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) 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.Internal.toNanosSaturated;
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.GwtIncompatible;
import com.google.common.base.Function;
import com.google.common.base.MoreObjects;
import com.google.common.base.Stopwatch;
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.ImmutableSet;
import com.google.common.collect.ImmutableSetMultimap;
import com.google.common.collect.Lists;
import com.google.common.collect.Maps;
import com.google.common.collect.MultimapBuilder;
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.util.concurrent.Service.State;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import com.google.errorprone.annotations.concurrent.GuardedBy;
import com.google.j2objc.annotations.WeakOuter;
import java.lang.ref.WeakReference;
import java.time.Duration;
import java.util.Collections;
import java.util.EnumSet;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
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;


A manager for monitoring and controlling a set of services. This class provides methods for starting, stopping and inspecting a collection of services. Additionally, users can monitor state transitions with the listener mechanism. 
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 startAsync, the listeners will be invoked when appropriate and awaitHealthy will still work as expected. 
Here is a simple example of how to use a ServiceManager to start a server. 

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
  }
 }



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
/**
 * 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
 */
@GwtIncompatible
public final class ServiceManager implements ServiceManagerBridge {
  private static final Logger logger = Logger.getLogger(ServiceManager.class.getName());
  private static final ListenerCallQueue.Event<Listener> HEALTHY_EVENT =
      new ListenerCallQueue.Event<Listener>() {
        @Override
        public void call(Listener listener) {
          listener.healthy();
        }

        @Override
        public String toString() {
          return "healthy()";
        }
      };
  private static final ListenerCallQueue.Event<Listener> STOPPED_EVENT =
      new ListenerCallQueue.Event<Listener>() {
        @Override
        public void call(Listener listener) {
          listener.stopped();
        }

        @Override
        public String toString() {
          return "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 service starts, or terminates), should attach service
listeners to each individual service. 
Author:Luke Sandberg
Since:15.0 (present as an interface in 14.0)
/**
   * 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)
   */
  public abstract static class Listener {
    
Called when the service initially becomes healthy.

This will be called at most once after all the services have entered the running state. If any services fail during start up or fail/terminate before all other services have started running then this method will not be called. 
/**
     * 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 terminated or failed. 
/**
     * 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 failed. 
Params:
  • service – The service that failed.
/**
     * 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 
service listeners. This is extracted into its own object so that ServiceListener could be made static and its instances can be safely constructed and added in the ServiceManager constructor without having to close over the partially constructed ServiceManager instance (i.e. avoid leaking a pointer to this). 
/**
   * 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.

Params:
  • services – The services to manage
Throws:
/**
   * 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<>(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 Listener to be 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 
started. 
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. 
RuntimeExceptions thrown by a listener will be caught and logged. Any exception thrown during Executor.execute (e.g., a RejectedExecutionException) will be caught and logged. 
When selecting an executor, note that directExecutor is dangerous in some cases. See the discussion in the ListenableFuture.addListener documentation. 
Params:
  • listener – the listener to run when the manager changes state
  • executor – the executor in which the listeners callback methods will be run.
/**
   * 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>When selecting an executor, note that {@code directExecutor} is dangerous in some cases. See
   * the discussion in the {@link ListenableFuture#addListener ListenableFuture.addListener}
   * documentation.
   *
   * @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);
  }

  
Initiates service startup on all the services being managed. It is only valid to call this method if all of the services are new. 
Throws:
Returns:this
/**
   * 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.
   */
  @CanIgnoreReturnValue
  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 ServiceManager to become healthy. The manager will become healthy after all the component services have reached the 
running state. 
Throws:
/**
   * 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 ServiceManager to become healthy for no more than the given time. The manager will become healthy after all the component services have reached the running state. 
Params:
  • timeout – the maximum time to wait
Throws:
Since:28.0
/**
   * 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
   * @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}.
   * @since 28.0
   */
  public void awaitHealthy(Duration timeout) throws TimeoutException {
    awaitHealthy(toNanosSaturated(timeout), TimeUnit.NANOSECONDS);
  }

  
Waits for the ServiceManager to become healthy for no more than the given time. The manager will become healthy after all the component services have reached the running state. 
Params:
  • timeout – the maximum time to wait
  • unit – the time unit of the timeout argument
Throws:
/**
   * 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}.
   */
  @SuppressWarnings("GoodTime") // should accept a java.time.Duration
  public void awaitHealthy(long timeout, TimeUnit unit) throws TimeoutException {
    state.awaitHealthy(timeout, unit);
  }

  
Initiates service shutdown if necessary on all the services being managed. 
Returns:this
/**
   * Initiates service {@linkplain Service#stopAsync shutdown} if necessary on all the services
   * being managed.
   *
   * @return this
   */
  @CanIgnoreReturnValue
  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 terminated or failed. 
/**
   * 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 
terminated or failed. 
Params:
  • timeout – the maximum time to wait
Throws:
  • TimeoutException – if not all of the services have stopped within the deadline
Since:28.0
/**
   * 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
   * @throws TimeoutException if not all of the services have stopped within the deadline
   * @since 28.0
   */
  public void awaitStopped(Duration timeout) throws TimeoutException {
    awaitStopped(toNanosSaturated(timeout), TimeUnit.NANOSECONDS);
  }

  
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 
terminated or failed. 
Params:
  • timeout – the maximum time to wait
  • unit – the time unit of the timeout argument
Throws:
  • TimeoutException – if not all of the services have stopped within the deadline
/**
   * 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
   */
  @SuppressWarnings("GoodTime") // should accept a java.time.Duration
  public void awaitStopped(long timeout, TimeUnit unit) throws TimeoutException {
    state.awaitStopped(timeout, unit);
  }

  
Returns true if all services are currently in the running state. 
Users who want more detailed information should use the servicesByState method to get detailed information about which services are not running. 
/**
   * 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.

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.

Since:29.0 (present with return type ImmutableMultimap since 14.0)
/**
   * 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.
   *
   * @since 29.0 (present with return type {@code ImmutableMultimap} since 14.0)
   */
  @Override
  public ImmutableSetMultimap<State, Service> servicesByState() {
    return state.servicesByState();
  }

  
Returns the service load times. This value will only return startup times for services that
have finished starting.

Returns:Map of services and their corresponding startup time in millis, the map entries will be
    ordered by startup time.
/**
   * 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 ServiceManager that needs to be accessed by instances of ServiceListener. 
/**
   * 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 =
        MultimapBuilder.enumKeys(State.class).linkedHashSetValues().build();

    @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.

ready: is set by markReady to indicate that all listeners have been correctly installed 
transitioned: is set by transitionService to indicate that some transition has been performed. 
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.

/**
     * These two booleans are used to mark the state as ready to start.
     *
     * <p>{@link #ready}: is set by {@link #markReady} to indicate that all listeners have been
     * correctly installed
     *
     * <p>{@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.

/**
     * 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 AwaitHealthGuard();

    @WeakOuter
    final class AwaitHealthGuard extends Monitor.Guard {
      AwaitHealthGuard() {
        super(ServiceManagerState.this.monitor);
      }

      @Override
      @GuardedBy("ServiceManagerState.this.monitor")
      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. 
/** Controls how long to wait for all services to reach a terminal state. */
    final Monitor.Guard stoppedGuard = new StoppedGuard();

    @WeakOuter
    final class StoppedGuard extends Monitor.Guard {
      StoppedGuard() {
        super(ServiceManagerState.this.monitor);
      }

      @Override
      @GuardedBy("ServiceManagerState.this.monitor")
      public boolean isSatisfied() {
        return states.count(TERMINATED) + states.count(FAILED) == numberOfServices;
      }
    }

    
The listeners to notify during a state transition. 
/** The listeners to notify during a state transition. */
    final ListenerCallQueue<Listener> listeners = new ListenerCallQueue<>();

    
It is implicitly assumed that all the services are NEW and that they will all remain NEW until all the Listeners are installed and markReady() is called. It is our caller's responsibility to only call markReady() if all services were new at the time this method was called and when all the listeners were installed. 
/**
     * 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 Service.startAsync(). 
/**
     * 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 State as ready to receive transitions. Returns true if no transitions have been observed yet. 
/**
     * 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) {
      listeners.addListener(listener, executor);
    }

    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(EnumSet.of(TERMINATED, FAILED)))));
        }
      } finally {
        monitor.leave();
      }
    }

    ImmutableSetMultimap<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);
          }
        }
      } 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.natural()
              .onResultOf(
                  new Function<Entry<Service, Long>, Long>() {
                    @Override
                    public Long apply(Entry<Service, Long> input) {
                      return input.getValue();
                    }
                  }));
      return ImmutableMap.copyOf(loadTimes);
    }

    
Updates the state with the given service transition.

This method performs the main logic of ServiceManager in the following steps.

    
  
  1. Update the servicesByState() 
  2. Update the startupTimers 
  3. Based on the new state queue listeners to run
  
  4. Run the listeners (outside of the lock)



/**
     * 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) {
          enqueueFailedEvent(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.
          enqueueHealthyEvent();
        } else if (states.count(TERMINATED) + states.count(FAILED) == numberOfServices) {
          enqueueStoppedEvent();
        }
      } finally {
        monitor.leave();
        // Run our executors outside of the lock
        dispatchListenerEvents();
      }
    }

    void enqueueStoppedEvent() {
      listeners.enqueue(STOPPED_EVENT);
    }

    void enqueueHealthyEvent() {
      listeners.enqueue(HEALTHY_EVENT);
    }

    void enqueueFailedEvent(final Service service) {
      listeners.enqueue(
          new ListenerCallQueue.Event<Listener>() {
            @Override
            public void call(Listener listener) {
              listener.failure(service);
            }

            @Override
            public String toString() {
              return "failed({service=" + service + "})";
            }
          });
    }

    
Attempts to execute all the listeners in listeners. 
/** Attempts to execute all the listeners in {@link #listeners}. */
    void dispatchListenerEvents() {
      checkState(
          !monitor.isOccupiedByCurrentThread(),
          "It is incorrect to execute listeners with the monitor held.");
      listeners.dispatch();
    }

    @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))));
        for (Service service : servicesByState.get(State.FAILED)) {
          exception.addSuppressed(new FailedService(service));
        }
        throw exception;
      }
    }
  }

  
A Service that wraps another service and times how long it takes for it to start and also calls the ServiceManagerState.transitionService(Service, State, State), to record the state transitions. 
/**
   * 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.
        boolean log = !(service instanceof NoOpService);
        /*
         * We have already exposed startup exceptions to the user in the form of suppressed
         * exceptions. We don't need to log those exceptions again.
         */
        log &= from != State.STARTING;
        if (log) {
          logger.log(
              Level.SEVERE,
              "Service " + service + " has failed in the " + from + " state.",
              failure);
        }
        state.transitionService(service, from, FAILED);
      }
    }
  }

  
A Service instance that does nothing. This is only useful as a placeholder to ensure that the ServiceManager functions properly even when it is managing no services. 
The use of this class is considered an implementation detail of ServiceManager and as such it is excluded from servicesByState, startupTimes, AbstractService.toString and all logging statements. 
/**
   * 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. 
/** This is never thrown but only used for logging. */
  private static final class EmptyServiceManagerWarning extends Throwable {}

  private static final class FailedService extends Throwable {
    FailedService(Service service) {
      super(
          service.toString(),
          service.failureCause(),
          false /* don't enable suppression */,
          false /* don't calculate a stack trace. */);
    }
  }
}