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 |
* increased. Attempts to <tt>put</tt> an element into a full queue |
24 |
* will result in the operation blocking; attempts to <tt>take</tt> 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 <tt>true</tt> 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}/../guide/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 |
private final E[] items; |
59 |
/** items index for next take, poll or remove */ |
60 |
private transient int takeIndex; |
61 |
/** items index for next put, offer, or add. */ |
62 |
private transient int putIndex; |
63 |
/** Number of items in the queue */ |
64 |
private int count; |
65 |
|
66 |
/* |
67 |
* Concurrency control uses the classic two-condition algorithm |
68 |
* found in any textbook. |
69 |
*/ |
70 |
|
71 |
/** Main lock guarding all access */ |
72 |
private final ReentrantLock lock; |
73 |
/** Condition for waiting takes */ |
74 |
private final Condition notEmpty; |
75 |
/** Condition for waiting puts */ |
76 |
private final Condition notFull; |
77 |
|
78 |
// Internal helper methods |
79 |
|
80 |
/** |
81 |
* Circularly increment i. |
82 |
*/ |
83 |
final int inc(int i) { |
84 |
return (++i == items.length)? 0 : i; |
85 |
} |
86 |
|
87 |
/** |
88 |
* Inserts element at current put position, advances, and signals. |
89 |
* Call only when holding lock. |
90 |
*/ |
91 |
private void insert(E x) { |
92 |
items[putIndex] = x; |
93 |
putIndex = inc(putIndex); |
94 |
++count; |
95 |
notEmpty.signal(); |
96 |
} |
97 |
|
98 |
/** |
99 |
* Extracts element at current take position, advances, and signals. |
100 |
* Call only when holding lock. |
101 |
*/ |
102 |
private E extract() { |
103 |
final E[] items = this.items; |
104 |
E x = items[takeIndex]; |
105 |
items[takeIndex] = null; |
106 |
takeIndex = inc(takeIndex); |
107 |
--count; |
108 |
notFull.signal(); |
109 |
return x; |
110 |
} |
111 |
|
112 |
/** |
113 |
* Utility for remove and iterator.remove: Delete item at position i. |
114 |
* Call only when holding lock. |
115 |
*/ |
116 |
void removeAt(int i) { |
117 |
final E[] items = this.items; |
118 |
// if removing front item, just advance |
119 |
if (i == takeIndex) { |
120 |
items[takeIndex] = null; |
121 |
takeIndex = inc(takeIndex); |
122 |
} else { |
123 |
// slide over all others up through putIndex. |
124 |
for (;;) { |
125 |
int nexti = inc(i); |
126 |
if (nexti != putIndex) { |
127 |
items[i] = items[nexti]; |
128 |
i = nexti; |
129 |
} else { |
130 |
items[i] = null; |
131 |
putIndex = i; |
132 |
break; |
133 |
} |
134 |
} |
135 |
} |
136 |
--count; |
137 |
notFull.signal(); |
138 |
} |
139 |
|
140 |
/** |
141 |
* Creates an <tt>ArrayBlockingQueue</tt> with the given (fixed) |
142 |
* capacity and default access policy. |
143 |
* |
144 |
* @param capacity the capacity of this queue |
145 |
* @throws IllegalArgumentException if <tt>capacity</tt> is less than 1 |
146 |
*/ |
147 |
public ArrayBlockingQueue(int capacity) { |
148 |
this(capacity, false); |
149 |
} |
150 |
|
151 |
/** |
152 |
* Creates an <tt>ArrayBlockingQueue</tt> with the given (fixed) |
153 |
* capacity and the specified access policy. |
154 |
* |
155 |
* @param capacity the capacity of this queue |
156 |
* @param fair if <tt>true</tt> then queue accesses for threads blocked |
157 |
* on insertion or removal, are processed in FIFO order; |
158 |
* if <tt>false</tt> the access order is unspecified. |
159 |
* @throws IllegalArgumentException if <tt>capacity</tt> is less than 1 |
160 |
*/ |
161 |
public ArrayBlockingQueue(int capacity, boolean fair) { |
162 |
if (capacity <= 0) |
163 |
throw new IllegalArgumentException(); |
164 |
this.items = (E[]) new Object[capacity]; |
165 |
lock = new ReentrantLock(fair); |
166 |
notEmpty = lock.newCondition(); |
167 |
notFull = lock.newCondition(); |
168 |
} |
169 |
|
170 |
/** |
171 |
* Creates an <tt>ArrayBlockingQueue</tt> with the given (fixed) |
172 |
* capacity, the specified access policy and initially containing the |
173 |
* elements of the given collection, |
174 |
* added in traversal order of the collection's iterator. |
175 |
* |
176 |
* @param capacity the capacity of this queue |
177 |
* @param fair if <tt>true</tt> then queue accesses for threads blocked |
178 |
* on insertion or removal, are processed in FIFO order; |
179 |
* if <tt>false</tt> the access order is unspecified. |
180 |
* @param c the collection of elements to initially contain |
181 |
* @throws IllegalArgumentException if <tt>capacity</tt> is less than |
182 |
* <tt>c.size()</tt>, or less than 1. |
183 |
* @throws NullPointerException if the specified collection or any |
184 |
* of its elements are null |
185 |
*/ |
186 |
public ArrayBlockingQueue(int capacity, boolean fair, |
187 |
Collection<? extends E> c) { |
188 |
this(capacity, fair); |
189 |
if (capacity < c.size()) |
190 |
throw new IllegalArgumentException(); |
191 |
|
192 |
for (Iterator<? extends E> it = c.iterator(); it.hasNext();) |
193 |
add(it.next()); |
194 |
} |
195 |
|
196 |
/** |
197 |
* Inserts the specified element at the tail of this queue if it is |
198 |
* possible to do so immediately without exceeding the queue's capacity, |
199 |
* returning <tt>true</tt> upon success and throwing an |
200 |
* <tt>IllegalStateException</tt> if this queue is full. |
201 |
* |
202 |
* @param e the element to add |
203 |
* @return <tt>true</tt> (as per the spec for {@link Collection#add}) |
204 |
* @throws IllegalStateException if this queue is full |
205 |
* @throws NullPointerException if the specified element is null |
206 |
*/ |
207 |
public boolean add(E e) { |
208 |
return super.add(e); |
209 |
} |
210 |
|
211 |
/** |
212 |
* Inserts the specified element at the tail of this queue if it is |
213 |
* possible to do so immediately without exceeding the queue's capacity, |
214 |
* returning <tt>true</tt> upon success and <tt>false</tt> if this queue |
215 |
* is full. This method is generally preferable to method {@link #add}, |
216 |
* which can fail to insert an element only by throwing an exception. |
217 |
* |
218 |
* @throws NullPointerException if the specified element is null |
219 |
*/ |
220 |
public boolean offer(E e) { |
221 |
if (e == null) throw new NullPointerException(); |
222 |
final ReentrantLock lock = this.lock; |
223 |
lock.lock(); |
224 |
try { |
225 |
if (count == items.length) |
226 |
return false; |
227 |
else { |
228 |
insert(e); |
229 |
return true; |
230 |
} |
231 |
} finally { |
232 |
lock.unlock(); |
233 |
} |
234 |
} |
235 |
|
236 |
/** |
237 |
* Inserts the specified element at the tail of this queue, waiting |
238 |
* for space to become available if the queue is full. |
239 |
* |
240 |
* @throws InterruptedException {@inheritDoc} |
241 |
* @throws NullPointerException {@inheritDoc} |
242 |
*/ |
243 |
public void put(E e) throws InterruptedException { |
244 |
if (e == null) throw new NullPointerException(); |
245 |
final E[] items = this.items; |
246 |
final ReentrantLock lock = this.lock; |
247 |
lock.lockInterruptibly(); |
248 |
try { |
249 |
try { |
250 |
while (count == items.length) |
251 |
notFull.await(); |
252 |
} catch (InterruptedException ie) { |
253 |
notFull.signal(); // propagate to non-interrupted thread |
254 |
throw ie; |
255 |
} |
256 |
insert(e); |
257 |
} finally { |
258 |
lock.unlock(); |
259 |
} |
260 |
} |
261 |
|
262 |
/** |
263 |
* Inserts the specified element at the tail of this queue, waiting |
264 |
* up to the specified wait time for space to become available if |
265 |
* the queue is full. |
266 |
* |
267 |
* @throws InterruptedException {@inheritDoc} |
268 |
* @throws NullPointerException {@inheritDoc} |
269 |
*/ |
270 |
public boolean offer(E e, long timeout, TimeUnit unit) |
271 |
throws InterruptedException { |
272 |
|
273 |
if (e == null) throw new NullPointerException(); |
274 |
final ReentrantLock lock = this.lock; |
275 |
lock.lockInterruptibly(); |
276 |
try { |
277 |
long nanos = unit.toNanos(timeout); |
278 |
for (;;) { |
279 |
if (count != items.length) { |
280 |
insert(e); |
281 |
return true; |
282 |
} |
283 |
if (nanos <= 0) |
284 |
return false; |
285 |
try { |
286 |
nanos = notFull.awaitNanos(nanos); |
287 |
} catch (InterruptedException ie) { |
288 |
notFull.signal(); // propagate to non-interrupted thread |
289 |
throw ie; |
290 |
} |
291 |
} |
292 |
} finally { |
293 |
lock.unlock(); |
294 |
} |
295 |
} |
296 |
|
297 |
public E poll() { |
298 |
final ReentrantLock lock = this.lock; |
299 |
lock.lock(); |
300 |
try { |
301 |
if (count == 0) |
302 |
return null; |
303 |
E x = extract(); |
304 |
return x; |
305 |
} finally { |
306 |
lock.unlock(); |
307 |
} |
308 |
} |
309 |
|
310 |
public E take() throws InterruptedException { |
311 |
final ReentrantLock lock = this.lock; |
312 |
lock.lockInterruptibly(); |
313 |
try { |
314 |
try { |
315 |
while (count == 0) |
316 |
notEmpty.await(); |
317 |
} catch (InterruptedException ie) { |
318 |
notEmpty.signal(); // propagate to non-interrupted thread |
319 |
throw ie; |
320 |
} |
321 |
E x = extract(); |
322 |
return x; |
323 |
} finally { |
324 |
lock.unlock(); |
325 |
} |
326 |
} |
327 |
|
328 |
public E poll(long timeout, TimeUnit unit) throws InterruptedException { |
329 |
final ReentrantLock lock = this.lock; |
330 |
lock.lockInterruptibly(); |
331 |
try { |
332 |
long nanos = unit.toNanos(timeout); |
333 |
for (;;) { |
334 |
if (count != 0) { |
335 |
E x = extract(); |
336 |
return x; |
337 |
} |
338 |
if (nanos <= 0) |
339 |
return null; |
340 |
try { |
341 |
nanos = notEmpty.awaitNanos(nanos); |
342 |
} catch (InterruptedException ie) { |
343 |
notEmpty.signal(); // propagate to non-interrupted thread |
344 |
throw ie; |
345 |
} |
346 |
|
347 |
} |
348 |
} finally { |
349 |
lock.unlock(); |
350 |
} |
351 |
} |
352 |
|
353 |
public E peek() { |
354 |
final ReentrantLock lock = this.lock; |
355 |
lock.lock(); |
356 |
try { |
357 |
return (count == 0) ? null : items[takeIndex]; |
358 |
} finally { |
359 |
lock.unlock(); |
360 |
} |
361 |
} |
362 |
|
363 |
// this doc comment is overridden to remove the reference to collections |
364 |
// greater in size than Integer.MAX_VALUE |
365 |
/** |
366 |
* Returns the number of elements in this queue. |
367 |
* |
368 |
* @return the number of elements in this queue |
369 |
*/ |
370 |
public int size() { |
371 |
final ReentrantLock lock = this.lock; |
372 |
lock.lock(); |
373 |
try { |
374 |
return count; |
375 |
} finally { |
376 |
lock.unlock(); |
377 |
} |
378 |
} |
379 |
|
380 |
// this doc comment is a modified copy of the inherited doc comment, |
381 |
// without the reference to unlimited queues. |
382 |
/** |
383 |
* Returns the number of additional elements that this queue can ideally |
384 |
* (in the absence of memory or resource constraints) accept without |
385 |
* blocking. This is always equal to the initial capacity of this queue |
386 |
* less the current <tt>size</tt> of this queue. |
387 |
* |
388 |
* <p>Note that you <em>cannot</em> always tell if an attempt to insert |
389 |
* an element will succeed by inspecting <tt>remainingCapacity</tt> |
390 |
* because it may be the case that another thread is about to |
391 |
* insert or remove an element. |
392 |
*/ |
393 |
public int remainingCapacity() { |
394 |
final ReentrantLock lock = this.lock; |
395 |
lock.lock(); |
396 |
try { |
397 |
return items.length - count; |
398 |
} finally { |
399 |
lock.unlock(); |
400 |
} |
401 |
} |
402 |
|
403 |
/** |
404 |
* Removes a single instance of the specified element from this queue, |
405 |
* if it is present. More formally, removes an element <tt>e</tt> such |
406 |
* that <tt>o.equals(e)</tt>, if this queue contains one or more such |
407 |
* elements. |
408 |
* Returns <tt>true</tt> if this queue contained the specified element |
409 |
* (or equivalently, if this queue changed as a result of the call). |
410 |
* |
411 |
* @param o element to be removed from this queue, if present |
412 |
* @return <tt>true</tt> if this queue changed as a result of the call |
413 |
*/ |
414 |
public boolean remove(Object o) { |
415 |
if (o == null) return false; |
416 |
final E[] items = this.items; |
417 |
final ReentrantLock lock = this.lock; |
418 |
lock.lock(); |
419 |
try { |
420 |
int i = takeIndex; |
421 |
int k = 0; |
422 |
for (;;) { |
423 |
if (k++ >= count) |
424 |
return false; |
425 |
if (o.equals(items[i])) { |
426 |
removeAt(i); |
427 |
return true; |
428 |
} |
429 |
i = inc(i); |
430 |
} |
431 |
|
432 |
} finally { |
433 |
lock.unlock(); |
434 |
} |
435 |
} |
436 |
|
437 |
/** |
438 |
* Returns <tt>true</tt> if this queue contains the specified element. |
439 |
* More formally, returns <tt>true</tt> if and only if this queue contains |
440 |
* at least one element <tt>e</tt> such that <tt>o.equals(e)</tt>. |
441 |
* |
442 |
* @param o object to be checked for containment in this queue |
443 |
* @return <tt>true</tt> if this queue contains the specified element |
444 |
*/ |
445 |
public boolean contains(Object o) { |
446 |
if (o == null) return false; |
447 |
final E[] items = this.items; |
448 |
final ReentrantLock lock = this.lock; |
449 |
lock.lock(); |
450 |
try { |
451 |
int i = takeIndex; |
452 |
int k = 0; |
453 |
while (k++ < count) { |
454 |
if (o.equals(items[i])) |
455 |
return true; |
456 |
i = inc(i); |
457 |
} |
458 |
return false; |
459 |
} finally { |
460 |
lock.unlock(); |
461 |
} |
462 |
} |
463 |
|
464 |
/** |
465 |
* Returns an array containing all of the elements in this queue, in |
466 |
* proper sequence. |
467 |
* |
468 |
* <p>The returned array will be "safe" in that no references to it are |
469 |
* maintained by this queue. (In other words, this method must allocate |
470 |
* a new array). The caller is thus free to modify the returned array. |
471 |
* |
472 |
* <p>This method acts as bridge between array-based and collection-based |
473 |
* APIs. |
474 |
* |
475 |
* @return an array containing all of the elements in this queue |
476 |
*/ |
477 |
public Object[] toArray() { |
478 |
final E[] items = this.items; |
479 |
final ReentrantLock lock = this.lock; |
480 |
lock.lock(); |
481 |
try { |
482 |
Object[] a = new Object[count]; |
483 |
int k = 0; |
484 |
int i = takeIndex; |
485 |
while (k < count) { |
486 |
a[k++] = items[i]; |
487 |
i = inc(i); |
488 |
} |
489 |
return a; |
490 |
} finally { |
491 |
lock.unlock(); |
492 |
} |
493 |
} |
494 |
|
495 |
/** |
496 |
* Returns an array containing all of the elements in this queue, in |
497 |
* proper sequence; the runtime type of the returned array is that of |
498 |
* the specified array. If the queue fits in the specified array, it |
499 |
* is returned therein. Otherwise, a new array is allocated with the |
500 |
* runtime type of the specified array and the size of this queue. |
501 |
* |
502 |
* <p>If this queue fits in the specified array with room to spare |
503 |
* (i.e., the array has more elements than this queue), the element in |
504 |
* the array immediately following the end of the queue is set to |
505 |
* <tt>null</tt>. |
506 |
* |
507 |
* <p>Like the {@link #toArray()} method, this method acts as bridge between |
508 |
* array-based and collection-based APIs. Further, this method allows |
509 |
* precise control over the runtime type of the output array, and may, |
510 |
* under certain circumstances, be used to save allocation costs. |
511 |
* |
512 |
* <p>Suppose <tt>x</tt> is a queue known to contain only strings. |
513 |
* The following code can be used to dump the queue into a newly |
514 |
* allocated array of <tt>String</tt>: |
515 |
* |
516 |
* <pre> |
517 |
* String[] y = x.toArray(new String[0]);</pre> |
518 |
* |
519 |
* Note that <tt>toArray(new Object[0])</tt> is identical in function to |
520 |
* <tt>toArray()</tt>. |
521 |
* |
522 |
* @param a the array into which the elements of the queue are to |
523 |
* be stored, if it is big enough; otherwise, a new array of the |
524 |
* same runtime type is allocated for this purpose |
525 |
* @return an array containing all of the elements in this queue |
526 |
* @throws ArrayStoreException if the runtime type of the specified array |
527 |
* is not a supertype of the runtime type of every element in |
528 |
* this queue |
529 |
* @throws NullPointerException if the specified array is null |
530 |
*/ |
531 |
public <T> T[] toArray(T[] a) { |
532 |
final E[] items = this.items; |
533 |
final ReentrantLock lock = this.lock; |
534 |
lock.lock(); |
535 |
try { |
536 |
if (a.length < count) |
537 |
a = (T[])java.lang.reflect.Array.newInstance( |
538 |
a.getClass().getComponentType(), |
539 |
count |
540 |
); |
541 |
|
542 |
int k = 0; |
543 |
int i = takeIndex; |
544 |
while (k < count) { |
545 |
a[k++] = (T)items[i]; |
546 |
i = inc(i); |
547 |
} |
548 |
if (a.length > count) |
549 |
a[count] = null; |
550 |
return a; |
551 |
} finally { |
552 |
lock.unlock(); |
553 |
} |
554 |
} |
555 |
|
556 |
public String toString() { |
557 |
final ReentrantLock lock = this.lock; |
558 |
lock.lock(); |
559 |
try { |
560 |
return super.toString(); |
561 |
} finally { |
562 |
lock.unlock(); |
563 |
} |
564 |
} |
565 |
|
566 |
/** |
567 |
* Atomically removes all of the elements from this queue. |
568 |
* The queue will be empty after this call returns. |
569 |
*/ |
570 |
public void clear() { |
571 |
final E[] items = this.items; |
572 |
final ReentrantLock lock = this.lock; |
573 |
lock.lock(); |
574 |
try { |
575 |
int i = takeIndex; |
576 |
int k = count; |
577 |
while (k-- > 0) { |
578 |
items[i] = null; |
579 |
i = inc(i); |
580 |
} |
581 |
count = 0; |
582 |
putIndex = 0; |
583 |
takeIndex = 0; |
584 |
notFull.signalAll(); |
585 |
} finally { |
586 |
lock.unlock(); |
587 |
} |
588 |
} |
589 |
|
590 |
/** |
591 |
* @throws UnsupportedOperationException {@inheritDoc} |
592 |
* @throws ClassCastException {@inheritDoc} |
593 |
* @throws NullPointerException {@inheritDoc} |
594 |
* @throws IllegalArgumentException {@inheritDoc} |
595 |
*/ |
596 |
public int drainTo(Collection<? super E> c) { |
597 |
if (c == null) |
598 |
throw new NullPointerException(); |
599 |
if (c == this) |
600 |
throw new IllegalArgumentException(); |
601 |
final E[] items = this.items; |
602 |
final ReentrantLock lock = this.lock; |
603 |
lock.lock(); |
604 |
try { |
605 |
int i = takeIndex; |
606 |
int n = 0; |
607 |
int max = count; |
608 |
while (n < max) { |
609 |
c.add(items[i]); |
610 |
items[i] = null; |
611 |
i = inc(i); |
612 |
++n; |
613 |
} |
614 |
if (n > 0) { |
615 |
count = 0; |
616 |
putIndex = 0; |
617 |
takeIndex = 0; |
618 |
notFull.signalAll(); |
619 |
} |
620 |
return n; |
621 |
} finally { |
622 |
lock.unlock(); |
623 |
} |
624 |
} |
625 |
|
626 |
/** |
627 |
* @throws UnsupportedOperationException {@inheritDoc} |
628 |
* @throws ClassCastException {@inheritDoc} |
629 |
* @throws NullPointerException {@inheritDoc} |
630 |
* @throws IllegalArgumentException {@inheritDoc} |
631 |
*/ |
632 |
public int drainTo(Collection<? super E> c, int maxElements) { |
633 |
if (c == null) |
634 |
throw new NullPointerException(); |
635 |
if (c == this) |
636 |
throw new IllegalArgumentException(); |
637 |
if (maxElements <= 0) |
638 |
return 0; |
639 |
final E[] items = this.items; |
640 |
final ReentrantLock lock = this.lock; |
641 |
lock.lock(); |
642 |
try { |
643 |
int i = takeIndex; |
644 |
int n = 0; |
645 |
int sz = count; |
646 |
int max = (maxElements < count)? maxElements : count; |
647 |
while (n < max) { |
648 |
c.add(items[i]); |
649 |
items[i] = null; |
650 |
i = inc(i); |
651 |
++n; |
652 |
} |
653 |
if (n > 0) { |
654 |
count -= n; |
655 |
takeIndex = i; |
656 |
notFull.signalAll(); |
657 |
} |
658 |
return n; |
659 |
} finally { |
660 |
lock.unlock(); |
661 |
} |
662 |
} |
663 |
|
664 |
|
665 |
/** |
666 |
* Returns an iterator over the elements in this queue in proper sequence. |
667 |
* The returned <tt>Iterator</tt> is a "weakly consistent" iterator that |
668 |
* will never throw {@link ConcurrentModificationException}, |
669 |
* and guarantees to traverse elements as they existed upon |
670 |
* construction of the iterator, and may (but is not guaranteed to) |
671 |
* reflect any modifications subsequent to construction. |
672 |
* |
673 |
* @return an iterator over the elements in this queue in proper sequence |
674 |
*/ |
675 |
public Iterator<E> iterator() { |
676 |
final ReentrantLock lock = this.lock; |
677 |
lock.lock(); |
678 |
try { |
679 |
return new Itr(); |
680 |
} finally { |
681 |
lock.unlock(); |
682 |
} |
683 |
} |
684 |
|
685 |
/** |
686 |
* Iterator for ArrayBlockingQueue |
687 |
*/ |
688 |
private class Itr implements Iterator<E> { |
689 |
/** |
690 |
* Index of element to be returned by next, |
691 |
* or a negative number if no such. |
692 |
*/ |
693 |
private int nextIndex; |
694 |
|
695 |
/** |
696 |
* nextItem holds on to item fields because once we claim |
697 |
* that an element exists in hasNext(), we must return it in |
698 |
* the following next() call even if it was in the process of |
699 |
* being removed when hasNext() was called. |
700 |
*/ |
701 |
private E nextItem; |
702 |
|
703 |
/** |
704 |
* Index of element returned by most recent call to next. |
705 |
* Reset to -1 if this element is deleted by a call to remove. |
706 |
*/ |
707 |
private int lastRet; |
708 |
|
709 |
Itr() { |
710 |
lastRet = -1; |
711 |
if (count == 0) |
712 |
nextIndex = -1; |
713 |
else { |
714 |
nextIndex = takeIndex; |
715 |
nextItem = items[takeIndex]; |
716 |
} |
717 |
} |
718 |
|
719 |
public boolean hasNext() { |
720 |
/* |
721 |
* No sync. We can return true by mistake here |
722 |
* only if this iterator passed across threads, |
723 |
* which we don't support anyway. |
724 |
*/ |
725 |
return nextIndex >= 0; |
726 |
} |
727 |
|
728 |
/** |
729 |
* Checks whether nextIndex is valid; if so setting nextItem. |
730 |
* Stops iterator when either hits putIndex or sees null item. |
731 |
*/ |
732 |
private void checkNext() { |
733 |
if (nextIndex == putIndex) { |
734 |
nextIndex = -1; |
735 |
nextItem = null; |
736 |
} else { |
737 |
nextItem = items[nextIndex]; |
738 |
if (nextItem == null) |
739 |
nextIndex = -1; |
740 |
} |
741 |
} |
742 |
|
743 |
public E next() { |
744 |
final ReentrantLock lock = ArrayBlockingQueue.this.lock; |
745 |
lock.lock(); |
746 |
try { |
747 |
if (nextIndex < 0) |
748 |
throw new NoSuchElementException(); |
749 |
lastRet = nextIndex; |
750 |
E x = nextItem; |
751 |
nextIndex = inc(nextIndex); |
752 |
checkNext(); |
753 |
return x; |
754 |
} finally { |
755 |
lock.unlock(); |
756 |
} |
757 |
} |
758 |
|
759 |
public void remove() { |
760 |
final ReentrantLock lock = ArrayBlockingQueue.this.lock; |
761 |
lock.lock(); |
762 |
try { |
763 |
int i = lastRet; |
764 |
if (i == -1) |
765 |
throw new IllegalStateException(); |
766 |
lastRet = -1; |
767 |
|
768 |
int ti = takeIndex; |
769 |
removeAt(i); |
770 |
// back up cursor (reset to front if was first element) |
771 |
nextIndex = (i == ti) ? takeIndex : i; |
772 |
checkNext(); |
773 |
} finally { |
774 |
lock.unlock(); |
775 |
} |
776 |
} |
777 |
} |
778 |
} |