| DelayQueue.java |
1 /*
2 * %W% %E%
3 *
4 * Copyright (c) 2006, Oracle and/or its affiliates. All rights reserved.
5 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
6 */
7
8 package java.util.concurrent;
9 import java.util.concurrent.locks.*;
10 import java.util.*;
11
12 /**
13 * An unbounded {@linkplain BlockingQueue blocking queue} of
14 * <tt>Delayed</tt> elements, in which an element can only be taken
15 * when its delay has expired. The <em>head</em> of the queue is that
16 * <tt>Delayed</tt> element whose delay expired furthest in the
17 * past. If no delay has expired there is no head and <tt>poll</tt>
18 * will return <tt>null</tt>. Expiration occurs when an element's
19 * <tt>getDelay(TimeUnit.NANOSECONDS)</tt> method returns a value less
20 * than or equal to zero. Even though unexpired elements cannot be
21 * removed using <tt>take</tt> or <tt>poll</tt>, they are otherwise
22 * treated as normal elements. For example, the <tt>size</tt> method
23 * returns the count of both expired and unexpired elements.
24 * This queue does not permit null elements.
25 *
26 * <p>This class and its iterator implement all of the
27 * <em>optional</em> methods of the {@link Collection} and {@link
28 * Iterator} interfaces.
29 *
30 * <p>This class is a member of the
31 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
32 * Java Collections Framework</a>.
33 *
34 * @since 1.5
35 * @author Doug Lea
36 * @param <E> the type of elements held in this collection
37 */
38
39 public class DelayQueue<E extends Delayed> extends AbstractQueue<E>
40 implements BlockingQueue<E> {
41
42 private transient final ReentrantLock lock = new ReentrantLock();
43 private transient final Condition available = lock.newCondition();
44 private final PriorityQueue<E> q = new PriorityQueue<E>();
45
46 /**
47 * Creates a new <tt>DelayQueue</tt> that is initially empty.
48 */
49 public DelayQueue() {}
50
51 /**
52 * Creates a <tt>DelayQueue</tt> initially containing the elements of the
53 * given collection of {@link Delayed} instances.
54 *
55 * @param c the collection of elements to initially contain
56 * @throws NullPointerException if the specified collection or any
57 * of its elements are null
58 */
59 public DelayQueue(Collection<? extends E> c) {
60 this.addAll(c);
61 }
62
63 /**
64 * Inserts the specified element into this delay queue.
65 *
66 * @param e the element to add
67 * @return <tt>true</tt> (as specified by {@link Collection#add})
68 * @throws NullPointerException if the specified element is null
69 */
70 public boolean add(E e) {
71 return offer(e);
72 }
73
74 /**
75 * Inserts the specified element into this delay queue.
76 *
77 * @param e the element to add
78 * @return <tt>true</tt>
79 * @throws NullPointerException if the specified element is null
80 */
81 public boolean offer(E e) {
82 final ReentrantLock lock = this.lock;
83 lock.lock();
84 try {
85 E first = q.peek();
86 q.offer(e);
87 if (first == null || e.compareTo(first) < 0)
88 available.signalAll();
89 return true;
90 } finally {
91 lock.unlock();
92 }
93 }
94
95 /**
96 * Inserts the specified element into this delay queue. As the queue is
97 * unbounded this method will never block.
98 *
99 * @param e the element to add
100 * @throws NullPointerException {@inheritDoc}
101 */
102 public void put(E e) {
103 offer(e);
104 }
105
106 /**
107 * Inserts the specified element into this delay queue. As the queue is
108 * unbounded this method will never block.
109 *
110 * @param e the element to add
111 * @param timeout This parameter is ignored as the method never blocks
112 * @param unit This parameter is ignored as the method never blocks
113 * @return <tt>true</tt>
114 * @throws NullPointerException {@inheritDoc}
115 */
116 public boolean offer(E e, long timeout, TimeUnit unit) {
117 return offer(e);
118 }
119
120 /**
121 * Retrieves and removes the head of this queue, or returns <tt>null</tt>
122 * if this queue has no elements with an expired delay.
123 *
124 * @return the head of this queue, or <tt>null</tt> if this
125 * queue has no elements with an expired delay
126 */
127 public E poll() {
128 final ReentrantLock lock = this.lock;
129 lock.lock();
130 try {
131 E first = q.peek();
132 if (first == null || first.getDelay(TimeUnit.NANOSECONDS) > 0)
133 return null;
134 else {
135 E x = q.poll();
136 assert x != null;
137 if (q.size() != 0)
138 available.signalAll();
139 return x;
140 }
141 } finally {
142 lock.unlock();
143 }
144 }
145
146 /**
147 * Retrieves and removes the head of this queue, waiting if necessary
148 * until an element with an expired delay is available on this queue.
149 *
150 * @return the head of this queue
151 * @throws InterruptedException {@inheritDoc}
152 */
153 public E take() throws InterruptedException {
154 final ReentrantLock lock = this.lock;
155 lock.lockInterruptibly();
156 try {
157 for (;;) {
158 E first = q.peek();
159 if (first == null) {
160 available.await();
161 } else {
162 long delay = first.getDelay(TimeUnit.NANOSECONDS);
163 if (delay > 0) {
164 long tl = available.awaitNanos(delay);
165 } else {
166 E x = q.poll();
167 assert x != null;
168 if (q.size() != 0)
169 available.signalAll(); // wake up other takers
170 return x;
171
172 }
173 }
174 }
175 } finally {
176 lock.unlock();
177 }
178 }
179
180 /**
181 * Retrieves and removes the head of this queue, waiting if necessary
182 * until an element with an expired delay is available on this queue,
183 * or the specified wait time expires.
184 *
185 * @return the head of this queue, or <tt>null</tt> if the
186 * specified waiting time elapses before an element with
187 * an expired delay becomes available
188 * @throws InterruptedException {@inheritDoc}
189 */
190 public E poll(long timeout, TimeUnit unit) throws InterruptedException {
191 long nanos = unit.toNanos(timeout);
192 final ReentrantLock lock = this.lock;
193 lock.lockInterruptibly();
194 try {
195 for (;;) {
196 E first = q.peek();
197 if (first == null) {
198 if (nanos <= 0)
199 return null;
200 else
201 nanos = available.awaitNanos(nanos);
202 } else {
203 long delay = first.getDelay(TimeUnit.NANOSECONDS);
204 if (delay > 0) {
205 if (nanos <= 0)
206 return null;
207 if (delay > nanos)
208 delay = nanos;
209 long timeLeft = available.awaitNanos(delay);
210 nanos -= delay - timeLeft;
211 } else {
212 E x = q.poll();
213 assert x != null;
214 if (q.size() != 0)
215 available.signalAll();
216 return x;
217 }
218 }
219 }
220 } finally {
221 lock.unlock();
222 }
223 }
224
225 /**
226 * Retrieves, but does not remove, the head of this queue, or
227 * returns <tt>null</tt> if this queue is empty. Unlike
228 * <tt>poll</tt>, if no expired elements are available in the queue,
229 * this method returns the element that will expire next,
230 * if one exists.
231 *
232 * @return the head of this queue, or <tt>null</tt> if this
233 * queue is empty.
234 */
235 public E peek() {
236 final ReentrantLock lock = this.lock;
237 lock.lock();
238 try {
239 return q.peek();
240 } finally {
241 lock.unlock();
242 }
243 }
244
245 public int size() {
246 final ReentrantLock lock = this.lock;
247 lock.lock();
248 try {
249 return q.size();
250 } finally {
251 lock.unlock();
252 }
253 }
254
255 /**
256 * @throws UnsupportedOperationException {@inheritDoc}
257 * @throws ClassCastException {@inheritDoc}
258 * @throws NullPointerException {@inheritDoc}
259 * @throws IllegalArgumentException {@inheritDoc}
260 */
261 public int drainTo(Collection<? super E> c) {
262 if (c == null)
263 throw new NullPointerException();
264 if (c == this)
265 throw new IllegalArgumentException();
266 final ReentrantLock lock = this.lock;
267 lock.lock();
268 try {
269 int n = 0;
270 for (;;) {
271 E first = q.peek();
272 if (first == null || first.getDelay(TimeUnit.NANOSECONDS) > 0)
273 break;
274 c.add(q.poll());
275 ++n;
276 }
277 if (n > 0)
278 available.signalAll();
279 return n;
280 } finally {
281 lock.unlock();
282 }
283 }
284
285 /**
286 * @throws UnsupportedOperationException {@inheritDoc}
287 * @throws ClassCastException {@inheritDoc}
288 * @throws NullPointerException {@inheritDoc}
289 * @throws IllegalArgumentException {@inheritDoc}
290 */
291 public int drainTo(Collection<? super E> c, int maxElements) {
292 if (c == null)
293 throw new NullPointerException();
294 if (c == this)
295 throw new IllegalArgumentException();
296 if (maxElements <= 0)
297 return 0;
298 final ReentrantLock lock = this.lock;
299 lock.lock();
300 try {
301 int n = 0;
302 while (n < maxElements) {
303 E first = q.peek();
304 if (first == null || first.getDelay(TimeUnit.NANOSECONDS) > 0)
305 break;
306 c.add(q.poll());
307 ++n;
308 }
309 if (n > 0)
310 available.signalAll();
311 return n;
312 } finally {
313 lock.unlock();
314 }
315 }
316
317 /**
318 * Atomically removes all of the elements from this delay queue.
319 * The queue will be empty after this call returns.
320 * Elements with an unexpired delay are not waited for; they are
321 * simply discarded from the queue.
322 */
323 public void clear() {
324 final ReentrantLock lock = this.lock;
325 lock.lock();
326 try {
327 q.clear();
328 } finally {
329 lock.unlock();
330 }
331 }
332
333 /**
334 * Always returns <tt>Integer.MAX_VALUE</tt> because
335 * a <tt>DelayQueue</tt> is not capacity constrained.
336 *
337 * @return <tt>Integer.MAX_VALUE</tt>
338 */
339 public int remainingCapacity() {
340 return Integer.MAX_VALUE;
341 }
342
343 /**
344 * Returns an array containing all of the elements in this queue.
345 * The returned array elements are in no particular order.
346 *
347 * <p>The returned array will be "safe" in that no references to it are
348 * maintained by this queue. (In other words, this method must allocate
349 * a new array). The caller is thus free to modify the returned array.
350 *
351 * <p>This method acts as bridge between array-based and collection-based
352 * APIs.
353 *
354 * @return an array containing all of the elements in this queue
355 */
356 public Object[] toArray() {
357 final ReentrantLock lock = this.lock;
358 lock.lock();
359 try {
360 return q.toArray();
361 } finally {
362 lock.unlock();
363 }
364 }
365
366 /**
367 * Returns an array containing all of the elements in this queue; the
368 * runtime type of the returned array is that of the specified array.
369 * The returned array elements are in no particular order.
370 * If the queue fits in the specified array, it is returned therein.
371 * Otherwise, a new array is allocated with the runtime type of the
372 * specified array and the size of this queue.
373 *
374 * <p>If this queue fits in the specified array with room to spare
375 * (i.e., the array has more elements than this queue), the element in
376 * the array immediately following the end of the queue is set to
377 * <tt>null</tt>.
378 *
379 * <p>Like the {@link #toArray()} method, this method acts as bridge between
380 * array-based and collection-based APIs. Further, this method allows
381 * precise control over the runtime type of the output array, and may,
382 * under certain circumstances, be used to save allocation costs.
383 *
384 * <p>The following code can be used to dump a delay queue into a newly
385 * allocated array of <tt>Delayed</tt>:
386 *
387 * <pre>
388 * Delayed[] a = q.toArray(new Delayed[0]);</pre>
389 *
390 * Note that <tt>toArray(new Object[0])</tt> is identical in function to
391 * <tt>toArray()</tt>.
392 *
393 * @param a the array into which the elements of the queue are to
394 * be stored, if it is big enough; otherwise, a new array of the
395 * same runtime type is allocated for this purpose
396 * @return an array containing all of the elements in this queue
397 * @throws ArrayStoreException if the runtime type of the specified array
398 * is not a supertype of the runtime type of every element in
399 * this queue
400 * @throws NullPointerException if the specified array is null
401 */
402 public <T> T[] toArray(T[] a) {
403 final ReentrantLock lock = this.lock;
404 lock.lock();
405 try {
406 return q.toArray(a);
407 } finally {
408 lock.unlock();
409 }
410 }
411
412 /**
413 * Removes a single instance of the specified element from this
414 * queue, if it is present, whether or not it has expired.
415 */
416 public boolean remove(Object o) {
417 final ReentrantLock lock = this.lock;
418 lock.lock();
419 try {
420 return q.remove(o);
421 } finally {
422 lock.unlock();
423 }
424 }
425
426 /**
427 * Returns an iterator over all the elements (both expired and
428 * unexpired) in this queue. The iterator does not return the
429 * elements in any particular order. The returned
430 * <tt>Iterator</tt> is a "weakly consistent" iterator that will
431 * never throw {@link ConcurrentModificationException}, and
432 * guarantees to traverse elements as they existed upon
433 * construction of the iterator, and may (but is not guaranteed
434 * to) reflect any modifications subsequent to construction.
435 *
436 * @return an iterator over the elements in this queue
437 */
438 public Iterator<E> iterator() {
439 return new Itr(toArray());
440 }
441
442 /**
443 * Snapshot iterator that works off copy of underlying q array.
444 */
445 private class Itr implements Iterator<E> {
446 final Object[] array; // Array of all elements
447 int cursor; // index of next element to return;
448 int lastRet; // index of last element, or -1 if no such
449
450 Itr(Object[] array) {
451 lastRet = -1;
452 this.array = array;
453 }
454
455 public boolean hasNext() {
456 return cursor < array.length;
457 }
458
459 public E next() {
460 if (cursor >= array.length)
461 throw new NoSuchElementException();
462 lastRet = cursor;
463 return (E)array[cursor++];
464 }
465
466 public void remove() {
467 if (lastRet < 0)
468 throw new IllegalStateException();
469 Object x = array[lastRet];
470 lastRet = -1;
471 // Traverse underlying queue to find == element,
472 // not just a .equals element.
473 lock.lock();
474 try {
475 for (Iterator it = q.iterator(); it.hasNext(); ) {
476 if (it.next() == x) {
477 it.remove();
478 return;
479 }
480 }
481 } finally {
482 lock.unlock();
483 }
484 }
485 }
486
487 }
488