1 |
/* |
2 |
* Written by Doug Lea and Martin Buchholz with assistance from members of |
3 |
* JCP JSR-166 Expert Group and released to the public domain, as explained |
4 |
* at http://creativecommons.org/licenses/publicdomain |
5 |
*/ |
6 |
|
7 |
package java.util.concurrent; |
8 |
|
9 |
import java.util.AbstractCollection; |
10 |
import java.util.ArrayList; |
11 |
import java.util.Collection; |
12 |
import java.util.Deque; |
13 |
import java.util.Iterator; |
14 |
import java.util.ConcurrentModificationException; |
15 |
import java.util.NoSuchElementException; |
16 |
import java.util.concurrent.atomic.AtomicReference; |
17 |
|
18 |
/** |
19 |
* An unbounded concurrent {@linkplain Deque deque} based on linked nodes. |
20 |
* Concurrent insertion, removal, and access operations execute safely |
21 |
* across multiple threads. |
22 |
* A {@code ConcurrentLinkedDeque} is an appropriate choice when |
23 |
* many threads will share access to a common collection. |
24 |
* Like most other concurrent collection implementations, this class |
25 |
* does not permit the use of {@code null} elements. |
26 |
* |
27 |
* <p>Iterators are <i>weakly consistent</i>, returning elements |
28 |
* reflecting the state of the deque at some point at or since the |
29 |
* creation of the iterator. They do <em>not</em> throw {@link |
30 |
* java.util.ConcurrentModificationException |
31 |
* ConcurrentModificationException}, and may proceed concurrently with |
32 |
* other operations. |
33 |
* |
34 |
* <p>Beware that, unlike in most collections, the {@code size} |
35 |
* method is <em>NOT</em> a constant-time operation. Because of the |
36 |
* asynchronous nature of these deques, determining the current number |
37 |
* of elements requires a traversal of the elements. |
38 |
* |
39 |
* <p>This class and its iterator implement all of the <em>optional</em> |
40 |
* methods of the {@link Deque} and {@link Iterator} interfaces. |
41 |
* |
42 |
* <p>Memory consistency effects: As with other concurrent collections, |
43 |
* actions in a thread prior to placing an object into a |
44 |
* {@code ConcurrentLinkedDeque} |
45 |
* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a> |
46 |
* actions subsequent to the access or removal of that element from |
47 |
* the {@code ConcurrentLinkedDeque} in another thread. |
48 |
* |
49 |
* <p>This class is a member of the |
50 |
* <a href="{@docRoot}/../technotes/guides/collections/index.html"> |
51 |
* Java Collections Framework</a>. |
52 |
* |
53 |
* @since 1.7 |
54 |
* @author Doug Lea |
55 |
* @author Martin Buchholz |
56 |
* @param <E> the type of elements held in this collection |
57 |
*/ |
58 |
|
59 |
public class ConcurrentLinkedDeque<E> |
60 |
extends AbstractCollection<E> |
61 |
implements Deque<E>, java.io.Serializable { |
62 |
|
63 |
/* |
64 |
* This is an implementation of a concurrent lock-free deque |
65 |
* supporting interior removes but not interior insertions, as |
66 |
* required to support the entire Deque interface. |
67 |
* |
68 |
* We extend the techniques developed for ConcurrentLinkedQueue and |
69 |
* LinkedTransferQueue (see the internal docs for those classes). |
70 |
* |
71 |
* The data structure is a symmetrical doubly-linked "GC-robust" |
72 |
* linked list of nodes. We minimize the number of volatile writes |
73 |
* using two techniques: advancing multiple hops with a single CAS |
74 |
* and mixing volatile and non-volatile writes of the same memory |
75 |
* locations. |
76 |
* |
77 |
* A node contains the expected E ("item") and links to predecessor |
78 |
* ("prev") and successor ("next") nodes: |
79 |
* |
80 |
* class Node<E> { volatile Node<E> prev, next; volatile E item; } |
81 |
* |
82 |
* A node p is considered "live" if it contains a non-null item |
83 |
* (p.item != null). When an item is CASed to null, the item is |
84 |
* atomically logically deleted from the collection. |
85 |
* |
86 |
* At any time, there is precisely one "first" node with a null |
87 |
* prev reference that terminates any chain of prev references |
88 |
* starting at a live node. Similarly there is precisely one |
89 |
* "last" node terminating any chain of next references starting at |
90 |
* a live node. The "first" and "last" nodes may or may not be live. |
91 |
* The "first" and "last" nodes are always mutually reachable. |
92 |
* |
93 |
* A new element is added atomically by CASing the null prev or |
94 |
* next reference in the first or last node to a fresh node |
95 |
* containing the element. |
96 |
* |
97 |
* A node is considered "active" if it is a live node, or the |
98 |
* first or last node. Active nodes cannot be unlinked. |
99 |
* |
100 |
* A "self-link" is a next or prev reference that is the same node: |
101 |
* p.prev == p or p.next == p |
102 |
* Self-links are used in the node unlinking process. Active nodes |
103 |
* never have self-links. |
104 |
* |
105 |
* A node p is active if and only if: |
106 |
* |
107 |
* p.item != null || |
108 |
* (p.prev == null && p.next != p) || |
109 |
* (p.next == null && p.prev != p) |
110 |
* |
111 |
* The deque object has two node references, "head" and "tail". |
112 |
* The head and tail are only approximations to the first and last |
113 |
* nodes of the deque. The first node can always be found by |
114 |
* following prev pointers from head; likewise for tail. However, |
115 |
* it is permissible for head and tail to be referring to deleted |
116 |
* nodes that have been unlinked and so may not be reachable from |
117 |
* any live node. |
118 |
* |
119 |
* There are 3 stages of node deletion; |
120 |
* "logical deletion", "unlinking", and "gc-unlinking". |
121 |
* |
122 |
* 1. "logical deletion" by CASing item to null atomically removes |
123 |
* the element from the collection, and makes the containing node |
124 |
* eligible for unlinking. |
125 |
* |
126 |
* 2. "unlinking" makes a deleted node unreachable from active |
127 |
* nodes, and thus eventually reclaimable by GC. Unlinked nodes |
128 |
* may remain reachable indefinitely from an iterator. |
129 |
* |
130 |
* Physical node unlinking is merely an optimization (albeit a |
131 |
* critical one), and so can be performed at our convenience. At |
132 |
* any time, the set of live nodes maintained by prev and next |
133 |
* links are identical, that is, the live nodes found via next |
134 |
* links from the first node is equal to the elements found via |
135 |
* prev links from the last node. However, this is not true for |
136 |
* nodes that have already been logically deleted - such nodes may |
137 |
* be reachable in one direction only. |
138 |
* |
139 |
* 3. "gc-unlinking" takes unlinking further by making active |
140 |
* nodes unreachable from deleted nodes, making it easier for the |
141 |
* GC to reclaim future deleted nodes. This step makes the data |
142 |
* structure "gc-robust", as first described in detail by Boehm |
143 |
* (http://portal.acm.org/citation.cfm?doid=503272.503282). |
144 |
* |
145 |
* GC-unlinked nodes may remain reachable indefinitely from an |
146 |
* iterator, but unlike unlinked nodes, are never reachable from |
147 |
* head or tail. |
148 |
* |
149 |
* Making the data structure GC-robust will eliminate the risk of |
150 |
* unbounded memory retention with conservative GCs and is likely |
151 |
* to improve performance with generational GCs. |
152 |
* |
153 |
* When a node is dequeued at either end, e.g. via poll(), we would |
154 |
* like to break any references from the node to active nodes. We |
155 |
* develop further the use of self-links that was very effective in |
156 |
* other concurrent collection classes. The idea is to replace |
157 |
* prev and next pointers with special values that are interpreted |
158 |
* to mean off-the-list-at-one-end. These are approximations, but |
159 |
* good enough to preserve the properties we want in our |
160 |
* traversals, e.g. we guarantee that a traversal will never visit |
161 |
* the same element twice, but we don't guarantee whether a |
162 |
* traversal that runs out of elements will be able to see more |
163 |
* elements later after enqueues at that end. Doing gc-unlinking |
164 |
* safely is particularly tricky, since any node can be in use |
165 |
* indefinitely (for example by an iterator). We must ensure that |
166 |
* the nodes pointed at by head/tail never get gc-unlinked, since |
167 |
* head/tail are needed to get "back on track" by other nodes that |
168 |
* are gc-unlinked. gc-unlinking accounts for much of the |
169 |
* implementation complexity. |
170 |
* |
171 |
* Since neither unlinking nor gc-unlinking are necessary for |
172 |
* correctness, there are many implementation choices regarding |
173 |
* frequency (eagerness) of these operations. Since volatile |
174 |
* reads are likely to be much cheaper than CASes, saving CASes by |
175 |
* unlinking multiple adjacent nodes at a time may be a win. |
176 |
* gc-unlinking can be performed rarely and still be effective, |
177 |
* since it is most important that long chains of deleted nodes |
178 |
* are occasionally broken. |
179 |
* |
180 |
* The actual representation we use is that p.next == p means to |
181 |
* goto the first node (which in turn is reached by following prev |
182 |
* pointers from head), and p.next == null && p.prev == p means |
183 |
* that the iteration is at an end and that p is a (final static) |
184 |
* dummy node, NEXT_TERMINATOR, and not the last active node. |
185 |
* Finishing the iteration when encountering such a TERMINATOR is |
186 |
* good enough for read-only traversals, so such traversals can use |
187 |
* p.next == null as the termination condition. When we need to |
188 |
* find the last (active) node, for enqueueing a new node, we need |
189 |
* to check whether we have reached a TERMINATOR node; if so, |
190 |
* restart traversal from tail. |
191 |
* |
192 |
* The implementation is completely directionally symmetrical, |
193 |
* except that most public methods that iterate through the list |
194 |
* follow next pointers ("forward" direction). |
195 |
* |
196 |
* There is one desirable property we would like to have, but |
197 |
* don't: it is possible, when an addFirst(A) is racing with |
198 |
* pollFirst() removing B, for an iterating observer to see A B C |
199 |
* and subsequently see A C, even though no interior removes are |
200 |
* ever performed. I believe this wart can only be removed at |
201 |
* significant runtime cost. |
202 |
* |
203 |
* Empirically, microbenchmarks suggest that this class adds about |
204 |
* 40% overhead relative to ConcurrentLinkedQueue, which feels as |
205 |
* good as we can hope for. |
206 |
*/ |
207 |
|
208 |
private static final long serialVersionUID = 876323262645176354L; |
209 |
|
210 |
/** |
211 |
* A node from which the first node on list (that is, the unique node p |
212 |
* with p.prev == null && p.next != p) can be reached in O(1) time. |
213 |
* Invariants: |
214 |
* - the first node is always O(1) reachable from head via prev links |
215 |
* - all live nodes are reachable from the first node via succ() |
216 |
* - head != null |
217 |
* - (tmp = head).next != tmp || tmp != head |
218 |
* - head is never gc-unlinked (but may be unlinked) |
219 |
* Non-invariants: |
220 |
* - head.item may or may not be null |
221 |
* - head may not be reachable from the first or last node, or from tail |
222 |
*/ |
223 |
private transient volatile Node<E> head; |
224 |
|
225 |
/** |
226 |
* A node from which the last node on list (that is, the unique node p |
227 |
* with p.next == null && p.prev != p) can be reached in O(1) time. |
228 |
* Invariants: |
229 |
* - the last node is always O(1) reachable from tail via next links |
230 |
* - all live nodes are reachable from the last node via pred() |
231 |
* - tail != null |
232 |
* - tail is never gc-unlinked (but may be unlinked) |
233 |
* Non-invariants: |
234 |
* - tail.item may or may not be null |
235 |
* - tail may not be reachable from the first or last node, or from head |
236 |
*/ |
237 |
private transient volatile Node<E> tail; |
238 |
|
239 |
private final static Node<Object> PREV_TERMINATOR, NEXT_TERMINATOR; |
240 |
|
241 |
static { |
242 |
PREV_TERMINATOR = new Node<Object>(null); |
243 |
PREV_TERMINATOR.next = PREV_TERMINATOR; |
244 |
NEXT_TERMINATOR = new Node<Object>(null); |
245 |
NEXT_TERMINATOR.prev = NEXT_TERMINATOR; |
246 |
} |
247 |
|
248 |
@SuppressWarnings("unchecked") |
249 |
Node<E> prevTerminator() { |
250 |
return (Node<E>) PREV_TERMINATOR; |
251 |
} |
252 |
|
253 |
@SuppressWarnings("unchecked") |
254 |
Node<E> nextTerminator() { |
255 |
return (Node<E>) NEXT_TERMINATOR; |
256 |
} |
257 |
|
258 |
static final class Node<E> { |
259 |
volatile Node<E> prev; |
260 |
volatile E item; |
261 |
volatile Node<E> next; |
262 |
|
263 |
/** |
264 |
* Constructs a new node. Uses relaxed write because item can |
265 |
* only be seen after publication via casNext or casPrev. |
266 |
*/ |
267 |
Node(E item) { |
268 |
UNSAFE.putObject(this, itemOffset, item); |
269 |
} |
270 |
|
271 |
boolean casItem(E cmp, E val) { |
272 |
return UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val); |
273 |
} |
274 |
|
275 |
void lazySetNext(Node<E> val) { |
276 |
UNSAFE.putOrderedObject(this, nextOffset, val); |
277 |
} |
278 |
|
279 |
boolean casNext(Node<E> cmp, Node<E> val) { |
280 |
return UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val); |
281 |
} |
282 |
|
283 |
void lazySetPrev(Node<E> val) { |
284 |
UNSAFE.putOrderedObject(this, prevOffset, val); |
285 |
} |
286 |
|
287 |
boolean casPrev(Node<E> cmp, Node<E> val) { |
288 |
return UNSAFE.compareAndSwapObject(this, prevOffset, cmp, val); |
289 |
} |
290 |
|
291 |
// Unsafe mechanics |
292 |
|
293 |
private static final sun.misc.Unsafe UNSAFE = |
294 |
sun.misc.Unsafe.getUnsafe(); |
295 |
private static final long prevOffset = |
296 |
objectFieldOffset(UNSAFE, "prev", Node.class); |
297 |
private static final long itemOffset = |
298 |
objectFieldOffset(UNSAFE, "item", Node.class); |
299 |
private static final long nextOffset = |
300 |
objectFieldOffset(UNSAFE, "next", Node.class); |
301 |
} |
302 |
|
303 |
/** |
304 |
* Links e as first element. |
305 |
*/ |
306 |
private void linkFirst(E e) { |
307 |
checkNotNull(e); |
308 |
final Node<E> newNode = new Node<E>(e); |
309 |
|
310 |
retry: |
311 |
for (;;) { |
312 |
for (Node<E> h = head, p = h;;) { |
313 |
Node<E> q = p.prev; |
314 |
if (q == null) { |
315 |
if (p.next == p) // PREV_TERMINATOR |
316 |
continue retry; |
317 |
// p is first node |
318 |
newNode.lazySetNext(p); // CAS piggyback |
319 |
if (p.casPrev(null, newNode)) { |
320 |
if (p != h) // hop two nodes at a time |
321 |
casHead(h, newNode); |
322 |
return; |
323 |
} else { |
324 |
p = p.prev; // lost CAS race to another thread |
325 |
} |
326 |
} |
327 |
else if (p == q) |
328 |
continue retry; |
329 |
else |
330 |
p = q; |
331 |
} |
332 |
} |
333 |
} |
334 |
|
335 |
/** |
336 |
* Links e as last element. |
337 |
*/ |
338 |
private void linkLast(E e) { |
339 |
checkNotNull(e); |
340 |
final Node<E> newNode = new Node<E>(e); |
341 |
|
342 |
retry: |
343 |
for (;;) { |
344 |
for (Node<E> t = tail, p = t;;) { |
345 |
Node<E> q = p.next; |
346 |
if (q == null) { |
347 |
if (p.prev == p) // NEXT_TERMINATOR |
348 |
continue retry; |
349 |
// p is last node |
350 |
newNode.lazySetPrev(p); // CAS piggyback |
351 |
if (p.casNext(null, newNode)) { |
352 |
if (p != t) // hop two nodes at a time |
353 |
casTail(t, newNode); |
354 |
return; |
355 |
} else { |
356 |
p = p.next; // lost CAS race to another thread |
357 |
} |
358 |
} |
359 |
else if (p == q) |
360 |
continue retry; |
361 |
else |
362 |
p = q; |
363 |
} |
364 |
} |
365 |
} |
366 |
|
367 |
private final static int HOPS = 2; |
368 |
|
369 |
/** |
370 |
* Unlinks non-null node x. |
371 |
*/ |
372 |
void unlink(Node<E> x) { |
373 |
// assert x != null; |
374 |
// assert x.item == null; |
375 |
// assert x != PREV_TERMINATOR; |
376 |
// assert x != NEXT_TERMINATOR; |
377 |
|
378 |
final Node<E> prev = x.prev; |
379 |
final Node<E> next = x.next; |
380 |
if (prev == null) { |
381 |
unlinkFirst(x, next); |
382 |
} else if (next == null) { |
383 |
unlinkLast(x, prev); |
384 |
} else { |
385 |
// Unlink interior node. |
386 |
// |
387 |
// This is the common case, since a series of polls at the |
388 |
// same end will be "interior" removes, except perhaps for |
389 |
// the first one, since end nodes cannot be unlinked. |
390 |
// |
391 |
// At any time, all active nodes are mutually reachable by |
392 |
// following a sequence of either next or prev pointers. |
393 |
// |
394 |
// Our strategy is to find the unique active predecessor |
395 |
// and successor of x. Try to fix up their links so that |
396 |
// they point to each other, leaving x unreachable from |
397 |
// active nodes. If successful, and if x has no live |
398 |
// predecessor/successor, we additionally try to gc-unlink, |
399 |
// leaving active nodes unreachable from x, by rechecking |
400 |
// that the status of predecessor and successor are |
401 |
// unchanged and ensuring that x is not reachable from |
402 |
// tail/head, before setting x's prev/next links to their |
403 |
// logical approximate replacements, self/TERMINATOR. |
404 |
Node<E> activePred, activeSucc; |
405 |
boolean isFirst, isLast; |
406 |
int hops = 1; |
407 |
|
408 |
// Find active predecessor |
409 |
for (Node<E> p = prev; ; ++hops) { |
410 |
if (p.item != null) { |
411 |
activePred = p; |
412 |
isFirst = false; |
413 |
break; |
414 |
} |
415 |
Node<E> q = p.prev; |
416 |
if (q == null) { |
417 |
if (p.next == p) |
418 |
return; |
419 |
activePred = p; |
420 |
isFirst = true; |
421 |
break; |
422 |
} |
423 |
else if (p == q) |
424 |
return; |
425 |
else |
426 |
p = q; |
427 |
} |
428 |
|
429 |
// Find active successor |
430 |
for (Node<E> p = next; ; ++hops) { |
431 |
if (p.item != null) { |
432 |
activeSucc = p; |
433 |
isLast = false; |
434 |
break; |
435 |
} |
436 |
Node<E> q = p.next; |
437 |
if (q == null) { |
438 |
if (p.prev == p) |
439 |
return; |
440 |
activeSucc = p; |
441 |
isLast = true; |
442 |
break; |
443 |
} |
444 |
else if (p == q) |
445 |
return; |
446 |
else |
447 |
p = q; |
448 |
} |
449 |
|
450 |
// TODO: better HOP heuristics |
451 |
if (hops < HOPS |
452 |
// always squeeze out interior deleted nodes |
453 |
&& (isFirst | isLast)) |
454 |
return; |
455 |
|
456 |
// Squeeze out deleted nodes between activePred and |
457 |
// activeSucc, including x. |
458 |
skipDeletedSuccessors(activePred); |
459 |
skipDeletedPredecessors(activeSucc); |
460 |
|
461 |
// Try to gc-unlink, if possible |
462 |
if ((isFirst | isLast) && |
463 |
|
464 |
// Recheck expected state of predecessor and successor |
465 |
(activePred.next == activeSucc) && |
466 |
(activeSucc.prev == activePred) && |
467 |
(isFirst ? activePred.prev == null : activePred.item != null) && |
468 |
(isLast ? activeSucc.next == null : activeSucc.item != null)) { |
469 |
|
470 |
// Ensure x is not reachable from head or tail |
471 |
updateHead(); |
472 |
updateTail(); |
473 |
|
474 |
// Finally, actually gc-unlink |
475 |
x.lazySetPrev(isFirst ? prevTerminator() : x); |
476 |
x.lazySetNext(isLast ? nextTerminator() : x); |
477 |
} |
478 |
} |
479 |
} |
480 |
|
481 |
/** |
482 |
* Unlinks non-null first node. |
483 |
*/ |
484 |
private void unlinkFirst(Node<E> first, Node<E> next) { |
485 |
// assert first != null && next != null && first.item == null; |
486 |
Node<E> o = null, p = next; |
487 |
for (int hops = 0; ; ++hops) { |
488 |
Node<E> q; |
489 |
if (p.item != null || (q = p.next) == null) { |
490 |
if (hops >= HOPS && p.prev != p && first.casNext(next, p)) { |
491 |
skipDeletedPredecessors(p); |
492 |
if (first.prev == null && |
493 |
(p.next == null || p.item != null) && |
494 |
p.prev == first) { |
495 |
|
496 |
updateHead(); |
497 |
updateTail(); |
498 |
o.lazySetNext(o); |
499 |
o.lazySetPrev(prevTerminator()); |
500 |
} |
501 |
} |
502 |
return; |
503 |
} |
504 |
else if (p == q) |
505 |
return; |
506 |
else { |
507 |
o = p; |
508 |
p = q; |
509 |
} |
510 |
} |
511 |
} |
512 |
|
513 |
/** |
514 |
* Unlinks non-null last node. |
515 |
*/ |
516 |
private void unlinkLast(Node<E> last, Node<E> prev) { |
517 |
// assert last != null && prev != null && last.item == null; |
518 |
Node<E> o = null, p = prev; |
519 |
for (int hops = 0; ; ++hops) { |
520 |
Node<E> q; |
521 |
if (p.item != null || (q = p.prev) == null) { |
522 |
if (hops >= HOPS && p.next != p && last.casPrev(prev, p)) { |
523 |
skipDeletedSuccessors(p); |
524 |
if (last.next == null && |
525 |
(p.prev == null || p.item != null) && |
526 |
p.next == last) { |
527 |
|
528 |
updateHead(); |
529 |
updateTail(); |
530 |
o.lazySetPrev(o); |
531 |
o.lazySetNext(nextTerminator()); |
532 |
} |
533 |
} |
534 |
return; |
535 |
} |
536 |
else if (p == q) |
537 |
return; |
538 |
else { |
539 |
o = p; |
540 |
p = q; |
541 |
} |
542 |
} |
543 |
} |
544 |
|
545 |
/** |
546 |
* Sets head to first node. Guarantees that any node which was |
547 |
* unlinked before a call to this method will be unreachable from |
548 |
* head after it returns. |
549 |
*/ |
550 |
private final void updateHead() { |
551 |
first(); |
552 |
} |
553 |
|
554 |
/** |
555 |
* Sets tail to last node. Guarantees that any node which was |
556 |
* unlinked before a call to this method will be unreachable from |
557 |
* tail after it returns. |
558 |
*/ |
559 |
private final void updateTail() { |
560 |
last(); |
561 |
} |
562 |
|
563 |
private void skipDeletedPredecessors(Node<E> x) { |
564 |
whileActive: |
565 |
do { |
566 |
Node<E> prev = x.prev; |
567 |
// assert prev != null; |
568 |
// assert x != NEXT_TERMINATOR; |
569 |
// assert x != PREV_TERMINATOR; |
570 |
Node<E> p = prev; |
571 |
findActive: |
572 |
for (;;) { |
573 |
if (p.item != null) |
574 |
break findActive; |
575 |
Node<E> q = p.prev; |
576 |
if (q == null) { |
577 |
if (p.next == p) |
578 |
continue whileActive; |
579 |
break findActive; |
580 |
} |
581 |
else if (p == q) |
582 |
continue whileActive; |
583 |
else |
584 |
p = q; |
585 |
} |
586 |
|
587 |
// found active CAS target |
588 |
if (prev == p || x.casPrev(prev, p)) |
589 |
return; |
590 |
|
591 |
} while (x.item != null || x.next == null); |
592 |
} |
593 |
|
594 |
private void skipDeletedSuccessors(Node<E> x) { |
595 |
whileActive: |
596 |
do { |
597 |
Node<E> next = x.next; |
598 |
// assert next != null; |
599 |
// assert x != NEXT_TERMINATOR; |
600 |
// assert x != PREV_TERMINATOR; |
601 |
Node<E> p = next; |
602 |
findActive: |
603 |
for (;;) { |
604 |
if (p.item != null) |
605 |
break findActive; |
606 |
Node<E> q = p.next; |
607 |
if (q == null) { |
608 |
if (p.prev == p) |
609 |
continue whileActive; |
610 |
break findActive; |
611 |
} |
612 |
else if (p == q) |
613 |
continue whileActive; |
614 |
else |
615 |
p = q; |
616 |
} |
617 |
|
618 |
// found active CAS target |
619 |
if (next == p || x.casNext(next, p)) |
620 |
return; |
621 |
|
622 |
} while (x.item != null || x.prev == null); |
623 |
} |
624 |
|
625 |
/** |
626 |
* Returns the successor of p, or the first node if p.next has been |
627 |
* linked to self, which will only be true if traversing with a |
628 |
* stale pointer that is now off the list. |
629 |
*/ |
630 |
final Node<E> succ(Node<E> p) { |
631 |
// TODO: should we skip deleted nodes here? |
632 |
Node<E> q = p.next; |
633 |
return (p == q) ? first() : q; |
634 |
} |
635 |
|
636 |
/** |
637 |
* Returns the predecessor of p, or the last node if p.prev has been |
638 |
* linked to self, which will only be true if traversing with a |
639 |
* stale pointer that is now off the list. |
640 |
*/ |
641 |
final Node<E> pred(Node<E> p) { |
642 |
Node<E> q = p.prev; |
643 |
return (p == q) ? last() : q; |
644 |
} |
645 |
|
646 |
/** |
647 |
* Returns the first node, the unique node p for which: |
648 |
* p.prev == null && p.next != p |
649 |
* The returned node may or may not be logically deleted. |
650 |
* Guarantees that head is set to the returned node. |
651 |
*/ |
652 |
Node<E> first() { |
653 |
retry: |
654 |
for (;;) { |
655 |
for (Node<E> h = head, p = h;;) { |
656 |
Node<E> q = p.prev; |
657 |
if (q == null) { |
658 |
if (p == h |
659 |
// It is possible that p is PREV_TERMINATOR, |
660 |
// but if so, the CAS is guaranteed to fail. |
661 |
|| casHead(h, p)) |
662 |
return p; |
663 |
else |
664 |
continue retry; |
665 |
} else if (p == q) { |
666 |
continue retry; |
667 |
} else { |
668 |
p = q; |
669 |
} |
670 |
} |
671 |
} |
672 |
} |
673 |
|
674 |
/** |
675 |
* Returns the last node, the unique node p for which: |
676 |
* p.next == null && p.prev != p |
677 |
* The returned node may or may not be logically deleted. |
678 |
* Guarantees that tail is set to the returned node. |
679 |
*/ |
680 |
Node<E> last() { |
681 |
retry: |
682 |
for (;;) { |
683 |
for (Node<E> t = tail, p = t;;) { |
684 |
Node<E> q = p.next; |
685 |
if (q == null) { |
686 |
if (p == t |
687 |
// It is possible that p is NEXT_TERMINATOR, |
688 |
// but if so, the CAS is guaranteed to fail. |
689 |
|| casTail(t, p)) |
690 |
return p; |
691 |
else |
692 |
continue retry; |
693 |
} else if (p == q) { |
694 |
continue retry; |
695 |
} else { |
696 |
p = q; |
697 |
} |
698 |
} |
699 |
} |
700 |
} |
701 |
|
702 |
// Minor convenience utilities |
703 |
|
704 |
/** |
705 |
* Throws NullPointerException if argument is null. |
706 |
* |
707 |
* @param v the element |
708 |
*/ |
709 |
private static void checkNotNull(Object v) { |
710 |
if (v == null) |
711 |
throw new NullPointerException(); |
712 |
} |
713 |
|
714 |
/** |
715 |
* Returns element unless it is null, in which case throws |
716 |
* NoSuchElementException. |
717 |
* |
718 |
* @param v the element |
719 |
* @return the element |
720 |
*/ |
721 |
private E screenNullResult(E v) { |
722 |
if (v == null) |
723 |
throw new NoSuchElementException(); |
724 |
return v; |
725 |
} |
726 |
|
727 |
/** |
728 |
* Creates an array list and fills it with elements of this list. |
729 |
* Used by toArray. |
730 |
* |
731 |
* @return the arrayList |
732 |
*/ |
733 |
private ArrayList<E> toArrayList() { |
734 |
ArrayList<E> list = new ArrayList<E>(); |
735 |
for (Node<E> p = first(); p != null; p = succ(p)) { |
736 |
E item = p.item; |
737 |
if (item != null) |
738 |
list.add(item); |
739 |
} |
740 |
return list; |
741 |
} |
742 |
|
743 |
/** |
744 |
* Constructs an empty deque. |
745 |
*/ |
746 |
public ConcurrentLinkedDeque() { |
747 |
head = tail = new Node<E>(null); |
748 |
} |
749 |
|
750 |
/** |
751 |
* Constructs a deque initially containing the elements of |
752 |
* the given collection, added in traversal order of the |
753 |
* collection's iterator. |
754 |
* |
755 |
* @param c the collection of elements to initially contain |
756 |
* @throws NullPointerException if the specified collection or any |
757 |
* of its elements are null |
758 |
*/ |
759 |
public ConcurrentLinkedDeque(Collection<? extends E> c) { |
760 |
// Copy c into a private chain of Nodes |
761 |
Node<E> h = null, t = null; |
762 |
for (E e : c) { |
763 |
checkNotNull(e); |
764 |
Node<E> newNode = new Node<E>(e); |
765 |
if (h == null) |
766 |
h = t = newNode; |
767 |
else { |
768 |
t.next = newNode; |
769 |
newNode.prev = t; |
770 |
t = newNode; |
771 |
} |
772 |
} |
773 |
if (h == null) |
774 |
h = t = new Node<E>(null); |
775 |
head = h; |
776 |
tail = t; |
777 |
} |
778 |
|
779 |
/** |
780 |
* Inserts the specified element at the front of this deque. |
781 |
* |
782 |
* @throws NullPointerException {@inheritDoc} |
783 |
*/ |
784 |
public void addFirst(E e) { |
785 |
linkFirst(e); |
786 |
} |
787 |
|
788 |
/** |
789 |
* Inserts the specified element at the end of this deque. |
790 |
* |
791 |
* <p>This method is equivalent to {@link #add}. |
792 |
* |
793 |
* @throws NullPointerException {@inheritDoc} |
794 |
*/ |
795 |
public void addLast(E e) { |
796 |
linkLast(e); |
797 |
} |
798 |
|
799 |
/** |
800 |
* Inserts the specified element at the front of this deque. |
801 |
* |
802 |
* @return {@code true} always |
803 |
* @throws NullPointerException {@inheritDoc} |
804 |
*/ |
805 |
public boolean offerFirst(E e) { |
806 |
linkFirst(e); |
807 |
return true; |
808 |
} |
809 |
|
810 |
/** |
811 |
* Inserts the specified element at the end of this deque. |
812 |
* |
813 |
* <p>This method is equivalent to {@link #add}. |
814 |
* |
815 |
* @return {@code true} always |
816 |
* @throws NullPointerException {@inheritDoc} |
817 |
*/ |
818 |
public boolean offerLast(E e) { |
819 |
linkLast(e); |
820 |
return true; |
821 |
} |
822 |
|
823 |
public E peekFirst() { |
824 |
for (Node<E> p = first(); p != null; p = succ(p)) { |
825 |
E item = p.item; |
826 |
if (item != null) |
827 |
return item; |
828 |
} |
829 |
return null; |
830 |
} |
831 |
|
832 |
public E peekLast() { |
833 |
for (Node<E> p = last(); p != null; p = pred(p)) { |
834 |
E item = p.item; |
835 |
if (item != null) |
836 |
return item; |
837 |
} |
838 |
return null; |
839 |
} |
840 |
|
841 |
/** |
842 |
* @throws NoSuchElementException {@inheritDoc} |
843 |
*/ |
844 |
public E getFirst() { |
845 |
return screenNullResult(peekFirst()); |
846 |
} |
847 |
|
848 |
/** |
849 |
* @throws NoSuchElementException {@inheritDoc} |
850 |
*/ |
851 |
public E getLast() { |
852 |
return screenNullResult(peekLast()); |
853 |
} |
854 |
|
855 |
public E pollFirst() { |
856 |
for (Node<E> p = first(); p != null; p = succ(p)) { |
857 |
E item = p.item; |
858 |
if (item != null && p.casItem(item, null)) { |
859 |
unlink(p); |
860 |
return item; |
861 |
} |
862 |
} |
863 |
return null; |
864 |
} |
865 |
|
866 |
public E pollLast() { |
867 |
for (Node<E> p = last(); p != null; p = pred(p)) { |
868 |
E item = p.item; |
869 |
if (item != null && p.casItem(item, null)) { |
870 |
unlink(p); |
871 |
return item; |
872 |
} |
873 |
} |
874 |
return null; |
875 |
} |
876 |
|
877 |
/** |
878 |
* @throws NoSuchElementException {@inheritDoc} |
879 |
*/ |
880 |
public E removeFirst() { |
881 |
return screenNullResult(pollFirst()); |
882 |
} |
883 |
|
884 |
/** |
885 |
* @throws NoSuchElementException {@inheritDoc} |
886 |
*/ |
887 |
public E removeLast() { |
888 |
return screenNullResult(pollLast()); |
889 |
} |
890 |
|
891 |
// *** Queue and stack methods *** |
892 |
|
893 |
/** |
894 |
* Inserts the specified element at the tail of this deque. |
895 |
* |
896 |
* @return {@code true} (as specified by {@link Queue#offer}) |
897 |
* @throws NullPointerException if the specified element is null |
898 |
*/ |
899 |
public boolean offer(E e) { |
900 |
return offerLast(e); |
901 |
} |
902 |
|
903 |
/** |
904 |
* Inserts the specified element at the tail of this deque. |
905 |
* |
906 |
* @return {@code true} (as specified by {@link Collection#add}) |
907 |
* @throws NullPointerException if the specified element is null |
908 |
*/ |
909 |
public boolean add(E e) { |
910 |
return offerLast(e); |
911 |
} |
912 |
|
913 |
public E poll() { return pollFirst(); } |
914 |
public E remove() { return removeFirst(); } |
915 |
public E peek() { return peekFirst(); } |
916 |
public E element() { return getFirst(); } |
917 |
public void push(E e) { addFirst(e); } |
918 |
public E pop() { return removeFirst(); } |
919 |
|
920 |
/** |
921 |
* Removes the first element {@code e} such that |
922 |
* {@code o.equals(e)}, if such an element exists in this deque. |
923 |
* If the deque does not contain the element, it is unchanged. |
924 |
* |
925 |
* @param o element to be removed from this deque, if present |
926 |
* @return {@code true} if the deque contained the specified element |
927 |
* @throws NullPointerException if the specified element is {@code null} |
928 |
*/ |
929 |
public boolean removeFirstOccurrence(Object o) { |
930 |
checkNotNull(o); |
931 |
for (Node<E> p = first(); p != null; p = succ(p)) { |
932 |
E item = p.item; |
933 |
if (item != null && o.equals(item) && p.casItem(item, null)) { |
934 |
unlink(p); |
935 |
return true; |
936 |
} |
937 |
} |
938 |
return false; |
939 |
} |
940 |
|
941 |
/** |
942 |
* Removes the last element {@code e} such that |
943 |
* {@code o.equals(e)}, if such an element exists in this deque. |
944 |
* If the deque does not contain the element, it is unchanged. |
945 |
* |
946 |
* @param o element to be removed from this deque, if present |
947 |
* @return {@code true} if the deque contained the specified element |
948 |
* @throws NullPointerException if the specified element is {@code null} |
949 |
*/ |
950 |
public boolean removeLastOccurrence(Object o) { |
951 |
checkNotNull(o); |
952 |
for (Node<E> p = last(); p != null; p = pred(p)) { |
953 |
E item = p.item; |
954 |
if (item != null && o.equals(item) && p.casItem(item, null)) { |
955 |
unlink(p); |
956 |
return true; |
957 |
} |
958 |
} |
959 |
return false; |
960 |
} |
961 |
|
962 |
/** |
963 |
* Returns {@code true} if this deque contains at least one |
964 |
* element {@code e} such that {@code o.equals(e)}. |
965 |
* |
966 |
* @param o element whose presence in this deque is to be tested |
967 |
* @return {@code true} if this deque contains the specified element |
968 |
*/ |
969 |
public boolean contains(Object o) { |
970 |
if (o == null) return false; |
971 |
for (Node<E> p = first(); p != null; p = succ(p)) { |
972 |
E item = p.item; |
973 |
if (item != null && o.equals(item)) |
974 |
return true; |
975 |
} |
976 |
return false; |
977 |
} |
978 |
|
979 |
/** |
980 |
* Returns {@code true} if this collection contains no elements. |
981 |
* |
982 |
* @return {@code true} if this collection contains no elements |
983 |
*/ |
984 |
public boolean isEmpty() { |
985 |
return peekFirst() == null; |
986 |
} |
987 |
|
988 |
/** |
989 |
* Returns the number of elements in this deque. If this deque |
990 |
* contains more than {@code Integer.MAX_VALUE} elements, it |
991 |
* returns {@code Integer.MAX_VALUE}. |
992 |
* |
993 |
* <p>Beware that, unlike in most collections, this method is |
994 |
* <em>NOT</em> a constant-time operation. Because of the |
995 |
* asynchronous nature of these deques, determining the current |
996 |
* number of elements requires traversing them all to count them. |
997 |
* Additionally, it is possible for the size to change during |
998 |
* execution of this method, in which case the returned result |
999 |
* will be inaccurate. Thus, this method is typically not very |
1000 |
* useful in concurrent applications. |
1001 |
* |
1002 |
* @return the number of elements in this deque |
1003 |
*/ |
1004 |
public int size() { |
1005 |
long count = 0; |
1006 |
for (Node<E> p = first(); p != null; p = succ(p)) |
1007 |
if (p.item != null) |
1008 |
++count; |
1009 |
return (count >= Integer.MAX_VALUE) ? Integer.MAX_VALUE : (int) count; |
1010 |
} |
1011 |
|
1012 |
/** |
1013 |
* Removes the first element {@code e} such that |
1014 |
* {@code o.equals(e)}, if such an element exists in this deque. |
1015 |
* If the deque does not contain the element, it is unchanged. |
1016 |
* |
1017 |
* @param o element to be removed from this deque, if present |
1018 |
* @return {@code true} if the deque contained the specified element |
1019 |
* @throws NullPointerException if the specified element is {@code null} |
1020 |
*/ |
1021 |
public boolean remove(Object o) { |
1022 |
return removeFirstOccurrence(o); |
1023 |
} |
1024 |
|
1025 |
/** |
1026 |
* Appends all of the elements in the specified collection to the end of |
1027 |
* this deque, in the order that they are returned by the specified |
1028 |
* collection's iterator. Attempts to {@code addAll} of a deque to |
1029 |
* itself result in {@code IllegalArgumentException}. |
1030 |
* |
1031 |
* @param c the elements to be inserted into this deque |
1032 |
* @return {@code true} if this deque changed as a result of the call |
1033 |
* @throws NullPointerException if the specified collection or any |
1034 |
* of its elements are null |
1035 |
* @throws IllegalArgumentException if the collection is this deque |
1036 |
*/ |
1037 |
public boolean addAll(Collection<? extends E> c) { |
1038 |
if (c == this) |
1039 |
// As historically specified in AbstractQueue#addAll |
1040 |
throw new IllegalArgumentException(); |
1041 |
|
1042 |
// Copy c into a private chain of Nodes |
1043 |
Node<E> splice = null, last = null; |
1044 |
for (E e : c) { |
1045 |
checkNotNull(e); |
1046 |
Node<E> newNode = new Node<E>(e); |
1047 |
if (splice == null) |
1048 |
splice = last = newNode; |
1049 |
else { |
1050 |
last.next = newNode; |
1051 |
newNode.prev = last; |
1052 |
last = newNode; |
1053 |
} |
1054 |
} |
1055 |
if (splice == null) |
1056 |
return false; |
1057 |
|
1058 |
// Atomically splice the chain as the tail of this collection |
1059 |
retry: |
1060 |
for (;;) { |
1061 |
for (Node<E> t = tail, p = t;;) { |
1062 |
Node<E> q = p.next; |
1063 |
if (q == null) { |
1064 |
if (p.prev == p) // NEXT_TERMINATOR |
1065 |
continue retry; |
1066 |
// p is last node |
1067 |
splice.lazySetPrev(p); // CAS piggyback |
1068 |
if (p.casNext(null, splice)) { |
1069 |
if (! casTail(t, last)) { |
1070 |
// Try a little harder to update tail, |
1071 |
// since we may be adding many elements. |
1072 |
t = tail; |
1073 |
if (last.next == null) |
1074 |
casTail(t, last); |
1075 |
} |
1076 |
return true; |
1077 |
} else { |
1078 |
p = p.next; // lost CAS race to another thread |
1079 |
} |
1080 |
} |
1081 |
else if (p == q) |
1082 |
continue retry; |
1083 |
else |
1084 |
p = q; |
1085 |
} |
1086 |
} |
1087 |
} |
1088 |
|
1089 |
/** |
1090 |
* Removes all of the elements from this deque. |
1091 |
*/ |
1092 |
public void clear() { |
1093 |
while (pollFirst() != null) |
1094 |
; |
1095 |
} |
1096 |
|
1097 |
/** |
1098 |
* Returns an array containing all of the elements in this deque, in |
1099 |
* proper sequence (from first to last element). |
1100 |
* |
1101 |
* <p>The returned array will be "safe" in that no references to it are |
1102 |
* maintained by this deque. (In other words, this method must allocate |
1103 |
* a new array). The caller is thus free to modify the returned array. |
1104 |
* |
1105 |
* <p>This method acts as bridge between array-based and collection-based |
1106 |
* APIs. |
1107 |
* |
1108 |
* @return an array containing all of the elements in this deque |
1109 |
*/ |
1110 |
public Object[] toArray() { |
1111 |
return toArrayList().toArray(); |
1112 |
} |
1113 |
|
1114 |
/** |
1115 |
* Returns an array containing all of the elements in this deque, |
1116 |
* in proper sequence (from first to last element); the runtime |
1117 |
* type of the returned array is that of the specified array. If |
1118 |
* the deque fits in the specified array, it is returned therein. |
1119 |
* Otherwise, a new array is allocated with the runtime type of |
1120 |
* the specified array and the size of this deque. |
1121 |
* |
1122 |
* <p>If this deque fits in the specified array with room to spare |
1123 |
* (i.e., the array has more elements than this deque), the element in |
1124 |
* the array immediately following the end of the deque is set to |
1125 |
* {@code null}. |
1126 |
* |
1127 |
* <p>Like the {@link #toArray()} method, this method acts as |
1128 |
* bridge between array-based and collection-based APIs. Further, |
1129 |
* this method allows precise control over the runtime type of the |
1130 |
* output array, and may, under certain circumstances, be used to |
1131 |
* save allocation costs. |
1132 |
* |
1133 |
* <p>Suppose {@code x} is a deque known to contain only strings. |
1134 |
* The following code can be used to dump the deque into a newly |
1135 |
* allocated array of {@code String}: |
1136 |
* |
1137 |
* <pre> |
1138 |
* String[] y = x.toArray(new String[0]);</pre> |
1139 |
* |
1140 |
* Note that {@code toArray(new Object[0])} is identical in function to |
1141 |
* {@code toArray()}. |
1142 |
* |
1143 |
* @param a the array into which the elements of the deque are to |
1144 |
* be stored, if it is big enough; otherwise, a new array of the |
1145 |
* same runtime type is allocated for this purpose |
1146 |
* @return an array containing all of the elements in this deque |
1147 |
* @throws ArrayStoreException if the runtime type of the specified array |
1148 |
* is not a supertype of the runtime type of every element in |
1149 |
* this deque |
1150 |
* @throws NullPointerException if the specified array is null |
1151 |
*/ |
1152 |
public <T> T[] toArray(T[] a) { |
1153 |
return toArrayList().toArray(a); |
1154 |
} |
1155 |
|
1156 |
/** |
1157 |
* Returns an iterator over the elements in this deque in proper sequence. |
1158 |
* The elements will be returned in order from first (head) to last (tail). |
1159 |
* |
1160 |
* <p>The returned {@code Iterator} is a "weakly consistent" iterator that |
1161 |
* will never throw {@link java.util.ConcurrentModificationException |
1162 |
* ConcurrentModificationException}, |
1163 |
* and guarantees to traverse elements as they existed upon |
1164 |
* construction of the iterator, and may (but is not guaranteed to) |
1165 |
* reflect any modifications subsequent to construction. |
1166 |
* |
1167 |
* @return an iterator over the elements in this deque in proper sequence |
1168 |
*/ |
1169 |
public Iterator<E> iterator() { |
1170 |
return new Itr(); |
1171 |
} |
1172 |
|
1173 |
/** |
1174 |
* Returns an iterator over the elements in this deque in reverse |
1175 |
* sequential order. The elements will be returned in order from |
1176 |
* last (tail) to first (head). |
1177 |
* |
1178 |
* <p>The returned {@code Iterator} is a "weakly consistent" iterator that |
1179 |
* will never throw {@link java.util.ConcurrentModificationException |
1180 |
* ConcurrentModificationException}, |
1181 |
* and guarantees to traverse elements as they existed upon |
1182 |
* construction of the iterator, and may (but is not guaranteed to) |
1183 |
* reflect any modifications subsequent to construction. |
1184 |
* |
1185 |
* @return an iterator over the elements in this deque in reverse order |
1186 |
*/ |
1187 |
public Iterator<E> descendingIterator() { |
1188 |
return new DescendingItr(); |
1189 |
} |
1190 |
|
1191 |
private abstract class AbstractItr implements Iterator<E> { |
1192 |
/** |
1193 |
* Next node to return item for. |
1194 |
*/ |
1195 |
private Node<E> nextNode; |
1196 |
|
1197 |
/** |
1198 |
* nextItem holds on to item fields because once we claim |
1199 |
* that an element exists in hasNext(), we must return it in |
1200 |
* the following next() call even if it was in the process of |
1201 |
* being removed when hasNext() was called. |
1202 |
*/ |
1203 |
private E nextItem; |
1204 |
|
1205 |
/** |
1206 |
* Node returned by most recent call to next. Needed by remove. |
1207 |
* Reset to null if this element is deleted by a call to remove. |
1208 |
*/ |
1209 |
private Node<E> lastRet; |
1210 |
|
1211 |
abstract Node<E> startNode(); |
1212 |
abstract Node<E> nextNode(Node<E> p); |
1213 |
|
1214 |
AbstractItr() { |
1215 |
advance(); |
1216 |
} |
1217 |
|
1218 |
/** |
1219 |
* Sets nextNode and nextItem to next valid node, or to null |
1220 |
* if no such. |
1221 |
*/ |
1222 |
private void advance() { |
1223 |
lastRet = nextNode; |
1224 |
|
1225 |
Node<E> p = (nextNode == null) ? startNode() : nextNode(nextNode); |
1226 |
for (;; p = nextNode(p)) { |
1227 |
if (p == null) { |
1228 |
// p might be active end or TERMINATOR node; both are OK |
1229 |
nextNode = null; |
1230 |
nextItem = null; |
1231 |
break; |
1232 |
} |
1233 |
E item = p.item; |
1234 |
if (item != null) { |
1235 |
nextNode = p; |
1236 |
nextItem = item; |
1237 |
break; |
1238 |
} |
1239 |
} |
1240 |
} |
1241 |
|
1242 |
public boolean hasNext() { |
1243 |
return nextItem != null; |
1244 |
} |
1245 |
|
1246 |
public E next() { |
1247 |
E item = nextItem; |
1248 |
if (item == null) throw new NoSuchElementException(); |
1249 |
advance(); |
1250 |
return item; |
1251 |
} |
1252 |
|
1253 |
public void remove() { |
1254 |
Node<E> l = lastRet; |
1255 |
if (l == null) throw new IllegalStateException(); |
1256 |
l.item = null; |
1257 |
unlink(l); |
1258 |
lastRet = null; |
1259 |
} |
1260 |
} |
1261 |
|
1262 |
/** Forward iterator */ |
1263 |
private class Itr extends AbstractItr { |
1264 |
Node<E> startNode() { return first(); } |
1265 |
Node<E> nextNode(Node<E> p) { return succ(p); } |
1266 |
} |
1267 |
|
1268 |
/** Descending iterator */ |
1269 |
private class DescendingItr extends AbstractItr { |
1270 |
Node<E> startNode() { return last(); } |
1271 |
Node<E> nextNode(Node<E> p) { return pred(p); } |
1272 |
} |
1273 |
|
1274 |
/** |
1275 |
* Saves the state to a stream (that is, serializes it). |
1276 |
* |
1277 |
* @serialData All of the elements (each an {@code E}) in |
1278 |
* the proper order, followed by a null |
1279 |
* @param s the stream |
1280 |
*/ |
1281 |
private void writeObject(java.io.ObjectOutputStream s) |
1282 |
throws java.io.IOException { |
1283 |
|
1284 |
// Write out any hidden stuff |
1285 |
s.defaultWriteObject(); |
1286 |
|
1287 |
// Write out all elements in the proper order. |
1288 |
for (Node<E> p = first(); p != null; p = succ(p)) { |
1289 |
Object item = p.item; |
1290 |
if (item != null) |
1291 |
s.writeObject(item); |
1292 |
} |
1293 |
|
1294 |
// Use trailing null as sentinel |
1295 |
s.writeObject(null); |
1296 |
} |
1297 |
|
1298 |
/** |
1299 |
* Reconstitutes the instance from a stream (that is, deserializes it). |
1300 |
* @param s the stream |
1301 |
*/ |
1302 |
private void readObject(java.io.ObjectInputStream s) |
1303 |
throws java.io.IOException, ClassNotFoundException { |
1304 |
s.defaultReadObject(); |
1305 |
|
1306 |
// Read in elements until trailing null sentinel found |
1307 |
Node<E> h = null, t = null; |
1308 |
Object item; |
1309 |
while ((item = s.readObject()) != null) { |
1310 |
@SuppressWarnings("unchecked") |
1311 |
Node<E> newNode = new Node<E>((E) item); |
1312 |
if (h == null) |
1313 |
h = t = newNode; |
1314 |
else { |
1315 |
t.next = newNode; |
1316 |
newNode.prev = t; |
1317 |
t = newNode; |
1318 |
} |
1319 |
} |
1320 |
if (h == null) |
1321 |
h = t = new Node<E>(null); |
1322 |
head = h; |
1323 |
tail = t; |
1324 |
} |
1325 |
|
1326 |
// Unsafe mechanics |
1327 |
|
1328 |
private static final sun.misc.Unsafe UNSAFE = |
1329 |
sun.misc.Unsafe.getUnsafe(); |
1330 |
private static final long headOffset = |
1331 |
objectFieldOffset(UNSAFE, "head", ConcurrentLinkedDeque.class); |
1332 |
private static final long tailOffset = |
1333 |
objectFieldOffset(UNSAFE, "tail", ConcurrentLinkedDeque.class); |
1334 |
|
1335 |
private boolean casHead(Node<E> cmp, Node<E> val) { |
1336 |
return UNSAFE.compareAndSwapObject(this, headOffset, cmp, val); |
1337 |
} |
1338 |
|
1339 |
private boolean casTail(Node<E> cmp, Node<E> val) { |
1340 |
return UNSAFE.compareAndSwapObject(this, tailOffset, cmp, val); |
1341 |
} |
1342 |
|
1343 |
static long objectFieldOffset(sun.misc.Unsafe UNSAFE, |
1344 |
String field, Class<?> klazz) { |
1345 |
try { |
1346 |
return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field)); |
1347 |
} catch (NoSuchFieldException e) { |
1348 |
// Convert Exception to corresponding Error |
1349 |
NoSuchFieldError error = new NoSuchFieldError(field); |
1350 |
error.initCause(e); |
1351 |
throw error; |
1352 |
} |
1353 |
} |
1354 |
} |