/*
* 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.google.common.eventbus;
import static com.google.common.base.Preconditions.checkNotNull;
import com.google.common.annotations.Beta;
import com.google.common.annotations.VisibleForTesting;
import com.google.common.base.Throwables;
import com.google.common.cache.CacheBuilder;
import com.google.common.cache.CacheLoader;
import com.google.common.cache.LoadingCache;
import com.google.common.collect.HashMultimap;
import com.google.common.collect.Multimap;
import com.google.common.collect.SetMultimap;
import com.google.common.reflect.TypeToken;
import 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.
*
* <p>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 <em>not</em> a general-purpose publish-subscribe system,
* nor is it intended for interprocess communication.
*
* <h2>Receiving Events</h2>
* <p>To receive events, an object should:
* <ol>
* <li>Expose a public method, known as the <i>event subscriber</i>, which accepts
* a single argument of the type of event desired;</li>
* <li>Mark it with a {@link Subscribe} annotation;</li>
* <li>Pass itself to an EventBus instance's {@link #register(Object)} method.
* </li>
* </ol>
*
* <h2>Posting Events</h2>
* <p>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.
*
* <p>Events are routed based on their type — an event will be delivered
* to any subscriber for any type to which the event is <em>assignable.</em> This
* includes implemented interfaces, all superclasses, and all interfaces
* implemented by superclasses.
*
* <p>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}.)
*
* <h2>Subscriber Methods</h2>
* <p>Event subscriber methods must accept only one argument: the event.
*
* <p>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.
*
* <p>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.
*
* <h2>Dead Events</h2>
* <p>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.
*
* <p>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.
*
* <p>This class is safe for concurrent use.
*
* <p>See the Guava User Guide article on <a href=
* "http://code.google.com/p/guava-libraries/wiki/EventBusExplained">
* {@code EventBus}</a>.
*
* @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<Class<?>, Set<Class<?>>> flattenHierarchyCache =
CacheBuilder.newBuilder()
.weakKeys()
.build(new CacheLoader<Class<?>, Set<Class<?>>>() {
@SuppressWarnings({"unchecked", "rawtypes"}) // safe cast
@Override
public Set<Class<?>> load(Class<?> concreteClass) {
return (Set) TypeToken.of(concreteClass).getTypes().rawTypes();
}
});
/**
* All registered event subscribers, indexed by event type.
*
* <p>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<Class<?>, 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<Queue<EventWithSubscriber>> eventsToDispatch =
new ThreadLocal<Queue<EventWithSubscriber>>() {
@Override protected Queue<EventWithSubscriber> initialValue() {
return new LinkedList<EventWithSubscriber>();
}
};
/** true if the current thread is currently dispatching an event */
private final ThreadLocal<Boolean> isDispatching =
new ThreadLocal<Boolean>() {
@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<Class<?>, 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<Class<?>, EventSubscriber> methodsInListener = finder.findAllSubscribers(object);
for (Entry<Class<?>, Collection<EventSubscriber>> entry :
methodsInListener.asMap().entrySet()) {
Class<?> eventType = entry.getKey();
Collection<EventSubscriber> eventMethodsInListener = entry.getValue();
subscribersByTypeLock.writeLock().lock();
try {
Set<EventSubscriber> 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.
*
* <p>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<Class<?>> dispatchTypes = flattenHierarchy(event.getClass());
boolean dispatched = false;
for (Class<?> eventType : dispatchTypes) {
subscribersByTypeLock.readLock().lock();
try {
Set<EventSubscriber> 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<EventWithSubscriber> 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<Class<?>> 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);
}
}
}