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/*
* 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.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.Objects;
import com.google.common.base.Stopwatch;
import com.google.common.base.Supplier;
import com.google.common.collect.Collections2;
import com.google.common.collect.ImmutableCollection;
import com.google.common.collect.ImmutableList;
import com.google.common.collect.ImmutableMap;
import com.google.common.collect.ImmutableMultimap;
import com.google.common.collect.ImmutableSet;
import com.google.common.collect.ImmutableSetMultimap;
import com.google.common.collect.Lists;
import com.google.common.collect.Maps;
import com.google.common.collect.Multimaps;
import com.google.common.collect.Multiset;
import com.google.common.collect.Ordering;
import com.google.common.collect.SetMultimap;
import com.google.common.collect.Sets;
import com.google.common.util.concurrent.Service.State;
import java.lang.ref.WeakReference;
import java.util.Collection;
import java.util.Collections;
import java.util.EnumMap;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Set;
import java.util.concurrent.Executor;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.annotation.concurrent.GuardedBy;
import javax.annotation.concurrent.Immutable;
/**
* A manager for monitoring and controlling a set of {@link 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 {@link Listener 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 {@link #startAsync}, the listeners will be invoked
* when appropriate and {@link #awaitHealthy} will still work as expected.
*
*
Here is a simple example of how to use a {@link ServiceManager} to start a server.
*
{@code
* class Server {
* public static void main(String[] args) {
* Set 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.sameThreadExecutor());
*
* 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
*/
@Beta
public final class ServiceManager {
private static final Logger logger = Logger.getLogger(ServiceManager.class.getName());
/**
* 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
* {@link Service service} starts, or terminates), should attach {@link 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.
*
*
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 services;
/**
* Constructs a new instance for managing the given services.
*
* @param services The services to manage
*
* @throws IllegalArgumentException if not all services are {@link State#NEW new} or if there are
* any duplicate services.
*/
public ServiceManager(Iterable services) {
ImmutableList 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.of(new NoOpService());
}
this.state = new ServiceManagerState(copy);
this.services = copy;
WeakReference stateReference =
new WeakReference(state);
for (Service service : copy) {
// We give each listener its own SynchronizedExecutor to ensure that the state transitions
// are run in the same order that they occur. The Service.Listener api guarantees us only
// that the transitions are submitted to the executor in the same order that they occur, so by
// synchronizing the executions of each listeners callbacks we can ensure that the entire
// execution of the listener occurs in the same order as the transitions themselves.
//
// This is necessary to prevent transitions being played back in the wrong order due to thread
// races to acquire the monitor in ServiceManagerState.
service.addListener(new ServiceListener(service, stateReference), new SynchronizedExecutor());
// 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#start started}.
*
* There is no guaranteed ordering of execution of listeners, but any listener added through
* this method is guaranteed to be called whenever there is a state change.
*
*
Exceptions thrown by a listener will be propagated up to the executor. Any exception thrown
* during {@code Executor.execute} (e.g., a {@code RejectedExecutionException} or an exception
* thrown by {@linkplain MoreExecutors#sameThreadExecutor inline execution}) will be caught and
* logged.
*
*
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#start started}.
*
*
There is no guaranteed ordering of execution of listeners, but any listener added through
* this method is guaranteed to be called whenever there is a state change.
*
*
Exceptions thrown by a listener will be will be caught and logged.
*
* @param listener the listener to run when the manager changes state
*/
public void addListener(Listener listener) {
state.addListener(listener, MoreExecutors.sameThreadExecutor());
}
/**
* Initiates service {@linkplain Service#start 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 {
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#stop 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 {@link Service.State#TERMINATED terminated} or
* {@link 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 {@link Service.State#TERMINATED terminated} or
* {@link 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.
*
*
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.
*/
public ImmutableMultimap 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 startupTimes() {
return state.startupTimes();
}
@Override public String toString() {
return Objects.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 servicesByState =
Multimaps.newSetMultimap(new EnumMap>(State.class),
new Supplier>() {
@Override public Set get() {
return Sets.newLinkedHashSet();
}
});
@GuardedBy("monitor")
final Multiset states = servicesByState.keys();
@GuardedBy("monitor")
final Map 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.
*
* 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 listeners = Lists.newArrayList();
/**
* The queue of listeners that are waiting to be executed.
*
* Enqueue operations should be protected by {@link #monitor} while dequeue operations are
* not protected. Holding {@link #monitor} while enqueuing ensures that listeners in the queue
* are in the correct order and {@link ExecutionQueue} ensures that they are executed in the
* correct order.
*/
@GuardedBy("monitor")
final ExecutionQueue queuedListeners = new ExecutionQueue();
/**
* 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 services) {
this.numberOfServices = services.size();
servicesByState.putAll(NEW, services);
for (Service service : services) {
startupTimers.put(service, Stopwatch.createUnstarted());
}
}
/**
* 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 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 ListenerExecutorPair(listener, executor));
}
} finally {
monitor.leave();
}
}
void awaitHealthy() {
monitor.enterWhenUninterruptibly(awaitHealthGuard);
try {
checkHealthy();
} finally {
monitor.leave();
}
}
void awaitHealthy(long timeout, TimeUnit unit) throws TimeoutException {
monitor.enter();
try {
if (!monitor.waitForUninterruptibly(awaitHealthGuard, timeout, unit)) {
throw new TimeoutException("Timeout waiting for the services to become healthy. The "
+ "following services have not started: "
+ Multimaps.filterKeys(servicesByState, in(ImmutableSet.of(NEW, STARTING))));
}
checkHealthy();
} finally {
monitor.leave();
}
}
void awaitStopped() {
monitor.enterWhenUninterruptibly(stoppedGuard);
monitor.leave();
}
void awaitStopped(long timeout, TimeUnit unit) throws TimeoutException {
monitor.enter();
try {
if (!monitor.waitForUninterruptibly(stoppedGuard, timeout, unit)) {
throw new TimeoutException("Timeout waiting for the services to stop. The following "
+ "services have not stopped: "
+ Multimaps.filterKeys(servicesByState,
not(in(ImmutableSet.of(TERMINATED, FAILED)))));
}
} finally {
monitor.leave();
}
}
ImmutableMultimap servicesByState() {
ImmutableSetMultimap.Builder builder = ImmutableSetMultimap.builder();
monitor.enter();
try {
for (Entry entry : servicesByState.entries()) {
if (!(entry.getValue() instanceof NoOpService)) {
builder.put(entry.getKey(), entry.getValue());
}
}
} finally {
monitor.leave();
}
return builder.build();
}
ImmutableMap startupTimes() {
List> loadTimes;
monitor.enter();
try {
loadTimes = Lists.newArrayListWithCapacity(
states.size() - states.count(NEW) + states.count(STARTING));
for (Entry entry : startupTimers.entrySet()) {
Service service = entry.getKey();
Stopwatch stopWatch = entry.getValue();
// N.B. we check the service state in the multimap rather than via Service.state() because
// the multimap is guaranteed to be in sync with our timers while the Service.state() is
// not. Due to happens-before ness of the monitor this 'weirdness' will not be observable
// by our caller.
if (!stopWatch.isRunning() && !servicesByState.containsEntry(NEW, service)
&& !(service instanceof NoOpService)) {
loadTimes.add(Maps.immutableEntry(service, stopWatch.elapsed(MILLISECONDS)));
}
}
} finally {
monitor.leave();
}
Collections.sort(loadTimes, Ordering.natural()
.onResultOf(new Function, Long>() {
@Override public Long apply(Map.Entry input) {
return input.getValue();
}
}));
ImmutableMap.Builder builder = ImmutableMap.builder();
for (Entry entry : loadTimes) {
builder.put(entry);
}
return builder.build();
}
/**
* Updates the state with the given service transition.
*
* This method performs the main logic of ServiceManager in the following steps.
*
* - Update the {@link #servicesByState()}
*
- Update the {@link #startupTimers}
*
- Based on the new state queue listeners to run
*
- Run the listeners (outside of the lock)
*
*/
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 (from == NEW) {
stopwatch.start();
}
if (to.compareTo(RUNNING) >= 0 && stopwatch.isRunning()) {
// N.B. if we miss the STARTING event then we will 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();
// no more listeners could possibly be called, so clear them out to save some memory.
listeners.clear();
}
} finally {
monitor.leave();
// Run our executors outside of the lock
executeListeners();
}
}
@GuardedBy("monitor")
void fireStoppedListeners() {
for (final ListenerExecutorPair pair : listeners) {
queuedListeners.add(new Runnable() {
@Override public void run() {
pair.listener.stopped();
}
}, pair.executor);
}
}
@GuardedBy("monitor")
void fireHealthyListeners() {
for (final ListenerExecutorPair pair : listeners) {
queuedListeners.add(new Runnable() {
@Override public void run() {
pair.listener.healthy();
}
}, pair.executor);
}
}
@GuardedBy("monitor")
void fireFailedListeners(final Service service) {
for (final ListenerExecutorPair pair : listeners) {
queuedListeners.add(new Runnable() {
@Override public void run() {
pair.listener.failure(service);
}
}, pair.executor);
}
}
/** Attempts to execute all the listeners in {@link #queuedListeners}. */
void executeListeners() {
checkState(!monitor.isOccupiedByCurrentThread(),
"It is incorrect to execute listeners with the monitor held.");
queuedListeners.execute();
}
@GuardedBy("monitor")
void checkHealthy() {
if (states.count(RUNNING) != numberOfServices) {
throw new IllegalStateException("Expected to be healthy after starting. "
+ "The following services are not running: " +
Multimaps.filterKeys(servicesByState, not(equalTo(RUNNING))));
}
}
}
/**
* 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 state;
ServiceListener(Service service, WeakReference 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);
}
}
}
/** Simple value object binding a listener to its executor. */
@Immutable private static final class ListenerExecutorPair {
final Listener listener;
final Executor executor;
ListenerExecutorPair(Listener listener, Executor executor) {
this.listener = listener;
this.executor = executor;
}
}
/**
* 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.
*
* 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 {}
/**
* A same-thread executor that executes all the runnables from within a synchronized block.
*
*
This ensures that tasks submitted to the executor run in the same order that they were
* submitted.
*/
private static final class SynchronizedExecutor implements Executor {
@Override public synchronized void execute(Runnable command) {
command.run();
}
}
}