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 22 * or visit www.oracle.com if you need additional information or have any 23 * questions. 24 */ 25 26package java.util; 27import java.util.Date; 28import 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 <tt>Timer</tt> 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 <tt>Timer</tt> 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 <tt>cancel</tt> method. 51 * 52 * <p>If the timer's task execution thread terminates unexpectedly, for 53 * example, because its <tt>stop</tt> method is invoked, any further 54 * attempt to schedule a task on the timer will result in an 55 * <tt>IllegalStateException</tt>, as if the timer's <tt>cancel</tt> 56 * method had been invoked. 57 * 58 * <p>This class is thread-safe: multiple threads can share a single 59 * <tt>Timer</tt> 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 <tt>Object.wait(long)</tt> 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 89public 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 <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 // 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 */ 481class 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 */ 569class 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} 721