1 |
/* |
2 |
* Written by Doug Lea with assistance from members of JCP JSR-166 |
3 |
* Expert Group and released to the public domain, as explained at |
4 |
* http://creativecommons.org/licenses/publicdomain |
5 |
*/ |
6 |
|
7 |
package java.util.concurrent; |
8 |
import java.util.concurrent.locks.*; |
9 |
import java.util.*; |
10 |
|
11 |
/** |
12 |
* A bounded {@linkplain BlockingQueue blocking queue} backed by an |
13 |
* array. This queue orders elements FIFO (first-in-first-out). The |
14 |
* <em>head</em> of the queue is that element that has been on the |
15 |
* queue the longest time. The <em>tail</em> of the queue is that |
16 |
* element that has been on the queue the shortest time. New elements |
17 |
* are inserted at the tail of the queue, and the queue retrieval |
18 |
* operations obtain elements at the head of the queue. |
19 |
* |
20 |
* <p>This is a classic "bounded buffer", in which a |
21 |
* fixed-sized array holds elements inserted by producers and |
22 |
* extracted by consumers. Once created, the capacity cannot be |
23 |
* changed. Attempts to {@code put} an element into a full queue |
24 |
* will result in the operation blocking; attempts to {@code take} an |
25 |
* element from an empty queue will similarly block. |
26 |
* |
27 |
* <p>This class supports an optional fairness policy for ordering |
28 |
* waiting producer and consumer threads. By default, this ordering |
29 |
* is not guaranteed. However, a queue constructed with fairness set |
30 |
* to {@code true} grants threads access in FIFO order. Fairness |
31 |
* generally decreases throughput but reduces variability and avoids |
32 |
* starvation. |
33 |
* |
34 |
* <p>This class and its iterator implement all of the |
35 |
* <em>optional</em> methods of the {@link Collection} and {@link |
36 |
* Iterator} interfaces. |
37 |
* |
38 |
* <p>This class is a member of the |
39 |
* <a href="{@docRoot}/../technotes/guides/collections/index.html"> |
40 |
* Java Collections Framework</a>. |
41 |
* |
42 |
* @since 1.5 |
43 |
* @author Doug Lea |
44 |
* @param <E> the type of elements held in this collection |
45 |
*/ |
46 |
public class ArrayBlockingQueue<E> extends AbstractQueue<E> |
47 |
implements BlockingQueue<E>, java.io.Serializable { |
48 |
|
49 |
/** |
50 |
* Serialization ID. This class relies on default serialization |
51 |
* even for the items array, which is default-serialized, even if |
52 |
* it is empty. Otherwise it could not be declared final, which is |
53 |
* necessary here. |
54 |
*/ |
55 |
private static final long serialVersionUID = -817911632652898426L; |
56 |
|
57 |
/** The queued items */ |
58 |
final Object[] items; |
59 |
|
60 |
/** items index for next take, poll, peek or remove */ |
61 |
int takeIndex; |
62 |
|
63 |
/** items index for next put, offer, or add */ |
64 |
int putIndex; |
65 |
|
66 |
/** Number of elements in the queue */ |
67 |
int count; |
68 |
|
69 |
/* |
70 |
* Concurrency control uses the classic two-condition algorithm |
71 |
* found in any textbook. |
72 |
*/ |
73 |
|
74 |
/** Main lock guarding all access */ |
75 |
final ReentrantLock lock; |
76 |
/** Condition for waiting takes */ |
77 |
private final Condition notEmpty; |
78 |
/** Condition for waiting puts */ |
79 |
private final Condition notFull; |
80 |
|
81 |
// Internal helper methods |
82 |
|
83 |
/** |
84 |
* Circularly increment i. |
85 |
*/ |
86 |
final int inc(int i) { |
87 |
return (++i == items.length) ? 0 : i; |
88 |
} |
89 |
|
90 |
/** |
91 |
* Circularly decrement i. |
92 |
*/ |
93 |
final int dec(int i) { |
94 |
return ((i == 0) ? items.length : i) - 1; |
95 |
} |
96 |
|
97 |
@SuppressWarnings("unchecked") |
98 |
static <E> E cast(Object item) { |
99 |
return (E) item; |
100 |
} |
101 |
|
102 |
/** |
103 |
* Returns item at index i. |
104 |
*/ |
105 |
final E itemAt(int i) { |
106 |
return this.<E>cast(items[i]); |
107 |
} |
108 |
|
109 |
/** |
110 |
* Throws NullPointerException if argument is null. |
111 |
* |
112 |
* @param v the element |
113 |
*/ |
114 |
private static void checkNotNull(Object v) { |
115 |
if (v == null) |
116 |
throw new NullPointerException(); |
117 |
} |
118 |
|
119 |
/** |
120 |
* Inserts element at current put position, advances, and signals. |
121 |
* Call only when holding lock. |
122 |
*/ |
123 |
private void insert(E x) { |
124 |
items[putIndex] = x; |
125 |
putIndex = inc(putIndex); |
126 |
++count; |
127 |
notEmpty.signal(); |
128 |
} |
129 |
|
130 |
/** |
131 |
* Extracts element at current take position, advances, and signals. |
132 |
* Call only when holding lock. |
133 |
*/ |
134 |
private E extract() { |
135 |
final Object[] items = this.items; |
136 |
E x = this.<E>cast(items[takeIndex]); |
137 |
items[takeIndex] = null; |
138 |
takeIndex = inc(takeIndex); |
139 |
--count; |
140 |
notFull.signal(); |
141 |
return x; |
142 |
} |
143 |
|
144 |
/** |
145 |
* Deletes item at position i. |
146 |
* Utility for remove and iterator.remove. |
147 |
* Call only when holding lock. |
148 |
*/ |
149 |
void removeAt(int i) { |
150 |
final Object[] items = this.items; |
151 |
// if removing front item, just advance |
152 |
if (i == takeIndex) { |
153 |
items[takeIndex] = null; |
154 |
takeIndex = inc(takeIndex); |
155 |
} else { |
156 |
// slide over all others up through putIndex. |
157 |
for (;;) { |
158 |
int nexti = inc(i); |
159 |
if (nexti != putIndex) { |
160 |
items[i] = items[nexti]; |
161 |
i = nexti; |
162 |
} else { |
163 |
items[i] = null; |
164 |
putIndex = i; |
165 |
break; |
166 |
} |
167 |
} |
168 |
} |
169 |
--count; |
170 |
notFull.signal(); |
171 |
} |
172 |
|
173 |
/** |
174 |
* Creates an {@code ArrayBlockingQueue} with the given (fixed) |
175 |
* capacity and default access policy. |
176 |
* |
177 |
* @param capacity the capacity of this queue |
178 |
* @throws IllegalArgumentException if {@code capacity < 1} |
179 |
*/ |
180 |
public ArrayBlockingQueue(int capacity) { |
181 |
this(capacity, false); |
182 |
} |
183 |
|
184 |
/** |
185 |
* Creates an {@code ArrayBlockingQueue} with the given (fixed) |
186 |
* capacity and the specified access policy. |
187 |
* |
188 |
* @param capacity the capacity of this queue |
189 |
* @param fair if {@code true} then queue accesses for threads blocked |
190 |
* on insertion or removal, are processed in FIFO order; |
191 |
* if {@code false} the access order is unspecified. |
192 |
* @throws IllegalArgumentException if {@code capacity < 1} |
193 |
*/ |
194 |
public ArrayBlockingQueue(int capacity, boolean fair) { |
195 |
if (capacity <= 0) |
196 |
throw new IllegalArgumentException(); |
197 |
this.items = new Object[capacity]; |
198 |
lock = new ReentrantLock(fair); |
199 |
notEmpty = lock.newCondition(); |
200 |
notFull = lock.newCondition(); |
201 |
} |
202 |
|
203 |
/** |
204 |
* Creates an {@code ArrayBlockingQueue} with the given (fixed) |
205 |
* capacity, the specified access policy and initially containing the |
206 |
* elements of the given collection, |
207 |
* added in traversal order of the collection's iterator. |
208 |
* |
209 |
* @param capacity the capacity of this queue |
210 |
* @param fair if {@code true} then queue accesses for threads blocked |
211 |
* on insertion or removal, are processed in FIFO order; |
212 |
* if {@code false} the access order is unspecified. |
213 |
* @param c the collection of elements to initially contain |
214 |
* @throws IllegalArgumentException if {@code capacity} is less than |
215 |
* {@code c.size()}, or less than 1. |
216 |
* @throws NullPointerException if the specified collection or any |
217 |
* of its elements are null |
218 |
*/ |
219 |
public ArrayBlockingQueue(int capacity, boolean fair, |
220 |
Collection<? extends E> c) { |
221 |
this(capacity, fair); |
222 |
|
223 |
final ReentrantLock lock = this.lock; |
224 |
lock.lock(); // Lock only for visibility, not mutual exclusion |
225 |
try { |
226 |
int i = 0; |
227 |
try { |
228 |
for (E e : c) { |
229 |
checkNotNull(e); |
230 |
items[i++] = e; |
231 |
} |
232 |
} catch (ArrayIndexOutOfBoundsException ex) { |
233 |
throw new IllegalArgumentException(); |
234 |
} |
235 |
count = i; |
236 |
putIndex = (i == capacity) ? 0 : i; |
237 |
} finally { |
238 |
lock.unlock(); |
239 |
} |
240 |
} |
241 |
|
242 |
/** |
243 |
* Inserts the specified element at the tail of this queue if it is |
244 |
* possible to do so immediately without exceeding the queue's capacity, |
245 |
* returning {@code true} upon success and throwing an |
246 |
* {@code IllegalStateException} if this queue is full. |
247 |
* |
248 |
* @param e the element to add |
249 |
* @return {@code true} (as specified by {@link Collection#add}) |
250 |
* @throws IllegalStateException if this queue is full |
251 |
* @throws NullPointerException if the specified element is null |
252 |
*/ |
253 |
public boolean add(E e) { |
254 |
return super.add(e); |
255 |
} |
256 |
|
257 |
/** |
258 |
* Inserts the specified element at the tail of this queue if it is |
259 |
* possible to do so immediately without exceeding the queue's capacity, |
260 |
* returning {@code true} upon success and {@code false} if this queue |
261 |
* is full. This method is generally preferable to method {@link #add}, |
262 |
* which can fail to insert an element only by throwing an exception. |
263 |
* |
264 |
* @throws NullPointerException if the specified element is null |
265 |
*/ |
266 |
public boolean offer(E e) { |
267 |
checkNotNull(e); |
268 |
final ReentrantLock lock = this.lock; |
269 |
lock.lock(); |
270 |
try { |
271 |
if (count == items.length) |
272 |
return false; |
273 |
else { |
274 |
insert(e); |
275 |
return true; |
276 |
} |
277 |
} finally { |
278 |
lock.unlock(); |
279 |
} |
280 |
} |
281 |
|
282 |
/** |
283 |
* Inserts the specified element at the tail of this queue, waiting |
284 |
* for space to become available if the queue is full. |
285 |
* |
286 |
* @throws InterruptedException {@inheritDoc} |
287 |
* @throws NullPointerException {@inheritDoc} |
288 |
*/ |
289 |
public void put(E e) throws InterruptedException { |
290 |
checkNotNull(e); |
291 |
final ReentrantLock lock = this.lock; |
292 |
lock.lockInterruptibly(); |
293 |
try { |
294 |
while (count == items.length) |
295 |
notFull.await(); |
296 |
insert(e); |
297 |
} finally { |
298 |
lock.unlock(); |
299 |
} |
300 |
} |
301 |
|
302 |
/** |
303 |
* Inserts the specified element at the tail of this queue, waiting |
304 |
* up to the specified wait time for space to become available if |
305 |
* the queue is full. |
306 |
* |
307 |
* @throws InterruptedException {@inheritDoc} |
308 |
* @throws NullPointerException {@inheritDoc} |
309 |
*/ |
310 |
public boolean offer(E e, long timeout, TimeUnit unit) |
311 |
throws InterruptedException { |
312 |
|
313 |
checkNotNull(e); |
314 |
long nanos = unit.toNanos(timeout); |
315 |
final ReentrantLock lock = this.lock; |
316 |
lock.lockInterruptibly(); |
317 |
try { |
318 |
while (count == items.length) { |
319 |
if (nanos <= 0) |
320 |
return false; |
321 |
nanos = notFull.awaitNanos(nanos); |
322 |
} |
323 |
insert(e); |
324 |
return true; |
325 |
} finally { |
326 |
lock.unlock(); |
327 |
} |
328 |
} |
329 |
|
330 |
public E poll() { |
331 |
final ReentrantLock lock = this.lock; |
332 |
lock.lock(); |
333 |
try { |
334 |
return (count == 0) ? null : extract(); |
335 |
} finally { |
336 |
lock.unlock(); |
337 |
} |
338 |
} |
339 |
|
340 |
public E take() throws InterruptedException { |
341 |
final ReentrantLock lock = this.lock; |
342 |
lock.lockInterruptibly(); |
343 |
try { |
344 |
while (count == 0) |
345 |
notEmpty.await(); |
346 |
return extract(); |
347 |
} finally { |
348 |
lock.unlock(); |
349 |
} |
350 |
} |
351 |
|
352 |
public E poll(long timeout, TimeUnit unit) throws InterruptedException { |
353 |
long nanos = unit.toNanos(timeout); |
354 |
final ReentrantLock lock = this.lock; |
355 |
lock.lockInterruptibly(); |
356 |
try { |
357 |
while (count == 0) { |
358 |
if (nanos <= 0) |
359 |
return null; |
360 |
nanos = notEmpty.awaitNanos(nanos); |
361 |
} |
362 |
return extract(); |
363 |
} finally { |
364 |
lock.unlock(); |
365 |
} |
366 |
} |
367 |
|
368 |
public E peek() { |
369 |
final ReentrantLock lock = this.lock; |
370 |
lock.lock(); |
371 |
try { |
372 |
return (count == 0) ? null : itemAt(takeIndex); |
373 |
} finally { |
374 |
lock.unlock(); |
375 |
} |
376 |
} |
377 |
|
378 |
// this doc comment is overridden to remove the reference to collections |
379 |
// greater in size than Integer.MAX_VALUE |
380 |
/** |
381 |
* Returns the number of elements in this queue. |
382 |
* |
383 |
* @return the number of elements in this queue |
384 |
*/ |
385 |
public int size() { |
386 |
final ReentrantLock lock = this.lock; |
387 |
lock.lock(); |
388 |
try { |
389 |
return count; |
390 |
} finally { |
391 |
lock.unlock(); |
392 |
} |
393 |
} |
394 |
|
395 |
// this doc comment is a modified copy of the inherited doc comment, |
396 |
// without the reference to unlimited queues. |
397 |
/** |
398 |
* Returns the number of additional elements that this queue can ideally |
399 |
* (in the absence of memory or resource constraints) accept without |
400 |
* blocking. This is always equal to the initial capacity of this queue |
401 |
* less the current {@code size} of this queue. |
402 |
* |
403 |
* <p>Note that you <em>cannot</em> always tell if an attempt to insert |
404 |
* an element will succeed by inspecting {@code remainingCapacity} |
405 |
* because it may be the case that another thread is about to |
406 |
* insert or remove an element. |
407 |
*/ |
408 |
public int remainingCapacity() { |
409 |
final ReentrantLock lock = this.lock; |
410 |
lock.lock(); |
411 |
try { |
412 |
return items.length - count; |
413 |
} finally { |
414 |
lock.unlock(); |
415 |
} |
416 |
} |
417 |
|
418 |
/** |
419 |
* Removes a single instance of the specified element from this queue, |
420 |
* if it is present. More formally, removes an element {@code e} such |
421 |
* that {@code o.equals(e)}, if this queue contains one or more such |
422 |
* elements. |
423 |
* Returns {@code true} if this queue contained the specified element |
424 |
* (or equivalently, if this queue changed as a result of the call). |
425 |
* |
426 |
* <p>Removal of interior elements in circular array based queues |
427 |
* is an intrinsically slow and disruptive operation, so should |
428 |
* be undertaken only in exceptional circumstances, ideally |
429 |
* only when the queue is known not to be accessible by other |
430 |
* threads. |
431 |
* |
432 |
* @param o element to be removed from this queue, if present |
433 |
* @return {@code true} if this queue changed as a result of the call |
434 |
*/ |
435 |
public boolean remove(Object o) { |
436 |
if (o == null) return false; |
437 |
final Object[] items = this.items; |
438 |
final ReentrantLock lock = this.lock; |
439 |
lock.lock(); |
440 |
try { |
441 |
for (int i = takeIndex, k = count; k > 0; i = inc(i), k--) { |
442 |
if (o.equals(items[i])) { |
443 |
removeAt(i); |
444 |
return true; |
445 |
} |
446 |
} |
447 |
return false; |
448 |
} finally { |
449 |
lock.unlock(); |
450 |
} |
451 |
} |
452 |
|
453 |
/** |
454 |
* Returns {@code true} if this queue contains the specified element. |
455 |
* More formally, returns {@code true} if and only if this queue contains |
456 |
* at least one element {@code e} such that {@code o.equals(e)}. |
457 |
* |
458 |
* @param o object to be checked for containment in this queue |
459 |
* @return {@code true} if this queue contains the specified element |
460 |
*/ |
461 |
public boolean contains(Object o) { |
462 |
if (o == null) return false; |
463 |
final Object[] items = this.items; |
464 |
final ReentrantLock lock = this.lock; |
465 |
lock.lock(); |
466 |
try { |
467 |
for (int i = takeIndex, k = count; k > 0; i = inc(i), k--) |
468 |
if (o.equals(items[i])) |
469 |
return true; |
470 |
return false; |
471 |
} finally { |
472 |
lock.unlock(); |
473 |
} |
474 |
} |
475 |
|
476 |
/** |
477 |
* Returns an array containing all of the elements in this queue, in |
478 |
* proper sequence. |
479 |
* |
480 |
* <p>The returned array will be "safe" in that no references to it are |
481 |
* maintained by this queue. (In other words, this method must allocate |
482 |
* a new array). The caller is thus free to modify the returned array. |
483 |
* |
484 |
* <p>This method acts as bridge between array-based and collection-based |
485 |
* APIs. |
486 |
* |
487 |
* @return an array containing all of the elements in this queue |
488 |
*/ |
489 |
public Object[] toArray() { |
490 |
final Object[] items = this.items; |
491 |
final ReentrantLock lock = this.lock; |
492 |
lock.lock(); |
493 |
try { |
494 |
final int count = this.count; |
495 |
Object[] a = new Object[count]; |
496 |
for (int i = takeIndex, k = 0; k < count; i = inc(i), k++) |
497 |
a[k] = items[i]; |
498 |
return a; |
499 |
} finally { |
500 |
lock.unlock(); |
501 |
} |
502 |
} |
503 |
|
504 |
/** |
505 |
* Returns an array containing all of the elements in this queue, in |
506 |
* proper sequence; the runtime type of the returned array is that of |
507 |
* the specified array. If the queue fits in the specified array, it |
508 |
* is returned therein. Otherwise, a new array is allocated with the |
509 |
* runtime type of the specified array and the size of this queue. |
510 |
* |
511 |
* <p>If this queue fits in the specified array with room to spare |
512 |
* (i.e., the array has more elements than this queue), the element in |
513 |
* the array immediately following the end of the queue is set to |
514 |
* {@code null}. |
515 |
* |
516 |
* <p>Like the {@link #toArray()} method, this method acts as bridge between |
517 |
* array-based and collection-based APIs. Further, this method allows |
518 |
* precise control over the runtime type of the output array, and may, |
519 |
* under certain circumstances, be used to save allocation costs. |
520 |
* |
521 |
* <p>Suppose {@code x} is a queue known to contain only strings. |
522 |
* The following code can be used to dump the queue into a newly |
523 |
* allocated array of {@code String}: |
524 |
* |
525 |
* <pre> |
526 |
* String[] y = x.toArray(new String[0]);</pre> |
527 |
* |
528 |
* Note that {@code toArray(new Object[0])} is identical in function to |
529 |
* {@code toArray()}. |
530 |
* |
531 |
* @param a the array into which the elements of the queue are to |
532 |
* be stored, if it is big enough; otherwise, a new array of the |
533 |
* same runtime type is allocated for this purpose |
534 |
* @return an array containing all of the elements in this queue |
535 |
* @throws ArrayStoreException if the runtime type of the specified array |
536 |
* is not a supertype of the runtime type of every element in |
537 |
* this queue |
538 |
* @throws NullPointerException if the specified array is null |
539 |
*/ |
540 |
@SuppressWarnings("unchecked") |
541 |
public <T> T[] toArray(T[] a) { |
542 |
final Object[] items = this.items; |
543 |
final ReentrantLock lock = this.lock; |
544 |
lock.lock(); |
545 |
try { |
546 |
final int count = this.count; |
547 |
final int len = a.length; |
548 |
if (len < count) |
549 |
a = (T[])java.lang.reflect.Array.newInstance( |
550 |
a.getClass().getComponentType(), count); |
551 |
for (int i = takeIndex, k = 0; k < count; i = inc(i), k++) |
552 |
a[k] = (T) items[i]; |
553 |
if (len > count) |
554 |
a[count] = null; |
555 |
return a; |
556 |
} finally { |
557 |
lock.unlock(); |
558 |
} |
559 |
} |
560 |
|
561 |
public String toString() { |
562 |
final ReentrantLock lock = this.lock; |
563 |
lock.lock(); |
564 |
try { |
565 |
int k = count; |
566 |
if (k == 0) |
567 |
return "[]"; |
568 |
|
569 |
StringBuilder sb = new StringBuilder(); |
570 |
sb.append('['); |
571 |
for (int i = takeIndex; ; i = inc(i)) { |
572 |
Object e = items[i]; |
573 |
sb.append(e == this ? "(this Collection)" : e); |
574 |
if (--k == 0) |
575 |
return sb.append(']').toString(); |
576 |
sb.append(',').append(' '); |
577 |
} |
578 |
} finally { |
579 |
lock.unlock(); |
580 |
} |
581 |
} |
582 |
|
583 |
/** |
584 |
* Atomically removes all of the elements from this queue. |
585 |
* The queue will be empty after this call returns. |
586 |
*/ |
587 |
public void clear() { |
588 |
final Object[] items = this.items; |
589 |
final ReentrantLock lock = this.lock; |
590 |
lock.lock(); |
591 |
try { |
592 |
for (int i = takeIndex, k = count; k > 0; i = inc(i), k--) |
593 |
items[i] = null; |
594 |
count = 0; |
595 |
putIndex = 0; |
596 |
takeIndex = 0; |
597 |
notFull.signalAll(); |
598 |
} finally { |
599 |
lock.unlock(); |
600 |
} |
601 |
} |
602 |
|
603 |
/** |
604 |
* @throws UnsupportedOperationException {@inheritDoc} |
605 |
* @throws ClassCastException {@inheritDoc} |
606 |
* @throws NullPointerException {@inheritDoc} |
607 |
* @throws IllegalArgumentException {@inheritDoc} |
608 |
*/ |
609 |
public int drainTo(Collection<? super E> c) { |
610 |
checkNotNull(c); |
611 |
if (c == this) |
612 |
throw new IllegalArgumentException(); |
613 |
final Object[] items = this.items; |
614 |
final ReentrantLock lock = this.lock; |
615 |
lock.lock(); |
616 |
try { |
617 |
int i = takeIndex; |
618 |
int n = 0; |
619 |
int max = count; |
620 |
while (n < max) { |
621 |
c.add(this.<E>cast(items[i])); |
622 |
items[i] = null; |
623 |
i = inc(i); |
624 |
++n; |
625 |
} |
626 |
if (n > 0) { |
627 |
count = 0; |
628 |
putIndex = 0; |
629 |
takeIndex = 0; |
630 |
notFull.signalAll(); |
631 |
} |
632 |
return n; |
633 |
} finally { |
634 |
lock.unlock(); |
635 |
} |
636 |
} |
637 |
|
638 |
/** |
639 |
* @throws UnsupportedOperationException {@inheritDoc} |
640 |
* @throws ClassCastException {@inheritDoc} |
641 |
* @throws NullPointerException {@inheritDoc} |
642 |
* @throws IllegalArgumentException {@inheritDoc} |
643 |
*/ |
644 |
public int drainTo(Collection<? super E> c, int maxElements) { |
645 |
checkNotNull(c); |
646 |
if (c == this) |
647 |
throw new IllegalArgumentException(); |
648 |
if (maxElements <= 0) |
649 |
return 0; |
650 |
final Object[] items = this.items; |
651 |
final ReentrantLock lock = this.lock; |
652 |
lock.lock(); |
653 |
try { |
654 |
int i = takeIndex; |
655 |
int n = 0; |
656 |
int sz = count; |
657 |
int max = (maxElements < count) ? maxElements : count; |
658 |
while (n < max) { |
659 |
c.add(this.<E>cast(items[i])); |
660 |
items[i] = null; |
661 |
i = inc(i); |
662 |
++n; |
663 |
} |
664 |
if (n > 0) { |
665 |
count -= n; |
666 |
takeIndex = i; |
667 |
notFull.signalAll(); |
668 |
} |
669 |
return n; |
670 |
} finally { |
671 |
lock.unlock(); |
672 |
} |
673 |
} |
674 |
|
675 |
/** |
676 |
* Returns an iterator over the elements in this queue in proper |
677 |
* sequence. The returned {@code Iterator} is "weakly |
678 |
* consistent" with respect to operations at the head and tail of |
679 |
* the queue, and will never throw {@link |
680 |
* ConcurrentModificationException}. It might return elements |
681 |
* that existed upon construction of the iterator but have since |
682 |
* been polled or taken, and might not return elements that have |
683 |
* since been added. Further, no consistency guarantees are made |
684 |
* with respect to "interior" removals occuring in concurrent |
685 |
* invocations of {@link Collection#remove(Object)} or {@link |
686 |
* Iterator#remove} occurring in other threads. |
687 |
* |
688 |
* <p>The returned iterator supports the optional {@link Iterator#remove} |
689 |
* operation. However, removal of interior elements in circular |
690 |
* array based queues is an intrinsically slow and disruptive |
691 |
* operation, so should be undertaken only in exceptional |
692 |
* circumstances, ideally only when the queue is known not to be |
693 |
* accessible by other threads. |
694 |
* |
695 |
* @return an iterator over the elements in this queue in proper sequence |
696 |
*/ |
697 |
public Iterator<E> iterator() { |
698 |
return new Itr(); |
699 |
} |
700 |
|
701 |
/** |
702 |
* Iterator for ArrayBlockingQueue. To maintain weak consistency |
703 |
* with respect to puts and takes, we (1) read ahead one slot, so |
704 |
* as to not report hasNext true but then not have an element to |
705 |
* return (2) ensure that each array slot is traversed at most |
706 |
* once (by tracking "remaining" elements); (3) skip over null |
707 |
* slots, which can occur if takes race ahead of iterators. |
708 |
* However, for circular array-based queues, we cannot rely on any |
709 |
* well established definition of what it means to be weakly |
710 |
* consistent with respect to interior removes since these may |
711 |
* require slot overwrites in the process of sliding elements to |
712 |
* cover gaps. So we settle for resiliency, operating on |
713 |
* established apparent nexts, which may miss some elements that |
714 |
* have moved between calls to next. |
715 |
*/ |
716 |
private class Itr implements Iterator<E> { |
717 |
private int remaining; // Number of elements yet to be returned |
718 |
private int nextIndex; // Index of element to be returned by next |
719 |
private E nextItem; // Element to be returned by next call to next |
720 |
private E lastItem; // Element returned by last call to next |
721 |
private int lastRet; // Index of last element returned, or -1 if none |
722 |
|
723 |
Itr() { |
724 |
final ReentrantLock lock = ArrayBlockingQueue.this.lock; |
725 |
lock.lock(); |
726 |
try { |
727 |
lastRet = -1; |
728 |
if ((remaining = count) > 0) |
729 |
nextItem = itemAt(nextIndex = takeIndex); |
730 |
} finally { |
731 |
lock.unlock(); |
732 |
} |
733 |
} |
734 |
|
735 |
public boolean hasNext() { |
736 |
return remaining > 0; |
737 |
} |
738 |
|
739 |
public E next() { |
740 |
final ReentrantLock lock = ArrayBlockingQueue.this.lock; |
741 |
lock.lock(); |
742 |
try { |
743 |
if (remaining <= 0) |
744 |
throw new NoSuchElementException(); |
745 |
lastRet = nextIndex; |
746 |
E x = lastItem = nextItem; |
747 |
while (--remaining > 0) { |
748 |
if ((nextItem = itemAt(nextIndex = inc(nextIndex))) != null) |
749 |
break; |
750 |
} |
751 |
return x; |
752 |
} finally { |
753 |
lock.unlock(); |
754 |
} |
755 |
} |
756 |
|
757 |
public void remove() { |
758 |
final ReentrantLock lock = ArrayBlockingQueue.this.lock; |
759 |
lock.lock(); |
760 |
try { |
761 |
int i = lastRet; |
762 |
if (i == -1) |
763 |
throw new IllegalStateException(); |
764 |
lastRet = -1; |
765 |
E x = lastItem; |
766 |
lastItem = null; |
767 |
// only remove if item still at index |
768 |
if (x == items[i]) { |
769 |
boolean removingHead = (i == takeIndex); |
770 |
removeAt(i); |
771 |
if (!removingHead) |
772 |
nextIndex = dec(nextIndex); |
773 |
} |
774 |
} finally { |
775 |
lock.unlock(); |
776 |
} |
777 |
} |
778 |
} |
779 |
|
780 |
} |