1 /*
2 * Copyright 1999-2007 Sun Microsystems, Inc. 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. Sun designates this
8 * particular file as subject to the "Classpath" exception as provided
9 * by Sun 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 Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
22 * CA 95054 USA or visit www.sun.com if you need additional information or
23 * have any questions.
24 */
25
26 package java.util;
27 import java.util.Date;
28
29 /**
30 * A facility for threads to schedule tasks for future execution in a
31 * background thread. Tasks may be scheduled for one-time execution, or for
32 * repeated execution at regular intervals.
33 *
34 * <p>Corresponding to each <tt>Timer</tt> object is a single background
35 * thread that is used to execute all of the timer's tasks, sequentially.
36 * Timer tasks should complete quickly. If a timer task takes excessive time
37 * to complete, it "hogs" the timer's task execution thread. This can, in
38 * turn, delay the execution of subsequent tasks, which may "bunch up" and
39 * execute in rapid succession when (and if) the offending task finally
40 * completes.
41 *
42 * <p>After the last live reference to a <tt>Timer</tt> object goes away
43 * <i>and</i> all outstanding tasks have completed execution, the timer's task
44 * execution thread terminates gracefully (and becomes subject to garbage
45 * collection). However, this can take arbitrarily long to occur. By
46 * default, the task execution thread does not run as a <i>daemon thread</i>,
47 * so it is capable of keeping an application from terminating. If a caller
48 * wants to terminate a timer's task execution thread rapidly, the caller
49 * should invoke the timer's <tt>cancel</tt> method.
50 *
51 * <p>If the timer's task execution thread terminates unexpectedly, for
52 * example, because its <tt>stop</tt> method is invoked, any further
53 * attempt to schedule a task on the timer will result in an
54 * <tt>IllegalStateException</tt>, as if the timer's <tt>cancel</tt>
55 * method had been invoked.
56 *
57 * <p>This class is thread-safe: multiple threads can share a single
58 * <tt>Timer</tt> object without the need for external synchronization.
59 *
60 * <p>This class does <i>not</i> offer real-time guarantees: it schedules
61 * tasks using the <tt>Object.wait(long)</tt> method.
62 *
63 * <p>Java 5.0 introduced the {@code java.util.concurrent} package and
64 * one of the concurrency utilities therein is the {@link
65 * java.util.concurrent.ScheduledThreadPoolExecutor
66 * ScheduledThreadPoolExecutor} which is a thread pool for repeatedly
67 * executing tasks at a given rate or delay. It is effectively a more
68 * versatile replacement for the {@code Timer}/{@code TimerTask}
69 * combination, as it allows multiple service threads, accepts various
70 * time units, and doesn't require subclassing {@code TimerTask} (just
71 * implement {@code Runnable}). Configuring {@code
72 * ScheduledThreadPoolExecutor} with one thread makes it equivalent to
73 * {@code Timer}.
74 *
75 * <p>Implementation note: This class scales to large numbers of concurrently
76 * scheduled tasks (thousands should present no problem). Internally,
77 * it uses a binary heap to represent its task queue, so the cost to schedule
78 * a task is O(log n), where n is the number of concurrently scheduled tasks.
79 *
80 * <p>Implementation note: All constructors start a timer thread.
81 *
82 * @author Josh Bloch
83 * @see TimerTask
84 * @see Object#wait(long)
85 * @since 1.3
86 */
87
88 public class Timer {
89 /**
90 * The timer task queue. This data structure is shared with the timer
91 * thread. The timer produces tasks, via its various schedule calls,
92 * and the timer thread consumes, executing timer tasks as appropriate,
93 * and removing them from the queue when they're obsolete.
94 */
95 private TaskQueue queue = new TaskQueue();
96
97 /**
98 * The timer thread.
99 */
100 private TimerThread thread = new TimerThread(queue);
101
102 /**
103 * This object causes the timer's task execution thread to exit
104 * gracefully when there are no live references to the Timer object and no
105 * tasks in the timer queue. It is used in preference to a finalizer on
106 * Timer as such a finalizer would be susceptible to a subclass's
107 * finalizer forgetting to call it.
108 */
109 private Object threadReaper = new Object() {
110 protected void finalize() throws Throwable {
111 synchronized(queue) {
112 thread.newTasksMayBeScheduled = false;
113 queue.notify(); // In case queue is empty.
114 }
115 }
116 };
117
118 /**
119 * This ID is used to generate thread names. (It could be replaced
120 * by an AtomicInteger as soon as they become available.)
121 */
122 private static int nextSerialNumber = 0;
123 private static synchronized int serialNumber() {
124 return nextSerialNumber++;
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 <tt>delay</tt> is negative, or
185 * <tt>delay + System.currentTimeMillis()</tt> 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 <tt>time.getTime()</tt> 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 <tt>Object.wait(long)</tt> 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 <tt>Object.wait(long)</tt> 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 <tt>Object.wait(long)</tt> 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 <tt>Object.wait(long)</tt> 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 <tt>time</tt> 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 synchronized(queue) {
391 if (!thread.newTasksMayBeScheduled)
392 throw new IllegalStateException("Timer already cancelled.");
393
394 synchronized(task.lock) {
395 if (task.state != TimerTask.VIRGIN)
396 throw new IllegalStateException(
397 "Task already scheduled or cancelled");
398 task.nextExecutionTime = time;
399 task.period = period;
400 task.state = TimerTask.SCHEDULED;
401 }
402
403 queue.add(task);
404 if (queue.getMin() == task)
405 queue.notify();
406 }
407 }
408
409 /**
410 * Terminates this timer, discarding any currently scheduled tasks.
411 * Does not interfere with a currently executing task (if it exists).
412 * Once a timer has been terminated, its execution thread terminates
413 * gracefully, and no more tasks may be scheduled on it.
414 *
415 * <p>Note that calling this method from within the run method of a
416 * timer task that was invoked by this timer absolutely guarantees that
417 * the ongoing task execution is the last task execution that will ever
418 * be performed by this timer.
419 *
420 * <p>This method may be called repeatedly; the second and subsequent
421 * calls have no effect.
422 */
423 public void cancel() {
424 synchronized(queue) {
425 thread.newTasksMayBeScheduled = false;
426 queue.clear();
427 queue.notify(); // In case queue was already empty.
428 }
429 }
430
431 /**
432 * Removes all cancelled tasks from this timer's task queue. <i>Calling
433 * this method has no effect on the behavior of the timer</i>, but
434 * eliminates the references to the cancelled tasks from the queue.
435 * If there are no external references to these tasks, they become
436 * eligible for garbage collection.
437 *
438 * <p>Most programs will have no need to call this method.
439 * It is designed for use by the rare application that cancels a large
440 * number of tasks. Calling this method trades time for space: the
441 * runtime of the method may be proportional to n + c log n, where n
442 * is the number of tasks in the queue and c is the number of cancelled
443 * tasks.
444 *
445 * <p>Note that it is permissible to call this method from within a
446 * a task scheduled on this timer.
447 *
448 * @return the number of tasks removed from the queue.
449 * @since 1.5
450 */
451 public int purge() {
452 int result = 0;
453
454 synchronized(queue) {
455 for (int i = queue.size(); i > 0; i--) {
456 if (queue.get(i).state == TimerTask.CANCELLED) {
457 queue.quickRemove(i);
458 result++;
459 }
460 }
461
462 if (result != 0)
463 queue.heapify();
464 }
465
466 return result;
467 }
468 }
469
470 /**
471 * This "helper class" implements the timer's task execution thread, which
472 * waits for tasks on the timer queue, executions them when they fire,
473 * reschedules repeating tasks, and removes cancelled tasks and spent
474 * non-repeating tasks from the queue.
475 */
476 class TimerThread extends Thread {
477 /**
478 * This flag is set to false by the reaper to inform us that there
479 * are no more live references to our Timer object. Once this flag
480 * is true and there are no more tasks in our queue, there is no
481 * work left for us to do, so we terminate gracefully. Note that
482 * this field is protected by queue's monitor!
483 */
484 boolean newTasksMayBeScheduled = true;
485
486 /**
487 * Our Timer's queue. We store this reference in preference to
488 * a reference to the Timer so the reference graph remains acyclic.
489 * Otherwise, the Timer would never be garbage-collected and this
490 * thread would never go away.
491 */
492 private TaskQueue queue;
493
494 TimerThread(TaskQueue queue) {
495 this.queue = queue;
496 }
497
498 public void run() {
499 try {
500 mainLoop();
501 } finally {
502 // Someone killed this Thread, behave as if Timer cancelled
503 synchronized(queue) {
504 newTasksMayBeScheduled = false;
505 queue.clear(); // Eliminate obsolete references
506 }
507 }
508 }
509
510 /**
511 * The main timer loop. (See class comment.)
512 */
513 private void mainLoop() {
514 while (true) {
515 try {
516 TimerTask task;
517 boolean taskFired;
518 synchronized(queue) {
519 // Wait for queue to become non-empty
520 while (queue.isEmpty() && newTasksMayBeScheduled)
521 queue.wait();
522 if (queue.isEmpty())
523 break; // Queue is empty and will forever remain; die
524
525 // Queue nonempty; look at first evt and do the right thing
526 long currentTime, executionTime;
527 task = queue.getMin();
528 synchronized(task.lock) {
529 if (task.state == TimerTask.CANCELLED) {
530 queue.removeMin();
531 continue; // No action required, poll queue again
532 }
533 currentTime = System.currentTimeMillis();
534 executionTime = task.nextExecutionTime;
535 if (taskFired = (executionTime<=currentTime)) {
536 if (task.period == 0) { // Non-repeating, remove
537 queue.removeMin();
538 task.state = TimerTask.EXECUTED;
539 } else { // Repeating task, reschedule
540 queue.rescheduleMin(
541 task.period<0 ? currentTime - task.period
542 : executionTime + task.period);
543 }
544 }
545 }
546 if (!taskFired) // Task hasn't yet fired; wait
547 queue.wait(executionTime - currentTime);
548 }
549 if (taskFired) // Task fired; run it, holding no locks
550 task.run();
551 } catch(InterruptedException e) {
552 }
553 }
554 }
555 }
556
557 /**
558 * This class represents a timer task queue: a priority queue of TimerTasks,
559 * ordered on nextExecutionTime. Each Timer object has one of these, which it
560 * shares with its TimerThread. Internally this class uses a heap, which
561 * offers log(n) performance for the add, removeMin and rescheduleMin
562 * operations, and constant time performance for the getMin operation.
563 */
564 class TaskQueue {
565 /**
566 * Priority queue represented as a balanced binary heap: the two children
567 * of queue[n] are queue[2*n] and queue[2*n+1]. The priority queue is
568 * ordered on the nextExecutionTime field: The TimerTask with the lowest
569 * nextExecutionTime is in queue[1] (assuming the queue is nonempty). For
570 * each node n in the heap, and each descendant of n, d,
571 * n.nextExecutionTime <= d.nextExecutionTime.
572 */
573 private TimerTask[] queue = new TimerTask[128];
574
575 /**
576 * The number of tasks in the priority queue. (The tasks are stored in
577 * queue[1] up to queue[size]).
578 */
579 private int size = 0;
580
581 /**
582 * Returns the number of tasks currently on the queue.
583 */
584 int size() {
585 return size;
586 }
587
588 /**
589 * Adds a new task to the priority queue.
590 */
591 void add(TimerTask task) {
592 // Grow backing store if necessary
593 if (size + 1 == queue.length)
594 queue = Arrays.copyOf(queue, 2*queue.length);
595
596 queue[++size] = task;
597 fixUp(size);
598 }
599
600 /**
601 * Return the "head task" of the priority queue. (The head task is an
602 * task with the lowest nextExecutionTime.)
603 */
604 TimerTask getMin() {
605 return queue[1];
606 }
607
608 /**
609 * Return the ith task in the priority queue, where i ranges from 1 (the
610 * head task, which is returned by getMin) to the number of tasks on the
611 * queue, inclusive.
612 */
613 TimerTask get(int i) {
614 return queue[i];
615 }
616
617 /**
618 * Remove the head task from the priority queue.
619 */
620 void removeMin() {
621 queue[1] = queue[size];
622 queue[size--] = null; // Drop extra reference to prevent memory leak
623 fixDown(1);
624 }
625
626 /**
627 * Removes the ith element from queue without regard for maintaining
628 * the heap invariant. Recall that queue is one-based, so
629 * 1 <= i <= size.
630 */
631 void quickRemove(int i) {
632 assert i <= size;
633
634 queue[i] = queue[size];
635 queue[size--] = null; // Drop extra ref to prevent memory leak
636 }
637
638 /**
639 * Sets the nextExecutionTime associated with the head task to the
640 * specified value, and adjusts priority queue accordingly.
641 */
642 void rescheduleMin(long newTime) {
643 queue[1].nextExecutionTime = newTime;
644 fixDown(1);
645 }
646
647 /**
648 * Returns true if the priority queue contains no elements.
649 */
650 boolean isEmpty() {
651 return size==0;
652 }
653
654 /**
655 * Removes all elements from the priority queue.
656 */
657 void clear() {
658 // Null out task references to prevent memory leak
659 for (int i=1; i<=size; i++)
660 queue[i] = null;
661
662 size = 0;
663 }
664
665 /**
666 * Establishes the heap invariant (described above) assuming the heap
667 * satisfies the invariant except possibly for the leaf-node indexed by k
668 * (which may have a nextExecutionTime less than its parent's).
669 *
670 * This method functions by "promoting" queue[k] up the hierarchy
671 * (by swapping it with its parent) repeatedly until queue[k]'s
672 * nextExecutionTime is greater than or equal to that of its parent.
673 */
674 private void fixUp(int k) {
675 while (k > 1) {
676 int j = k >> 1;
677 if (queue[j].nextExecutionTime <= queue[k].nextExecutionTime)
678 break;
679 TimerTask tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
680 k = j;
681 }
682 }
683
684 /**
685 * Establishes the heap invariant (described above) in the subtree
686 * rooted at k, which is assumed to satisfy the heap invariant except
687 * possibly for node k itself (which may have a nextExecutionTime greater
688 * than its children's).
689 *
690 * This method functions by "demoting" queue[k] down the hierarchy
691 * (by swapping it with its smaller child) repeatedly until queue[k]'s
692 * nextExecutionTime is less than or equal to those of its children.
693 */
694 private void fixDown(int k) {
695 int j;
696 while ((j = k << 1) <= size && j > 0) {
697 if (j < size &&
698 queue[j].nextExecutionTime > queue[j+1].nextExecutionTime)
699 j++; // j indexes smallest kid
700 if (queue[k].nextExecutionTime <= queue[j].nextExecutionTime)
701 break;
702 TimerTask tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
703 k = j;
704 }
705 }
706
707 /**
708 * Establishes the heap invariant (described above) in the entire tree,
709 * assuming nothing about the order of the elements prior to the call.
710 */
711 void heapify() {
712 for (int i = size/2; i >= 1; i--)
713 fixDown(i);
714 }
715 }
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