001 /*
002 * Copyright (C) 2011 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License");
005 * you may not use this file except in compliance with the License.
006 * You may obtain a copy of the License at
007 *
008 * http://www.apache.org/licenses/LICENSE-2.0
009 *
010 * Unless required by applicable law or agreed to in writing, software
011 * distributed under the License is distributed on an "AS IS" BASIS,
012 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013 * See the License for the specific language governing permissions and
014 * limitations under the License.
015 */
016
017 package com.google.common.util.concurrent;
018
019 import com.google.common.annotations.Beta;
020 import com.google.common.base.Preconditions;
021 import com.google.common.base.Throwables;
022
023 import java.util.concurrent.Callable;
024 import java.util.concurrent.Executor;
025 import java.util.concurrent.Executors;
026 import java.util.concurrent.Future;
027 import java.util.concurrent.ScheduledExecutorService;
028 import java.util.concurrent.TimeUnit;
029 import java.util.concurrent.locks.ReentrantLock;
030 import java.util.logging.Level;
031 import java.util.logging.Logger;
032
033 import javax.annotation.concurrent.GuardedBy;
034
035 /**
036 * Base class for services that can implement {@link #startUp} and {@link #shutDown} but while in
037 * the "running" state need to perform a periodic task. Subclasses can implement {@link #startUp},
038 * {@link #shutDown} and also a {@link #runOneIteration} method that will be executed periodically.
039 *
040 * <p>This class uses the {@link ScheduledExecutorService} returned from {@link #executor} to run
041 * the {@link #startUp} and {@link #shutDown} methods and also uses that service to schedule the
042 * {@link #runOneIteration} that will be executed periodically as specified by its
043 * {@link Scheduler}. When this service is asked to stop via {@link #stop} or {@link #stopAndWait},
044 * it will cancel the periodic task (but not interrupt it) and wait for it to stop before running
045 * the {@link #shutDown} method.
046 *
047 * <p>Subclasses are guaranteed that the life cycle methods ({@link #runOneIteration}, {@link
048 * #startUp} and {@link #shutDown}) will never run concurrently. Notably, if any execution of {@link
049 * #runOneIteration} takes longer than its schedule defines, then subsequent executions may start
050 * late. Also, all life cycle methods are executed with a lock held, so subclasses can safely
051 * modify shared state without additional synchronization necessary for visibility to later
052 * executions of the life cycle methods.
053 *
054 * <h3>Usage Example</h3>
055 *
056 * Here is a sketch of a service which crawls a website and uses the scheduling capabilities to
057 * rate limit itself. <pre> {@code
058 * class CrawlingService extends AbstractScheduledService {
059 * private Set<Uri> visited;
060 * private Queue<Uri> toCrawl;
061 * protected void startUp() throws Exception {
062 * toCrawl = readStartingUris();
063 * }
064 *
065 * protected void runOneIteration() throws Exception {
066 * Uri uri = toCrawl.remove();
067 * Collection<Uri> newUris = crawl(uri);
068 * visited.add(uri);
069 * for (Uri newUri : newUris) {
070 * if (!visited.contains(newUri)) { toCrawl.add(newUri); }
071 * }
072 * }
073 *
074 * protected void shutDown() throws Exception {
075 * saveUris(toCrawl);
076 * }
077 *
078 * protected Scheduler scheduler() {
079 * return Scheduler.newFixedRateSchedule(0, 1, TimeUnit.SECONDS);
080 * }
081 * }}</pre>
082 *
083 * This class uses the life cycle methods to read in a list of starting URIs and save the set of
084 * outstanding URIs when shutting down. Also, it takes advantage of the scheduling functionality to
085 * rate limit the number of queries we perform.
086 *
087 * @author Luke Sandberg
088 * @since 11.0
089 */
090 @Beta
091 public abstract class AbstractScheduledService implements Service {
092 private static final Logger logger = Logger.getLogger(AbstractScheduledService.class.getName());
093
094 /**
095 * A scheduler defines the policy for how the {@link AbstractScheduledService} should run its
096 * task.
097 *
098 * <p>Consider using the {@link #newFixedDelaySchedule} and {@link #newFixedRateSchedule} factory
099 * methods, these provide {@link Scheduler} instances for the common use case of running the
100 * service with a fixed schedule. If more flexibility is needed then consider subclassing the
101 * {@link CustomScheduler} abstract class in preference to creating your own {@link Scheduler}
102 * implementation.
103 *
104 * @author Luke Sandberg
105 * @since 11.0
106 */
107 public abstract static class Scheduler {
108 /**
109 * Returns a {@link Scheduler} that schedules the task using the
110 * {@link ScheduledExecutorService#scheduleWithFixedDelay} method.
111 *
112 * @param initialDelay the time to delay first execution
113 * @param delay the delay between the termination of one execution and the commencement of the
114 * next
115 * @param unit the time unit of the initialDelay and delay parameters
116 */
117 public static Scheduler newFixedDelaySchedule(final long initialDelay, final long delay,
118 final TimeUnit unit) {
119 return new Scheduler() {
120
121 @Override
122 public Future<?> schedule(AbstractService service, ScheduledExecutorService executor,
123 Runnable task) {
124 return executor.scheduleWithFixedDelay(task, initialDelay, delay, unit);
125 }
126 };
127 }
128
129 /**
130 * Returns a {@link Scheduler} that schedules the task using the
131 * {@link ScheduledExecutorService#scheduleAtFixedRate} method.
132 *
133 * @param initialDelay the time to delay first execution
134 * @param period the period between successive executions of the task
135 * @param unit the time unit of the initialDelay and period parameters
136 */
137 public static Scheduler newFixedRateSchedule(final long initialDelay, final long period,
138 final TimeUnit unit) {
139 return new Scheduler() {
140
141 @Override
142 public Future<?> schedule(AbstractService service, ScheduledExecutorService executor,
143 Runnable task) {
144 return executor.scheduleAtFixedRate(task, initialDelay, period, unit);
145 }
146 };
147 }
148
149 /** Schedules the task to run on the provided executor on behalf of the service. */
150 abstract Future<?> schedule(AbstractService service, ScheduledExecutorService executor,
151 Runnable runnable);
152
153 private Scheduler() {}
154 }
155
156 /* use AbstractService for state management */
157 private final AbstractService delegate = new AbstractService() {
158
159 // A handle to the running task so that we can stop it when a shutdown has been requested.
160 // These two fields are volatile because their values will be accessed from multiple threads.
161 private volatile Future<?> runningTask;
162 private volatile ScheduledExecutorService executorService;
163
164 // This lock protects the task so we can ensure that none of the template methods (startUp,
165 // shutDown or runOneIteration) run concurrently with one another.
166 private final ReentrantLock lock = new ReentrantLock();
167
168 private final Runnable task = new Runnable() {
169 public void run() {
170 lock.lock();
171 try {
172 AbstractScheduledService.this.runOneIteration();
173 } catch (Throwable t) {
174 try {
175 shutDown();
176 } catch (Exception ignored) {
177 logger.log(Level.WARNING,
178 "Error while attempting to shut down the service after failure.", ignored);
179 }
180 notifyFailed(t);
181 throw Throwables.propagate(t);
182 } finally {
183 lock.unlock();
184 }
185 }
186 };
187
188 @Override
189 protected final void doStart() {
190 executorService = executor();
191 executorService.execute(new Runnable() {
192 public void run() {
193 lock.lock();
194 try {
195 startUp();
196 runningTask = scheduler().schedule(delegate, executorService, task);
197 notifyStarted();
198 } catch (Throwable t) {
199 notifyFailed(t);
200 throw Throwables.propagate(t);
201 } finally {
202 lock.unlock();
203 }
204 }
205 });
206 }
207
208 @Override
209 protected final void doStop() {
210 runningTask.cancel(false);
211 executorService.execute(new Runnable() {
212 public void run() {
213 try {
214 lock.lock();
215 try {
216 if (state() != State.STOPPING) {
217 // This means that the state has changed since we were scheduled. This implies that
218 // an execution of runOneIteration has thrown an exception and we have transitioned
219 // to a failed state, also this means that shutDown has already been called, so we
220 // do not want to call it again.
221 return;
222 }
223 shutDown();
224 } finally {
225 lock.unlock();
226 }
227 notifyStopped();
228 } catch (Throwable t) {
229 notifyFailed(t);
230 throw Throwables.propagate(t);
231 }
232 }
233 });
234 }
235 };
236
237 /**
238 * Run one iteration of the scheduled task. If any invocation of this method throws an exception,
239 * the service will transition to the {@link Service.State#FAILED} state and this method will no
240 * longer be called.
241 */
242 protected abstract void runOneIteration() throws Exception;
243
244 /**
245 * Start the service.
246 *
247 * <p>By default this method does nothing.
248 */
249 protected void startUp() throws Exception {}
250
251 /**
252 * Stop the service. This is guaranteed not to run concurrently with {@link #runOneIteration}.
253 *
254 * <p>By default this method does nothing.
255 */
256 protected void shutDown() throws Exception {}
257
258 /**
259 * Returns the {@link Scheduler} object used to configure this service. This method will only be
260 * called once.
261 */
262 protected abstract Scheduler scheduler();
263
264 /**
265 * Returns the {@link ScheduledExecutorService} that will be used to execute the {@link #startUp},
266 * {@link #runOneIteration} and {@link #shutDown} methods. The executor will not be
267 * {@link ScheduledExecutorService#shutdown} when this service stops. Subclasses may override this
268 * method to use a custom {@link ScheduledExecutorService} instance.
269 *
270 * <p>By default this returns a new {@link ScheduledExecutorService} with a single thread thread
271 * pool. This method will only be called once.
272 */
273 protected ScheduledExecutorService executor() {
274 return Executors.newSingleThreadScheduledExecutor();
275 }
276
277
278 @Override
279 public String toString() {
280 return getClass().getSimpleName() + " [" + state() + "]";
281 }
282
283 // We override instead of using ForwardingService so that these can be final.
284
285 public final ListenableFuture<State> start() {
286 return delegate.start();
287 }
288
289 public final State startAndWait() {
290 return delegate.startAndWait();
291 }
292
293 public final boolean isRunning() {
294 return delegate.isRunning();
295 }
296
297 public final State state() {
298 return delegate.state();
299 }
300
301 public final ListenableFuture<State> stop() {
302 return delegate.stop();
303 }
304
305 public final State stopAndWait() {
306 return delegate.stopAndWait();
307 }
308
309 public final void addListener(Listener listener, Executor executor) {
310 delegate.addListener(listener, executor);
311 }
312
313 /**
314 * A {@link Scheduler} that provides a convenient way for the {@link AbstractScheduledService} to
315 * use a dynamically changing schedule. After every execution of the task, assuming it hasn't
316 * been cancelled, the {@link #getNextSchedule} method will be called.
317 *
318 * @author Luke Sandberg
319 * @since 11.0
320 */
321 @Beta
322 public abstract static class CustomScheduler extends Scheduler {
323
324 /**
325 * A callable class that can reschedule itself using a {@link CustomScheduler}.
326 */
327 private class ReschedulableCallable extends ForwardingFuture<Void> implements Callable<Void> {
328
329 /** The underlying task. */
330 private final Runnable wrappedRunnable;
331
332 /** The executor on which this Callable will be scheduled. */
333 private final ScheduledExecutorService executor;
334
335 /**
336 * The service that is managing this callable. This is used so that failure can be
337 * reported properly.
338 */
339 private final AbstractService service;
340
341 /**
342 * This lock is used to ensure safe and correct cancellation, it ensures that a new task is
343 * not scheduled while a cancel is ongoing. Also it protects the currentFuture variable to
344 * ensure that it is assigned atomically with being scheduled.
345 */
346 private final ReentrantLock lock = new ReentrantLock();
347
348 /** The future that represents the next execution of this task.*/
349 @GuardedBy("lock")
350 private Future<Void> currentFuture;
351
352 ReschedulableCallable(AbstractService service, ScheduledExecutorService executor,
353 Runnable runnable) {
354 this.wrappedRunnable = runnable;
355 this.executor = executor;
356 this.service = service;
357 }
358
359
360 public Void call() throws Exception {
361 wrappedRunnable.run();
362 reschedule();
363 return null;
364 }
365
366 /**
367 * Atomically reschedules this task and assigns the new future to {@link #currentFuture}.
368 */
369 public void reschedule() {
370 // We reschedule ourselves with a lock held for two reasons. 1. we want to make sure that
371 // cancel calls cancel on the correct future. 2. we want to make sure that the assignment
372 // to currentFuture doesn't race with itself so that currentFuture is assigned in the
373 // correct order.
374 lock.lock();
375 try {
376 if (currentFuture == null || !currentFuture.isCancelled()) {
377 final Schedule schedule = CustomScheduler.this.getNextSchedule();
378 currentFuture = executor.schedule(this, schedule.delay, schedule.unit);
379 }
380 } catch (Throwable e) {
381 // If an exception is thrown by the subclass then we need to make sure that the service
382 // notices and transitions to the FAILED state. We do it by calling notifyFailed directly
383 // because the service does not monitor the state of the future so if the exception is not
384 // caught and forwarded to the service the task would stop executing but the service would
385 // have no idea.
386 service.notifyFailed(e);
387 } finally {
388 lock.unlock();
389 }
390 }
391
392 // N.B. Only protect cancel and isCancelled because those are the only methods that are
393 // invoked by the AbstractScheduledService.
394
395 @Override
396 public boolean cancel(boolean mayInterruptIfRunning) {
397 // Ensure that a task cannot be rescheduled while a cancel is ongoing.
398 lock.lock();
399 try {
400 return currentFuture.cancel(mayInterruptIfRunning);
401 } finally {
402 lock.unlock();
403 }
404 }
405
406
407 @Override
408 protected Future<Void> delegate() {
409 throw new UnsupportedOperationException("Only cancel is supported by this future");
410 }
411 }
412
413
414 @Override
415 final Future<?> schedule(AbstractService service, ScheduledExecutorService executor,
416 Runnable runnable) {
417 ReschedulableCallable task = new ReschedulableCallable(service, executor, runnable);
418 task.reschedule();
419 return task;
420 }
421
422 /**
423 * A value object that represents an absolute delay until a task should be invoked.
424 *
425 * @author Luke Sandberg
426 * @since 11.0
427 */
428 @Beta
429 protected static final class Schedule {
430
431 private final long delay;
432 private final TimeUnit unit;
433
434 /**
435 * @param delay the time from now to delay execution
436 * @param unit the time unit of the delay parameter
437 */
438 public Schedule(long delay, TimeUnit unit) {
439 this.delay = delay;
440 this.unit = Preconditions.checkNotNull(unit);
441 }
442 }
443
444 /**
445 * Calculates the time at which to next invoke the task.
446 *
447 * <p>This is guaranteed to be called immediately after the task has completed an iteration and
448 * on the same thread as the previous execution of {@link
449 * AbstractScheduledService#runOneIteration}.
450 *
451 * @return a schedule that defines the delay before the next execution.
452 */
453 protected abstract Schedule getNextSchedule() throws Exception;
454 }
455 }