1 /*
2 * Copyright (c) 1999, 2008, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation. Oracle designates this
8 * particular file as subject to the "Classpath" exception as provided
9 * by Oracle in the LICENSE file that accompanied this code.
10 *
11 * This code is distributed in the hope that it will be useful, but WITHOUT
12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
14 * version 2 for more details (a copy is included in the LICENSE file that
15 * accompanied this code).
16 *
17 * You should have received a copy of the GNU General Public License version
18 * 2 along with this work; if not, write to the Free Software Foundation,
19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
20 *
21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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23 * questions.
24 */
25
26 package java.util;
27 import java.util.Date;
28 import java.util.concurrent.atomic.AtomicInteger;
29
30 /**
31 * A facility for threads to schedule tasks for future execution in a
32 * background thread. Tasks may be scheduled for one-time execution, or for
33 * repeated execution at regular intervals.
34 *
35 * <p>Corresponding to each {@code Timer} object is a single background
36 * thread that is used to execute all of the timer's tasks, sequentially.
37 * Timer tasks should complete quickly. If a timer task takes excessive time
38 * to complete, it "hogs" the timer's task execution thread. This can, in
39 * turn, delay the execution of subsequent tasks, which may "bunch up" and
40 * execute in rapid succession when (and if) the offending task finally
41 * completes.
42 *
43 * <p>After the last live reference to a {@code Timer} object goes away
44 * <i>and</i> all outstanding tasks have completed execution, the timer's task
45 * execution thread terminates gracefully (and becomes subject to garbage
46 * collection). However, this can take arbitrarily long to occur. By
47 * default, the task execution thread does not run as a <i>daemon thread</i>,
48 * so it is capable of keeping an application from terminating. If a caller
49 * wants to terminate a timer's task execution thread rapidly, the caller
50 * should invoke the timer's {@code cancel} method.
51 *
52 * <p>If the timer's task execution thread terminates unexpectedly, for
53 * example, because its {@code stop} method is invoked, any further
54 * attempt to schedule a task on the timer will result in an
55 * {@code IllegalStateException}, as if the timer's {@code cancel}
56 * method had been invoked.
57 *
58 * <p>This class is thread-safe: multiple threads can share a single
59 * {@code Timer} object without the need for external synchronization.
60 *
61 * <p>This class does <i>not</i> offer real-time guarantees: it schedules
62 * tasks using the {@code Object.wait(long)} method.
63 *
64 * <p>Java 5.0 introduced the {@code java.util.concurrent} package and
65 * one of the concurrency utilities therein is the {@link
66 * java.util.concurrent.ScheduledThreadPoolExecutor
67 * ScheduledThreadPoolExecutor} which is a thread pool for repeatedly
68 * executing tasks at a given rate or delay. It is effectively a more
69 * versatile replacement for the {@code Timer}/{@code TimerTask}
70 * combination, as it allows multiple service threads, accepts various
71 * time units, and doesn't require subclassing {@code TimerTask} (just
72 * implement {@code Runnable}). Configuring {@code
73 * ScheduledThreadPoolExecutor} with one thread makes it equivalent to
74 * {@code Timer}.
75 *
76 * <p>Implementation note: This class scales to large numbers of concurrently
77 * scheduled tasks (thousands should present no problem). Internally,
78 * it uses a binary heap to represent its task queue, so the cost to schedule
79 * a task is O(log n), where n is the number of concurrently scheduled tasks.
80 *
81 * <p>Implementation note: All constructors start a timer thread.
82 *
83 * @author Josh Bloch
84 * @see TimerTask
85 * @see Object#wait(long)
86 * @since 1.3
87 */
88
89 public class Timer {
90 /**
91 * The timer task queue. This data structure is shared with the timer
92 * thread. The timer produces tasks, via its various schedule calls,
93 * and the timer thread consumes, executing timer tasks as appropriate,
94 * and removing them from the queue when they're obsolete.
95 */
96 private final TaskQueue queue = new TaskQueue();
97
98 /**
99 * The timer thread.
100 */
101 private final TimerThread thread = new TimerThread(queue);
102
103 /**
104 * This object causes the timer's task execution thread to exit
105 * gracefully when there are no live references to the Timer object and no
106 * tasks in the timer queue. It is used in preference to a finalizer on
107 * Timer as such a finalizer would be susceptible to a subclass's
108 * finalizer forgetting to call it.
109 */
110 private final Object threadReaper = new Object() {
111 protected void finalize() throws Throwable {
112 synchronized(queue) {
113 thread.newTasksMayBeScheduled = false;
114 queue.notify(); // In case queue is empty.
115 }
116 }
117 };
118
119 /**
120 * This ID is used to generate thread names.
121 */
122 private final static AtomicInteger nextSerialNumber = new AtomicInteger(0);
123 private static int serialNumber() {
124 return nextSerialNumber.getAndIncrement();
125 }
126
127 /**
128 * Creates a new timer. The associated thread does <i>not</i>
129 * {@linkplain Thread#setDaemon run as a daemon}.
130 */
131 public Timer() {
132 this("Timer-" + serialNumber());
133 }
134
135 /**
136 * Creates a new timer whose associated thread may be specified to
137 * {@linkplain Thread#setDaemon run as a daemon}.
138 * A daemon thread is called for if the timer will be used to
139 * schedule repeating "maintenance activities", which must be
140 * performed as long as the application is running, but should not
141 * prolong the lifetime of the application.
142 *
143 * @param isDaemon true if the associated thread should run as a daemon.
144 */
145 public Timer(boolean isDaemon) {
146 this("Timer-" + serialNumber(), isDaemon);
147 }
148
149 /**
150 * Creates a new timer whose associated thread has the specified name.
151 * The associated thread does <i>not</i>
152 * {@linkplain Thread#setDaemon run as a daemon}.
153 *
154 * @param name the name of the associated thread
155 * @throws NullPointerException if {@code name} is null
156 * @since 1.5
157 */
158 public Timer(String name) {
159 thread.setName(name);
160 thread.start();
161 }
162
163 /**
164 * Creates a new timer whose associated thread has the specified name,
165 * and may be specified to
166 * {@linkplain Thread#setDaemon run as a daemon}.
167 *
168 * @param name the name of the associated thread
169 * @param isDaemon true if the associated thread should run as a daemon
170 * @throws NullPointerException if {@code name} is null
171 * @since 1.5
172 */
173 public Timer(String name, boolean isDaemon) {
174 thread.setName(name);
175 thread.setDaemon(isDaemon);
176 thread.start();
177 }
178
179 /**
180 * Schedules the specified task for execution after the specified delay.
181 *
182 * @param task task to be scheduled.
183 * @param delay delay in milliseconds before task is to be executed.
184 * @throws IllegalArgumentException if {@code delay} is negative, or
185 * {@code delay + System.currentTimeMillis()} is negative.
186 * @throws IllegalStateException if task was already scheduled or
187 * cancelled, timer was cancelled, or timer thread terminated.
188 * @throws NullPointerException if {@code task} is null
189 */
190 public void schedule(TimerTask task, long delay) {
191 if (delay < 0)
192 throw new IllegalArgumentException("Negative delay.");
193 sched(task, System.currentTimeMillis()+delay, 0);
194 }
195
196 /**
197 * Schedules the specified task for execution at the specified time. If
198 * the time is in the past, the task is scheduled for immediate execution.
199 *
200 * @param task task to be scheduled.
201 * @param time time at which task is to be executed.
202 * @throws IllegalArgumentException if {@code time.getTime()} is negative.
203 * @throws IllegalStateException if task was already scheduled or
204 * cancelled, timer was cancelled, or timer thread terminated.
205 * @throws NullPointerException if {@code task} or {@code time} is null
206 */
207 public void schedule(TimerTask task, Date time) {
208 sched(task, time.getTime(), 0);
209 }
210
211 /**
212 * Schedules the specified task for repeated <i>fixed-delay execution</i>,
213 * beginning after the specified delay. Subsequent executions take place
214 * at approximately regular intervals separated by the specified period.
215 *
216 * <p>In fixed-delay execution, each execution is scheduled relative to
217 * the actual execution time of the previous execution. If an execution
218 * is delayed for any reason (such as garbage collection or other
219 * background activity), subsequent executions will be delayed as well.
220 * In the long run, the frequency of execution will generally be slightly
221 * lower than the reciprocal of the specified period (assuming the system
222 * clock underlying {@code Object.wait(long)} is accurate).
223 *
224 * <p>Fixed-delay execution is appropriate for recurring activities
225 * that require "smoothness." In other words, it is appropriate for
226 * activities where it is more important to keep the frequency accurate
227 * in the short run than in the long run. This includes most animation
228 * tasks, such as blinking a cursor at regular intervals. It also includes
229 * tasks wherein regular activity is performed in response to human
230 * input, such as automatically repeating a character as long as a key
231 * is held down.
232 *
233 * @param task task to be scheduled.
234 * @param delay delay in milliseconds before task is to be executed.
235 * @param period time in milliseconds between successive task executions.
236 * @throws IllegalArgumentException if {@code delay < 0}, or
237 * {@code delay + System.currentTimeMillis() < 0}, or
238 * {@code period <= 0}
239 * @throws IllegalStateException if task was already scheduled or
240 * cancelled, timer was cancelled, or timer thread terminated.
241 * @throws NullPointerException if {@code task} is null
242 */
243 public void schedule(TimerTask task, long delay, long period) {
244 if (delay < 0)
245 throw new IllegalArgumentException("Negative delay.");
246 if (period <= 0)
247 throw new IllegalArgumentException("Non-positive period.");
248 sched(task, System.currentTimeMillis()+delay, -period);
249 }
250
251 /**
252 * Schedules the specified task for repeated <i>fixed-delay execution</i>,
253 * beginning at the specified time. Subsequent executions take place at
254 * approximately regular intervals, separated by the specified period.
255 *
256 * <p>In fixed-delay execution, each execution is scheduled relative to
257 * the actual execution time of the previous execution. If an execution
258 * is delayed for any reason (such as garbage collection or other
259 * background activity), subsequent executions will be delayed as well.
260 * In the long run, the frequency of execution will generally be slightly
261 * lower than the reciprocal of the specified period (assuming the system
262 * clock underlying {@code Object.wait(long)} is accurate). As a
263 * consequence of the above, if the scheduled first time is in the past,
264 * it is scheduled for immediate execution.
265 *
266 * <p>Fixed-delay execution is appropriate for recurring activities
267 * that require "smoothness." In other words, it is appropriate for
268 * activities where it is more important to keep the frequency accurate
269 * in the short run than in the long run. This includes most animation
270 * tasks, such as blinking a cursor at regular intervals. It also includes
271 * tasks wherein regular activity is performed in response to human
272 * input, such as automatically repeating a character as long as a key
273 * is held down.
274 *
275 * @param task task to be scheduled.
276 * @param firstTime First time at which task is to be executed.
277 * @param period time in milliseconds between successive task executions.
278 * @throws IllegalArgumentException if {@code firstTime.getTime() < 0}, or
279 * {@code period <= 0}
280 * @throws IllegalStateException if task was already scheduled or
281 * cancelled, timer was cancelled, or timer thread terminated.
282 * @throws NullPointerException if {@code task} or {@code firstTime} is null
283 */
284 public void schedule(TimerTask task, Date firstTime, long period) {
285 if (period <= 0)
286 throw new IllegalArgumentException("Non-positive period.");
287 sched(task, firstTime.getTime(), -period);
288 }
289
290 /**
291 * Schedules the specified task for repeated <i>fixed-rate execution</i>,
292 * beginning after the specified delay. Subsequent executions take place
293 * at approximately regular intervals, separated by the specified period.
294 *
295 * <p>In fixed-rate execution, each execution is scheduled relative to the
296 * scheduled execution time of the initial execution. If an execution is
297 * delayed for any reason (such as garbage collection or other background
298 * activity), two or more executions will occur in rapid succession to
299 * "catch up." In the long run, the frequency of execution will be
300 * exactly the reciprocal of the specified period (assuming the system
301 * clock underlying {@code Object.wait(long)} is accurate).
302 *
303 * <p>Fixed-rate execution is appropriate for recurring activities that
304 * are sensitive to <i>absolute</i> time, such as ringing a chime every
305 * hour on the hour, or running scheduled maintenance every day at a
306 * particular time. It is also appropriate for recurring activities
307 * where the total time to perform a fixed number of executions is
308 * important, such as a countdown timer that ticks once every second for
309 * ten seconds. Finally, fixed-rate execution is appropriate for
310 * scheduling multiple repeating timer tasks that must remain synchronized
311 * with respect to one another.
312 *
313 * @param task task to be scheduled.
314 * @param delay delay in milliseconds before task is to be executed.
315 * @param period time in milliseconds between successive task executions.
316 * @throws IllegalArgumentException if {@code delay < 0}, or
317 * {@code delay + System.currentTimeMillis() < 0}, or
318 * {@code period <= 0}
319 * @throws IllegalStateException if task was already scheduled or
320 * cancelled, timer was cancelled, or timer thread terminated.
321 * @throws NullPointerException if {@code task} is null
322 */
323 public void scheduleAtFixedRate(TimerTask task, long delay, long period) {
324 if (delay < 0)
325 throw new IllegalArgumentException("Negative delay.");
326 if (period <= 0)
327 throw new IllegalArgumentException("Non-positive period.");
328 sched(task, System.currentTimeMillis()+delay, period);
329 }
330
331 /**
332 * Schedules the specified task for repeated <i>fixed-rate execution</i>,
333 * beginning at the specified time. Subsequent executions take place at
334 * approximately regular intervals, separated by the specified period.
335 *
336 * <p>In fixed-rate execution, each execution is scheduled relative to the
337 * scheduled execution time of the initial execution. If an execution is
338 * delayed for any reason (such as garbage collection or other background
339 * activity), two or more executions will occur in rapid succession to
340 * "catch up." In the long run, the frequency of execution will be
341 * exactly the reciprocal of the specified period (assuming the system
342 * clock underlying {@code Object.wait(long)} is accurate). As a
343 * consequence of the above, if the scheduled first time is in the past,
344 * then any "missed" executions will be scheduled for immediate "catch up"
345 * execution.
346 *
347 * <p>Fixed-rate execution is appropriate for recurring activities that
348 * are sensitive to <i>absolute</i> time, such as ringing a chime every
349 * hour on the hour, or running scheduled maintenance every day at a
350 * particular time. It is also appropriate for recurring activities
351 * where the total time to perform a fixed number of executions is
352 * important, such as a countdown timer that ticks once every second for
353 * ten seconds. Finally, fixed-rate execution is appropriate for
354 * scheduling multiple repeating timer tasks that must remain synchronized
355 * with respect to one another.
356 *
357 * @param task task to be scheduled.
358 * @param firstTime First time at which task is to be executed.
359 * @param period time in milliseconds between successive task executions.
360 * @throws IllegalArgumentException if {@code firstTime.getTime() < 0} or
361 * {@code period <= 0}
362 * @throws IllegalStateException if task was already scheduled or
363 * cancelled, timer was cancelled, or timer thread terminated.
364 * @throws NullPointerException if {@code task} or {@code firstTime} is null
365 */
366 public void scheduleAtFixedRate(TimerTask task, Date firstTime,
367 long period) {
368 if (period <= 0)
369 throw new IllegalArgumentException("Non-positive period.");
370 sched(task, firstTime.getTime(), period);
371 }
372
373 /**
374 * Schedule the specified timer task for execution at the specified
375 * time with the specified period, in milliseconds. If period is
376 * positive, the task is scheduled for repeated execution; if period is
377 * zero, the task is scheduled for one-time execution. Time is specified
378 * in Date.getTime() format. This method checks timer state, task state,
379 * and initial execution time, but not period.
380 *
381 * @throws IllegalArgumentException if {@code time} is negative.
382 * @throws IllegalStateException if task was already scheduled or
383 * cancelled, timer was cancelled, or timer thread terminated.
384 * @throws NullPointerException if {@code task} is null
385 */
386 private void sched(TimerTask task, long time, long period) {
387 if (time < 0)
388 throw new IllegalArgumentException("Illegal execution time.");
389
390 // Constrain value of period sufficiently to prevent numeric
391 // overflow while still being effectively infinitely large.
392 if (Math.abs(period) > (Long.MAX_VALUE >> 1))
393 period >>= 1;
394
395 synchronized(queue) {
396 if (!thread.newTasksMayBeScheduled)
397 throw new IllegalStateException("Timer already cancelled.");
398
399 synchronized(task.lock) {
400 if (task.state != TimerTask.VIRGIN)
401 throw new IllegalStateException(
402 "Task already scheduled or cancelled");
403 task.nextExecutionTime = time;
404 task.period = period;
405 task.state = TimerTask.SCHEDULED;
406 }
407
408 queue.add(task);
409 if (queue.getMin() == task)
410 queue.notify();
411 }
412 }
413
414 /**
415 * Terminates this timer, discarding any currently scheduled tasks.
416 * Does not interfere with a currently executing task (if it exists).
417 * Once a timer has been terminated, its execution thread terminates
418 * gracefully, and no more tasks may be scheduled on it.
419 *
420 * <p>Note that calling this method from within the run method of a
421 * timer task that was invoked by this timer absolutely guarantees that
422 * the ongoing task execution is the last task execution that will ever
423 * be performed by this timer.
424 *
425 * <p>This method may be called repeatedly; the second and subsequent
426 * calls have no effect.
427 */
428 public void cancel() {
429 synchronized(queue) {
430 thread.newTasksMayBeScheduled = false;
431 queue.clear();
432 queue.notify(); // In case queue was already empty.
433 }
434 }
435
436 /**
437 * Removes all cancelled tasks from this timer's task queue. <i>Calling
438 * this method has no effect on the behavior of the timer</i>, but
439 * eliminates the references to the cancelled tasks from the queue.
440 * If there are no external references to these tasks, they become
441 * eligible for garbage collection.
442 *
443 * <p>Most programs will have no need to call this method.
444 * It is designed for use by the rare application that cancels a large
445 * number of tasks. Calling this method trades time for space: the
446 * runtime of the method may be proportional to n + c log n, where n
447 * is the number of tasks in the queue and c is the number of cancelled
448 * tasks.
449 *
450 * <p>Note that it is permissible to call this method from within a
451 * a task scheduled on this timer.
452 *
453 * @return the number of tasks removed from the queue.
454 * @since 1.5
455 */
456 public int purge() {
457 int result = 0;
458
459 synchronized(queue) {
460 for (int i = queue.size(); i > 0; i--) {
461 if (queue.get(i).state == TimerTask.CANCELLED) {
462 queue.quickRemove(i);
463 result++;
464 }
465 }
466
467 if (result != 0)
468 queue.heapify();
469 }
470
471 return result;
472 }
473 }
474
475 /**
476 * This "helper class" implements the timer's task execution thread, which
477 * waits for tasks on the timer queue, executions them when they fire,
478 * reschedules repeating tasks, and removes cancelled tasks and spent
479 * non-repeating tasks from the queue.
480 */
481 class TimerThread extends Thread {
482 /**
483 * This flag is set to false by the reaper to inform us that there
484 * are no more live references to our Timer object. Once this flag
485 * is true and there are no more tasks in our queue, there is no
486 * work left for us to do, so we terminate gracefully. Note that
487 * this field is protected by queue's monitor!
488 */
489 boolean newTasksMayBeScheduled = true;
490
491 /**
492 * Our Timer's queue. We store this reference in preference to
493 * a reference to the Timer so the reference graph remains acyclic.
494 * Otherwise, the Timer would never be garbage-collected and this
495 * thread would never go away.
496 */
497 private TaskQueue queue;
498
499 TimerThread(TaskQueue queue) {
500 this.queue = queue;
501 }
502
503 public void run() {
504 try {
505 mainLoop();
506 } finally {
507 // Someone killed this Thread, behave as if Timer cancelled
508 synchronized(queue) {
509 newTasksMayBeScheduled = false;
510 queue.clear(); // Eliminate obsolete references
511 }
512 }
513 }
514
515 /**
516 * The main timer loop. (See class comment.)
517 */
518 private void mainLoop() {
519 while (true) {
520 try {
521 TimerTask task;
522 boolean taskFired;
523 synchronized(queue) {
524 // Wait for queue to become non-empty
525 while (queue.isEmpty() && newTasksMayBeScheduled)
526 queue.wait();
527 if (queue.isEmpty())
528 break; // Queue is empty and will forever remain; die
529
530 // Queue nonempty; look at first evt and do the right thing
531 long currentTime, executionTime;
532 task = queue.getMin();
533 synchronized(task.lock) {
534 if (task.state == TimerTask.CANCELLED) {
535 queue.removeMin();
536 continue; // No action required, poll queue again
537 }
538 currentTime = System.currentTimeMillis();
539 executionTime = task.nextExecutionTime;
540 if (taskFired = (executionTime<=currentTime)) {
541 if (task.period == 0) { // Non-repeating, remove
542 queue.removeMin();
543 task.state = TimerTask.EXECUTED;
544 } else { // Repeating task, reschedule
545 queue.rescheduleMin(
546 task.period<0 ? currentTime - task.period
547 : executionTime + task.period);
548 }
549 }
550 }
551 if (!taskFired) // Task hasn't yet fired; wait
552 queue.wait(executionTime - currentTime);
553 }
554 if (taskFired) // Task fired; run it, holding no locks
555 task.run();
556 } catch(InterruptedException e) {
557 }
558 }
559 }
560 }
561
562 /**
563 * This class represents a timer task queue: a priority queue of TimerTasks,
564 * ordered on nextExecutionTime. Each Timer object has one of these, which it
565 * shares with its TimerThread. Internally this class uses a heap, which
566 * offers log(n) performance for the add, removeMin and rescheduleMin
567 * operations, and constant time performance for the getMin operation.
568 */
569 class TaskQueue {
570 /**
571 * Priority queue represented as a balanced binary heap: the two children
572 * of queue[n] are queue[2*n] and queue[2*n+1]. The priority queue is
573 * ordered on the nextExecutionTime field: The TimerTask with the lowest
574 * nextExecutionTime is in queue[1] (assuming the queue is nonempty). For
575 * each node n in the heap, and each descendant of n, d,
576 * n.nextExecutionTime <= d.nextExecutionTime.
577 */
578 private TimerTask[] queue = new TimerTask[128];
579
580 /**
581 * The number of tasks in the priority queue. (The tasks are stored in
582 * queue[1] up to queue[size]).
583 */
584 private int size = 0;
585
586 /**
587 * Returns the number of tasks currently on the queue.
588 */
589 int size() {
590 return size;
591 }
592
593 /**
594 * Adds a new task to the priority queue.
595 */
596 void add(TimerTask task) {
597 // Grow backing store if necessary
598 if (size + 1 == queue.length)
599 queue = Arrays.copyOf(queue, 2*queue.length);
600
601 queue[++size] = task;
602 fixUp(size);
603 }
604
605 /**
606 * Return the "head task" of the priority queue. (The head task is an
607 * task with the lowest nextExecutionTime.)
608 */
609 TimerTask getMin() {
610 return queue[1];
611 }
612
613 /**
614 * Return the ith task in the priority queue, where i ranges from 1 (the
615 * head task, which is returned by getMin) to the number of tasks on the
616 * queue, inclusive.
617 */
618 TimerTask get(int i) {
619 return queue[i];
620 }
621
622 /**
623 * Remove the head task from the priority queue.
624 */
625 void removeMin() {
626 queue[1] = queue[size];
627 queue[size--] = null; // Drop extra reference to prevent memory leak
628 fixDown(1);
629 }
630
631 /**
632 * Removes the ith element from queue without regard for maintaining
633 * the heap invariant. Recall that queue is one-based, so
634 * 1 <= i <= size.
635 */
636 void quickRemove(int i) {
637 assert i <= size;
638
639 queue[i] = queue[size];
640 queue[size--] = null; // Drop extra ref to prevent memory leak
641 }
642
643 /**
644 * Sets the nextExecutionTime associated with the head task to the
645 * specified value, and adjusts priority queue accordingly.
646 */
647 void rescheduleMin(long newTime) {
648 queue[1].nextExecutionTime = newTime;
649 fixDown(1);
650 }
651
652 /**
653 * Returns true if the priority queue contains no elements.
654 */
655 boolean isEmpty() {
656 return size==0;
657 }
658
659 /**
660 * Removes all elements from the priority queue.
661 */
662 void clear() {
663 // Null out task references to prevent memory leak
664 for (int i=1; i<=size; i++)
665 queue[i] = null;
666
667 size = 0;
668 }
669
670 /**
671 * Establishes the heap invariant (described above) assuming the heap
672 * satisfies the invariant except possibly for the leaf-node indexed by k
673 * (which may have a nextExecutionTime less than its parent's).
674 *
675 * This method functions by "promoting" queue[k] up the hierarchy
676 * (by swapping it with its parent) repeatedly until queue[k]'s
677 * nextExecutionTime is greater than or equal to that of its parent.
678 */
679 private void fixUp(int k) {
680 while (k > 1) {
681 int j = k >> 1;
682 if (queue[j].nextExecutionTime <= queue[k].nextExecutionTime)
683 break;
684 TimerTask tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
685 k = j;
686 }
687 }
688
689 /**
690 * Establishes the heap invariant (described above) in the subtree
691 * rooted at k, which is assumed to satisfy the heap invariant except
692 * possibly for node k itself (which may have a nextExecutionTime greater
693 * than its children's).
694 *
695 * This method functions by "demoting" queue[k] down the hierarchy
696 * (by swapping it with its smaller child) repeatedly until queue[k]'s
697 * nextExecutionTime is less than or equal to those of its children.
698 */
699 private void fixDown(int k) {
700 int j;
701 while ((j = k << 1) <= size && j > 0) {
702 if (j < size &&
703 queue[j].nextExecutionTime > queue[j+1].nextExecutionTime)
704 j++; // j indexes smallest kid
705 if (queue[k].nextExecutionTime <= queue[j].nextExecutionTime)
706 break;
707 TimerTask tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
708 k = j;
709 }
710 }
711
712 /**
713 * Establishes the heap invariant (described above) in the entire tree,
714 * assuming nothing about the order of the elements prior to the call.
715 */
716 void heapify() {
717 for (int i = size/2; i >= 1; i--)
718 fixDown(i);
719 }
720 }