001/*
002 * Copyright (C) 2012 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
005 * in compliance with the License. You may obtain a copy of the License at
006 *
007 * http://www.apache.org/licenses/LICENSE-2.0
008 *
009 * Unless required by applicable law or agreed to in writing, software distributed under the License
010 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
011 * or implied. See the License for the specific language governing permissions and limitations under
012 * the License.
013 */
014
015package com.google.common.util.concurrent;
016
017import static com.google.common.base.Preconditions.checkArgument;
018import static com.google.common.base.Preconditions.checkNotNull;
019import static com.google.common.base.Preconditions.checkState;
020import static com.google.common.base.Predicates.equalTo;
021import static com.google.common.base.Predicates.in;
022import static com.google.common.base.Predicates.instanceOf;
023import static com.google.common.base.Predicates.not;
024import static com.google.common.util.concurrent.Internal.toNanosSaturated;
025import static com.google.common.util.concurrent.MoreExecutors.directExecutor;
026import static com.google.common.util.concurrent.Service.State.FAILED;
027import static com.google.common.util.concurrent.Service.State.NEW;
028import static com.google.common.util.concurrent.Service.State.RUNNING;
029import static com.google.common.util.concurrent.Service.State.STARTING;
030import static com.google.common.util.concurrent.Service.State.STOPPING;
031import static com.google.common.util.concurrent.Service.State.TERMINATED;
032import static java.util.concurrent.TimeUnit.MILLISECONDS;
033
034import com.google.common.annotations.GwtIncompatible;
035import com.google.common.annotations.J2ktIncompatible;
036import com.google.common.base.Function;
037import com.google.common.base.MoreObjects;
038import com.google.common.base.Stopwatch;
039import com.google.common.collect.Collections2;
040import com.google.common.collect.ImmutableCollection;
041import com.google.common.collect.ImmutableList;
042import com.google.common.collect.ImmutableMap;
043import com.google.common.collect.ImmutableSet;
044import com.google.common.collect.ImmutableSetMultimap;
045import com.google.common.collect.Lists;
046import com.google.common.collect.Maps;
047import com.google.common.collect.MultimapBuilder;
048import com.google.common.collect.Multimaps;
049import com.google.common.collect.Multiset;
050import com.google.common.collect.Ordering;
051import com.google.common.collect.SetMultimap;
052import com.google.common.util.concurrent.Service.State;
053import com.google.errorprone.annotations.CanIgnoreReturnValue;
054import com.google.errorprone.annotations.concurrent.GuardedBy;
055import com.google.j2objc.annotations.J2ObjCIncompatible;
056import com.google.j2objc.annotations.WeakOuter;
057import java.lang.ref.WeakReference;
058import java.time.Duration;
059import java.util.Collections;
060import java.util.EnumSet;
061import java.util.IdentityHashMap;
062import java.util.List;
063import java.util.Map.Entry;
064import java.util.concurrent.Executor;
065import java.util.concurrent.TimeUnit;
066import java.util.concurrent.TimeoutException;
067import java.util.logging.Level;
068
069/**
070 * A manager for monitoring and controlling a set of {@linkplain Service services}. This class
071 * provides methods for {@linkplain #startAsync() starting}, {@linkplain #stopAsync() stopping} and
072 * {@linkplain #servicesByState inspecting} a collection of {@linkplain Service services}.
073 * Additionally, users can monitor state transitions with the {@linkplain Listener listener}
074 * mechanism.
075 *
076 * <p>While it is recommended that service lifecycles be managed via this class, state transitions
077 * initiated via other mechanisms do not impact the correctness of its methods. For example, if the
078 * services are started by some mechanism besides {@link #startAsync}, the listeners will be invoked
079 * when appropriate and {@link #awaitHealthy} will still work as expected.
080 *
081 * <p>Here is a simple example of how to use a {@code ServiceManager} to start a server.
082 *
083 * <pre>{@code
084 * class Server {
085 *   public static void main(String[] args) {
086 *     Set<Service> services = ...;
087 *     ServiceManager manager = new ServiceManager(services);
088 *     manager.addListener(new Listener() {
089 *         public void stopped() {}
090 *         public void healthy() {
091 *           // Services have been initialized and are healthy, start accepting requests...
092 *         }
093 *         public void failure(Service service) {
094 *           // Something failed, at this point we could log it, notify a load balancer, or take
095 *           // some other action.  For now we will just exit.
096 *           System.exit(1);
097 *         }
098 *       },
099 *       MoreExecutors.directExecutor());
100 *
101 *     Runtime.getRuntime().addShutdownHook(new Thread() {
102 *       public void run() {
103 *         // Give the services 5 seconds to stop to ensure that we are responsive to shutdown
104 *         // requests.
105 *         try {
106 *           manager.stopAsync().awaitStopped(5, TimeUnit.SECONDS);
107 *         } catch (TimeoutException timeout) {
108 *           // stopping timed out
109 *         }
110 *       }
111 *     });
112 *     manager.startAsync();  // start all the services asynchronously
113 *   }
114 * }
115 * }</pre>
116 *
117 * <p>This class uses the ServiceManager's methods to start all of its services, to respond to
118 * service failure and to ensure that when the JVM is shutting down all the services are stopped.
119 *
120 * @author Luke Sandberg
121 * @since 14.0
122 */
123@J2ktIncompatible
124@GwtIncompatible
125public final class ServiceManager implements ServiceManagerBridge {
126  private static final LazyLogger logger = new LazyLogger(ServiceManager.class);
127  private static final ListenerCallQueue.Event<Listener> HEALTHY_EVENT =
128      new ListenerCallQueue.Event<Listener>() {
129        @Override
130        public void call(Listener listener) {
131          listener.healthy();
132        }
133
134        @Override
135        public String toString() {
136          return "healthy()";
137        }
138      };
139  private static final ListenerCallQueue.Event<Listener> STOPPED_EVENT =
140      new ListenerCallQueue.Event<Listener>() {
141        @Override
142        public void call(Listener listener) {
143          listener.stopped();
144        }
145
146        @Override
147        public String toString() {
148          return "stopped()";
149        }
150      };
151
152  /**
153   * A listener for the aggregate state changes of the services that are under management. Users
154   * that need to listen to more fine-grained events (such as when each particular {@linkplain
155   * Service service} starts, or terminates), should attach {@linkplain Service.Listener service
156   * listeners} to each individual service.
157   *
158   * @author Luke Sandberg
159   * @since 15.0 (present as an interface in 14.0)
160   */
161  public abstract static class Listener {
162    /** Constructor for use by subclasses. */
163    public Listener() {}
164
165    /**
166     * Called when the service initially becomes healthy.
167     *
168     * <p>This will be called at most once after all the services have entered the {@linkplain
169     * State#RUNNING running} state. If any services fail during start up or {@linkplain
170     * State#FAILED fail}/{@linkplain State#TERMINATED terminate} before all other services have
171     * started {@linkplain State#RUNNING running} then this method will not be called.
172     */
173    public void healthy() {}
174
175    /**
176     * Called when the all of the component services have reached a terminal state, either
177     * {@linkplain State#TERMINATED terminated} or {@linkplain State#FAILED failed}.
178     */
179    public void stopped() {}
180
181    /**
182     * Called when a component service has {@linkplain State#FAILED failed}.
183     *
184     * @param service The service that failed.
185     */
186    public void failure(Service service) {}
187  }
188
189  /**
190   * An encapsulation of all of the state that is accessed by the {@linkplain ServiceListener
191   * service listeners}. This is extracted into its own object so that {@link ServiceListener} could
192   * be made {@code static} and its instances can be safely constructed and added in the {@link
193   * ServiceManager} constructor without having to close over the partially constructed {@link
194   * ServiceManager} instance (i.e. avoid leaking a pointer to {@code this}).
195   */
196  private final ServiceManagerState state;
197
198  private final ImmutableList<Service> services;
199
200  /**
201   * Constructs a new instance for managing the given services.
202   *
203   * @param services The services to manage
204   * @throws IllegalArgumentException if not all services are {@linkplain State#NEW new} or if there
205   *     are any duplicate services.
206   */
207  public ServiceManager(Iterable<? extends Service> services) {
208    ImmutableList<Service> copy = ImmutableList.copyOf(services);
209    if (copy.isEmpty()) {
210      // Having no services causes the manager to behave strangely. Notably, listeners are never
211      // fired. To avoid this we substitute a placeholder service.
212      logger
213          .get()
214          .log(
215              Level.WARNING,
216              "ServiceManager configured with no services.  Is your application configured"
217                  + " properly?",
218              new EmptyServiceManagerWarning());
219      copy = ImmutableList.<Service>of(new NoOpService());
220    }
221    this.state = new ServiceManagerState(copy);
222    this.services = copy;
223    WeakReference<ServiceManagerState> stateReference = new WeakReference<>(state);
224    for (Service service : copy) {
225      service.addListener(new ServiceListener(service, stateReference), directExecutor());
226      // We check the state after adding the listener as a way to ensure that our listener was added
227      // to a NEW service.
228      checkArgument(service.state() == NEW, "Can only manage NEW services, %s", service);
229    }
230    // We have installed all of our listeners and after this point any state transition should be
231    // correct.
232    this.state.markReady();
233  }
234
235  /**
236   * Registers a {@link Listener} to be {@linkplain Executor#execute executed} on the given
237   * executor. The listener will not have previous state changes replayed, so it is suggested that
238   * listeners are added before any of the managed services are {@linkplain Service#startAsync
239   * started}.
240   *
241   * <p>{@code addListener} guarantees execution ordering across calls to a given listener but not
242   * across calls to multiple listeners. Specifically, a given listener will have its callbacks
243   * invoked in the same order as the underlying service enters those states. Additionally, at most
244   * one of the listener's callbacks will execute at once. However, multiple listeners' callbacks
245   * may execute concurrently, and listeners may execute in an order different from the one in which
246   * they were registered.
247   *
248   * <p>RuntimeExceptions thrown by a listener will be caught and logged. Any exception thrown
249   * during {@code Executor.execute} (e.g., a {@code RejectedExecutionException}) will be caught and
250   * logged.
251   *
252   * <p>When selecting an executor, note that {@code directExecutor} is dangerous in some cases. See
253   * the discussion in the {@link ListenableFuture#addListener ListenableFuture.addListener}
254   * documentation.
255   *
256   * @param listener the listener to run when the manager changes state
257   * @param executor the executor in which the listeners callback methods will be run.
258   */
259  public void addListener(Listener listener, Executor executor) {
260    state.addListener(listener, executor);
261  }
262
263  /**
264   * Initiates service {@linkplain Service#startAsync startup} on all the services being managed. It
265   * is only valid to call this method if all of the services are {@linkplain State#NEW new}.
266   *
267   * @return this
268   * @throws IllegalStateException if any of the Services are not {@link State#NEW new} when the
269   *     method is called.
270   */
271  @CanIgnoreReturnValue
272  public ServiceManager startAsync() {
273    for (Service service : services) {
274      checkState(service.state() == NEW, "Not all services are NEW, cannot start %s", this);
275    }
276    for (Service service : services) {
277      try {
278        state.tryStartTiming(service);
279        service.startAsync();
280      } catch (IllegalStateException e) {
281        // This can happen if the service has already been started or stopped (e.g. by another
282        // service or listener). Our contract says it is safe to call this method if
283        // all services were NEW when it was called, and this has already been verified above, so we
284        // don't propagate the exception.
285        logger.get().log(Level.WARNING, "Unable to start Service " + service, e);
286      }
287    }
288    return this;
289  }
290
291  /**
292   * Waits for the {@link ServiceManager} to become {@linkplain #isHealthy() healthy}. The manager
293   * will become healthy after all the component services have reached the {@linkplain State#RUNNING
294   * running} state.
295   *
296   * @throws IllegalStateException if the service manager reaches a state from which it cannot
297   *     become {@linkplain #isHealthy() healthy}.
298   */
299  public void awaitHealthy() {
300    state.awaitHealthy();
301  }
302
303  /**
304   * Waits for the {@link ServiceManager} to become {@linkplain #isHealthy() healthy} for no more
305   * than the given time. The manager will become healthy after all the component services have
306   * reached the {@linkplain State#RUNNING running} state.
307   *
308   * @param timeout the maximum time to wait
309   * @throws TimeoutException if not all of the services have finished starting within the deadline
310   * @throws IllegalStateException if the service manager reaches a state from which it cannot
311   *     become {@linkplain #isHealthy() healthy}.
312   * @since 28.0 (but only since 33.4.0 in the Android flavor)
313   */
314  public void awaitHealthy(Duration timeout) throws TimeoutException {
315    awaitHealthy(toNanosSaturated(timeout), TimeUnit.NANOSECONDS);
316  }
317
318  /**
319   * Waits for the {@link ServiceManager} to become {@linkplain #isHealthy() healthy} for no more
320   * than the given time. The manager will become healthy after all the component services have
321   * reached the {@linkplain State#RUNNING running} state.
322   *
323   * @param timeout the maximum time to wait
324   * @param unit the time unit of the timeout argument
325   * @throws TimeoutException if not all of the services have finished starting within the deadline
326   * @throws IllegalStateException if the service manager reaches a state from which it cannot
327   *     become {@linkplain #isHealthy() healthy}.
328   */
329  @SuppressWarnings("GoodTime") // should accept a java.time.Duration
330  public void awaitHealthy(long timeout, TimeUnit unit) throws TimeoutException {
331    state.awaitHealthy(timeout, unit);
332  }
333
334  /**
335   * Initiates service {@linkplain Service#stopAsync shutdown} if necessary on all the services
336   * being managed.
337   *
338   * @return this
339   */
340  @CanIgnoreReturnValue
341  public ServiceManager stopAsync() {
342    for (Service service : services) {
343      service.stopAsync();
344    }
345    return this;
346  }
347
348  /**
349   * Waits for the all the services to reach a terminal state. After this method returns all
350   * services will either be {@linkplain Service.State#TERMINATED terminated} or {@linkplain
351   * Service.State#FAILED failed}.
352   */
353  public void awaitStopped() {
354    state.awaitStopped();
355  }
356
357  /**
358   * Waits for the all the services to reach a terminal state for no more than the given time. After
359   * this method returns all services will either be {@linkplain Service.State#TERMINATED
360   * terminated} or {@linkplain Service.State#FAILED failed}.
361   *
362   * @param timeout the maximum time to wait
363   * @throws TimeoutException if not all of the services have stopped within the deadline
364   * @since 28.0 (but only since 33.4.0 in the Android flavor)
365   */
366  public void awaitStopped(Duration timeout) throws TimeoutException {
367    awaitStopped(toNanosSaturated(timeout), TimeUnit.NANOSECONDS);
368  }
369
370  /**
371   * Waits for the all the services to reach a terminal state for no more than the given time. After
372   * this method returns all services will either be {@linkplain Service.State#TERMINATED
373   * terminated} or {@linkplain Service.State#FAILED failed}.
374   *
375   * @param timeout the maximum time to wait
376   * @param unit the time unit of the timeout argument
377   * @throws TimeoutException if not all of the services have stopped within the deadline
378   */
379  @SuppressWarnings("GoodTime") // should accept a java.time.Duration
380  public void awaitStopped(long timeout, TimeUnit unit) throws TimeoutException {
381    state.awaitStopped(timeout, unit);
382  }
383
384  /**
385   * Returns true if all services are currently in the {@linkplain State#RUNNING running} state.
386   *
387   * <p>Users who want more detailed information should use the {@link #servicesByState} method to
388   * get detailed information about which services are not running.
389   */
390  public boolean isHealthy() {
391    for (Service service : services) {
392      if (!service.isRunning()) {
393        return false;
394      }
395    }
396    return true;
397  }
398
399  /**
400   * Provides a snapshot of the current state of all the services under management.
401   *
402   * <p>N.B. This snapshot is guaranteed to be consistent, i.e. the set of states returned will
403   * correspond to a point in time view of the services.
404   *
405   * @since 29.0 (present with return type {@code ImmutableMultimap} since 14.0)
406   */
407  @Override
408  public ImmutableSetMultimap<State, Service> servicesByState() {
409    return state.servicesByState();
410  }
411
412  /**
413   * Returns the service load times. This value will only return startup times for services that
414   * have finished starting.
415   *
416   * @return Map of services and their corresponding startup time in millis, the map entries will be
417   *     ordered by startup time.
418   */
419  public ImmutableMap<Service, Long> startupTimes() {
420    return state.startupTimes();
421  }
422
423  /**
424   * Returns the service load times. This value will only return startup times for services that
425   * have finished starting.
426   *
427   * @return Map of services and their corresponding startup time, the map entries will be ordered
428   *     by startup time.
429   * @since 31.0 (but only since 33.4.0 in the Android flavor)
430   */
431  @J2ObjCIncompatible
432  public ImmutableMap<Service, Duration> startupDurations() {
433    return ImmutableMap.copyOf(
434        Maps.<Service, Long, Duration>transformValues(startupTimes(), Duration::ofMillis));
435  }
436
437  @Override
438  public String toString() {
439    return MoreObjects.toStringHelper(ServiceManager.class)
440        .add("services", Collections2.filter(services, not(instanceOf(NoOpService.class))))
441        .toString();
442  }
443
444  /**
445   * An encapsulation of all the mutable state of the {@link ServiceManager} that needs to be
446   * accessed by instances of {@link ServiceListener}.
447   */
448  private static final class ServiceManagerState {
449    final Monitor monitor = new Monitor();
450
451    @GuardedBy("monitor")
452    final SetMultimap<State, Service> servicesByState =
453        MultimapBuilder.enumKeys(State.class).linkedHashSetValues().build();
454
455    @GuardedBy("monitor")
456    final Multiset<State> states = servicesByState.keys();
457
458    @GuardedBy("monitor")
459    final IdentityHashMap<Service, Stopwatch> startupTimers = new IdentityHashMap<>();
460
461    /**
462     * These two booleans are used to mark the state as ready to start.
463     *
464     * <p>{@link #ready}: is set by {@link #markReady} to indicate that all listeners have been
465     * correctly installed
466     *
467     * <p>{@link #transitioned}: is set by {@link #transitionService} to indicate that some
468     * transition has been performed.
469     *
470     * <p>Together, they allow us to enforce that all services have their listeners installed prior
471     * to any service performing a transition, then we can fail in the ServiceManager constructor
472     * rather than in a Service.Listener callback.
473     */
474    @GuardedBy("monitor")
475    boolean ready;
476
477    @GuardedBy("monitor")
478    boolean transitioned;
479
480    final int numberOfServices;
481
482    /**
483     * Controls how long to wait for all the services to either become healthy or reach a state from
484     * which it is guaranteed that it can never become healthy.
485     */
486    final Monitor.Guard awaitHealthGuard = new AwaitHealthGuard();
487
488    @WeakOuter
489    final class AwaitHealthGuard extends Monitor.Guard {
490      AwaitHealthGuard() {
491        super(ServiceManagerState.this.monitor);
492      }
493
494      @Override
495      @GuardedBy("ServiceManagerState.this.monitor")
496      public boolean isSatisfied() {
497        // All services have started or some service has terminated/failed.
498        return states.count(RUNNING) == numberOfServices
499            || states.contains(STOPPING)
500            || states.contains(TERMINATED)
501            || states.contains(FAILED);
502      }
503    }
504
505    /** Controls how long to wait for all services to reach a terminal state. */
506    final Monitor.Guard stoppedGuard = new StoppedGuard();
507
508    @WeakOuter
509    final class StoppedGuard extends Monitor.Guard {
510      StoppedGuard() {
511        super(ServiceManagerState.this.monitor);
512      }
513
514      @Override
515      @GuardedBy("ServiceManagerState.this.monitor")
516      public boolean isSatisfied() {
517        return states.count(TERMINATED) + states.count(FAILED) == numberOfServices;
518      }
519    }
520
521    /** The listeners to notify during a state transition. */
522    final ListenerCallQueue<Listener> listeners = new ListenerCallQueue<>();
523
524    /**
525     * It is implicitly assumed that all the services are NEW and that they will all remain NEW
526     * until all the Listeners are installed and {@link #markReady()} is called. It is our caller's
527     * responsibility to only call {@link #markReady()} if all services were new at the time this
528     * method was called and when all the listeners were installed.
529     */
530    ServiceManagerState(ImmutableCollection<Service> services) {
531      this.numberOfServices = services.size();
532      servicesByState.putAll(NEW, services);
533    }
534
535    /**
536     * Attempts to start the timer immediately prior to the service being started via {@link
537     * Service#startAsync()}.
538     */
539    void tryStartTiming(Service service) {
540      monitor.enter();
541      try {
542        Stopwatch stopwatch = startupTimers.get(service);
543        if (stopwatch == null) {
544          startupTimers.put(service, Stopwatch.createStarted());
545        }
546      } finally {
547        monitor.leave();
548      }
549    }
550
551    /**
552     * Marks the {@link State} as ready to receive transitions. Returns true if no transitions have
553     * been observed yet.
554     */
555    void markReady() {
556      monitor.enter();
557      try {
558        if (!transitioned) {
559          // nothing has transitioned since construction, good.
560          ready = true;
561        } else {
562          // This should be an extremely rare race condition.
563          List<Service> servicesInBadStates = Lists.newArrayList();
564          for (Service service : servicesByState().values()) {
565            if (service.state() != NEW) {
566              servicesInBadStates.add(service);
567            }
568          }
569          throw new IllegalArgumentException(
570              "Services started transitioning asynchronously before "
571                  + "the ServiceManager was constructed: "
572                  + servicesInBadStates);
573        }
574      } finally {
575        monitor.leave();
576      }
577    }
578
579    void addListener(Listener listener, Executor executor) {
580      listeners.addListener(listener, executor);
581    }
582
583    void awaitHealthy() {
584      monitor.enterWhenUninterruptibly(awaitHealthGuard);
585      try {
586        checkHealthy();
587      } finally {
588        monitor.leave();
589      }
590    }
591
592    void awaitHealthy(long timeout, TimeUnit unit) throws TimeoutException {
593      monitor.enter();
594      try {
595        if (!monitor.waitForUninterruptibly(awaitHealthGuard, timeout, unit)) {
596          throw new TimeoutException(
597              "Timeout waiting for the services to become healthy. The "
598                  + "following services have not started: "
599                  + Multimaps.filterKeys(servicesByState, in(ImmutableSet.of(NEW, STARTING))));
600        }
601        checkHealthy();
602      } finally {
603        monitor.leave();
604      }
605    }
606
607    void awaitStopped() {
608      monitor.enterWhenUninterruptibly(stoppedGuard);
609      monitor.leave();
610    }
611
612    void awaitStopped(long timeout, TimeUnit unit) throws TimeoutException {
613      monitor.enter();
614      try {
615        if (!monitor.waitForUninterruptibly(stoppedGuard, timeout, unit)) {
616          throw new TimeoutException(
617              "Timeout waiting for the services to stop. The following "
618                  + "services have not stopped: "
619                  + Multimaps.filterKeys(servicesByState, not(in(EnumSet.of(TERMINATED, FAILED)))));
620        }
621      } finally {
622        monitor.leave();
623      }
624    }
625
626    ImmutableSetMultimap<State, Service> servicesByState() {
627      ImmutableSetMultimap.Builder<State, Service> builder = ImmutableSetMultimap.builder();
628      monitor.enter();
629      try {
630        for (Entry<State, Service> entry : servicesByState.entries()) {
631          if (!(entry.getValue() instanceof NoOpService)) {
632            builder.put(entry);
633          }
634        }
635      } finally {
636        monitor.leave();
637      }
638      return builder.build();
639    }
640
641    ImmutableMap<Service, Long> startupTimes() {
642      List<Entry<Service, Long>> loadTimes;
643      monitor.enter();
644      try {
645        loadTimes = Lists.newArrayListWithCapacity(startupTimers.size());
646        // N.B. There will only be an entry in the map if the service has started
647        for (Entry<Service, Stopwatch> entry : startupTimers.entrySet()) {
648          Service service = entry.getKey();
649          Stopwatch stopwatch = entry.getValue();
650          if (!stopwatch.isRunning() && !(service instanceof NoOpService)) {
651            loadTimes.add(Maps.immutableEntry(service, stopwatch.elapsed(MILLISECONDS)));
652          }
653        }
654      } finally {
655        monitor.leave();
656      }
657      Collections.sort(
658          loadTimes,
659          Ordering.natural()
660              .onResultOf(
661                  new Function<Entry<Service, Long>, Long>() {
662                    @Override
663                    public Long apply(Entry<Service, Long> input) {
664                      return input.getValue();
665                    }
666                  }));
667      return ImmutableMap.copyOf(loadTimes);
668    }
669
670    /**
671     * Updates the state with the given service transition.
672     *
673     * <p>This method performs the main logic of ServiceManager in the following steps.
674     *
675     * <ol>
676     *   <li>Update the {@link #servicesByState()}
677     *   <li>Update the {@link #startupTimers}
678     *   <li>Based on the new state queue listeners to run
679     *   <li>Run the listeners (outside of the lock)
680     * </ol>
681     */
682    void transitionService(final Service service, State from, State to) {
683      checkNotNull(service);
684      checkArgument(from != to);
685      monitor.enter();
686      try {
687        transitioned = true;
688        if (!ready) {
689          return;
690        }
691        // Update state.
692        checkState(
693            servicesByState.remove(from, service),
694            "Service %s not at the expected location in the state map %s",
695            service,
696            from);
697        checkState(
698            servicesByState.put(to, service),
699            "Service %s in the state map unexpectedly at %s",
700            service,
701            to);
702        // Update the timer
703        Stopwatch stopwatch = startupTimers.get(service);
704        if (stopwatch == null) {
705          // This means the service was started by some means other than ServiceManager.startAsync
706          stopwatch = Stopwatch.createStarted();
707          startupTimers.put(service, stopwatch);
708        }
709        if (to.compareTo(RUNNING) >= 0 && stopwatch.isRunning()) {
710          // N.B. if we miss the STARTING event then we may never record a startup time.
711          stopwatch.stop();
712          if (!(service instanceof NoOpService)) {
713            logger.get().log(Level.FINE, "Started {0} in {1}.", new Object[] {service, stopwatch});
714          }
715        }
716        // Queue our listeners
717
718        // Did a service fail?
719        if (to == FAILED) {
720          enqueueFailedEvent(service);
721        }
722
723        if (states.count(RUNNING) == numberOfServices) {
724          // This means that the manager is currently healthy. N.B. If other threads call isHealthy
725          // they are not guaranteed to get 'true', because any service could fail right now.
726          enqueueHealthyEvent();
727        } else if (states.count(TERMINATED) + states.count(FAILED) == numberOfServices) {
728          enqueueStoppedEvent();
729        }
730      } finally {
731        monitor.leave();
732        // Run our executors outside of the lock
733        dispatchListenerEvents();
734      }
735    }
736
737    void enqueueStoppedEvent() {
738      listeners.enqueue(STOPPED_EVENT);
739    }
740
741    void enqueueHealthyEvent() {
742      listeners.enqueue(HEALTHY_EVENT);
743    }
744
745    void enqueueFailedEvent(final Service service) {
746      listeners.enqueue(
747          new ListenerCallQueue.Event<Listener>() {
748            @Override
749            public void call(Listener listener) {
750              listener.failure(service);
751            }
752
753            @Override
754            public String toString() {
755              return "failed({service=" + service + "})";
756            }
757          });
758    }
759
760    /** Attempts to execute all the listeners in {@link #listeners}. */
761    void dispatchListenerEvents() {
762      checkState(
763          !monitor.isOccupiedByCurrentThread(),
764          "It is incorrect to execute listeners with the monitor held.");
765      listeners.dispatch();
766    }
767
768    @GuardedBy("monitor")
769    void checkHealthy() {
770      if (states.count(RUNNING) != numberOfServices) {
771        IllegalStateException exception =
772            new IllegalStateException(
773                "Expected to be healthy after starting. The following services are not running: "
774                    + Multimaps.filterKeys(servicesByState, not(equalTo(RUNNING))));
775        for (Service service : servicesByState.get(State.FAILED)) {
776          exception.addSuppressed(new FailedService(service));
777        }
778        throw exception;
779      }
780    }
781  }
782
783  /**
784   * A {@link Service} that wraps another service and times how long it takes for it to start and
785   * also calls the {@link ServiceManagerState#transitionService(Service, State, State)}, to record
786   * the state transitions.
787   */
788  private static final class ServiceListener extends Service.Listener {
789    final Service service;
790    // We store the state in a weak reference to ensure that if something went wrong while
791    // constructing the ServiceManager we don't pointlessly keep updating the state.
792    final WeakReference<ServiceManagerState> state;
793
794    ServiceListener(Service service, WeakReference<ServiceManagerState> state) {
795      this.service = service;
796      this.state = state;
797    }
798
799    @Override
800    public void starting() {
801      ServiceManagerState state = this.state.get();
802      if (state != null) {
803        state.transitionService(service, NEW, STARTING);
804        if (!(service instanceof NoOpService)) {
805          logger.get().log(Level.FINE, "Starting {0}.", service);
806        }
807      }
808    }
809
810    @Override
811    public void running() {
812      ServiceManagerState state = this.state.get();
813      if (state != null) {
814        state.transitionService(service, STARTING, RUNNING);
815      }
816    }
817
818    @Override
819    public void stopping(State from) {
820      ServiceManagerState state = this.state.get();
821      if (state != null) {
822        state.transitionService(service, from, STOPPING);
823      }
824    }
825
826    @Override
827    public void terminated(State from) {
828      ServiceManagerState state = this.state.get();
829      if (state != null) {
830        if (!(service instanceof NoOpService)) {
831          logger
832              .get()
833              .log(
834                  Level.FINE,
835                  "Service {0} has terminated. Previous state was: {1}",
836                  new Object[] {service, from});
837        }
838        state.transitionService(service, from, TERMINATED);
839      }
840    }
841
842    @Override
843    public void failed(State from, Throwable failure) {
844      ServiceManagerState state = this.state.get();
845      if (state != null) {
846        // Log before the transition, so that if the process exits in response to server failure,
847        // there is a higher likelihood that the cause will be in the logs.
848        boolean log = !(service instanceof NoOpService);
849        /*
850         * We have already exposed startup exceptions to the user in the form of suppressed
851         * exceptions. We don't need to log those exceptions again.
852         */
853        log &= from != State.STARTING;
854        if (log) {
855          logger
856              .get()
857              .log(
858                  Level.SEVERE,
859                  "Service " + service + " has failed in the " + from + " state.",
860                  failure);
861        }
862        state.transitionService(service, from, FAILED);
863      }
864    }
865  }
866
867  /**
868   * A {@link Service} instance that does nothing. This is only useful as a placeholder to ensure
869   * that the {@link ServiceManager} functions properly even when it is managing no services.
870   *
871   * <p>The use of this class is considered an implementation detail of ServiceManager and as such
872   * it is excluded from {@link #servicesByState}, {@link #startupTimes}, {@link #toString} and all
873   * logging statements.
874   */
875  private static final class NoOpService extends AbstractService {
876    @Override
877    protected void doStart() {
878      notifyStarted();
879    }
880
881    @Override
882    protected void doStop() {
883      notifyStopped();
884    }
885  }
886
887  /** This is never thrown but only used for logging. */
888  private static final class EmptyServiceManagerWarning extends Throwable {}
889
890  private static final class FailedService extends Throwable {
891    FailedService(Service service) {
892      super(
893          service.toString(),
894          service.failureCause(),
895          false /* don't enable suppression */,
896          false /* don't calculate a stack trace. */);
897    }
898  }
899}