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概要

本章对Java.util.concurrent包中的ConcurrentHashMap类进行详细的介绍。内容包括:
ConcurrentLinkedQueue介绍
ConcurrentLinkedQueue原理和数据结构

ConcurrentLinkedQueue函数列表
ConcurrentLinkedQueue源码分析(JDK1.7.0_40版本)
ConcurrentLinkedQueue示例

转载请注明出处:http://www.cnblogs.com/skywang12345/p/3498995.html

 

ConcurrentLinkedQueue介绍

ConcurrentLinkedQueue是线程安全的队列,它适用于“高并发”的场景。
它是一个基于链接节点的无界线程安全队列,按照 FIFO(先进先出)原则对元素进行排序。队列元素中不可以放置null元素(内部实现的特殊节点除外)。

 

ConcurrentLinkedQueue原理和数据结构

ConcurrentLinkedQueue的数据结构,如下图所示:

说明
1. ConcurrentLinkedQueue继承于AbstractQueue。
2. ConcurrentLinkedQueue内部是通过链表来实现的。它同时包含链表的头节点head和尾节点tail。ConcurrentLinkedQueue按照 FIFO(先进先出)原则对元素进行排序。元素都是从尾部插入到链表,从头部开始返回。
3. ConcurrentLinkedQueue的链表Node中的next的类型是volatile,而且链表数据item的类型也是volatile。关于volatile,我们知道它的语义包含:“即对一个volatile变量的读,总是能看到(任意线程)对这个volatile变量最后的写入”。ConcurrentLinkedQueue就是通过volatile来实现多线程对竞争资源的互斥访问的。

 

ConcurrentLinkedQueue函数列表

// 创建一个最初为空的 ConcurrentLinkedQueue。
ConcurrentLinkedQueue()
// 创建一个最初包含给定 collection 元素的 ConcurrentLinkedQueue,按照此 collection 迭代器的遍历顺序来添加元素。
ConcurrentLinkedQueue(Collection<? extends E> c)

// 将指定元素插入此队列的尾部。
boolean add(E e)
// 如果此队列包含指定元素,则返回 true。
boolean contains(Object o)
// 如果此队列不包含任何元素,则返回 true。
boolean isEmpty()
// 返回在此队列元素上以恰当顺序进行迭代的迭代器。
Iterator<E> iterator()
// 将指定元素插入此队列的尾部。
boolean offer(E e)
// 获取但不移除此队列的头;如果此队列为空,则返回 null。
E peek()
// 获取并移除此队列的头,如果此队列为空,则返回 null。
E poll()
// 从队列中移除指定元素的单个实例(如果存在)。
boolean remove(Object o)
// 返回此队列中的元素数量。
int size()
// 返回以恰当顺序包含此队列所有元素的数组。
Object[] toArray()
// 返回以恰当顺序包含此队列所有元素的数组;返回数组的运行时类型是指定数组的运行时类型。
<T> T[] toArray(T[] a)

 

ConcurrentLinkedQueue源码分析(JDK1.7.0_40版本)

ConcurrentLinkedQueue的完整源码如下:

  1 /*
  2  * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
  3  *
  4  *
  5  *
  6  *
  7  *
  8  *
  9  *
 10  *
 11  *
 12  *
 13  *
 14  *
 15  *
 16  *
 17  *
 18  *
 19  *
 20  *
 21  *
 22  *
 23  */
 24 
 25 /*
 26  *
 27  *
 28  *
 29  *
 30  *
 31  * Written by Doug Lea and Martin Buchholz with assistance from members of
 32  * JCP JSR-166 Expert Group and released to the public domain, as explained
 33  * at http://creativecommons.org/publicdomain/zero/1.0/
 34  */
 35 
 36 package java.util.concurrent;
 37 
 38 import java.util.AbstractQueue;
 39 import java.util.ArrayList;
 40 import java.util.Collection;
 41 import java.util.Iterator;
 42 import java.util.NoSuchElementException;
 43 import java.util.Queue;
 44 
 45 /**
 46  * An unbounded thread-safe {@linkplain Queue queue} based on linked nodes.
 47  * This queue orders elements FIFO (first-in-first-out).
 48  * The <em>head</em> of the queue is that element that has been on the
 49  * queue the longest time.
 50  * The <em>tail</em> of the queue is that element that has been on the
 51  * queue the shortest time. New elements
 52  * are inserted at the tail of the queue, and the queue retrieval
 53  * operations obtain elements at the head of the queue.
 54  * A {@code ConcurrentLinkedQueue} is an appropriate choice when
 55  * many threads will share access to a common collection.
 56  * Like most other concurrent collection implementations, this class
 57  * does not permit the use of {@code null} elements.
 58  *
 59  * <p>This implementation employs an efficient &quot;wait-free&quot;
 60  * algorithm based on one described in <a
 61  * href="http://www.cs.rochester.edu/u/michael/PODC96.html"> Simple,
 62  * Fast, and Practical Non-Blocking and Blocking Concurrent Queue
 63  * Algorithms</a> by Maged M. Michael and Michael L. Scott.
 64  *
 65  * <p>Iterators are <i>weakly consistent</i>, returning elements
 66  * reflecting the state of the queue at some point at or since the
 67  * creation of the iterator.  They do <em>not</em> throw {@link
 68  * java.util.ConcurrentModificationException}, and may proceed concurrently
 69  * with other operations.  Elements contained in the queue since the creation
 70  * of the iterator will be returned exactly once.
 71  *
 72  * <p>Beware that, unlike in most collections, the {@code size} method
 73  * is <em>NOT</em> a constant-time operation. Because of the
 74  * asynchronous nature of these queues, determining the current number
 75  * of elements requires a traversal of the elements, and so may report
 76  * inaccurate results if this collection is modified during traversal.
 77  * Additionally, the bulk operations {@code addAll},
 78  * {@code removeAll}, {@code retainAll}, {@code containsAll},
 79  * {@code equals}, and {@code toArray} are <em>not</em> guaranteed
 80  * to be performed atomically. For example, an iterator operating
 81  * concurrently with an {@code addAll} operation might view only some
 82  * of the added elements.
 83  *
 84  * <p>This class and its iterator implement all of the <em>optional</em>
 85  * methods of the {@link Queue} and {@link Iterator} interfaces.
 86  *
 87  * <p>Memory consistency effects: As with other concurrent
 88  * collections, actions in a thread prior to placing an object into a
 89  * {@code ConcurrentLinkedQueue}
 90  * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
 91  * actions subsequent to the access or removal of that element from
 92  * the {@code ConcurrentLinkedQueue} in another thread.
 93  *
 94  * <p>This class is a member of the
 95  * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 96  * Java Collections Framework</a>.
 97  *
 98  * @since 1.5
 99  * @author Doug Lea
100  * @param <E> the type of elements held in this collection
101  *
102  */
103 public class ConcurrentLinkedQueue<E> extends AbstractQueue<E>
104         implements Queue<E>, java.io.Serializable {
105     private static final long serialVersionUID = 196745693267521676L;
106 
107     /*
108      * This is a modification of the Michael & Scott algorithm,
109      * adapted for a garbage-collected environment, with support for
110      * interior node deletion (to support remove(Object)).  For
111      * explanation, read the paper.
112      *
113      * Note that like most non-blocking algorithms in this package,
114      * this implementation relies on the fact that in garbage
115      * collected systems, there is no possibility of ABA problems due
116      * to recycled nodes, so there is no need to use "counted
117      * pointers" or related techniques seen in versions used in
118      * non-GC'ed settings.
119      *
120      * The fundamental invariants are:
121      * - There is exactly one (last) Node with a null next reference,
122      *   which is CASed when enqueueing.  This last Node can be
123      *   reached in O(1) time from tail, but tail is merely an
124      *   optimization - it can always be reached in O(N) time from
125      *   head as well.
126      * - The elements contained in the queue are the non-null items in
127      *   Nodes that are reachable from head.  CASing the item
128      *   reference of a Node to null atomically removes it from the
129      *   queue.  Reachability of all elements from head must remain
130      *   true even in the case of concurrent modifications that cause
131      *   head to advance.  A dequeued Node may remain in use
132      *   indefinitely due to creation of an Iterator or simply a
133      *   poll() that has lost its time slice.
134      *
135      * The above might appear to imply that all Nodes are GC-reachable
136      * from a predecessor dequeued Node.  That would cause two problems:
137      * - allow a rogue Iterator to cause unbounded memory retention
138      * - cause cross-generational linking of old Nodes to new Nodes if
139      *   a Node was tenured while live, which generational GCs have a
140      *   hard time dealing with, causing repeated major collections.
141      * However, only non-deleted Nodes need to be reachable from
142      * dequeued Nodes, and reachability does not necessarily have to
143      * be of the kind understood by the GC.  We use the trick of
144      * linking a Node that has just been dequeued to itself.  Such a
145      * self-link implicitly means to advance to head.
146      *
147      * Both head and tail are permitted to lag.  In fact, failing to
148      * update them every time one could is a significant optimization
149      * (fewer CASes). As with LinkedTransferQueue (see the internal
150      * documentation for that class), we use a slack threshold of two;
151      * that is, we update head/tail when the current pointer appears
152      * to be two or more steps away from the first/last node.
153      *
154      * Since head and tail are updated concurrently and independently,
155      * it is possible for tail to lag behind head (why not)?
156      *
157      * CASing a Node's item reference to null atomically removes the
158      * element from the queue.  Iterators skip over Nodes with null
159      * items.  Prior implementations of this class had a race between
160      * poll() and remove(Object) where the same element would appear
161      * to be successfully removed by two concurrent operations.  The
162      * method remove(Object) also lazily unlinks deleted Nodes, but
163      * this is merely an optimization.
164      *
165      * When constructing a Node (before enqueuing it) we avoid paying
166      * for a volatile write to item by using Unsafe.putObject instead
167      * of a normal write.  This allows the cost of enqueue to be
168      * "one-and-a-half" CASes.
169      *
170      * Both head and tail may or may not point to a Node with a
171      * non-null item.  If the queue is empty, all items must of course
172      * be null.  Upon creation, both head and tail refer to a dummy
173      * Node with null item.  Both head and tail are only updated using
174      * CAS, so they never regress, although again this is merely an
175      * optimization.
176      */
177 
178     private static class Node<E> {
179         volatile E item;
180         volatile Node<E> next;
181 
182         /**
183          * Constructs a new node.  Uses relaxed write because item can
184          * only be seen after publication via casNext.
185          */
186         Node(E item) {
187             UNSAFE.putObject(this, itemOffset, item);
188         }
189 
190         boolean casItem(E cmp, E val) {
191             return UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val);
192         }
193 
194         void lazySetNext(Node<E> val) {
195             UNSAFE.putOrderedObject(this, nextOffset, val);
196         }
197 
198         boolean casNext(Node<E> cmp, Node<E> val) {
199             return UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val);
200         }
201 
202         // Unsafe mechanics
203 
204         private static final sun.misc.Unsafe UNSAFE;
205         private static final long itemOffset;
206         private static final long nextOffset;
207 
208         static {
209             try {
210                 UNSAFE = sun.misc.Unsafe.getUnsafe();
211                 Class k = Node.class;
212                 itemOffset = UNSAFE.objectFieldOffset
213                     (k.getDeclaredField("item"));
214                 nextOffset = UNSAFE.objectFieldOffset
215                     (k.getDeclaredField("next"));
216             } catch (Exception e) {
217                 throw new Error(e);
218             }
219         }
220     }
221 
222     /**
223      * A node from which the first live (non-deleted) node (if any)
224      * can be reached in O(1) time.
225      * Invariants:
226      * - all live nodes are reachable from head via succ()
227      * - head != null
228      * - (tmp = head).next != tmp || tmp != head
229      * Non-invariants:
230      * - head.item may or may not be null.
231      * - it is permitted for tail to lag behind head, that is, for tail
232      *   to not be reachable from head!
233      */
234     private transient volatile Node<E> head;
235 
236     /**
237      * A node from which the last node on list (that is, the unique
238      * node with node.next == null) can be reached in O(1) time.
239      * Invariants:
240      * - the last node is always reachable from tail via succ()
241      * - tail != null
242      * Non-invariants:
243      * - tail.item may or may not be null.
244      * - it is permitted for tail to lag behind head, that is, for tail
245      *   to not be reachable from head!
246      * - tail.next may or may not be self-pointing to tail.
247      */
248     private transient volatile Node<E> tail;
249 
250 
251     /**
252      * Creates a {@code ConcurrentLinkedQueue} that is initially empty.
253      */
254     public ConcurrentLinkedQueue() {
255         head = tail = new Node<E>(null);
256     }
257 
258     /**
259      * Creates a {@code ConcurrentLinkedQueue}
260      * initially containing the elements of the given collection,
261      * added in traversal order of the collection's iterator.
262      *
263      * @param c the collection of elements to initially contain
264      * @throws NullPointerException if the specified collection or any
265      *         of its elements are null
266      */
267     public ConcurrentLinkedQueue(Collection<? extends E> c) {
268         Node<E> h = null, t = null;
269         for (E e : c) {
270             checkNotNull(e);
271             Node<E> newNode = new Node<E>(e);
272             if (h == null)
273                 h = t = newNode;
274             else {
275                 t.lazySetNext(newNode);
276                 t = newNode;
277             }
278         }
279         if (h == null)
280             h = t = new Node<E>(null);
281         head = h;
282         tail = t;
283     }
284 
285     // Have to override just to update the javadoc
286 
287     /**
288      * Inserts the specified element at the tail of this queue.
289      * As the queue is unbounded, this method will never throw
290      * {@link IllegalStateException} or return {@code false}.
291      *
292      * @return {@code true} (as specified by {@link Collection#add})
293      * @throws NullPointerException if the specified element is null
294      */
295     public boolean add(E e) {
296         return offer(e);
297     }
298 
299     /**
300      * Try to CAS head to p. If successful, repoint old head to itself
301      * as sentinel for succ(), below.
302      */
303     final void updateHead(Node<E> h, Node<E> p) {
304         if (h != p && casHead(h, p))
305             h.lazySetNext(h);
306     }
307 
308     /**
309      * Returns the successor of p, or the head node if p.next has been
310      * linked to self, which will only be true if traversing with a
311      * stale pointer that is now off the list.
312      */
313     final Node<E> succ(Node<E> p) {
314         Node<E> next = p.next;
315         return (p == next) ? head : next;
316     }
317 
318     /**
319      * Inserts the specified element at the tail of this queue.
320      * As the queue is unbounded, this method will never return {@code false}.
321      *
322      * @return {@code true} (as specified by {@link Queue#offer})
323      * @throws NullPointerException if the specified element is null
324      */
325     public boolean offer(E e) {
326         checkNotNull(e);
327         final Node<E> newNode = new Node<E>(e);
328 
329         for (Node<E> t = tail, p = t;;) {
330             Node<E> q = p.next;
331             if (q == null) {
332                 // p is last node
333                 if (p.casNext(null, newNode)) {
334                     // Successful CAS is the linearization point
335                     // for e to become an element of this queue,
336                     // and for newNode to become "live".
337                     if (p != t) // hop two nodes at a time
338                         casTail(t, newNode);  // Failure is OK.
339                     return true;
340                 }
341                 // Lost CAS race to another thread; re-read next
342             }
343             else if (p == q)
344                 // We have fallen off list.  If tail is unchanged, it
345                 // will also be off-list, in which case we need to
346                 // jump to head, from which all live nodes are always
347                 // reachable.  Else the new tail is a better bet.
348                 p = (t != (t = tail)) ? t : head;
349             else
350                 // Check for tail updates after two hops.
351                 p = (p != t && t != (t = tail)) ? t : q;
352         }
353     }
354 
355     public E poll() {
356         restartFromHead:
357         for (;;) {
358             for (Node<E> h = head, p = h, q;;) {
359                 E item = p.item;
360 
361                 if (item != null && p.casItem(item, null)) {
362                     // Successful CAS is the linearization point
363                     // for item to be removed from this queue.
364                     if (p != h) // hop two nodes at a time
365                         updateHead(h, ((q = p.next) != null) ? q : p);
366                     return item;
367                 }
368                 else if ((q = p.next) == null) {
369                     updateHead(h, p);
370                     return null;
371                 }
372                 else if (p == q)
373                     continue restartFromHead;
374                 else
375                     p = q;
376             }
377         }
378     }
379 
380     public E peek() {
381         restartFromHead:
382         for (;;) {
383             for (Node<E> h = head, p = h, q;;) {
384                 E item = p.item;
385                 if (item != null || (q = p.next) == null) {
386                     updateHead(h, p);
387                     return item;
388                 }
389                 else if (p == q)
390                     continue restartFromHead;
391                 else
392                     p = q;
393             }
394         }
395     }
396 
397     /**
398      * Returns the first live (non-deleted) node on list, or null if none.
399      * This is yet another variant of poll/peek; here returning the
400      * first node, not element.  We could make peek() a wrapper around
401      * first(), but that would cost an extra volatile read of item,
402      * and the need to add a retry loop to deal with the possibility
403      * of losing a race to a concurrent poll().
404      */
405     Node<E> first() {
406         restartFromHead:
407         for (;;) {
408             for (Node<E> h = head, p = h, q;;) {
409                 boolean hasItem = (p.item != null);
410                 if (hasItem || (q = p.next) == null) {
411                     updateHead(h, p);
412                     return hasItem ? p : null;
413                 }
414                 else if (p == q)
415                     continue restartFromHead;
416                 else
417                     p = q;
418             }
419         }
420     }
421 
422     /**
423      * Returns {@code true} if this queue contains no elements.
424      *
425      * @return {@code true} if this queue contains no elements
426      */
427     public boolean isEmpty() {
428         return first() == null;
429     }
430 
431     /**
432      * Returns the number of elements in this queue.  If this queue
433      * contains more than {@code Integer.MAX_VALUE} elements, returns
434      * {@code Integer.MAX_VALUE}.
435      *
436      * <p>Beware that, unlike in most collections, this method is
437      * <em>NOT</em> a constant-time operation. Because of the
438      * asynchronous nature of these queues, determining the current
439      * number of elements requires an O(n) traversal.
440      * Additionally, if elements are added or removed during execution
441      * of this method, the returned result may be inaccurate.  Thus,
442      * this method is typically not very useful in concurrent
443      * applications.
444      *
445      * @return the number of elements in this queue
446      */
447     public int size() {
448         int count = 0;
449         for (Node<E> p = first(); p != null; p = succ(p))
450             if (p.item != null)
451                 // Collection.size() spec says to max out
452                 if (++count == Integer.MAX_VALUE)
453                     break;
454         return count;
455     }
456 
457     /**
458      * Returns {@code true} if this queue contains the specified element.
459      * More formally, returns {@code true} if and only if this queue contains
460      * at least one element {@code e} such that {@code o.equals(e)}.
461      *
462      * @param o object to be checked for containment in this queue
463      * @return {@code true} if this queue contains the specified element
464      */
465     public boolean contains(Object o) {
466         if (o == null) return false;
467         for (Node<E> p = first(); p != null; p = succ(p)) {
468             E item = p.item;
469             if (item != null && o.equals(item))
470                 return true;
471         }
472         return false;
473     }
474 
475     /**
476      * Removes a single instance of the specified element from this queue,
477      * if it is present.  More formally, removes an element {@code e} such
478      * that {@code o.equals(e)}, if this queue contains one or more such
479      * elements.
480      * Returns {@code true} if this queue contained the specified element
481      * (or equivalently, if this queue changed as a result of the call).
482      *
483      * @param o element to be removed from this queue, if present
484      * @return {@code true} if this queue changed as a result of the call
485      */
486     public boolean remove(Object o) {
487         if (o == null) return false;
488         Node<E> pred = null;
489         for (Node<E> p = first(); p != null; p = succ(p)) {
490             E item = p.item;
491             if (item != null &&
492                 o.equals(item) &&
493                 p.casItem(item, null)) {
494                 Node<E> next = succ(p);
495                 if (pred != null && next != null)
496                     pred.casNext(p, next);
497                 return true;
498             }
499             pred = p;
500         }
501         return false;
502     }
503 
504     /**
505      * Appends all of the elements in the specified collection to the end of
506      * this queue, in the order that they are returned by the specified
507      * collection's iterator.  Attempts to {@code addAll} of a queue to
508      * itself result in {@code IllegalArgumentException}.
509      *
510      * @param c the elements to be inserted into this queue
511      * @return {@code true} if this queue changed as a result of the call
512      * @throws NullPointerException if the specified collection or any
513      *         of its elements are null
514      * @throws IllegalArgumentException if the collection is this queue
515      */
516     public boolean addAll(Collection<? extends E> c) {
517         if (c == this)
518             // As historically specified in AbstractQueue#addAll
519             throw new IllegalArgumentException();
520 
521         // Copy c into a private chain of Nodes
522         Node<E> beginningOfTheEnd = null, last = null;
523         for (E e : c) {
524             checkNotNull(e);
525             Node<E> newNode = new Node<E>(e);
526             if (beginningOfTheEnd == null)
527                 beginningOfTheEnd = last = newNode;
528             else {
529                 last.lazySetNext(newNode);
530                 last = newNode;
531             }
532         }
533         if (beginningOfTheEnd == null)
534             return false;
535 
536         // Atomically append the chain at the tail of this collection
537         for (Node<E> t = tail, p = t;;) {
538             Node<E> q = p.next;
539             if (q == null) {
540                 // p is last node
541                 if (p.casNext(null, beginningOfTheEnd)) {
542                     // Successful CAS is the linearization point
543                     // for all elements to be added to this queue.
544                     if (!casTail(t, last)) {
545                         // Try a little harder to update tail,
546                         // since we may be adding many elements.
547                         t = tail;
548                         if (last.next == null)
549                             casTail(t, last);
550                     }
551                     return true;
552                 }
553                 // Lost CAS race to another thread; re-read next
554             }
555             else if (p == q)
556                 // We have fallen off list.  If tail is unchanged, it
557                 // will also be off-list, in which case we need to
558                 // jump to head, from which all live nodes are always
559                 // reachable.  Else the new tail is a better bet.
560                 p = (t != (t = tail)) ? t : head;
561             else
562                 // Check for tail updates after two hops.
563                 p = (p != t && t != (t = tail)) ? t : q;
564         }
565     }
566 
567     /**
568      * Returns an array containing all of the elements in this queue, in
569      * proper sequence.
570      *
571      * <p>The returned array will be "safe" in that no references to it are
572      * maintained by this queue.  (In other words, this method must allocate
573      * a new array).  The caller is thus free to modify the returned array.
574      *
575      * <p>This method acts as bridge between array-based and collection-based
576      * APIs.
577      *
578      * @return an array containing all of the elements in this queue
579      */
580     public Object[] toArray() {
581         // Use ArrayList to deal with resizing.
582         ArrayList<E> al = new ArrayList<E>();
583         for (Node<E> p = first(); p != null; p = succ(p)) {
584             E item = p.item;
585             if (item != null)
586                 al.add(item);
587         }
588         return al.toArray();
589     }
590 
591     /**
592      * Returns an array containing all of the elements in this queue, in
593      * proper sequence; the runtime type of the returned array is that of
594      * the specified array.  If the queue fits in the specified array, it
595      * is returned therein.  Otherwise, a new array is allocated with the
596      * runtime type of the specified array and the size of this queue.
597      *
598      * <p>If this queue fits in the specified array with room to spare
599      * (i.e., the array has more elements than this queue), the element in
600      * the array immediately following the end of the queue is set to
601      * {@code null}.
602      *
603      * <p>Like the {@link #toArray()} method, this method acts as bridge between
604      * array-based and collection-based APIs.  Further, this method allows
605      * precise control over the runtime type of the output array, and may,
606      * under certain circumstances, be used to save allocation costs.
607      *
608      * <p>Suppose {@code x} is a queue known to contain only strings.
609      * The following code can be used to dump the queue into a newly
610      * allocated array of {@code String}:
611      *
612      * <pre>
613      *     String[] y = x.toArray(new String[0]);</pre>
614      *
615      * Note that {@code toArray(new Object[0])} is identical in function to
616      * {@code toArray()}.
617      *
618      * @param a the array into which the elements of the queue are to
619      *          be stored, if it is big enough; otherwise, a new array of the
620      *          same runtime type is allocated for this purpose
621      * @return an array containing all of the elements in this queue
622      * @throws ArrayStoreException if the runtime type of the specified array
623      *         is not a supertype of the runtime type of every element in
624      *         this queue
625      * @throws NullPointerException if the specified array is null
626      */
627     @SuppressWarnings("unchecked")
628     public <T> T[] toArray(T[] a) {
629         // try to use sent-in array
630         int k = 0;
631         Node<E> p;
632         for (p = first(); p != null && k < a.length; p = succ(p)) {
633             E item = p.item;
634             if (item != null)
635                 a[k++] = (T)item;
636         }
637         if (p == null) {
638             if (k < a.length)
639                 a[k] = null;
640             return a;
641         }
642 
643         // If won't fit, use ArrayList version
644         ArrayList<E> al = new ArrayList<E>();
645         for (Node<E> q = first(); q != null; q = succ(q)) {
646             E item = q.item;
647             if (item != null)
648                 al.add(item);
649         }
650         return al.toArray(a);
651     }
652 
653     /**
654      * Returns an iterator over the elements in this queue in proper sequence.
655      * The elements will be returned in order from first (head) to last (tail).
656      *
657      * <p>The returned iterator is a "weakly consistent" iterator that
658      * will never throw {@link java.util.ConcurrentModificationException
659      * ConcurrentModificationException}, and guarantees to traverse
660      * elements as they existed upon construction of the iterator, and
661      * may (but is not guaranteed to) reflect any modifications
662      * subsequent to construction.
663      *
664      * @return an iterator over the elements in this queue in proper sequence
665      */
666     public Iterator<E> iterator() {
667         return new Itr();
668     }
669 
670     private class Itr implements Iterator<E> {
671         /**
672          * Next node to return item for.
673          */
674         private Node<E> nextNode;
675 
676         /**
677          * nextItem holds on to item fields because once we claim
678          * that an element exists in hasNext(), we must return it in
679          * the following next() call even if it was in the process of
680          * being removed when hasNext() was called.
681          */
682         private E nextItem;
683 
684         /**
685          * Node of the last returned item, to support remove.
686          */
687         private Node<E> lastRet;
688 
689         Itr() {
690             advance();
691         }
692 
693         /**
694          * Moves to next valid node and returns item to return for
695          * next(), or null if no such.
696          */
697         private E advance() {
698             lastRet = nextNode;
699             E x = nextItem;
700 
701             Node<E> pred, p;
702             if (nextNode == null) {
703                 p = first();
704                 pred = null;
705             } else {
706                 pred = nextNode;
707                 p = succ(nextNode);
708             }
709 
710             for (;;) {
711                 if (p == null) {
712                     nextNode = null;
713                     nextItem = null;
714                     return x;
715                 }
716                 E item = p.item;
717                 if (item != null) {
718                     nextNode = p;
719                     nextItem = item;
720                     return x;
721                 } else {
722                     // skip over nulls
723                     Node<E> next = succ(p);
724                     if (pred != null && next != null)
725                         pred.casNext(p, next);
726                     p = next;
727                 }
728             }
729         }
730 
731         public boolean hasNext() {
732             return nextNode != null;
733         }
734 
735         public E next() {
736             if (nextNode == null) throw new NoSuchElementException();
737             return advance();
738         }
739 
740         public void remove() {
741             Node<E> l = lastRet;
742             if (l == null) throw new IllegalStateException();
743             // rely on a future traversal to relink.
744             l.item = null;
745             lastRet = null;
746         }
747     }
748 
749     /**
750      * Saves the state to a stream (that is, serializes it).
751      *
752      * @serialData All of the elements (each an {@code E}) in
753      * the proper order, followed by a null
754      * @param s the stream
755      */
756     private void writeObject(java.io.ObjectOutputStream s)
757         throws java.io.IOException {
758 
759         // Write out any hidden stuff
760         s.defaultWriteObject();
761 
762         // Write out all elements in the proper order.
763         for (Node<E> p = first(); p != null; p = succ(p)) {
764             Object item = p.item;
765             if (item != null)
766                 s.writeObject(item);
767         }
768 
769         // Use trailing null as sentinel
770         s.writeObject(null);
771     }
772 
773     /**
774      * Reconstitutes the instance from a stream (that is, deserializes it).
775      * @param s the stream
776      */
777     private void readObject(java.io.ObjectInputStream s)
778         throws java.io.IOException, ClassNotFoundException {
779         s.defaultReadObject();
780 
781         // Read in elements until trailing null sentinel found
782         Node<E> h = null, t = null;
783         Object item;
784         while ((item = s.readObject()) != null) {
785             @SuppressWarnings("unchecked")
786             Node<E> newNode = new Node<E>((E) item);
787             if (h == null)
788                 h = t = newNode;
789             else {
790                 t.lazySetNext(newNode);
791                 t = newNode;
792             }
793         }
794         if (h == null)
795             h = t = new Node<E>(null);
796         head = h;
797         tail = t;
798     }
799 
800     /**
801      * Throws NullPointerException if argument is null.
802      *
803      * @param v the element
804      */
805     private static void checkNotNull(Object v) {
806         if (v == null)
807             throw new NullPointerException();
808     }
809 
810     private boolean casTail(Node<E> cmp, Node<E> val) {
811         return UNSAFE.compareAndSwapObject(this, tailOffset, cmp, val);
812     }
813 
814     private boolean casHead(Node<E> cmp, Node<E> val) {
815         return UNSAFE.compareAndSwapObject(this, headOffset, cmp, val);
816     }
817 
818     // Unsafe mechanics
819 
820     private static final sun.misc.Unsafe UNSAFE;
821     private static final long headOffset;
822     private static final long tailOffset;
823     static {
824         try {
825             UNSAFE = sun.misc.Unsafe.getUnsafe();
826             Class k = ConcurrentLinkedQueue.class;
827             headOffset = UNSAFE.objectFieldOffset
828                 (k.getDeclaredField("head"));
829             tailOffset = UNSAFE.objectFieldOffset
830                 (k.getDeclaredField("tail"));
831         } catch (Exception e) {
832             throw new Error(e);
833         }
834     }
835 }
View Code

 

下面从ConcurrentLinkedQueue的创建,添加,删除这几个方面对它进行分析。

1 创建

下面以ConcurrentLinkedQueue()来进行说明。

public ConcurrentLinkedQueue() {
    head = tail = new Node<E>(null);
}

说明:在构造函数中,新建了一个“内容为null的节点”,并设置表头head和表尾tail的值为新节点。

head和tail的定义如下:

private transient volatile Node<E> head;
private transient volatile Node<E> tail;

head和tail都是volatile类型,他们具有volatile赋予的含义:“即对一个volatile变量的读,总是能看到(任意线程)对这个volatile变量最后的写入”。


Node的声明如下:

private static class Node<E> {
    volatile E item;
    volatile Node<E> next;

    Node(E item) {
        UNSAFE.putObject(this, itemOffset, item);
    }

    boolean casItem(E cmp, E val) {
        return UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val);
    }

    void lazySetNext(Node<E> val) {
        UNSAFE.putOrderedObject(this, nextOffset, val);
    }

    boolean casNext(Node<E> cmp, Node<E> val) {
        return UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val);
    }

    // Unsafe mechanics
    private static final sun.misc.Unsafe UNSAFE;
    private static final long itemOffset;
    private static final long nextOffset;

    static {
        try {
            UNSAFE = sun.misc.Unsafe.getUnsafe();
            Class k = Node.class;
            itemOffset = UNSAFE.objectFieldOffset
                (k.getDeclaredField("item"));
            nextOffset = UNSAFE.objectFieldOffset
                (k.getDeclaredField("next"));
        } catch (Exception e) {
            throw new Error(e);
        }
    }
}

说明
Node是个单向链表节点,next用于指向下一个Node,item用于存储数据。Node中操作节点数据的API,都是通过Unsafe机制的CAS函数实现的;例如casNext()是通过CAS函数“比较并设置节点的下一个节点”。

 

2. 添加

下面以add(E e)为例对ConcurrentLinkedQueue中的添加进行说明。

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

说明:add()实际上是调用的offer()来完成添加操作的。

offer()的源码如下:

public boolean offer(E e) {
    // 检查e是不是null,是的话抛出NullPointerException异常。
    checkNotNull(e);
    // 创建新的节点
    final Node<E> newNode = new Node<E>(e);

    // 将“新的节点”添加到链表的末尾。
    for (Node<E> t = tail, p = t;;) {
        Node<E> q = p.next;
        // 情况1:q为空
        if (q == null) {
            // CAS操作:如果“p的下一个节点为null”(即p为尾节点),则设置p的下一个节点为newNode。
            // 如果该CAS操作成功的话,则比较“p和t”(若p不等于t,则设置newNode为新的尾节点),然后返回true。
            // 如果该CAS操作失败,这意味着“其它线程对尾节点进行了修改”,则重新循环。
            if (p.casNext(null, newNode)) {
                if (p != t) // hop two nodes at a time
                    casTail(t, newNode);  // Failure is OK.
                return true;
            }
        }
        // 情况2:p和q相等
        else if (p == q)
            p = (t != (t = tail)) ? t : head;
        // 情况3:其它
        else
            p = (p != t && t != (t = tail)) ? t : q;
    }
}

说明offer(E e)的作用就是将元素e添加到链表的末尾。offer()比较的地方是理解for循环,下面区分3种情况对for进行分析。

情况1 -- q为空。这意味着q是尾节点的下一个节点。此时,通过p.casNext(null, newNode)将“p的下一个节点设为newNode”,若设置成功的话,则比较“p和t”(若p不等于t,则设置newNode为新的尾节点),然后返回true。否则的话(意味着“其它线程对尾节点进行了修改”),什么也不做,继续进行for循环。
p.casNext(null, newNode),是调用CAS对p进行操作。若“p的下一个节点等于null”,则设置“p的下一个节点等于newNode”;设置成功的话,返回true,失败的话返回false。

情况2 -- p和q相等。这种情况什么时候会发生呢?通过“情况3”,我们知道,经过“情况3”的处理后,p的值可能等于q。
此时,若尾节点没有发生变化的话,那么,应该是头节点发生了变化,则设置p为头节点,然后重新遍历链表;否则(尾节点变化的话),则设置p为尾节点。

情况3 -- 其它。
我们将p = (p != t && t != (t = tail)) ? t : q;转换成如下代码。

if (p==t) {
    p = q;
} else {
    Node<E> tmp=t;
    t = tail;
    if (tmp==t) {
        p=q;
    } else {
        p=t;
    }
}

如果p和t相等,则设置p为q。否则的话,判断“尾节点是否发生变化”,没有变化的话,则设置p为q;否则,设置p为尾节点。


checkNotNull()的源码如下:

private static void checkNotNull(Object v) {
    if (v == null)
        throw new NullPointerException();
}

 

3. 删除

下面以poll()为例对ConcurrentLinkedQueue中的删除进行说明。

public E poll() {
    // 设置“标记”
    restartFromHead:
    for (;;) {
        for (Node<E> h = head, p = h, q;;) {
            E item = p.item;

            // 情况1
            // 表头的数据不为null,并且“设置表头的数据为null”这个操作成功的话;
            // 则比较“p和h”(若p!=h,即表头发生了变化,则更新表头,即设置表头为p),然后返回原表头的item值。
            if (item != null && p.casItem(item, null)) {
                if (p != h) // hop two nodes at a time
                    updateHead(h, ((q = p.next) != null) ? q : p);
                return item;
            }
            // 情况2
            // 表头的下一个节点为null,即链表只有一个“内容为null的表头节点”。则更新表头为p,并返回null。
            else if ((q = p.next) == null) {
                updateHead(h, p);
                return null;
            }
            // 情况3
            // 这可能到由于“情况4”的发生导致p=q,在该情况下跳转到restartFromHead标记重新操作。
            else if (p == q)
                continue restartFromHead;
            // 情况4
            // 设置p为q
            else
                p = q;
        }
    }
}

说明poll()的作用就是删除链表的表头节点,并返回被删节点对应的值。poll()的实现原理和offer()比较类似,下面根将or循环划分为4种情况进行分析。

情况1:“表头节点的数据”不为null,并且“设置表头节点的数据为null”这个操作成功。
p.casItem(item, null) -- 调用CAS函数,比较“节点p的数据值”与item是否相等,是的话,设置节点p的数据值为null。
在情况1发生时,先比较“p和h”,若p!=h,即表头发生了变化,则调用updateHead()更新表头;然后返回删除节点的item值。
updateHead()的源码如下:

final void updateHead(Node<E> h, Node<E> p) {
    if (h != p && casHead(h, p))
        h.lazySetNext(h);
}

说明:updateHead()的最终目的是更新表头为p,并设置h的下一个节点为h本身。
casHead(h,p)是通过CAS函数设置表头,若表头等于h的话,则设置表头为p。
lazySetNext()的源码如下:

void lazySetNext(Node<E> val) {
    UNSAFE.putOrderedObject(this, nextOffset, val);
}

putOrderedObject()函数,我们在前面一章“TODO”中介绍过。h.lazySetNext(h)的作用是通过CAS函数设置h的下一个节点为h自身,该设置可能会延迟执行。

情况2:如果表头的下一个节点为null,即链表只有一个“内容为null的表头节点”。
则调用updateHead(h, p),将表头更新p;然后返回null。

情况3:p=q
在“情况4”的发生后,会导致p=q;此时,“情况3”就会发生。当“情况3”发生后,它会跳转到restartFromHead标记重新操作。

情况4:其它情况。
设置p=q。

 

ConcurrentLinkedQueue示例

 1 import java.util.*;
 2 import java.util.concurrent.*;
 3 
 4 /*
 5  *   ConcurrentLinkedQueue是“线程安全”的队列,而LinkedList是非线程安全的。
 6  *
 7  *   下面是“多个线程同时操作并且遍历queue”的示例
 8  *   (01) 当queue是ConcurrentLinkedQueue对象时,程序能正常运行。
 9  *   (02) 当queue是LinkedList对象时,程序会产生ConcurrentModificationException异常。
10  *
11  * @author skywang
12  */
13 public class ConcurrentLinkedQueueDemo1 {
14 
15     // TODO: queue是LinkedList对象时,程序会出错。
16     //private static Queue<String> queue = new LinkedList<String>();
17     private static Queue<String> queue = new ConcurrentLinkedQueue<String>();
18     public static void main(String[] args) {
19     
20         // 同时启动两个线程对queue进行操作!
21         new MyThread("ta").start();
22         new MyThread("tb").start();
23     }
24 
25     private static void printAll() {
26         String value;
27         Iterator iter = queue.iterator();
28         while(iter.hasNext()) {
29             value = (String)iter.next();
30             System.out.print(value+", ");
31         }
32         System.out.println();
33     }
34 
35     private static class MyThread extends Thread {
36         MyThread(String name) {
37             super(name);
38         }
39         @Override
40         public void run() {
41                 int i = 0;
42             while (i++ < 6) {
43                 // “线程名” + "-" + "序号"
44                 String val = Thread.currentThread().getName()+i;
45                 queue.add(val);
46                 // 通过“Iterator”遍历queue。
47                 printAll();
48             }
49         }
50     }
51 }

(某一次)运行结果

ta1, ta1, tb1, tb1,

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

结果说明如果将源码中的queue改成LinkedList对象时,程序会产生ConcurrentModificationException异常。

 


更多内容

1. Java多线程系列--“JUC集合”01之 框架 

2. Java多线程系列目录(共xx篇)

 

posted on 2014-02-03 20:04 如果天空不死 阅读(...) 评论(...) 编辑 收藏