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
26 package java.util;
27 import java.util.Date;
28 import java.util.concurrent.atomic.AtomicInteger;
29
30 /**
31 * A facility for threads to schedule tasks for future execution in a
32 * background thread. Tasks may be scheduled for one-time execution, or for
33 * repeated execution at regular intervals.
34 *
35 * <p>Corresponding to each <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 *
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(
|
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
26 package java.util;
27 import java.util.Date;
28 import java.util.concurrent.atomic.AtomicInteger;
29
30 /**
31 * A facility for threads to schedule tasks for future execution in a
32 * background thread. Tasks may be scheduled for one-time execution, or for
33 * repeated execution at regular intervals.
34 *
35 * <p>Corresponding to each {@code Timer} object is a single background
36 * thread that is used to execute all of the timer's tasks, sequentially.
37 * Timer tasks should complete quickly. If a timer task takes excessive time
38 * to complete, it "hogs" the timer's task execution thread. This can, in
39 * turn, delay the execution of subsequent tasks, which may "bunch up" and
40 * execute in rapid succession when (and if) the offending task finally
41 * completes.
42 *
43 * <p>After the last live reference to a {@code Timer} object goes away
44 * <i>and</i> all outstanding tasks have completed execution, the timer's task
45 * execution thread terminates gracefully (and becomes subject to garbage
46 * collection). However, this can take arbitrarily long to occur. By
47 * default, the task execution thread does not run as a <i>daemon thread</i>,
48 * so it is capable of keeping an application from terminating. If a caller
49 * wants to terminate a timer's task execution thread rapidly, the caller
50 * should invoke the timer's {@code cancel} method.
51 *
52 * <p>If the timer's task execution thread terminates unexpectedly, for
53 * example, because its {@code stop} method is invoked, any further
54 * attempt to schedule a task on the timer will result in an
55 * {@code IllegalStateException}, as if the timer's {@code cancel}
56 * method had been invoked.
57 *
58 * <p>This class is thread-safe: multiple threads can share a single
59 * {@code Timer} object without the need for external synchronization.
60 *
61 * <p>This class does <i>not</i> offer real-time guarantees: it schedules
62 * tasks using the {@code Object.wait(long)} method.
63 *
64 * <p>Java 5.0 introduced the {@code java.util.concurrent} package and
65 * one of the concurrency utilities therein is the {@link
66 * java.util.concurrent.ScheduledThreadPoolExecutor
67 * ScheduledThreadPoolExecutor} which is a thread pool for repeatedly
68 * executing tasks at a given rate or delay. It is effectively a more
69 * versatile replacement for the {@code Timer}/{@code TimerTask}
70 * combination, as it allows multiple service threads, accepts various
71 * time units, and doesn't require subclassing {@code TimerTask} (just
72 * implement {@code Runnable}). Configuring {@code
73 * ScheduledThreadPoolExecutor} with one thread makes it equivalent to
74 * {@code Timer}.
75 *
76 * <p>Implementation note: This class scales to large numbers of concurrently
77 * scheduled tasks (thousands should present no problem). Internally,
78 * it uses a binary heap to represent its task queue, so the cost to schedule
79 * a task is O(log n), where n is the number of concurrently scheduled tasks.
80 *
81 * <p>Implementation note: All constructors start a timer thread.
82 *
164 * Creates a new timer whose associated thread has the specified name,
165 * and may be specified to
166 * {@linkplain Thread#setDaemon run as a daemon}.
167 *
168 * @param name the name of the associated thread
169 * @param isDaemon true if the associated thread should run as a daemon
170 * @throws NullPointerException if {@code name} is null
171 * @since 1.5
172 */
173 public Timer(String name, boolean isDaemon) {
174 thread.setName(name);
175 thread.setDaemon(isDaemon);
176 thread.start();
177 }
178
179 /**
180 * Schedules the specified task for execution after the specified delay.
181 *
182 * @param task task to be scheduled.
183 * @param delay delay in milliseconds before task is to be executed.
184 * @throws IllegalArgumentException if {@code delay} is negative, or
185 * {@code delay + System.currentTimeMillis()} is negative.
186 * @throws IllegalStateException if task was already scheduled or
187 * cancelled, timer was cancelled, or timer thread terminated.
188 * @throws NullPointerException if {@code task} is null
189 */
190 public void schedule(TimerTask task, long delay) {
191 if (delay < 0)
192 throw new IllegalArgumentException("Negative delay.");
193 sched(task, System.currentTimeMillis()+delay, 0);
194 }
195
196 /**
197 * Schedules the specified task for execution at the specified time. If
198 * the time is in the past, the task is scheduled for immediate execution.
199 *
200 * @param task task to be scheduled.
201 * @param time time at which task is to be executed.
202 * @throws IllegalArgumentException if {@code time.getTime()} is negative.
203 * @throws IllegalStateException if task was already scheduled or
204 * cancelled, timer was cancelled, or timer thread terminated.
205 * @throws NullPointerException if {@code task} or {@code time} is null
206 */
207 public void schedule(TimerTask task, Date time) {
208 sched(task, time.getTime(), 0);
209 }
210
211 /**
212 * Schedules the specified task for repeated <i>fixed-delay execution</i>,
213 * beginning after the specified delay. Subsequent executions take place
214 * at approximately regular intervals separated by the specified period.
215 *
216 * <p>In fixed-delay execution, each execution is scheduled relative to
217 * the actual execution time of the previous execution. If an execution
218 * is delayed for any reason (such as garbage collection or other
219 * background activity), subsequent executions will be delayed as well.
220 * In the long run, the frequency of execution will generally be slightly
221 * lower than the reciprocal of the specified period (assuming the system
222 * clock underlying {@code Object.wait(long)} is accurate).
223 *
224 * <p>Fixed-delay execution is appropriate for recurring activities
225 * that require "smoothness." In other words, it is appropriate for
226 * activities where it is more important to keep the frequency accurate
227 * in the short run than in the long run. This includes most animation
228 * tasks, such as blinking a cursor at regular intervals. It also includes
229 * tasks wherein regular activity is performed in response to human
230 * input, such as automatically repeating a character as long as a key
231 * is held down.
232 *
233 * @param task task to be scheduled.
234 * @param delay delay in milliseconds before task is to be executed.
235 * @param period time in milliseconds between successive task executions.
236 * @throws IllegalArgumentException if {@code delay < 0}, or
237 * {@code delay + System.currentTimeMillis() < 0}, or
238 * {@code period <= 0}
239 * @throws IllegalStateException if task was already scheduled or
240 * cancelled, timer was cancelled, or timer thread terminated.
241 * @throws NullPointerException if {@code task} is null
242 */
243 public void schedule(TimerTask task, long delay, long period) {
244 if (delay < 0)
245 throw new IllegalArgumentException("Negative delay.");
246 if (period <= 0)
247 throw new IllegalArgumentException("Non-positive period.");
248 sched(task, System.currentTimeMillis()+delay, -period);
249 }
250
251 /**
252 * Schedules the specified task for repeated <i>fixed-delay execution</i>,
253 * beginning at the specified time. Subsequent executions take place at
254 * approximately regular intervals, separated by the specified period.
255 *
256 * <p>In fixed-delay execution, each execution is scheduled relative to
257 * the actual execution time of the previous execution. If an execution
258 * is delayed for any reason (such as garbage collection or other
259 * background activity), subsequent executions will be delayed as well.
260 * In the long run, the frequency of execution will generally be slightly
261 * lower than the reciprocal of the specified period (assuming the system
262 * clock underlying {@code Object.wait(long)} is accurate). As a
263 * consequence of the above, if the scheduled first time is in the past,
264 * it is scheduled for immediate execution.
265 *
266 * <p>Fixed-delay execution is appropriate for recurring activities
267 * that require "smoothness." In other words, it is appropriate for
268 * activities where it is more important to keep the frequency accurate
269 * in the short run than in the long run. This includes most animation
270 * tasks, such as blinking a cursor at regular intervals. It also includes
271 * tasks wherein regular activity is performed in response to human
272 * input, such as automatically repeating a character as long as a key
273 * is held down.
274 *
275 * @param task task to be scheduled.
276 * @param firstTime First time at which task is to be executed.
277 * @param period time in milliseconds between successive task executions.
278 * @throws IllegalArgumentException if {@code firstTime.getTime() < 0}, or
279 * {@code period <= 0}
280 * @throws IllegalStateException if task was already scheduled or
281 * cancelled, timer was cancelled, or timer thread terminated.
282 * @throws NullPointerException if {@code task} or {@code firstTime} is null
283 */
284 public void schedule(TimerTask task, Date firstTime, long period) {
285 if (period <= 0)
286 throw new IllegalArgumentException("Non-positive period.");
287 sched(task, firstTime.getTime(), -period);
288 }
289
290 /**
291 * Schedules the specified task for repeated <i>fixed-rate execution</i>,
292 * beginning after the specified delay. Subsequent executions take place
293 * at approximately regular intervals, separated by the specified period.
294 *
295 * <p>In fixed-rate execution, each execution is scheduled relative to the
296 * scheduled execution time of the initial execution. If an execution is
297 * delayed for any reason (such as garbage collection or other background
298 * activity), two or more executions will occur in rapid succession to
299 * "catch up." In the long run, the frequency of execution will be
300 * exactly the reciprocal of the specified period (assuming the system
301 * clock underlying {@code Object.wait(long)} is accurate).
302 *
303 * <p>Fixed-rate execution is appropriate for recurring activities that
304 * are sensitive to <i>absolute</i> time, such as ringing a chime every
305 * hour on the hour, or running scheduled maintenance every day at a
306 * particular time. It is also appropriate for recurring activities
307 * where the total time to perform a fixed number of executions is
308 * important, such as a countdown timer that ticks once every second for
309 * ten seconds. Finally, fixed-rate execution is appropriate for
310 * scheduling multiple repeating timer tasks that must remain synchronized
311 * with respect to one another.
312 *
313 * @param task task to be scheduled.
314 * @param delay delay in milliseconds before task is to be executed.
315 * @param period time in milliseconds between successive task executions.
316 * @throws IllegalArgumentException if {@code delay < 0}, or
317 * {@code delay + System.currentTimeMillis() < 0}, or
318 * {@code period <= 0}
319 * @throws IllegalStateException if task was already scheduled or
320 * cancelled, timer was cancelled, or timer thread terminated.
321 * @throws NullPointerException if {@code task} is null
322 */
323 public void scheduleAtFixedRate(TimerTask task, long delay, long period) {
324 if (delay < 0)
325 throw new IllegalArgumentException("Negative delay.");
326 if (period <= 0)
327 throw new IllegalArgumentException("Non-positive period.");
328 sched(task, System.currentTimeMillis()+delay, period);
329 }
330
331 /**
332 * Schedules the specified task for repeated <i>fixed-rate execution</i>,
333 * beginning at the specified time. Subsequent executions take place at
334 * approximately regular intervals, separated by the specified period.
335 *
336 * <p>In fixed-rate execution, each execution is scheduled relative to the
337 * scheduled execution time of the initial execution. If an execution is
338 * delayed for any reason (such as garbage collection or other background
339 * activity), two or more executions will occur in rapid succession to
340 * "catch up." In the long run, the frequency of execution will be
341 * exactly the reciprocal of the specified period (assuming the system
342 * clock underlying {@code Object.wait(long)} is accurate). As a
343 * consequence of the above, if the scheduled first time is in the past,
344 * then any "missed" executions will be scheduled for immediate "catch up"
345 * execution.
346 *
347 * <p>Fixed-rate execution is appropriate for recurring activities that
348 * are sensitive to <i>absolute</i> time, such as ringing a chime every
349 * hour on the hour, or running scheduled maintenance every day at a
350 * particular time. It is also appropriate for recurring activities
351 * where the total time to perform a fixed number of executions is
352 * important, such as a countdown timer that ticks once every second for
353 * ten seconds. Finally, fixed-rate execution is appropriate for
354 * scheduling multiple repeating timer tasks that must remain synchronized
355 * with respect to one another.
356 *
357 * @param task task to be scheduled.
358 * @param firstTime First time at which task is to be executed.
359 * @param period time in milliseconds between successive task executions.
360 * @throws IllegalArgumentException if {@code firstTime.getTime() < 0} or
361 * {@code period <= 0}
362 * @throws IllegalStateException if task was already scheduled or
363 * cancelled, timer was cancelled, or timer thread terminated.
364 * @throws NullPointerException if {@code task} or {@code firstTime} is null
365 */
366 public void scheduleAtFixedRate(TimerTask task, Date firstTime,
367 long period) {
368 if (period <= 0)
369 throw new IllegalArgumentException("Non-positive period.");
370 sched(task, firstTime.getTime(), period);
371 }
372
373 /**
374 * Schedule the specified timer task for execution at the specified
375 * time with the specified period, in milliseconds. If period is
376 * positive, the task is scheduled for repeated execution; if period is
377 * zero, the task is scheduled for one-time execution. Time is specified
378 * in Date.getTime() format. This method checks timer state, task state,
379 * and initial execution time, but not period.
380 *
381 * @throws IllegalArgumentException if {@code time} is negative.
382 * @throws IllegalStateException if task was already scheduled or
383 * cancelled, timer was cancelled, or timer thread terminated.
384 * @throws NullPointerException if {@code task} is null
385 */
386 private void sched(TimerTask task, long time, long period) {
387 if (time < 0)
388 throw new IllegalArgumentException("Illegal execution time.");
389
390 // Constrain value of period sufficiently to prevent numeric
391 // overflow while still being effectively infinitely large.
392 if (Math.abs(period) > (Long.MAX_VALUE >> 1))
393 period >>= 1;
394
395 synchronized(queue) {
396 if (!thread.newTasksMayBeScheduled)
397 throw new IllegalStateException("Timer already cancelled.");
398
399 synchronized(task.lock) {
400 if (task.state != TimerTask.VIRGIN)
401 throw new IllegalStateException(
|