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/*
* Copyright (C) 2011 The Guava Authors
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.google.common.util.concurrent;
import static com.google.common.base.Preconditions.checkArgument;
import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.util.concurrent.MoreExecutors.directExecutor;
import com.google.common.annotations.Beta;
import com.google.common.base.Supplier;
import com.google.j2objc.annotations.WeakOuter;
import java.util.concurrent.Callable;
import java.util.concurrent.Executor;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.ThreadFactory;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
import java.util.concurrent.locks.ReentrantLock;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.annotation.concurrent.GuardedBy;
/**
* Base class for services that can implement {@link #startUp} and {@link #shutDown} but while in
* the "running" state need to perform a periodic task. Subclasses can implement {@link #startUp},
* {@link #shutDown} and also a {@link #runOneIteration} method that will be executed periodically.
*
* This class uses the {@link ScheduledExecutorService} returned from {@link #executor} to run
* the {@link #startUp} and {@link #shutDown} methods and also uses that service to schedule the
* {@link #runOneIteration} that will be executed periodically as specified by its
* {@link Scheduler}. When this service is asked to stop via {@link #stopAsync} it will cancel the
* periodic task (but not interrupt it) and wait for it to stop before running the
* {@link #shutDown} method.
*
*
Subclasses are guaranteed that the life cycle methods ({@link #runOneIteration}, {@link
* #startUp} and {@link #shutDown}) will never run concurrently. Notably, if any execution of {@link
* #runOneIteration} takes longer than its schedule defines, then subsequent executions may start
* late. Also, all life cycle methods are executed with a lock held, so subclasses can safely
* modify shared state without additional synchronization necessary for visibility to later
* executions of the life cycle methods.
*
*
Usage Example
*
* Here is a sketch of a service which crawls a website and uses the scheduling capabilities to
* rate limit itself.
{@code
* class CrawlingService extends AbstractScheduledService {
* private Set visited;
* private Queue toCrawl;
* protected void startUp() throws Exception {
* toCrawl = readStartingUris();
* }
*
* protected void runOneIteration() throws Exception {
* Uri uri = toCrawl.remove();
* Collection newUris = crawl(uri);
* visited.add(uri);
* for (Uri newUri : newUris) {
* if (!visited.contains(newUri)) { toCrawl.add(newUri); }
* }
* }
*
* protected void shutDown() throws Exception {
* saveUris(toCrawl);
* }
*
* protected Scheduler scheduler() {
* return Scheduler.newFixedRateSchedule(0, 1, TimeUnit.SECONDS);
* }
* }}
*
* This class uses the life cycle methods to read in a list of starting URIs and save the set of
* outstanding URIs when shutting down. Also, it takes advantage of the scheduling functionality to
* rate limit the number of queries we perform.
*
* @author Luke Sandberg
* @since 11.0
*/
@Beta
public abstract class AbstractScheduledService implements Service {
private static final Logger logger = Logger.getLogger(AbstractScheduledService.class.getName());
/**
* A scheduler defines the policy for how the {@link AbstractScheduledService} should run its
* task.
*
*
Consider using the {@link #newFixedDelaySchedule} and {@link #newFixedRateSchedule} factory
* methods, these provide {@link Scheduler} instances for the common use case of running the
* service with a fixed schedule. If more flexibility is needed then consider subclassing
* {@link CustomScheduler}.
*
* @author Luke Sandberg
* @since 11.0
*/
public abstract static class Scheduler {
/**
* Returns a {@link Scheduler} that schedules the task using the
* {@link ScheduledExecutorService#scheduleWithFixedDelay} method.
*
* @param initialDelay the time to delay first execution
* @param delay the delay between the termination of one execution and the commencement of the
* next
* @param unit the time unit of the initialDelay and delay parameters
*/
public static Scheduler newFixedDelaySchedule(final long initialDelay, final long delay,
final TimeUnit unit) {
checkNotNull(unit);
checkArgument(delay > 0, "delay must be > 0, found %s", delay);
return new Scheduler() {
@Override
public Future schedule(AbstractService service, ScheduledExecutorService executor,
Runnable task) {
return executor.scheduleWithFixedDelay(task, initialDelay, delay, unit);
}
};
}
/**
* Returns a {@link Scheduler} that schedules the task using the
* {@link ScheduledExecutorService#scheduleAtFixedRate} method.
*
* @param initialDelay the time to delay first execution
* @param period the period between successive executions of the task
* @param unit the time unit of the initialDelay and period parameters
*/
public static Scheduler newFixedRateSchedule(final long initialDelay, final long period,
final TimeUnit unit) {
checkNotNull(unit);
checkArgument(period > 0, "period must be > 0, found %s", period);
return new Scheduler() {
@Override
public Future schedule(AbstractService service, ScheduledExecutorService executor,
Runnable task) {
return executor.scheduleAtFixedRate(task, initialDelay, period, unit);
}
};
}
/** Schedules the task to run on the provided executor on behalf of the service. */
abstract Future schedule(AbstractService service, ScheduledExecutorService executor,
Runnable runnable);
private Scheduler() {}
}
/* use AbstractService for state management */
private final AbstractService delegate = new ServiceDelegate();
@WeakOuter
private final class ServiceDelegate extends AbstractService {
// A handle to the running task so that we can stop it when a shutdown has been requested.
// These two fields are volatile because their values will be accessed from multiple threads.
private volatile Future runningTask;
private volatile ScheduledExecutorService executorService;
// This lock protects the task so we can ensure that none of the template methods (startUp,
// shutDown or runOneIteration) run concurrently with one another.
// TODO(lukes): why don't we use ListenableFuture to sequence things? Then we could drop the
// lock.
private final ReentrantLock lock = new ReentrantLock();
@WeakOuter
class Task implements Runnable {
@Override public void run() {
lock.lock();
try {
if (runningTask.isCancelled()) {
// task may have been cancelled while blocked on the lock.
return;
}
AbstractScheduledService.this.runOneIteration();
} catch (Throwable t) {
try {
shutDown();
} catch (Exception ignored) {
logger.log(Level.WARNING,
"Error while attempting to shut down the service after failure.", ignored);
}
notifyFailed(t);
runningTask.cancel(false); // prevent future invocations.
} finally {
lock.unlock();
}
}
}
private final Runnable task = new Task();
@Override protected final void doStart() {
executorService = MoreExecutors.renamingDecorator(executor(), new Supplier() {
@Override public String get() {
return serviceName() + " " + state();
}
});
executorService.execute(new Runnable() {
@Override public void run() {
lock.lock();
try {
startUp();
runningTask = scheduler().schedule(delegate, executorService, task);
notifyStarted();
} catch (Throwable t) {
notifyFailed(t);
if (runningTask != null) {
// prevent the task from running if possible
runningTask.cancel(false);
}
} finally {
lock.unlock();
}
}
});
}
@Override protected final void doStop() {
runningTask.cancel(false);
executorService.execute(new Runnable() {
@Override public void run() {
try {
lock.lock();
try {
if (state() != State.STOPPING) {
// This means that the state has changed since we were scheduled. This implies that
// an execution of runOneIteration has thrown an exception and we have transitioned
// to a failed state, also this means that shutDown has already been called, so we
// do not want to call it again.
return;
}
shutDown();
} finally {
lock.unlock();
}
notifyStopped();
} catch (Throwable t) {
notifyFailed(t);
}
}
});
}
@Override public String toString() {
return AbstractScheduledService.this.toString();
}
}
/** Constructor for use by subclasses. */
protected AbstractScheduledService() {}
/**
* Run one iteration of the scheduled task. If any invocation of this method throws an exception,
* the service will transition to the {@link Service.State#FAILED} state and this method will no
* longer be called.
*/
protected abstract void runOneIteration() throws Exception;
/**
* Start the service.
*
* By default this method does nothing.
*/
protected void startUp() throws Exception {}
/**
* Stop the service. This is guaranteed not to run concurrently with {@link #runOneIteration}.
*
*
By default this method does nothing.
*/
protected void shutDown() throws Exception {}
/**
* Returns the {@link Scheduler} object used to configure this service. This method will only be
* called once.
*/
protected abstract Scheduler scheduler();
/**
* Returns the {@link ScheduledExecutorService} that will be used to execute the {@link #startUp},
* {@link #runOneIteration} and {@link #shutDown} methods. If this method is overridden the
* executor will not be {@linkplain ScheduledExecutorService#shutdown shutdown} when this
* service {@linkplain Service.State#TERMINATED terminates} or
* {@linkplain Service.State#TERMINATED fails}. Subclasses may override this method to supply a
* custom {@link ScheduledExecutorService} instance. This method is guaranteed to only be called
* once.
*
*
By default this returns a new {@link ScheduledExecutorService} with a single thread thread
* pool that sets the name of the thread to the {@linkplain #serviceName() service name}.
* Also, the pool will be {@linkplain ScheduledExecutorService#shutdown() shut down} when the
* service {@linkplain Service.State#TERMINATED terminates} or
* {@linkplain Service.State#TERMINATED fails}.
*/
protected ScheduledExecutorService executor() {
@WeakOuter class ThreadFactoryImpl implements ThreadFactory {
@Override public Thread newThread(Runnable runnable) {
return MoreExecutors.newThread(serviceName(), runnable);
}
}
final ScheduledExecutorService executor =
Executors.newSingleThreadScheduledExecutor(new ThreadFactoryImpl());
// Add a listener to shutdown the executor after the service is stopped. This ensures that the
// JVM shutdown will not be prevented from exiting after this service has stopped or failed.
// Technically this listener is added after start() was called so it is a little gross, but it
// is called within doStart() so we know that the service cannot terminate or fail concurrently
// with adding this listener so it is impossible to miss an event that we are interested in.
addListener(new Listener() {
@Override public void terminated(State from) {
executor.shutdown();
}
@Override public void failed(State from, Throwable failure) {
executor.shutdown();
}
}, directExecutor());
return executor;
}
/**
* Returns the name of this service. {@link AbstractScheduledService} may include the name in
* debugging output.
*
* @since 14.0
*/
protected String serviceName() {
return getClass().getSimpleName();
}
@Override public String toString() {
return serviceName() + " [" + state() + "]";
}
@Override public final boolean isRunning() {
return delegate.isRunning();
}
@Override public final State state() {
return delegate.state();
}
/**
* @since 13.0
*/
@Override public final void addListener(Listener listener, Executor executor) {
delegate.addListener(listener, executor);
}
/**
* @since 14.0
*/
@Override public final Throwable failureCause() {
return delegate.failureCause();
}
/**
* @since 15.0
*/
@Override public final Service startAsync() {
delegate.startAsync();
return this;
}
/**
* @since 15.0
*/
@Override public final Service stopAsync() {
delegate.stopAsync();
return this;
}
/**
* @since 15.0
*/
@Override public final void awaitRunning() {
delegate.awaitRunning();
}
/**
* @since 15.0
*/
@Override public final void awaitRunning(long timeout, TimeUnit unit) throws TimeoutException {
delegate.awaitRunning(timeout, unit);
}
/**
* @since 15.0
*/
@Override public final void awaitTerminated() {
delegate.awaitTerminated();
}
/**
* @since 15.0
*/
@Override public final void awaitTerminated(long timeout, TimeUnit unit) throws TimeoutException {
delegate.awaitTerminated(timeout, unit);
}
/**
* A {@link Scheduler} that provides a convenient way for the {@link AbstractScheduledService} to
* use a dynamically changing schedule. After every execution of the task, assuming it hasn't
* been cancelled, the {@link #getNextSchedule} method will be called.
*
* @author Luke Sandberg
* @since 11.0
*/
@Beta
public abstract static class CustomScheduler extends Scheduler {
/**
* A callable class that can reschedule itself using a {@link CustomScheduler}.
*/
private class ReschedulableCallable extends ForwardingFuture implements Callable {
/** The underlying task. */
private final Runnable wrappedRunnable;
/** The executor on which this Callable will be scheduled. */
private final ScheduledExecutorService executor;
/**
* The service that is managing this callable. This is used so that failure can be
* reported properly.
*/
private final AbstractService service;
/**
* This lock is used to ensure safe and correct cancellation, it ensures that a new task is
* not scheduled while a cancel is ongoing. Also it protects the currentFuture variable to
* ensure that it is assigned atomically with being scheduled.
*/
private final ReentrantLock lock = new ReentrantLock();
/** The future that represents the next execution of this task.*/
@GuardedBy("lock")
private Future currentFuture;
ReschedulableCallable(AbstractService service, ScheduledExecutorService executor,
Runnable runnable) {
this.wrappedRunnable = runnable;
this.executor = executor;
this.service = service;
}
@Override
public Void call() throws Exception {
wrappedRunnable.run();
reschedule();
return null;
}
/**
* Atomically reschedules this task and assigns the new future to {@link #currentFuture}.
*/
public void reschedule() {
// invoke the callback outside the lock, prevents some shenanigans.
Schedule schedule;
try {
schedule = CustomScheduler.this.getNextSchedule();
} catch (Throwable t) {
service.notifyFailed(t);
return;
}
// We reschedule ourselves with a lock held for two reasons. 1. we want to make sure that
// cancel calls cancel on the correct future. 2. we want to make sure that the assignment
// to currentFuture doesn't race with itself so that currentFuture is assigned in the
// correct order.
Throwable scheduleFailure = null;
lock.lock();
try {
if (currentFuture == null || !currentFuture.isCancelled()) {
currentFuture = executor.schedule(this, schedule.delay, schedule.unit);
}
} catch (Throwable e) {
// If an exception is thrown by the subclass then we need to make sure that the service
// notices and transitions to the FAILED state. We do it by calling notifyFailed directly
// because the service does not monitor the state of the future so if the exception is not
// caught and forwarded to the service the task would stop executing but the service would
// have no idea.
// TODO(lukes): consider building everything in terms of ListenableScheduledFuture then
// the AbstractService could monitor the future directly. Rescheduling is still hard...
// but it would help with some of these lock ordering issues.
scheduleFailure = e;
} finally {
lock.unlock();
}
// Call notifyFailed outside the lock to avoid lock ordering issues.
if (scheduleFailure != null) {
service.notifyFailed(scheduleFailure);
}
}
// N.B. Only protect cancel and isCancelled because those are the only methods that are
// invoked by the AbstractScheduledService.
@Override
public boolean cancel(boolean mayInterruptIfRunning) {
// Ensure that a task cannot be rescheduled while a cancel is ongoing.
lock.lock();
try {
return currentFuture.cancel(mayInterruptIfRunning);
} finally {
lock.unlock();
}
}
@Override
public boolean isCancelled() {
lock.lock();
try {
return currentFuture.isCancelled();
} finally {
lock.unlock();
}
}
@Override
protected Future delegate() {
throw new UnsupportedOperationException(
"Only cancel and isCancelled is supported by this future");
}
}
@Override
final Future schedule(AbstractService service, ScheduledExecutorService executor,
Runnable runnable) {
ReschedulableCallable task = new ReschedulableCallable(service, executor, runnable);
task.reschedule();
return task;
}
/**
* A value object that represents an absolute delay until a task should be invoked.
*
* @author Luke Sandberg
* @since 11.0
*/
@Beta
protected static final class Schedule {
private final long delay;
private final TimeUnit unit;
/**
* @param delay the time from now to delay execution
* @param unit the time unit of the delay parameter
*/
public Schedule(long delay, TimeUnit unit) {
this.delay = delay;
this.unit = checkNotNull(unit);
}
}
/**
* Calculates the time at which to next invoke the task.
*
* This is guaranteed to be called immediately after the task has completed an iteration and
* on the same thread as the previous execution of {@link
* AbstractScheduledService#runOneIteration}.
*
* @return a schedule that defines the delay before the next execution.
*/
protected abstract Schedule getNextSchedule() throws Exception;
}
}