001/* Timer.java -- Timer that runs TimerTasks at a later time.
002   Copyright (C) 2000, 2001, 2005 Free Software Foundation, Inc.
003
004This file is part of GNU Classpath.
005
006GNU Classpath is free software; you can redistribute it and/or modify
007it under the terms of the GNU General Public License as published by
008the Free Software Foundation; either version 2, or (at your option)
009any later version.
010
011GNU Classpath is distributed in the hope that it will be useful, but
012WITHOUT ANY WARRANTY; without even the implied warranty of
013MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
014General Public License for more details.
015
016You should have received a copy of the GNU General Public License
017along with GNU Classpath; see the file COPYING.  If not, write to the
018Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
01902110-1301 USA.
020
021Linking this library statically or dynamically with other modules is
022making a combined work based on this library.  Thus, the terms and
023conditions of the GNU General Public License cover the whole
024combination.
025
026As a special exception, the copyright holders of this library give you
027permission to link this library with independent modules to produce an
028executable, regardless of the license terms of these independent
029modules, and to copy and distribute the resulting executable under
030terms of your choice, provided that you also meet, for each linked
031independent module, the terms and conditions of the license of that
032module.  An independent module is a module which is not derived from
033or based on this library.  If you modify this library, you may extend
034this exception to your version of the library, but you are not
035obligated to do so.  If you do not wish to do so, delete this
036exception statement from your version. */
037
038package java.util;
039
040/**
041 * Timer that can run TimerTasks at a later time.
042 * TimerTasks can be scheduled for one time execution at some time in the
043 * future. They can be scheduled to be rescheduled at a time period after the
044 * task was last executed. Or they can be scheduled to be executed repeatedly
045 * at a fixed rate.
046 * <p>
047 * The normal scheduling will result in a more or less even delay in time
048 * between successive executions, but the executions could drift in time if
049 * the task (or other tasks) takes a long time to execute. Fixed delay
050 * scheduling guarantees more or less that the task will be executed at a
051 * specific time, but if there is ever a delay in execution then the period
052 * between successive executions will be shorter. The first method of
053 * repeated scheduling is preferred for repeated tasks in response to user
054 * interaction, the second method of repeated scheduling is preferred for tasks
055 * that act like alarms.
056 * <p>
057 * The Timer keeps a binary heap as a task priority queue which means that
058 * scheduling and serving of a task in a queue of n tasks costs O(log n).
059 *
060 * @see TimerTask
061 * @since 1.3
062 * @author Mark Wielaard (mark@klomp.org)
063 */
064public class Timer
065{
066  /**
067   * Priority Task Queue.
068   * TimerTasks are kept in a binary heap.
069   * The scheduler calls sleep() on the queue when it has nothing to do or
070   * has to wait. A sleeping scheduler can be notified by calling interrupt()
071   * which is automatically called by the enqueue(), cancel() and
072   * timerFinalized() methods.
073   */
074  private static final class TaskQueue
075  {
076    /** Default size of this queue */
077    private static final int DEFAULT_SIZE = 32;
078
079    /** Whether to return null when there is nothing in the queue */
080    private boolean nullOnEmpty;
081
082    /**
083     * The heap containing all the scheduled TimerTasks
084     * sorted by the TimerTask.scheduled field.
085     * Null when the stop() method has been called.
086     */
087    private TimerTask heap[];
088
089    /**
090     * The actual number of elements in the heap
091     * Can be less then heap.length.
092     * Note that heap[0] is used as a sentinel.
093     */
094    private int elements;
095
096    /**
097     * Creates a TaskQueue of default size without any elements in it.
098     */
099    public TaskQueue()
100    {
101      heap = new TimerTask[DEFAULT_SIZE];
102      elements = 0;
103      nullOnEmpty = false;
104    }
105
106    /**
107     * Adds a TimerTask at the end of the heap.
108     * Grows the heap if necessary by doubling the heap in size.
109     */
110    private void add(TimerTask task)
111    {
112      elements++;
113      if (elements == heap.length)
114        {
115          TimerTask new_heap[] = new TimerTask[heap.length * 2];
116          System.arraycopy(heap, 0, new_heap, 0, heap.length);
117          heap = new_heap;
118        }
119      heap[elements] = task;
120    }
121
122    /**
123     * Removes the last element from the heap.
124     * Shrinks the heap in half if
125     * elements+DEFAULT_SIZE/2 <= heap.length/4.
126     */
127    private void remove()
128    {
129      // clear the entry first
130      heap[elements] = null;
131      elements--;
132      if (elements + DEFAULT_SIZE / 2 <= (heap.length / 4))
133        {
134          TimerTask new_heap[] = new TimerTask[heap.length / 2];
135          System.arraycopy(heap, 0, new_heap, 0, elements + 1);
136          heap = new_heap;
137        }
138    }
139
140    /**
141     * Adds a task to the queue and puts it at the correct place
142     * in the heap.
143     */
144    public synchronized void enqueue(TimerTask task)
145    {
146      // Check if it is legal to add another element
147      if (heap == null)
148        {
149          throw new IllegalStateException
150            ("cannot enqueue when stop() has been called on queue");
151        }
152
153      heap[0] = task;           // sentinel
154      add(task);                // put the new task at the end
155      // Now push the task up in the heap until it has reached its place
156      int child = elements;
157      int parent = child / 2;
158      while (heap[parent].scheduled > task.scheduled)
159        {
160          heap[child] = heap[parent];
161          child = parent;
162          parent = child / 2;
163        }
164      // This is the correct place for the new task
165      heap[child] = task;
166      heap[0] = null;           // clear sentinel
167      // Maybe sched() is waiting for a new element
168      this.notify();
169    }
170
171    /**
172     * Returns the top element of the queue.
173     * Can return null when no task is in the queue.
174     */
175    private TimerTask top()
176    {
177      if (elements == 0)
178        {
179          return null;
180        }
181      else
182        {
183          return heap[1];
184        }
185    }
186
187    /**
188     * Returns the top task in the Queue.
189     * Removes the element from the heap and reorders the heap first.
190     * Can return null when there is nothing in the queue.
191     */
192    public synchronized TimerTask serve()
193    {
194      // The task to return
195      TimerTask task = null;
196
197      while (task == null)
198        {
199          // Get the next task
200          task = top();
201
202          // return null when asked to stop
203          // or if asked to return null when the queue is empty
204          if ((heap == null) || (task == null && nullOnEmpty))
205            {
206              return null;
207            }
208
209          // Do we have a task?
210          if (task != null)
211            {
212              // The time to wait until the task should be served
213              long time = task.scheduled - System.currentTimeMillis();
214              if (time > 0)
215                {
216                  // This task should not yet be served
217                  // So wait until this task is ready
218                  // or something else happens to the queue
219                  task = null;  // set to null to make sure we call top()
220                  try
221                    {
222                      this.wait(time);
223                    }
224                  catch (InterruptedException _)
225                    {
226                    }
227                }
228            }
229          else
230            {
231              // wait until a task is added
232              // or something else happens to the queue
233              try
234                {
235                  this.wait();
236                }
237              catch (InterruptedException _)
238                {
239                }
240            }
241        }
242
243      // reconstruct the heap
244      TimerTask lastTask = heap[elements];
245      remove();
246
247      // drop lastTask at the beginning and move it down the heap
248      int parent = 1;
249      int child = 2;
250      heap[1] = lastTask;
251      while (child <= elements)
252        {
253          if (child < elements)
254            {
255              if (heap[child].scheduled > heap[child + 1].scheduled)
256                {
257                  child++;
258                }
259            }
260
261          if (lastTask.scheduled <= heap[child].scheduled)
262            break;              // found the correct place (the parent) - done
263
264          heap[parent] = heap[child];
265          parent = child;
266          child = parent * 2;
267        }
268
269      // this is the correct new place for the lastTask
270      heap[parent] = lastTask;
271
272      // return the task
273      return task;
274    }
275
276    /**
277     * When nullOnEmpty is true the serve() method will return null when
278     * there are no tasks in the queue, otherwise it will wait until
279     * a new element is added to the queue. It is used to indicate to
280     * the scheduler that no new tasks will ever be added to the queue.
281     */
282    public synchronized void setNullOnEmpty(boolean nullOnEmpty)
283    {
284      this.nullOnEmpty = nullOnEmpty;
285      this.notify();
286    }
287
288    /**
289     * When this method is called the current and all future calls to
290     * serve() will return null. It is used to indicate to the Scheduler
291     * that it should stop executing since no more tasks will come.
292     */
293    public synchronized void stop()
294    {
295      this.heap = null;
296      this.elements = 0;
297      this.notify();
298    }
299
300    /**
301     * Remove all canceled tasks from the queue.
302     */
303    public synchronized int purge()
304    {
305      int removed = 0;
306      // Null out any elements that are canceled.  Skip element 0 as
307      // it is the sentinel.
308      for (int i = elements; i > 0; --i)
309        {
310          if (heap[i].scheduled < 0)
311            {
312              ++removed;
313
314              // Remove an element by pushing the appropriate child
315              // into place, and then iterating to the bottom of the
316              // tree.
317              int index = i;
318              while (heap[index] != null)
319                {
320                  int child = 2 * index;
321                  if (child >= heap.length)
322                    {
323                      // Off end; we're done.
324                      heap[index] = null;
325                      break;
326                    }
327
328                  if (child + 1 >= heap.length || heap[child + 1] == null)
329                    {
330                      // Nothing -- we're done.
331                    }
332                  else if (heap[child] == null
333                           || (heap[child].scheduled
334                               > heap[child + 1].scheduled))
335                    ++child;
336                  heap[index] = heap[child];
337                  index = child;
338                }
339            }
340        }
341
342      // Make a new heap if we shrank enough.
343      int newLen = heap.length;
344      while (elements - removed + DEFAULT_SIZE / 2 <= newLen / 4)
345        newLen /= 2;
346      if (newLen != heap.length)
347        {
348          TimerTask[] newHeap = new TimerTask[newLen];
349          System.arraycopy(heap, 0, newHeap, 0, elements + 1);
350          heap = newHeap;
351        }
352
353      return removed;
354    }
355  }                             // TaskQueue
356
357  /**
358   * The scheduler that executes all the tasks on a particular TaskQueue,
359   * reschedules any repeating tasks and that waits when no task has to be
360   * executed immediately. Stops running when canceled or when the parent
361   * Timer has been finalized and no more tasks have to be executed.
362   */
363  private static final class Scheduler implements Runnable
364  {
365    // The priority queue containing all the TimerTasks.
366    private TaskQueue queue;
367
368    /**
369     * Creates a new Scheduler that will schedule the tasks on the
370     * given TaskQueue.
371     */
372    public Scheduler(TaskQueue queue)
373    {
374      this.queue = queue;
375    }
376
377    public void run()
378    {
379      TimerTask task;
380      while ((task = queue.serve()) != null)
381        {
382          // If this task has not been canceled
383          if (task.scheduled >= 0)
384            {
385
386              // Mark execution time
387              task.lastExecutionTime = task.scheduled;
388
389              // Repeatable task?
390              if (task.period < 0)
391                {
392                  // Last time this task is executed
393                  task.scheduled = -1;
394                }
395
396              // Run the task
397              try
398                {
399                  task.run();
400                }
401              catch (ThreadDeath death)
402                {
403                  // If an exception escapes, the Timer becomes invalid.
404                  queue.stop();
405                  throw death;
406                }
407              catch (Throwable t)
408                {
409                  // If an exception escapes, the Timer becomes invalid.
410                  queue.stop();
411                }
412            }
413
414          // Calculate next time and possibly re-enqueue.
415          if (task.scheduled >= 0)
416            {
417              if (task.fixed)
418                {
419                  task.scheduled += task.period;
420                }
421              else
422                {
423                  task.scheduled = task.period + System.currentTimeMillis();
424                }
425
426              try
427                {
428                  queue.enqueue(task);
429                }
430              catch (IllegalStateException ise)
431                {
432                  // Ignore. Apparently the Timer queue has been stopped.
433                }
434            }
435        }
436    }
437  }                             // Scheduler
438
439  // Number of Timers created.
440  // Used for creating nice Thread names.
441  private static int nr;
442
443  // The queue that all the tasks are put in.
444  // Given to the scheduler
445  private TaskQueue queue;
446
447  // The Scheduler that does all the real work
448  private Scheduler scheduler;
449
450  // Used to run the scheduler.
451  // Also used to checked if the Thread is still running by calling
452  // thread.isAlive(). Sometimes a Thread is suddenly killed by the system
453  // (if it belonged to an Applet).
454  private Thread thread;
455
456  // When cancelled we don't accept any more TimerTasks.
457  private boolean canceled;
458
459  /**
460   * Creates a new Timer with a non daemon Thread as Scheduler, with normal
461   * priority and a default name.
462   */
463  public Timer()
464  {
465    this(false);
466  }
467
468  /**
469   * Creates a new Timer with a daemon Thread as scheduler if daemon is true,
470   * with normal priority and a default name.
471   */
472  public Timer(boolean daemon)
473  {
474    this(daemon, Thread.NORM_PRIORITY);
475  }
476
477  /**
478   * Create a new Timer whose Thread has the indicated name.  It will have
479   * normal priority and will not be a daemon thread.
480   * @param name the name of the Thread
481   * @since 1.5
482   */
483  public Timer(String name)
484  {
485    this(false, Thread.NORM_PRIORITY, name);
486  }
487
488  /**
489   * Create a new Timer whose Thread has the indicated name.  It will have
490   * normal priority.  The boolean argument controls whether or not it
491   * will be a daemon thread.
492   * @param name the name of the Thread
493   * @param daemon true if the Thread should be a daemon thread
494   * @since 1.5
495   */
496  public Timer(String name, boolean daemon)
497  {
498    this(daemon, Thread.NORM_PRIORITY, name);
499  }
500
501  /**
502   * Creates a new Timer with a daemon Thread as scheduler if daemon is true,
503   * with the priority given and a default name.
504   */
505  private Timer(boolean daemon, int priority)
506  {
507    this(daemon, priority, "Timer-" + (++nr));
508  }
509
510  /**
511   * Creates a new Timer with a daemon Thread as scheduler if daemon is true,
512   * with the priority and name given.E
513   */
514  private Timer(boolean daemon, int priority, String name)
515  {
516    canceled = false;
517    queue = new TaskQueue();
518    scheduler = new Scheduler(queue);
519    thread = new Thread(scheduler, name);
520    thread.setDaemon(daemon);
521    thread.setPriority(priority);
522    thread.start();
523  }
524
525  /**
526   * Cancels the execution of the scheduler. If a task is executing it will
527   * normally finish execution, but no other tasks will be executed and no
528   * more tasks can be scheduled.
529   */
530  public void cancel()
531  {
532    canceled = true;
533    queue.stop();
534  }
535
536  /**
537   * Schedules the task at Time time, repeating every period
538   * milliseconds if period is positive and at a fixed rate if fixed is true.
539   *
540   * @exception IllegalArgumentException if time is negative
541   * @exception IllegalStateException if the task was already scheduled or
542   * canceled or this Timer is canceled or the scheduler thread has died
543   */
544  private void schedule(TimerTask task, long time, long period, boolean fixed)
545  {
546    if (time < 0)
547      throw new IllegalArgumentException("negative time");
548
549    if (task.scheduled == 0 && task.lastExecutionTime == -1)
550      {
551        task.scheduled = time;
552        task.period = period;
553        task.fixed = fixed;
554      }
555    else
556      {
557        throw new IllegalStateException
558          ("task was already scheduled or canceled");
559      }
560
561    if (!this.canceled && this.thread != null)
562      {
563        queue.enqueue(task);
564      }
565    else
566      {
567        throw new IllegalStateException
568          ("timer was canceled or scheduler thread has died");
569      }
570  }
571
572  private static void positiveDelay(long delay)
573  {
574    if (delay < 0)
575      {
576        throw new IllegalArgumentException("delay is negative");
577      }
578  }
579
580  private static void positivePeriod(long period)
581  {
582    if (period < 0)
583      {
584        throw new IllegalArgumentException("period is negative");
585      }
586  }
587
588  /**
589   * Schedules the task at the specified data for one time execution.
590   *
591   * @exception IllegalArgumentException if date.getTime() is negative
592   * @exception IllegalStateException if the task was already scheduled or
593   * canceled or this Timer is canceled or the scheduler thread has died
594   */
595  public void schedule(TimerTask task, Date date)
596  {
597    long time = date.getTime();
598    schedule(task, time, -1, false);
599  }
600
601  /**
602   * Schedules the task at the specified date and reschedules the task every
603   * period milliseconds after the last execution of the task finishes until
604   * this timer or the task is canceled.
605   *
606   * @exception IllegalArgumentException if period or date.getTime() is
607   * negative
608   * @exception IllegalStateException if the task was already scheduled or
609   * canceled or this Timer is canceled or the scheduler thread has died
610   */
611  public void schedule(TimerTask task, Date date, long period)
612  {
613    positivePeriod(period);
614    long time = date.getTime();
615    schedule(task, time, period, false);
616  }
617
618  /**
619   * Schedules the task after the specified delay milliseconds for one time
620   * execution.
621   *
622   * @exception IllegalArgumentException if delay or
623   * System.currentTimeMillis + delay is negative
624   * @exception IllegalStateException if the task was already scheduled or
625   * canceled or this Timer is canceled or the scheduler thread has died
626   */
627  public void schedule(TimerTask task, long delay)
628  {
629    positiveDelay(delay);
630    long time = System.currentTimeMillis() + delay;
631    schedule(task, time, -1, false);
632  }
633
634  /**
635   * Schedules the task after the delay milliseconds and reschedules the
636   * task every period milliseconds after the last execution of the task
637   * finishes until this timer or the task is canceled.
638   *
639   * @exception IllegalArgumentException if delay or period is negative
640   * @exception IllegalStateException if the task was already scheduled or
641   * canceled or this Timer is canceled or the scheduler thread has died
642   */
643  public void schedule(TimerTask task, long delay, long period)
644  {
645    positiveDelay(delay);
646    positivePeriod(period);
647    long time = System.currentTimeMillis() + delay;
648    schedule(task, time, period, false);
649  }
650
651  /**
652   * Schedules the task at the specified date and reschedules the task at a
653   * fixed rate every period milliseconds until this timer or the task is
654   * canceled.
655   *
656   * @exception IllegalArgumentException if period or date.getTime() is
657   * negative
658   * @exception IllegalStateException if the task was already scheduled or
659   * canceled or this Timer is canceled or the scheduler thread has died
660   */
661  public void scheduleAtFixedRate(TimerTask task, Date date, long period)
662  {
663    positivePeriod(period);
664    long time = date.getTime();
665    schedule(task, time, period, true);
666  }
667
668  /**
669   * Schedules the task after the delay milliseconds and reschedules the task
670   * at a fixed rate every period milliseconds until this timer or the task
671   * is canceled.
672   *
673   * @exception IllegalArgumentException if delay or
674   * System.currentTimeMillis + delay is negative
675   * @exception IllegalStateException if the task was already scheduled or
676   * canceled or this Timer is canceled or the scheduler thread has died
677   */
678  public void scheduleAtFixedRate(TimerTask task, long delay, long period)
679  {
680    positiveDelay(delay);
681    positivePeriod(period);
682    long time = System.currentTimeMillis() + delay;
683    schedule(task, time, period, true);
684  }
685
686  /**
687   * Tells the scheduler that the Timer task died
688   * so there will be no more new tasks scheduled.
689   */
690  protected void finalize() throws Throwable
691  {
692    queue.setNullOnEmpty(true);
693  }
694
695  /**
696   * Removes all cancelled tasks from the queue.
697   * @return the number of tasks removed
698   * @since 1.5
699   */
700  public int purge()
701  {
702    return queue.purge();
703  }
704}