https://github.com/google/guava/guava: Guava is a suite of core and expanded libraries that include utility classes, google's collections, io classes, and much much more. 
/*
 * 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.Internal.toNanosSaturated;
import static com.google.common.util.concurrent.MoreExecutors.directExecutor;

import com.google.common.annotations.GwtIncompatible;
import com.google.common.base.Supplier;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import com.google.errorprone.annotations.concurrent.GuardedBy;
import com.google.j2objc.annotations.WeakOuter;
import java.time.Duration;
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 org.checkerframework.checker.nullness.qual.Nullable;


Base class for services that can implement startUp and shutDown but while in the "running" state need to perform a periodic task. Subclasses can implement startUp, shutDown and also a runOneIteration method that will be executed periodically. 
This class uses the ScheduledExecutorService returned from executor to run the startUp and shutDown methods and also uses that service to schedule the runOneIteration that will be executed periodically as specified by its Scheduler. When this service is asked to stop via stopAsync it will cancel the periodic task (but not interrupt it) and wait for it to stop before running the shutDown method. 
Subclasses are guaranteed that the life cycle methods (runOneIteration, startUp and shutDown) will never run concurrently. Notably, if any execution of 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.


class CrawlingService extends AbstractScheduledService {
  private Set<Uri> visited;
  private Queue<Uri> toCrawl;
  protected void startUp() throws Exception {
    toCrawl = readStartingUris();
  }
  protected void runOneIteration() throws Exception {
    Uri uri = toCrawl.remove();
    Collection<Uri> 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
/**
 * 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.
 *
 * <p>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.
 *
 * <p>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.
 *
 * <h3>Usage Example</h3>
 *
 * <p>Here is a sketch of a service which crawls a website and uses the scheduling capabilities to
 * rate limit itself.
 *
 * <pre>{@code
 * class CrawlingService extends AbstractScheduledService {
 *   private Set<Uri> visited;
 *   private Queue<Uri> toCrawl;
 *   protected void startUp() throws Exception {
 *     toCrawl = readStartingUris();
 *   }
 *
 *   protected void runOneIteration() throws Exception {
 *     Uri uri = toCrawl.remove();
 *     Collection<Uri> 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);
 *   }
 * }
 * }</pre>
 *
 * <p>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
 */
@GwtIncompatible
public abstract class AbstractScheduledService implements Service {
  private static final Logger logger = Logger.getLogger(AbstractScheduledService.class.getName());

  
A scheduler defines the policy for how the AbstractScheduledService should run its task. 
Consider using the newFixedDelaySchedule and newFixedRateSchedule factory methods, these provide Scheduler instances for the common use case of running the service with a fixed schedule. If more flexibility is needed then consider subclassing CustomScheduler. 
Author:Luke Sandberg
Since:11.0
/**
   * A scheduler defines the policy for how the {@link AbstractScheduledService} should run its
   * task.
   *
   * <p>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 Scheduler that schedules the task using the ScheduledExecutorService.scheduleWithFixedDelay method. 
Params:
  • initialDelay – the time to delay first execution
  • delay – the delay between the termination of one execution and the commencement of the
    next
Since:28.0
/**
     * 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
     * @since 28.0
     */
    public static Scheduler newFixedDelaySchedule(Duration initialDelay, Duration delay) {
      return newFixedDelaySchedule(
          toNanosSaturated(initialDelay), toNanosSaturated(delay), TimeUnit.NANOSECONDS);
    }

    
Returns a Scheduler that schedules the task using the ScheduledExecutorService.scheduleWithFixedDelay method. 
Params:
  • initialDelay – the time to delay first execution
  • delay – the delay between the termination of one execution and the commencement of the
    next
  • unit – the time unit of the initialDelay and delay parameters
/**
     * 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
     */
    @SuppressWarnings("GoodTime") // should accept a java.time.Duration
    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 Scheduler that schedules the task using the ScheduledExecutorService.scheduleAtFixedRate method. 
Params:
  • initialDelay – the time to delay first execution
  • period – the period between successive executions of the task
Since:28.0
/**
     * 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
     * @since 28.0
     */
    public static Scheduler newFixedRateSchedule(Duration initialDelay, Duration period) {
      return newFixedRateSchedule(
          toNanosSaturated(initialDelay), toNanosSaturated(period), TimeUnit.NANOSECONDS);
    }

    
Returns a Scheduler that schedules the task using the ScheduledExecutorService.scheduleAtFixedRate method. 
Params:
  • initialDelay – the time to delay first execution
  • period – the period between successive executions of the task
  • unit – the time unit of the initialDelay and period parameters
/**
     * 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
     */
    @SuppressWarnings("GoodTime") // should accept a java.time.Duration
    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. 
/** 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 @Nullable Future<?> runningTask;
    private volatile @Nullable 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<String>() {
                @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. 
/** 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 State.FAILED state and this method will no longer be called. 
/**
   * 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.

/**
   * Start the service.
   *
   * <p>By default this method does nothing.
   */
  protected void startUp() throws Exception {}

  
Stop the service. This is guaranteed not to run concurrently with runOneIteration. 
By default this method does nothing.

/**
   * Stop the service. This is guaranteed not to run concurrently with {@link #runOneIteration}.
   *
   * <p>By default this method does nothing.
   */
  protected void shutDown() throws Exception {}

  
Returns the Scheduler object used to configure this service. This method will only be called once. 
/**
   * Returns the {@link Scheduler} object used to configure this service. This method will only be
   * called once.
   */
  // TODO(cpovirk): @ForOverride
  protected abstract Scheduler scheduler();

  
Returns the ScheduledExecutorService that will be used to execute the startUp, runOneIteration and shutDown methods. If this method is overridden the executor will not be shutdown when this service terminates or 
fails. Subclasses may override this method to supply a custom ScheduledExecutorService instance. This method is guaranteed to only be called once. 
By default this returns a new ScheduledExecutorService with a single thread thread pool that sets the name of the thread to the service name. Also, the pool will be shut down when the service terminates or 
fails. 
/**
   * 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.
   *
   * <p>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. AbstractScheduledService may include the name in debugging output. 
Since:14.0
/**
   * 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
/** @since 13.0 */
  @Override
  public final void addListener(Listener listener, Executor executor) {
    delegate.addListener(listener, executor);
  }

  
Since:14.0
/** @since 14.0 */
  @Override
  public final Throwable failureCause() {
    return delegate.failureCause();
  }

  
Since:15.0
/** @since 15.0 */
  @CanIgnoreReturnValue
  @Override
  public final Service startAsync() {
    delegate.startAsync();
    return this;
  }

  
Since:15.0
/** @since 15.0 */
  @CanIgnoreReturnValue
  @Override
  public final Service stopAsync() {
    delegate.stopAsync();
    return this;
  }

  
Since:15.0
/** @since 15.0 */
  @Override
  public final void awaitRunning() {
    delegate.awaitRunning();
  }

  
Since:28.0
/** @since 28.0 */
  @Override
  public final void awaitRunning(Duration timeout) throws TimeoutException {
    Service.super.awaitRunning(timeout);
  }

  
Since:15.0
/** @since 15.0 */
  @Override
  public final void awaitRunning(long timeout, TimeUnit unit) throws TimeoutException {
    delegate.awaitRunning(timeout, unit);
  }

  
Since:15.0
/** @since 15.0 */
  @Override
  public final void awaitTerminated() {
    delegate.awaitTerminated();
  }

  
Since:28.0
/** @since 28.0 */
  @Override
  public final void awaitTerminated(Duration timeout) throws TimeoutException {
    Service.super.awaitTerminated(timeout);
  }

  
Since:15.0
/** @since 15.0 */
  @Override
  public final void awaitTerminated(long timeout, TimeUnit unit) throws TimeoutException {
    delegate.awaitTerminated(timeout, unit);
  }

  
A Scheduler that provides a convenient way for the AbstractScheduledService to use a dynamically changing schedule. After every execution of the task, assuming it hasn't been cancelled, the getNextSchedule method will be called. 
Author:Luke Sandberg
Since:11.0
/**
   * 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
   */
  public abstract static class CustomScheduler extends Scheduler {

    
A callable class that can reschedule itself using a CustomScheduler. 
/** A callable class that can reschedule itself using a {@link CustomScheduler}. */
    private class ReschedulableCallable extends ForwardingFuture<Void> implements Callable<Void> {

      
The underlying task. 
/** The underlying task. */
      private final Runnable wrappedRunnable;

      
The executor on which this Callable will be scheduled. 
/** 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.

/**
       * 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.

/**
       * 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. 
/** The future that represents the next execution of this task. */
      @GuardedBy("lock")
      private @Nullable Future<Void> 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 currentFuture. 
/** 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<Void> 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
/**
     * A value object that represents an absolute delay until a task should be invoked.
     *
     * @author Luke Sandberg
     * @since 11.0
     */
    protected static final class Schedule {

      private final long delay;
      private final TimeUnit unit;

      
Params:
  • delay – the time from now to delay execution
  • unit – the time unit of the delay parameter
/**
       * @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 AbstractScheduledService.runOneIteration. 
Returns:a schedule that defines the delay before the next execution.
/**
     * Calculates the time at which to next invoke the task.
     *
     * <p>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.
     */
    // TODO(cpovirk): @ForOverride
    protected abstract Schedule getNextSchedule() throws Exception;
  }
}