Java tutorial
/* * 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 com.google.j2objc.annotations.WeakOuter; 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 AwaitHealthGuard(); @WeakOuter final class AwaitHealthGuard extends Monitor.Guard { AwaitHealthGuard() { super(ServiceManagerState.this.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 StoppedGuard(); @WeakOuter final class StoppedGuard extends Monitor.Guard { StoppedGuard() { super(ServiceManagerState.this.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); } } } 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(Map.Entry<Service, Long> input) { return input.getValue(); } })); return ImmutableMap.copyOf(loadTimes); } /** * 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 { } }