// 容量
private final int capacity;
// 互斥锁 , 互斥锁对应的“非空条件notEmpty”, 互斥锁对应的“未满条件notFull”
final ReentrantLock lock = new ReentrantLock();
private final Condition notEmpty = lock.newCondition();
private final Condition notFull = lock.newCondition();

说明：lock是互斥锁，用于控制多线程对LinkedBlockingDeque中元素的互斥访问；而notEmpty和notFull是与lock绑定的条件，它们用于实现对多线程更精确的控制。

双向链表的节点Node的定义如下：

static final class Node<E> {
E item;       // 数据
Node<E> prev; // 前一节点
Node<E> next; // 后一节点

Node(E x) { item = x; }
}

2. 添加

下面以offer(E e)为例，对LinkedBlockingDeque的添加方法进行说明。

public boolean offer(E e) {
return offerLast(e);
}

offer()实际上是调用offerLast()将元素添加到队列的末尾。

offerLast()的源码如下：

public boolean offerLast(E e) {
if (e == null) throw new NullPointerException();
// 新建节点
Node<E> node = new Node<E>(e);
final ReentrantLock lock = this.lock;
// 获取锁
lock.lock();
try {
// 将“新节点”添加到双向链表的末尾
return linkLast(node);
} finally {
// 释放锁
lock.unlock();
}
}

说明：offerLast()的作用，是新建节点并将该节点插入到双向链表的末尾。它在插入节点前，会获取锁；操作完毕，再释放锁。

linkLast()的源码如下：

private boolean linkLast(Node<E> node) {
// 如果“双向链表的节点数量” > “容量”，则返回false，表示插入失败。
if (count >= capacity)
return false;
// 将“node添加到链表末尾”，并设置node为新的尾节点
Node<E> l = last;
node.prev = l;
last = node;
if (first == null)
first = node;
else
l.next = node;
// 将“节点数量”+1
++count;
// 插入节点之后，唤醒notEmpty上的等待线程。
notEmpty.signal();
return true;
}

说明：linkLast()的作用，是将节点插入到双向队列的末尾；插入节点之后，唤醒notEmpty上的等待线程。

3. 删除

下面以take()为例，对LinkedBlockingDeque的取出方法进行说明。

public E take() throws InterruptedException {
return takeFirst();
}

take()实际上是调用takeFirst()队列的第一个元素。

takeFirst()的源码如下：

public E takeFirst() throws InterruptedException {
final ReentrantLock lock = this.lock;
// 获取锁
lock.lock();
try {
E x;
// 若“队列为空”，则一直等待。否则，通过unlinkFirst()删除第一个节点。
while ( (x = unlinkFirst()) == null)
notEmpty.await();
return x;
} finally {
// 释放锁
lock.unlock();
}
}

说明：takeFirst()的作用，是删除双向链表的第一个节点，并返回节点对应的值。它在插入节点前，会获取锁；操作完毕，再释放锁。

unlinkFirst()的源码如下：

private E unlinkFirst() {
// assert lock.isHeldByCurrentThread();
Node<E> f = first;
if (f == null)
return null;
// 删除并更新“第一个节点”
Node<E> n = f.next;
E item = f.item;
f.item = null;
f.next = f; // help GC
first = n;
if (n == null)
last = null;
else
n.prev = null;
// 将“节点数量”-1
--count;
// 删除节点之后，唤醒notFull上的等待线程。
notFull.signal();
return item;
}

说明：unlinkFirst()的作用，是将双向队列的第一个节点删除；删除节点之后，唤醒notFull上的等待线程。

4. 遍历

下面对LinkedBlockingDeque的遍历方法进行说明。

public Iterator<E> iterator() {
return new Itr();
}

iterator()实际上是返回一个Iter对象。

Itr类的定义如下：

private class Itr extends AbstractItr {
// “双向队列”的表头
Node<E> firstNode() { return first; }
// 获取“节点n的下一个节点”
Node<E> nextNode(Node<E> n) { return n.next; }
}

Itr继承于AbstractItr，而AbstractItr的定义如下：

private abstract class AbstractItr implements Iterator<E> {
// next是下一次调用next()会返回的节点。
Node<E> next;
// nextItem是next()返回节点对应的数据。
E nextItem;
// 上一次next()返回的节点。
private Node<E> lastRet;
// 返回第一个节点
abstract Node<E> firstNode();
// 返回下一个节点
abstract Node<E> nextNode(Node<E> n);

AbstractItr() {
final ReentrantLock lock = LinkedBlockingDeque.this.lock;
// 获取“LinkedBlockingDeque的互斥锁”
lock.lock();
try {
// 获取“双向队列”的表头
next = firstNode();
// 获取表头对应的数据
nextItem = (next == null) ? null : next.item;
} finally {
// 释放“LinkedBlockingDeque的互斥锁”
lock.unlock();
}
}

// 获取n的后继节点
private Node<E> succ(Node<E> n) {
// Chains of deleted nodes ending in null or self-links
// are possible if multiple interior nodes are removed.
for (;;) {
Node<E> s = nextNode(n);
if (s == null)
return null;
else if (s.item != null)
return s;
else if (s == n)
return firstNode();
else
n = s;
}
}

// 更新next和nextItem。
void advance() {
final ReentrantLock lock = LinkedBlockingDeque.this.lock;
lock.lock();
try {
// assert next != null;
next = succ(next);
nextItem = (next == null) ? null : next.item;
} finally {
lock.unlock();
}
}

// 返回“下一个节点是否为null”
public boolean hasNext() {
return next != null;
}

// 返回下一个节点
public E next() {
if (next == null)
throw new NoSuchElementException();
lastRet = next;
E x = nextItem;
advance();
return x;
}

// 删除下一个节点
public void remove() {
Node<E> n = lastRet;
if (n == null)
throw new IllegalStateException();
lastRet = null;
final ReentrantLock lock = LinkedBlockingDeque.this.lock;
lock.lock();
try {
if (n.item != null)
unlink(n);
} finally {
lock.unlock();
}
}
}

LinkedBlockingDeque示例

import java.util.*;
import java.util.concurrent.*;

/*
*   LinkedBlockingDeque是“线程安全”的队列，而LinkedList是非线程安全的。
*
*   下面是“多个线程同时操作并且遍历queue”的示例
*   (01) 当queue是LinkedBlockingDeque对象时，程序能正常运行。
*   (02) 当queue是LinkedList对象时，程序会产生ConcurrentModificationException异常。
*
* @author skywang
*/
public class LinkedBlockingDequeDemo1 {

// TODO: queue是LinkedList对象时，程序会出错。
//private static Queue<String> queue = new LinkedList<String>();
private static Queue<String> queue = new LinkedBlockingDeque<String>();
public static void main(String[] args) {

// 同时启动两个线程对queue进行操作！
new MyThread("ta").start();
new MyThread("tb").start();
}

private static void printAll() {
String value;
Iterator iter = queue.iterator();
while(iter.hasNext()) {
value = (String)iter.next();
System.out.print(value+", ");
}
System.out.println();
}

private static class MyThread extends Thread {
MyThread(String name) {
super(name);
}
@Override
public void run() {
int i = 0;
while (i++ < 6) {
// “线程名” + "-" + "序号"
String val = Thread.currentThread().getName()+i;
queue.add(val);
// 通过“Iterator”遍历queue。
printAll();
}
}
}
}

(某一次)运行结果：

ta1, ta1, tb1, tb1,

ta1, ta1, tb1, tb1, tb2, tb2, ta2,
ta2,
ta1, ta1, tb1, tb1, tb2, tb2, ta2, ta2, tb3, tb3, ta3,
ta3, ta1,
tb1, ta1, tb2, tb1, ta2, tb2, tb3, ta2, ta3, tb3, tb4, ta3, ta4,
tb4, ta1, ta4, tb1, tb5,
tb2, ta1, ta2, tb1, tb3, tb2, ta3, ta2, tb4, tb3, ta4, ta3, tb5, tb4, ta5,
ta4, ta1, tb5, tb1, ta5, tb2, tb6,
ta2, ta1, tb3, tb1, ta3, tb2, tb4, ta2, ta4, tb3, tb5, ta3, ta5, tb4, tb6, ta4, ta6,
tb5, ta5, tb6, ta6,

结果说明：示例程序中，启动两个线程(线程ta和线程tb)分别对LinkedBlockingDeque进行操作。以线程ta而言，它会先获取“线程名”+“序号”，然后将该字符串添加到LinkedBlockingDeque中；接着，遍历并输出LinkedBlockingDeque中的全部元素。 线程tb的操作和线程ta一样，只不过线程tb的名字和线程ta的名字不同。
当queue是LinkedBlockingDeque对象时，程序能正常运行。如果将queue改为LinkedList时，程序会产生ConcurrentModificationException异常。

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