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