diff --git a/api/src/main/java/com/google/common/eventbus/EventBus.java b/api/src/main/java/com/google/common/eventbus/EventBus.java
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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.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.
+ *
+ *
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:
+ * - Expose a public method, known as the event handler, which accepts
+ * a single argument of the type of event desired;
+ * - Mark it with a {@link Subscribe} annotation;
+ * - Pass itself to an EventBus instance's {@link #register(Object)} method.
+ *
+ *
+ *
+ * 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 handler 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 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}.)
+ *
+ *
Handler Methods
+ * Event handler methods must accept only one argument: the event.
+ *
+ * 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.
+ *
+ *
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.
+ *
+ *
Dead Events
+ * 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.
+ *
+ * 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.
+ *
+ *
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 handlers, 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 #handlersByTypeLock}.
+ */
+ private final SetMultimap, EventHandler> handlersByType =
+ HashMultimap.create();
+ private final ReadWriteLock handlersByTypeLock = new ReentrantReadWriteLock();
+
+ /**
+ * 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> eventsToDispatch =
+ new ThreadLocal>() {
+ @Override protected Queue initialValue() {
+ return new LinkedList();
+ }
+ };
+
+ /**
+ * 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() + "." + checkNotNull(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) {
+ Multimap, EventHandler> methodsInListener =
+ finder.findAllHandlers(object);
+ handlersByTypeLock.writeLock().lock();
+ try {
+ handlersByType.putAll(methodsInListener);
+ } finally {
+ handlersByTypeLock.writeLock().unlock();
+ }
+ }
+
+ /**
+ * 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, EventHandler> methodsInListener = finder.findAllHandlers(object);
+ for (Entry, Collection> entry : methodsInListener.asMap().entrySet()) {
+ Class> eventType = entry.getKey();
+ Collection eventMethodsInListener = entry.getValue();
+
+ handlersByTypeLock.writeLock().lock();
+ try {
+ Set currentHandlers = handlersByType.get(eventType);
+ if (!currentHandlers.containsAll(eventMethodsInListener)) {
+ throw new IllegalArgumentException(
+ "missing event handler for an annotated method. Is " + object + " registered?");
+ }
+ currentHandlers.removeAll(eventMethodsInListener);
+ } finally {
+ handlersByTypeLock.writeLock().unlock();
+ }
+ }
+ }
+
+ /**
+ * 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.
+ *
+ * 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.
+ */
+ public void post(Object event) {
+ Set> dispatchTypes = flattenHierarchy(event.getClass());
+
+ boolean dispatched = false;
+ for (Class> eventType : dispatchTypes) {
+ handlersByTypeLock.readLock().lock();
+ try {
+ Set wrappers = handlersByType.get(eventType);
+
+ if (!wrappers.isEmpty()) {
+ dispatched = true;
+ for (EventHandler wrapper : wrappers) {
+ enqueueEvent(event, wrapper);
+ }
+ }
+ } finally {
+ handlersByTypeLock.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, 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() {
+ try {
+ Queue events = eventsToDispatch.get();
+ EventWithHandler eventWithHandler;
+ while ((eventWithHandler = events.poll()) != null) {
+ dispatch(eventWithHandler.event, eventWithHandler.handler);
+ }
+ } finally {
+ eventsToDispatch.remove();
+ }
+ }
+
+ /**
+ * 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);
+ }
+ }
+
+ /**
+ * 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 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 = checkNotNull(event);
+ this.handler = checkNotNull(handler);
+ }
+ }
+}