util/concurrent/ServiceManager.java

/*
 * Copyright (C) 2012 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.base.Preconditions.checkState;
import static com.google.common.base.Predicates.equalTo;
import static com.google.common.base.Predicates.in;
import static com.google.common.base.Predicates.instanceOf;
import static com.google.common.base.Predicates.not;
import static com.google.common.util.concurrent.MoreExecutors.directExecutor;
import static com.google.common.util.concurrent.Service.State.FAILED;
import static com.google.common.util.concurrent.Service.State.NEW;
import static com.google.common.util.concurrent.Service.State.RUNNING;
import static com.google.common.util.concurrent.Service.State.STARTING;
import static com.google.common.util.concurrent.Service.State.STOPPING;
import static com.google.common.util.concurrent.Service.State.TERMINATED;
import static java.util.concurrent.TimeUnit.MILLISECONDS;

import com.google.common.annotations.Beta;
import com.google.common.base.Function;
import com.google.common.base.MoreObjects;
import com.google.common.base.Stopwatch;
import com.google.common.base.Supplier;
import com.google.common.collect.Collections2;
import com.google.common.collect.ImmutableCollection;
import com.google.common.collect.ImmutableList;
import com.google.common.collect.ImmutableMap;
import com.google.common.collect.ImmutableMultimap;
import com.google.common.collect.ImmutableSet;
import com.google.common.collect.ImmutableSetMultimap;
import com.google.common.collect.Lists;
import com.google.common.collect.Maps;
import com.google.common.collect.Multimaps;
import com.google.common.collect.Multiset;
import com.google.common.collect.Ordering;
import com.google.common.collect.SetMultimap;
import com.google.common.collect.Sets;
import com.google.common.util.concurrent.ListenerCallQueue.Callback;
import com.google.common.util.concurrent.Service.State;

import java.lang.ref.WeakReference;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.EnumMap;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Set;
import java.util.concurrent.Executor;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
import java.util.logging.Level;
import java.util.logging.Logger;

import javax.annotation.concurrent.GuardedBy;

/**
 * A manager for monitoring and controlling a set of {@linkplain Service services}. This class
 * provides methods for {@linkplain #startAsync() starting}, {@linkplain #stopAsync() stopping} and
 * {@linkplain #servicesByState inspecting} a collection of {@linkplain Service services}.
 * Additionally, users can monitor state transitions with the {@linkplain Listener listener}
 * mechanism.
 *
 * <p>While it is recommended that service lifecycles be managed via this class, state transitions
 * initiated via other mechanisms do not impact the correctness of its methods. For example, if the
 * services are started by some mechanism besides {@link #startAsync}, the listeners will be invoked
 * when appropriate and {@link #awaitHealthy} will still work as expected.
 *
 * <p>Here is a simple example of how to use a {@code ServiceManager} to start a server.
 * <pre>   {@code
 * class Server {
 *   public static void main(String[] args) {
 *     Set<Service> services = ...;
 *     ServiceManager manager = new ServiceManager(services);
 *     manager.addListener(new Listener() {
 *         public void stopped() {}
 *         public void healthy() {
 *           // Services have been initialized and are healthy, start accepting requests...
 *         }
 *         public void failure(Service service) {
 *           // Something failed, at this point we could log it, notify a load balancer, or take
 *           // some other action.  For now we will just exit.
 *           System.exit(1);
 *         }
 *       },
 *       MoreExecutors.directExecutor());
 *
 *     Runtime.getRuntime().addShutdownHook(new Thread() {
 *       public void run() {
 *         // Give the services 5 seconds to stop to ensure that we are responsive to shutdown
 *         // requests.
 *         try {
 *           manager.stopAsync().awaitStopped(5, TimeUnit.SECONDS);
 *         } catch (TimeoutException timeout) {
 *           // stopping timed out
 *         }
 *       }
 *     });
 *     manager.startAsync();  // start all the services asynchronously
 *   }
 * }}</pre>
 *
 * <p>This class uses the ServiceManager's methods to start all of its services, to respond to
 * service failure and to ensure that when the JVM is shutting down all the services are stopped.
 *
 * @author Luke Sandberg
 * @since 14.0
 */
@Beta
public final class ServiceManager {
  private static final Logger logger = Logger.getLogger(ServiceManager.class.getName());
  private static final Callback<Listener> HEALTHY_CALLBACK = new Callback<Listener>("healthy()") {
    @Override void call(Listener listener) {
      listener.healthy();
    }
  };
  private static final Callback<Listener> STOPPED_CALLBACK = new Callback<Listener>("stopped()") {
    @Override void call(Listener listener) {
      listener.stopped();
    }
  };

  /**
   * A listener for the aggregate state changes of the services that are under management. Users
   * that need to listen to more fine-grained events (such as when each particular {@linkplain
   * Service service} starts, or terminates), should attach {@linkplain Service.Listener service
   * listeners} to each individual service.
   *
   * @author Luke Sandberg
   * @since 15.0 (present as an interface in 14.0)
   */
  @Beta  // Should come out of Beta when ServiceManager does
  public abstract static class Listener {
    /**
     * Called when the service initially becomes healthy.
     *
     * <p>This will be called at most once after all the services have entered the
     * {@linkplain State#RUNNING running} state. If any services fail during start up or
     * {@linkplain State#FAILED fail}/{@linkplain State#TERMINATED terminate} before all other
     * services have started {@linkplain State#RUNNING running} then this method will not be called.
     */
    public void healthy() {}

    /**
     * Called when the all of the component services have reached a terminal state, either
     * {@linkplain State#TERMINATED terminated} or {@linkplain State#FAILED failed}.
     */
    public void stopped() {}

    /**
     * Called when a component service has {@linkplain State#FAILED failed}.
     *
     * @param service The service that failed.
     */
    public void failure(Service service) {}
  }

  /**
   * An encapsulation of all of the state that is accessed by the {@linkplain ServiceListener
   * service listeners}.  This is extracted into its own object so that {@link ServiceListener}
   * could be made {@code static} and its instances can be safely constructed and added in the
   * {@link ServiceManager} constructor without having to close over the partially constructed
   * {@link ServiceManager} instance (i.e. avoid leaking a pointer to {@code this}).
   */
  private final ServiceManagerState state;
  private final ImmutableList<Service> services;

  /**
   * Constructs a new instance for managing the given services.
   *
   * @param services The services to manage
   *
   * @throws IllegalArgumentException if not all services are {@linkplain State#NEW new} or if there
   * are any duplicate services.
   */
  public ServiceManager(Iterable<? extends Service> services) {
    ImmutableList<Service> copy = ImmutableList.copyOf(services);
    if (copy.isEmpty()) {
      // Having no services causes the manager to behave strangely. Notably, listeners are never
      // fired.  To avoid this we substitute a placeholder service.
      logger.log(Level.WARNING,
          "ServiceManager configured with no services.  Is your application configured properly?",
          new EmptyServiceManagerWarning());
      copy = ImmutableList.<Service>of(new NoOpService());
    }
    this.state = new ServiceManagerState(copy);
    this.services = copy;
    WeakReference<ServiceManagerState> stateReference =
        new WeakReference<ServiceManagerState>(state);
    for (Service service : copy) {
      service.addListener(new ServiceListener(service, stateReference), directExecutor());
      // We check the state after adding the listener as a way to ensure that our listener was added
      // to a NEW service.
      checkArgument(service.state() == NEW, "Can only manage NEW services, %s", service);
    }
    // We have installed all of our listeners and after this point any state transition should be
    // correct.
    this.state.markReady();
  }

  /**
   * Registers a {@link Listener} to be {@linkplain Executor#execute executed} on the given
   * executor. The listener will not have previous state changes replayed, so it is
   * suggested that listeners are added before any of the managed services are
   * {@linkplain Service#startAsync started}.
   *
   * <p>{@code addListener} guarantees execution ordering across calls to a given listener but not
   * across calls to multiple listeners. Specifically, a given listener will have its callbacks
   * invoked in the same order as the underlying service enters those states. Additionally, at most
   * one of the listener's callbacks will execute at once. However, multiple listeners' callbacks
   * may execute concurrently, and listeners may execute in an order different from the one in which
   * they were registered.
   *
   * <p>RuntimeExceptions thrown by a listener will be caught and logged. Any exception thrown
   * during {@code Executor.execute} (e.g., a {@code RejectedExecutionException}) will be caught and
   * logged.
   *
   * <p> For fast, lightweight listeners that would be safe to execute in any thread, consider
   * calling {@link #addListener(Listener)}.
   *
   * @param listener the listener to run when the manager changes state
   * @param executor the executor in which the listeners callback methods will be run.
   */
  public void addListener(Listener listener, Executor executor) {
    state.addListener(listener, executor);
  }

  /**
   * Registers a {@link Listener} to be run when this {@link ServiceManager} changes state. The
   * listener will not have previous state changes replayed, so it is suggested that listeners are
   * added before any of the managed services are {@linkplain Service#startAsync started}.
   *
   * <p>{@code addListener} guarantees execution ordering across calls to a given listener but not
   * across calls to multiple listeners. Specifically, a given listener will have its callbacks
   * invoked in the same order as the underlying service enters those states. Additionally, at most
   * one of the listener's callbacks will execute at once. However, multiple listeners' callbacks
   * may execute concurrently, and listeners may execute in an order different from the one in which
   * they were registered.
   *
   * <p>RuntimeExceptions thrown by a listener will be caught and logged.
   *
   * @param listener the listener to run when the manager changes state
   */
  public void addListener(Listener listener) {
    state.addListener(listener, directExecutor());
  }

  /**
   * Initiates service {@linkplain Service#startAsync startup} on all the services being managed.
   * It is only valid to call this method if all of the services are {@linkplain State#NEW new}.
   *
   * @return this
   * @throws IllegalStateException if any of the Services are not {@link State#NEW new} when the
   *     method is called.
   */
  public ServiceManager startAsync() {
    for (Service service : services) {
      State state = service.state();
      checkState(state == NEW, "Service %s is %s, cannot start it.", service, state);
    }
    for (Service service : services) {
      try {
        state.tryStartTiming(service);
        service.startAsync();
      } catch (IllegalStateException e) {
        // This can happen if the service has already been started or stopped (e.g. by another
        // service or listener). Our contract says it is safe to call this method if
        // all services were NEW when it was called, and this has already been verified above, so we
        // don't propagate the exception.
        logger.log(Level.WARNING, "Unable to start Service " + service, e);
      }
    }
    return this;
  }

  /**
   * Waits for the {@link ServiceManager} to become {@linkplain #isHealthy() healthy}.  The manager
   * will become healthy after all the component services have reached the {@linkplain State#RUNNING
   * running} state.
   *
   * @throws IllegalStateException if the service manager reaches a state from which it cannot
   *     become {@linkplain #isHealthy() healthy}.
   */
  public void awaitHealthy() {
    state.awaitHealthy();
  }

  /**
   * Waits for the {@link ServiceManager} to become {@linkplain #isHealthy() healthy} for no more
   * than the given time.  The manager will become healthy after all the component services have
   * reached the {@linkplain State#RUNNING running} state.
   *
   * @param timeout the maximum time to wait
   * @param unit the time unit of the timeout argument
   * @throws TimeoutException if not all of the services have finished starting within the deadline
   * @throws IllegalStateException if the service manager reaches a state from which it cannot
   *     become {@linkplain #isHealthy() healthy}.
   */
  public void awaitHealthy(long timeout, TimeUnit unit) throws TimeoutException {
    state.awaitHealthy(timeout, unit);
  }

  /**
   * Initiates service {@linkplain Service#stopAsync shutdown} if necessary on all the services
   * being managed.
   *
   * @return this
   */
  public ServiceManager stopAsync() {
    for (Service service : services) {
      service.stopAsync();
    }
    return this;
  }

  /**
   * Waits for the all the services to reach a terminal state. After this method returns all
   * services will either be {@linkplain Service.State#TERMINATED terminated} or {@linkplain
   * Service.State#FAILED failed}.
   */
  public void awaitStopped() {
    state.awaitStopped();
  }

  /**
   * Waits for the all the services to reach a terminal state for no more than the given time. After
   * this method returns all services will either be {@linkplain Service.State#TERMINATED
   * terminated} or {@linkplain Service.State#FAILED failed}.
   *
   * @param timeout the maximum time to wait
   * @param unit the time unit of the timeout argument
   * @throws TimeoutException if not all of the services have stopped within the deadline
   */
  public void awaitStopped(long timeout, TimeUnit unit) throws TimeoutException {
    state.awaitStopped(timeout, unit);
  }

  /**
   * Returns true if all services are currently in the {@linkplain State#RUNNING running} state.
   *
   * <p>Users who want more detailed information should use the {@link #servicesByState} method to
   * get detailed information about which services are not running.
   */
  public boolean isHealthy() {
    for (Service service : services) {
      if (!service.isRunning()) {
        return false;
      }
    }
    return true;
  }

  /**
   * Provides a snapshot of the current state of all the services under management.
   *
   * <p>N.B. This snapshot is guaranteed to be consistent, i.e. the set of states returned will
   * correspond to a point in time view of the services.
   */
  public ImmutableMultimap<State, Service> servicesByState() {
    return state.servicesByState();
  }

  /**
   * Returns the service load times. This value will only return startup times for services that
   * have finished starting.
   *
   * @return Map of services and their corresponding startup time in millis, the map entries will be
   *     ordered by startup time.
   */
  public ImmutableMap<Service, Long> startupTimes() {
    return state.startupTimes();
  }

  @Override public String toString() {
    return MoreObjects.toStringHelper(ServiceManager.class)
        .add("services", Collections2.filter(services, not(instanceOf(NoOpService.class))))
        .toString();
  }

  /**
   * An encapsulation of all the mutable state of the {@link ServiceManager} that needs to be
   * accessed by instances of {@link ServiceListener}.
   */
  private static final class ServiceManagerState {
    final Monitor monitor = new Monitor();

    @GuardedBy("monitor")
    final SetMultimap<State, Service> servicesByState =
        Multimaps.newSetMultimap(new EnumMap<State, Collection<Service>>(State.class),
            new Supplier<Set<Service>>() {
              @Override public Set<Service> get() {
                return Sets.newLinkedHashSet();
              }
            });

    @GuardedBy("monitor")
    final Multiset<State> states = servicesByState.keys();

    @GuardedBy("monitor")
    final Map<Service, Stopwatch> startupTimers = Maps.newIdentityHashMap();

    /**
     * These two booleans are used to mark the state as ready to start.
     * {@link #ready}: is set by {@link #markReady} to indicate that all listeners have been
     *     correctly installed
     * {@link #transitioned}: is set by {@link #transitionService} to indicate that some transition
     *     has been performed.
     *
     * <p>Together, they allow us to enforce that all services have their listeners installed prior
     * to any service performing a transition, then we can fail in the ServiceManager constructor
     * rather than in a Service.Listener callback.
     */
    @GuardedBy("monitor")
    boolean ready;

    @GuardedBy("monitor")
    boolean transitioned;

    final int numberOfServices;

    /**
     * Controls how long to wait for all the services to either become healthy or reach a
     * state from which it is guaranteed that it can never become healthy.
     */
    final Monitor.Guard awaitHealthGuard = new Monitor.Guard(monitor) {
      @Override public boolean isSatisfied() {
        // All services have started or some service has terminated/failed.
        return states.count(RUNNING) == numberOfServices
            || states.contains(STOPPING)
            || states.contains(TERMINATED)
            || states.contains(FAILED);
      }
    };

    /**
     * Controls how long to wait for all services to reach a terminal state.
     */
    final Monitor.Guard stoppedGuard = new Monitor.Guard(monitor) {
      @Override public boolean isSatisfied() {
        return states.count(TERMINATED) + states.count(FAILED) == numberOfServices;
      }
    };

    /** The listeners to notify during a state transition. */
    @GuardedBy("monitor")
    final List<ListenerCallQueue<Listener>> listeners =
        Collections.synchronizedList(new ArrayList<ListenerCallQueue<Listener>>());

    /**
     * It is implicitly assumed that all the services are NEW and that they will all remain NEW
     * until all the Listeners are installed and {@link #markReady()} is called.  It is our caller's
     * responsibility to only call {@link #markReady()} if all services were new at the time this
     * method was called and when all the listeners were installed.
     */
    ServiceManagerState(ImmutableCollection<Service> services) {
      this.numberOfServices = services.size();
      servicesByState.putAll(NEW, services);
    }

    /**
     * Attempts to start the timer immediately prior to the service being started via
     * {@link Service#startAsync()}.
     */
    void tryStartTiming(Service service) {
      monitor.enter();
      try {
        Stopwatch stopwatch = startupTimers.get(service);
        if (stopwatch == null) {
          startupTimers.put(service, Stopwatch.createStarted());
        }
      } finally {
        monitor.leave();
      }
    }

    /**
     * Marks the {@link State} as ready to receive transitions. Returns true if no transitions have
     * been observed yet.
     */
    void markReady() {
      monitor.enter();
      try {
        if (!transitioned) {
          // nothing has transitioned since construction, good.
          ready = true;
        } else {
          // This should be an extremely rare race condition.
          List<Service> servicesInBadStates = Lists.newArrayList();
          for (Service service : servicesByState().values()) {
            if (service.state() != NEW) {
              servicesInBadStates.add(service);
            }
          }
          throw new IllegalArgumentException("Services started transitioning asynchronously before "
              + "the ServiceManager was constructed: " + servicesInBadStates);
        }
      } finally {
        monitor.leave();
      }
    }

    void addListener(Listener listener, Executor executor) {
      checkNotNull(listener, "listener");
      checkNotNull(executor, "executor");
      monitor.enter();
      try {
        // no point in adding a listener that will never be called
        if (!stoppedGuard.isSatisfied()) {
          listeners.add(new ListenerCallQueue<Listener>(listener, executor));
        }
      } finally {
        monitor.leave();
      }
    }

    void awaitHealthy() {
      monitor.enterWhenUninterruptibly(awaitHealthGuard);
      try {
        checkHealthy();
      } finally {
        monitor.leave();
      }
    }

    void awaitHealthy(long timeout, TimeUnit unit) throws TimeoutException {
      monitor.enter();
      try {
        if (!monitor.waitForUninterruptibly(awaitHealthGuard, timeout, unit)) {
          throw new TimeoutException("Timeout waiting for the services to become healthy. The "
              + "following services have not started: "
              + Multimaps.filterKeys(servicesByState, in(ImmutableSet.of(NEW, STARTING))));
        }
        checkHealthy();
      } finally {
        monitor.leave();
      }
    }

    void awaitStopped() {
      monitor.enterWhenUninterruptibly(stoppedGuard);
      monitor.leave();
    }

    void awaitStopped(long timeout, TimeUnit unit) throws TimeoutException {
      monitor.enter();
      try {
        if (!monitor.waitForUninterruptibly(stoppedGuard, timeout, unit)) {
          throw new TimeoutException("Timeout waiting for the services to stop. The following "
              + "services have not stopped: "
              + Multimaps.filterKeys(servicesByState,
                  not(in(ImmutableSet.of(TERMINATED, FAILED)))));
        }
      } finally {
        monitor.leave();
      }
    }

    ImmutableMultimap<State, Service> servicesByState() {
      ImmutableSetMultimap.Builder<State, Service> builder = ImmutableSetMultimap.builder();
      monitor.enter();
      try {
        for (Entry<State, Service> entry : servicesByState.entries()) {
          if (!(entry.getValue() instanceof NoOpService)) {
            builder.put(entry.getKey(), entry.getValue());
          }
        }
      } finally {
        monitor.leave();
      }
      return builder.build();
    }

    ImmutableMap<Service, Long> startupTimes() {
      List<Entry<Service, Long>> loadTimes;
      monitor.enter();
      try {
        loadTimes = Lists.newArrayListWithCapacity(startupTimers.size());
        // N.B. There will only be an entry in the map if the service has started
        for (Entry<Service, Stopwatch> entry : startupTimers.entrySet()) {
          Service service = entry.getKey();
          Stopwatch stopWatch = entry.getValue();
          if (!stopWatch.isRunning() && !(service instanceof NoOpService)) {
            loadTimes.add(Maps.immutableEntry(service, stopWatch.elapsed(MILLISECONDS)));
          }
        }
      } finally {
        monitor.leave();
      }
      Collections.sort(loadTimes, Ordering.<Long>natural()
          .onResultOf(new Function<Entry<Service, Long>, Long>() {
            @Override public Long apply(Map.Entry<Service, Long> input) {
              return input.getValue();
            }
          }));
      ImmutableMap.Builder<Service, Long> builder = ImmutableMap.builder();
      for (Entry<Service, Long> entry : loadTimes) {
        builder.put(entry);
      }
      return builder.build();
    }

    /**
     * Updates the state with the given service transition.
     *
     * <p>This method performs the main logic of ServiceManager in the following steps.
     * <ol>
     *      <li>Update the {@link #servicesByState()}
     *      <li>Update the {@link #startupTimers}
     *      <li>Based on the new state queue listeners to run
     *      <li>Run the listeners (outside of the lock)
     * </ol>
     */
    void transitionService(final Service service, State from, State to) {
      checkNotNull(service);
      checkArgument(from != to);
      monitor.enter();
      try {
        transitioned = true;
        if (!ready) {
          return;
        }
        // Update state.
        checkState(servicesByState.remove(from, service),
            "Service %s not at the expected location in the state map %s", service, from);
        checkState(servicesByState.put(to, service),
            "Service %s in the state map unexpectedly at %s", service, to);
        // Update the timer
        Stopwatch stopwatch = startupTimers.get(service);
        if (stopwatch == null) {
          // This means the service was started by some means other than ServiceManager.startAsync
          stopwatch = Stopwatch.createStarted();
          startupTimers.put(service, stopwatch);
        }
        if (to.compareTo(RUNNING) >= 0 && stopwatch.isRunning()) {
          // N.B. if we miss the STARTING event then we may never record a startup time.
          stopwatch.stop();
          if (!(service instanceof NoOpService)) {
            logger.log(Level.FINE, "Started {0} in {1}.", new Object[] {service, stopwatch});
          }
        }
        // Queue our listeners

        // Did a service fail?
        if (to == FAILED) {
          fireFailedListeners(service);
        }

        if (states.count(RUNNING) == numberOfServices) {
          // This means that the manager is currently healthy. N.B. If other threads call isHealthy
          // they are not guaranteed to get 'true', because any service could fail right now.
          fireHealthyListeners();
        } else if (states.count(TERMINATED) + states.count(FAILED) == numberOfServices) {
          fireStoppedListeners();
        }
      } finally {
        monitor.leave();
        // Run our executors outside of the lock
        executeListeners();
      }
    }

    @GuardedBy("monitor")
    void fireStoppedListeners() {
      STOPPED_CALLBACK.enqueueOn(listeners);
    }

    @GuardedBy("monitor")
    void fireHealthyListeners() {
      HEALTHY_CALLBACK.enqueueOn(listeners);
    }

    @GuardedBy("monitor")
    void fireFailedListeners(final Service service) {
      new Callback<Listener>("failed({service=" + service + "})") {
        @Override void call(Listener listener) {
          listener.failure(service);
        }
      }.enqueueOn(listeners);
    }

    /** Attempts to execute all the listeners in {@link #listeners}. */
    void executeListeners() {
      checkState(!monitor.isOccupiedByCurrentThread(),
          "It is incorrect to execute listeners with the monitor held.");
      // iterate by index to avoid concurrent modification exceptions
      for (int i = 0; i < listeners.size(); i++) {
        listeners.get(i).execute();
      }
    }

    @GuardedBy("monitor")
    void checkHealthy() {
      if (states.count(RUNNING) != numberOfServices) {
        IllegalStateException exception = new IllegalStateException(
            "Expected to be healthy after starting. The following services are not running: "
                + Multimaps.filterKeys(servicesByState, not(equalTo(RUNNING))));
        throw exception;
      }
    }
  }

  /**
   * A {@link Service} that wraps another service and times how long it takes for it to start and
   * also calls the {@link ServiceManagerState#transitionService(Service, State, State)},
   * to record the state transitions.
   */
  private static final class ServiceListener extends Service.Listener {
    final Service service;
    // We store the state in a weak reference to ensure that if something went wrong while
    // constructing the ServiceManager we don't pointlessly keep updating the state.
    final WeakReference<ServiceManagerState> state;

    ServiceListener(Service service, WeakReference<ServiceManagerState> state) {
      this.service = service;
      this.state = state;
    }

    @Override public void starting() {
      ServiceManagerState state = this.state.get();
      if (state != null) {
        state.transitionService(service, NEW, STARTING);
        if (!(service instanceof NoOpService)) {
          logger.log(Level.FINE, "Starting {0}.", service);
        }
      }
    }

    @Override public void running() {
      ServiceManagerState state = this.state.get();
      if (state != null) {
        state.transitionService(service, STARTING, RUNNING);
      }
    }

    @Override public void stopping(State from) {
      ServiceManagerState state = this.state.get();
      if (state != null) {
        state.transitionService(service, from, STOPPING);
      }
    }

    @Override public void terminated(State from) {
      ServiceManagerState state = this.state.get();
      if (state != null) {
        if (!(service instanceof NoOpService)) {
          logger.log(Level.FINE, "Service {0} has terminated. Previous state was: {1}",
              new Object[] {service, from});
        }
        state.transitionService(service, from, TERMINATED);
      }
    }

    @Override public void failed(State from, Throwable failure) {
      ServiceManagerState state = this.state.get();
      if (state != null) {
        // Log before the transition, so that if the process exits in response to server failure,
        // there is a higher likelihood that the cause will be in the logs.
        if (!(service instanceof NoOpService)) {
          logger.log(Level.SEVERE, "Service " + service + " has failed in the " + from + " state.",
              failure);
        }
        state.transitionService(service, from, FAILED);
      }
    }
  }

  /**
   * A {@link Service} instance that does nothing.  This is only useful as a placeholder to
   * ensure that the {@link ServiceManager} functions properly even when it is managing no services.
   *
   * <p>The use of this class is considered an implementation detail of ServiceManager and as such
   * it is excluded from {@link #servicesByState}, {@link #startupTimes}, {@link #toString} and all
   * logging statements.
   */
  private static final class NoOpService extends AbstractService {
    @Override protected void doStart() { notifyStarted(); }
    @Override protected void doStop() { notifyStopped(); }
  }

  /** This is never thrown but only used for logging. */
  private static final class EmptyServiceManagerWarning extends Throwable {}
}