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27 import java.util.Date;
28 import java.util.concurrent.atomic.AtomicInteger;
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.
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
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.
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.
58 * <p>This class is thread-safe: multiple threads can share a single
59 * <tt>Timer</tt> object without the need for external synchronization.
61 * <p>This class does <i>not</i> offer real-time guarantees: it schedules
62 * tasks using the <tt>Object.wait(long)</tt> method.
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
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.
81 * <p>Implementation note: All constructors start a timer thread.
85 * @see Object#wait(long)
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.
96 private final TaskQueue queue = new TaskQueue();
101 private final TimerThread thread = new TimerThread(queue);
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.
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.
120 * This ID is used to generate thread names.
122 private final static AtomicInteger nextSerialNumber = new AtomicInteger(0);
123 private static int serialNumber() {
124 return nextSerialNumber.getAndIncrement();
128 * Creates a new timer. The associated thread does <i>not</i>
129 * {@linkplain Thread#setDaemon run as a daemon}.
132 this("Timer-" + serialNumber());
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.
143 * @param isDaemon true if the associated thread should run as a daemon.
145 public Timer(boolean isDaemon) {
146 this("Timer-" + serialNumber(), isDaemon);
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}.
154 * @param name the name of the associated thread
155 * @throws NullPointerException if {@code name} is null
158 public Timer(String name) {
159 thread.setName(name);
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}.
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
173 public Timer(String name, boolean isDaemon) {
174 thread.setName(name);
175 thread.setDaemon(isDaemon);
180 * Schedules the specified task for execution after the specified delay.
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
190 public void schedule(TimerTask task, long delay) {
192 throw new IllegalArgumentException("Negative delay.");
193 sched(task, System.currentTimeMillis()+delay, 0);
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.
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
207 public void schedule(TimerTask task, Date time) {
208 sched(task, time.getTime(), 0);
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.
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).
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
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
243 public void schedule(TimerTask task, long delay, long period) {
245 throw new IllegalArgumentException("Negative delay.");
247 throw new IllegalArgumentException("Non-positive period.");
248 sched(task, System.currentTimeMillis()+delay, -period);
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.
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.
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
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
284 public void schedule(TimerTask task, Date firstTime, long period) {
286 throw new IllegalArgumentException("Non-positive period.");
287 sched(task, firstTime.getTime(), -period);
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.
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).
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.
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
323 public void scheduleAtFixedRate(TimerTask task, long delay, long period) {
325 throw new IllegalArgumentException("Negative delay.");
327 throw new IllegalArgumentException("Non-positive period.");
328 sched(task, System.currentTimeMillis()+delay, period);
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.
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"
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.
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
366 public void scheduleAtFixedRate(TimerTask task, Date firstTime,
369 throw new IllegalArgumentException("Non-positive period.");
370 sched(task, firstTime.getTime(), period);
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.
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
386 private void sched(TimerTask task, long time, long period) {
388 throw new IllegalArgumentException("Illegal execution time.");
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))
395 synchronized(queue) {
396 if (!thread.newTasksMayBeScheduled)
397 throw new IllegalStateException("Timer already cancelled.");
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;
409 if (queue.getMin() == task)
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.
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.
425 * <p>This method may be called repeatedly; the second and subsequent
426 * calls have no effect.
428 public void cancel() {
429 synchronized(queue) {
430 thread.newTasksMayBeScheduled = false;
432 queue.notify(); // In case queue was already empty.
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.
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
450 * <p>Note that it is permissible to call this method from within a
451 * a task scheduled on this timer.
453 * @return the number of tasks removed from the queue.
459 synchronized(queue) {
460 for (int i = queue.size(); i > 0; i--) {
461 if (queue.get(i).state == TimerTask.CANCELLED) {
462 queue.quickRemove(i);
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.
481 class TimerThread extends Thread {
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!
489 boolean newTasksMayBeScheduled = true;
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.
497 private TaskQueue queue;
499 TimerThread(TaskQueue queue) {
507 // Someone killed this Thread, behave as if Timer cancelled
508 synchronized(queue) {
509 newTasksMayBeScheduled = false;
510 queue.clear(); // Eliminate obsolete references
516 * The main timer loop. (See class comment.)
518 private void mainLoop() {
523 synchronized(queue) {
524 // Wait for queue to become non-empty
525 while (queue.isEmpty() && newTasksMayBeScheduled)
528 break; // Queue is empty and will forever remain; die
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) {
536 continue; // No action required, poll queue again
538 currentTime = System.currentTimeMillis();
539 executionTime = task.nextExecutionTime;
540 if (taskFired = (executionTime<=currentTime)) {
541 if (task.period == 0) { // Non-repeating, remove
543 task.state = TimerTask.EXECUTED;
544 } else { // Repeating task, reschedule
546 task.period<0 ? currentTime - task.period
547 : executionTime + task.period);
551 if (!taskFired) // Task hasn't yet fired; wait
552 queue.wait(executionTime - currentTime);
554 if (taskFired) // Task fired; run it, holding no locks
556 } catch(InterruptedException e) {
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.
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.
578 private TimerTask[] queue = new TimerTask[128];
581 * The number of tasks in the priority queue. (The tasks are stored in
582 * queue[1] up to queue[size]).
584 private int size = 0;
587 * Returns the number of tasks currently on the queue.
594 * Adds a new task to the priority queue.
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);
601 queue[++size] = task;
606 * Return the "head task" of the priority queue. (The head task is an
607 * task with the lowest nextExecutionTime.)
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
618 TimerTask get(int i) {
623 * Remove the head task from the priority queue.
626 queue[1] = queue[size];
627 queue[size--] = null; // Drop extra reference to prevent memory leak
632 * Removes the ith element from queue without regard for maintaining
633 * the heap invariant. Recall that queue is one-based, so
636 void quickRemove(int i) {
639 queue[i] = queue[size];
640 queue[size--] = null; // Drop extra ref to prevent memory leak
644 * Sets the nextExecutionTime associated with the head task to the
645 * specified value, and adjusts priority queue accordingly.
647 void rescheduleMin(long newTime) {
648 queue[1].nextExecutionTime = newTime;
653 * Returns true if the priority queue contains no elements.
660 * Removes all elements from the priority queue.
663 // Null out task references to prevent memory leak
664 for (int i=1; i<=size; i++)
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).
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.
679 private void fixUp(int k) {
682 if (queue[j].nextExecutionTime <= queue[k].nextExecutionTime)
684 TimerTask tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
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).
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.
699 private void fixDown(int k) {
701 while ((j = k << 1) <= size && j > 0) {
703 queue[j].nextExecutionTime > queue[j+1].nextExecutionTime)
704 j++; // j indexes smallest kid
705 if (queue[k].nextExecutionTime <= queue[j].nextExecutionTime)
707 TimerTask tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
713 * Establishes the heap invariant (described above) in the entire tree,
714 * assuming nothing about the order of the elements prior to the call.
717 for (int i = size/2; i >= 1; i--)