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/*
 * Copyright (C) 2007 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.datastax.driver.$internal.com.google.common.eventbus;

import static com.datastax.driver.$internal.com.google.common.base.Preconditions.checkNotNull;

import com.datastax.driver.$internal.com.google.common.annotations.Beta;
import com.datastax.driver.$internal.com.google.common.annotations.VisibleForTesting;
import com.datastax.driver.$internal.com.google.common.base.Throwables;
import com.datastax.driver.$internal.com.google.common.cache.CacheBuilder;
import com.datastax.driver.$internal.com.google.common.cache.CacheLoader;
import com.datastax.driver.$internal.com.google.common.cache.LoadingCache;
import com.datastax.driver.$internal.com.google.common.collect.HashMultimap;
import com.datastax.driver.$internal.com.google.common.collect.Multimap;
import com.datastax.driver.$internal.com.google.common.collect.SetMultimap;
import com.datastax.driver.$internal.com.google.common.reflect.TypeToken;
import com.datastax.driver.$internal.com.google.common.util.concurrent.UncheckedExecutionException;

import java.lang.reflect.InvocationTargetException;
import java.util.Collection;
import java.util.LinkedList;
import java.util.Map.Entry;
import java.util.Queue;
import java.util.Set;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;
import java.util.logging.Level;
import java.util.logging.Logger;

/**
 * Dispatches events to listeners, and provides ways for listeners to register
 * themselves.
 *
 * 

The EventBus allows publish-subscribe-style communication between * components without requiring the components to explicitly register with one * another (and thus be aware of each other). It is designed exclusively to * replace traditional Java in-process event distribution using explicit * registration. It is not a general-purpose publish-subscribe system, * nor is it intended for interprocess communication. * *

Receiving Events

*

To receive events, an object should: *

    *
  1. Expose a public method, known as the event subscriber, which accepts * a single argument of the type of event desired;
  2. *
  3. Mark it with a {@link Subscribe} annotation;
  4. *
  5. Pass itself to an EventBus instance's {@link #register(Object)} method. *
  6. *
* *

Posting Events

*

To post an event, simply provide the event object to the * {@link #post(Object)} method. The EventBus instance will determine the type * of event and route it to all registered listeners. * *

Events are routed based on their type — an event will be delivered * to any subscriber for any type to which the event is assignable. This * includes implemented interfaces, all superclasses, and all interfaces * implemented by superclasses. * *

When {@code post} is called, all registered subscribers for an event are run * in sequence, so subscribers should be reasonably quick. If an event may trigger * an extended process (such as a database load), spawn a thread or queue it for * later. (For a convenient way to do this, use an {@link AsyncEventBus}.) * *

Subscriber Methods

*

Event subscriber methods must accept only one argument: the event. * *

Subscribers should not, in general, throw. If they do, the EventBus will * catch and log the exception. This is rarely the right solution for error * handling and should not be relied upon; it is intended solely to help find * problems during development. * *

The EventBus guarantees that it will not call a subscriber method from * multiple threads simultaneously, unless the method explicitly allows it by * bearing the {@link AllowConcurrentEvents} annotation. If this annotation is * not present, subscriber methods need not worry about being reentrant, unless * also called from outside the EventBus. * *

Dead Events

*

If an event is posted, but no registered subscribers can accept it, it is * considered "dead." To give the system a second chance to handle dead events, * they are wrapped in an instance of {@link DeadEvent} and reposted. * *

If a subscriber for a supertype of all events (such as Object) is registered, * no event will ever be considered dead, and no DeadEvents will be generated. * Accordingly, while DeadEvent extends {@link Object}, a subscriber registered to * receive any Object will never receive a DeadEvent. * *

This class is safe for concurrent use. * *

See the Guava User Guide article on * {@code EventBus}. * * @author Cliff Biffle * @since 10.0 */ @Beta public class EventBus { /** * A thread-safe cache for flattenHierarchy(). The Class class is immutable. This cache is shared * across all EventBus instances, which greatly improves performance if multiple such instances * are created and objects of the same class are posted on all of them. */ private static final LoadingCache, Set>> flattenHierarchyCache = CacheBuilder.newBuilder() .weakKeys() .build(new CacheLoader, Set>>() { @SuppressWarnings({"unchecked", "rawtypes"}) // safe cast @Override public Set> load(Class concreteClass) { return (Set) TypeToken.of(concreteClass).getTypes().rawTypes(); } }); /** * All registered event subscribers, indexed by event type. * *

This SetMultimap is NOT safe for concurrent use; all access should be * made after acquiring a read or write lock via {@link #subscribersByTypeLock}. */ private final SetMultimap, EventSubscriber> subscribersByType = HashMultimap.create(); private final ReadWriteLock subscribersByTypeLock = new ReentrantReadWriteLock(); /** * Strategy for finding subscriber methods in registered objects. Currently, * only the {@link AnnotatedSubscriberFinder} is supported, but this is * encapsulated for future expansion. */ private final SubscriberFindingStrategy finder = new AnnotatedSubscriberFinder(); /** queues of events for the current thread to dispatch */ private final ThreadLocal> eventsToDispatch = new ThreadLocal>() { @Override protected Queue initialValue() { return new LinkedList(); } }; /** true if the current thread is currently dispatching an event */ private final ThreadLocal isDispatching = new ThreadLocal() { @Override protected Boolean initialValue() { return false; } }; private SubscriberExceptionHandler subscriberExceptionHandler; /** * Creates a new EventBus named "default". */ public EventBus() { this("default"); } /** * Creates a new EventBus with the given {@code identifier}. * * @param identifier a brief name for this bus, for logging purposes. Should * be a valid Java identifier. */ public EventBus(String identifier) { this(new LoggingSubscriberExceptionHandler(identifier)); } /** * Creates a new EventBus with the given {@link SubscriberExceptionHandler}. * * @param subscriberExceptionHandler Handler for subscriber exceptions. * @since 16.0 */ public EventBus(SubscriberExceptionHandler subscriberExceptionHandler) { this.subscriberExceptionHandler = checkNotNull(subscriberExceptionHandler); } /** * Registers all subscriber methods on {@code object} to receive events. * Subscriber methods are selected and classified using this EventBus's * {@link SubscriberFindingStrategy}; the default strategy is the * {@link AnnotatedSubscriberFinder}. * * @param object object whose subscriber methods should be registered. */ public void register(Object object) { Multimap, EventSubscriber> methodsInListener = finder.findAllSubscribers(object); subscribersByTypeLock.writeLock().lock(); try { subscribersByType.putAll(methodsInListener); } finally { subscribersByTypeLock.writeLock().unlock(); } } /** * Unregisters all subscriber methods on a registered {@code object}. * * @param object object whose subscriber methods should be unregistered. * @throws IllegalArgumentException if the object was not previously registered. */ public void unregister(Object object) { Multimap, EventSubscriber> methodsInListener = finder.findAllSubscribers(object); for (Entry, Collection> entry : methodsInListener.asMap().entrySet()) { Class eventType = entry.getKey(); Collection eventMethodsInListener = entry.getValue(); subscribersByTypeLock.writeLock().lock(); try { Set currentSubscribers = subscribersByType.get(eventType); if (!currentSubscribers.containsAll(eventMethodsInListener)) { throw new IllegalArgumentException( "missing event subscriber for an annotated method. Is " + object + " registered?"); } currentSubscribers.removeAll(eventMethodsInListener); } finally { subscribersByTypeLock.writeLock().unlock(); } } } /** * Posts an event to all registered subscribers. This method will return * successfully after the event has been posted to all subscribers, and * regardless of any exceptions thrown by subscribers. * *

If no subscribers have been subscribed for {@code event}'s class, and * {@code event} is not already a {@link DeadEvent}, it will be wrapped in a * DeadEvent and reposted. * * @param event event to post. */ public void post(Object event) { Set> dispatchTypes = flattenHierarchy(event.getClass()); boolean dispatched = false; for (Class eventType : dispatchTypes) { subscribersByTypeLock.readLock().lock(); try { Set wrappers = subscribersByType.get(eventType); if (!wrappers.isEmpty()) { dispatched = true; for (EventSubscriber wrapper : wrappers) { enqueueEvent(event, wrapper); } } } finally { subscribersByTypeLock.readLock().unlock(); } } if (!dispatched && !(event instanceof DeadEvent)) { post(new DeadEvent(this, event)); } dispatchQueuedEvents(); } /** * Queue the {@code event} for dispatch during * {@link #dispatchQueuedEvents()}. Events are queued in-order of occurrence * so they can be dispatched in the same order. */ void enqueueEvent(Object event, EventSubscriber subscriber) { eventsToDispatch.get().offer(new EventWithSubscriber(event, subscriber)); } /** * Drain the queue of events to be dispatched. As the queue is being drained, * new events may be posted to the end of the queue. */ void dispatchQueuedEvents() { // don't dispatch if we're already dispatching, that would allow reentrancy // and out-of-order events. Instead, leave the events to be dispatched // after the in-progress dispatch is complete. if (isDispatching.get()) { return; } isDispatching.set(true); try { Queue events = eventsToDispatch.get(); EventWithSubscriber eventWithSubscriber; while ((eventWithSubscriber = events.poll()) != null) { dispatch(eventWithSubscriber.event, eventWithSubscriber.subscriber); } } finally { isDispatching.remove(); eventsToDispatch.remove(); } } /** * Dispatches {@code event} to the subscriber in {@code wrapper}. This method * is an appropriate override point for subclasses that wish to make * event delivery asynchronous. * * @param event event to dispatch. * @param wrapper wrapper that will call the subscriber. */ void dispatch(Object event, EventSubscriber wrapper) { try { wrapper.handleEvent(event); } catch (InvocationTargetException e) { try { subscriberExceptionHandler.handleException( e.getCause(), new SubscriberExceptionContext( this, event, wrapper.getSubscriber(), wrapper.getMethod())); } catch (Throwable t) { // If the exception handler throws, log it. There isn't much else to do! Logger.getLogger(EventBus.class.getName()).log(Level.SEVERE, String.format( "Exception %s thrown while handling exception: %s", t, e.getCause()), t); } } } /** * Flattens a class's type hierarchy into a set of Class objects. The set * will include all superclasses (transitively), and all interfaces * implemented by these superclasses. * * @param concreteClass class whose type hierarchy will be retrieved. * @return {@code clazz}'s complete type hierarchy, flattened and uniqued. */ @VisibleForTesting Set> flattenHierarchy(Class concreteClass) { try { return flattenHierarchyCache.getUnchecked(concreteClass); } catch (UncheckedExecutionException e) { throw Throwables.propagate(e.getCause()); } } /** * Simple logging handler for subscriber exceptions. */ private static final class LoggingSubscriberExceptionHandler implements SubscriberExceptionHandler { /** * Logger for event dispatch failures. Named by the fully-qualified name of * this class, followed by the identifier provided at construction. */ private final Logger logger; /** * @param identifier a brief name for this bus, for logging purposes. Should * be a valid Java identifier. */ public LoggingSubscriberExceptionHandler(String identifier) { logger = Logger.getLogger( EventBus.class.getName() + "." + checkNotNull(identifier)); } @Override public void handleException(Throwable exception, SubscriberExceptionContext context) { logger.log(Level.SEVERE, "Could not dispatch event: " + context.getSubscriber() + " to " + context.getSubscriberMethod(), exception.getCause()); } } /** simple struct representing an event and it's subscriber */ static class EventWithSubscriber { final Object event; final EventSubscriber subscriber; public EventWithSubscriber(Object event, EventSubscriber subscriber) { this.event = checkNotNull(event); this.subscriber = checkNotNull(subscriber); } } }





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