/*
* 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.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.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.ListenerCallQueue.Callback;
import com.google.common.util.concurrent.Service.State;
import java.lang.ref.WeakReference;
import java.util.ArrayList;
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;
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 =
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.
* {@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(EnumSet.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 {}
}