/*
* 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 com.google.common.annotations.Beta;
import com.google.common.annotations.VisibleForTesting;
import com.google.common.base.Supplier;
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.Multimap;
import com.google.common.collect.Multimaps;
import com.google.common.collect.SetMultimap;
import com.google.common.reflect.TypeToken;
import java.lang.reflect.InvocationTargetException;
import java.util.Collection;
import java.util.Map.Entry;
import java.util.Set;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentLinkedQueue;
import java.util.concurrent.CopyOnWriteArraySet;
import java.util.concurrent.ExecutionException;
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>
* To receive events, an object should:<ol>
* <li>Expose a public method, known as the <i>event handler</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>
* 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 handler 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 handlers for an event are run
* in sequence, so handlers 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>Handler Methods</h2>
* Event handler methods must accept only one argument: the event.
*
* <p>Handlers 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 handler method from
* multiple threads simultaneously, unless the method explicitly allows it by
* bearing the {@link AllowConcurrentEvents} annotation. If this annotation is
* not present, handler methods need not worry about being reentrant, unless
* also called from outside the EventBus.
*
* <h2>Dead Events</h2>
* If an event is posted, but no registered handlers 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 handler 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 handler 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 {
/**
* All registered event handlers, indexed by event type.
*/
private final SetMultimap<Class<?>, EventHandler> handlersByType =
Multimaps.newSetMultimap(new ConcurrentHashMap<Class<?>, Collection<EventHandler>>(),
new Supplier<Set<EventHandler>>() {
@Override
public Set<EventHandler> get() {
return newHandlerSet();
}
});
/**
* 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;
/**
* Strategy for finding handler methods in registered objects. Currently,
* only the {@link AnnotatedHandlerFinder} is supported, but this is
* encapsulated for future expansion.
*/
private final HandlerFindingStrategy finder = new AnnotatedHandlerFinder();
/** queues of events for the current thread to dispatch */
private final ThreadLocal<ConcurrentLinkedQueue<EventWithHandler>>
eventsToDispatch =
new ThreadLocal<ConcurrentLinkedQueue<EventWithHandler>>() {
@Override protected ConcurrentLinkedQueue<EventWithHandler> initialValue() {
return new ConcurrentLinkedQueue<EventWithHandler>();
}
};
/** true if the current thread is currently dispatching an event */
private final ThreadLocal<Boolean> isDispatching =
new ThreadLocal<Boolean>() {
@Override protected Boolean initialValue() {
return false;
}
};
/**
* A thread-safe cache for flattenHierarchy(). The Class class is immutable.
*/
private 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) throws Exception {
return (Set) TypeToken.of(concreteClass).getTypes().rawTypes();
}
});
/**
* 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) {
logger = Logger.getLogger(EventBus.class.getName() + "." + identifier);
}
/**
* Registers all handler methods on {@code object} to receive events.
* Handler methods are selected and classified using this EventBus's
* {@link HandlerFindingStrategy}; the default strategy is the
* {@link AnnotatedHandlerFinder}.
*
* @param object object whose handler methods should be registered.
*/
public void register(Object object) {
handlersByType.putAll(finder.findAllHandlers(object));
}
/**
* Unregisters all handler methods on a registered {@code object}.
*
* @param object object whose handler methods should be unregistered.
* @throws IllegalArgumentException if the object was not previously registered.
*/
public void unregister(Object object) {
Multimap<Class<?>, EventHandler> methodsInListener = finder.findAllHandlers(object);
for (Entry<Class<?>, Collection<EventHandler>> entry : methodsInListener.asMap().entrySet()) {
Set<EventHandler> currentHandlers = getHandlersForEventType(entry.getKey());
Collection<EventHandler> eventMethodsInListener = entry.getValue();
if (currentHandlers == null || !currentHandlers.containsAll(entry.getValue())) {
throw new IllegalArgumentException(
"missing event handler for an annotated method. Is " + object + " registered?");
}
currentHandlers.removeAll(eventMethodsInListener);
}
}
/**
* Posts an event to all registered handlers. This method will return
* successfully after the event has been posted to all handlers, and
* regardless of any exceptions thrown by handlers.
*
* <p>If no handlers 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.
*/
@SuppressWarnings("deprecation") // only deprecated for external subclasses
public void post(Object event) {
Set<Class<?>> dispatchTypes = flattenHierarchy(event.getClass());
boolean dispatched = false;
for (Class<?> eventType : dispatchTypes) {
Set<EventHandler> wrappers = getHandlersForEventType(eventType);
if (wrappers != null && !wrappers.isEmpty()) {
dispatched = true;
for (EventHandler wrapper : wrappers) {
enqueueEvent(event, wrapper);
}
}
}
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, EventHandler handler) {
eventsToDispatch.get().offer(new EventWithHandler(event, handler));
}
/**
* 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 {
while (true) {
EventWithHandler eventWithHandler = eventsToDispatch.get().poll();
if (eventWithHandler == null) {
break;
}
dispatch(eventWithHandler.event, eventWithHandler.handler);
}
} finally {
isDispatching.set(false);
}
}
/**
* Dispatches {@code event} to the handler 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 handler.
*/
void dispatch(Object event, EventHandler wrapper) {
try {
wrapper.handleEvent(event);
} catch (InvocationTargetException e) {
logger.log(Level.SEVERE,
"Could not dispatch event: " + event + " to handler " + wrapper, e);
}
}
/**
* Retrieves a mutable set of the currently registered handlers for
* {@code type}. If no handlers are currently registered for {@code type},
* this method may either return {@code null} or an empty set.
*
* @param type type of handlers to retrieve.
* @return currently registered handlers, or {@code null}.
*/
Set<EventHandler> getHandlersForEventType(Class<?> type) {
return handlersByType.get(type);
}
/**
* Creates a new Set for insertion into the handler map. This is provided
* as an override point for subclasses. The returned set should support
* concurrent access.
*
* @return a new, mutable set for handlers.
*/
Set<EventHandler> newHandlerSet() {
return new CopyOnWriteArraySet<EventHandler>();
}
/**
* 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.get(concreteClass);
} catch (ExecutionException e) {
throw Throwables.propagate(e.getCause());
}
}
/** simple struct representing an event and it's handler */
static class EventWithHandler {
final Object event;
final EventHandler handler;
public EventWithHandler(Object event, EventHandler handler) {
this.event = event;
this.handler = handler;
}
}
}