001 /*
002 * Copyright (C) 2007 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License");
005 * you may not use this file except in compliance with the License.
006 * You may obtain a copy of the License at
007 *
008 * http://www.apache.org/licenses/LICENSE-2.0
009 *
010 * Unless required by applicable law or agreed to in writing, software
011 * distributed under the License is distributed on an "AS IS" BASIS,
012 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013 * See the License for the specific language governing permissions and
014 * limitations under the License.
015 */
016
017 package com.google.common.eventbus;
018
019 import com.google.common.annotations.Beta;
020 import com.google.common.annotations.VisibleForTesting;
021 import com.google.common.base.Supplier;
022 import com.google.common.base.Throwables;
023 import com.google.common.cache.CacheBuilder;
024 import com.google.common.cache.CacheLoader;
025 import com.google.common.cache.LoadingCache;
026 import com.google.common.collect.Multimap;
027 import com.google.common.collect.Multimaps;
028 import com.google.common.collect.SetMultimap;
029 import com.google.common.reflect.TypeToken;
030
031 import java.lang.reflect.InvocationTargetException;
032 import java.util.Collection;
033 import java.util.Map.Entry;
034 import java.util.Set;
035 import java.util.concurrent.ConcurrentHashMap;
036 import java.util.concurrent.ConcurrentLinkedQueue;
037 import java.util.concurrent.CopyOnWriteArraySet;
038 import java.util.concurrent.ExecutionException;
039 import java.util.logging.Level;
040 import java.util.logging.Logger;
041
042 /**
043 * Dispatches events to listeners, and provides ways for listeners to register
044 * themselves.
045 *
046 * <p>The EventBus allows publish-subscribe-style communication between
047 * components without requiring the components to explicitly register with one
048 * another (and thus be aware of each other). It is designed exclusively to
049 * replace traditional Java in-process event distribution using explicit
050 * registration. It is <em>not</em> a general-purpose publish-subscribe system,
051 * nor is it intended for interprocess communication.
052 *
053 * <h2>Receiving Events</h2>
054 * To receive events, an object should:<ol>
055 * <li>Expose a public method, known as the <i>event handler</i>, which accepts
056 * a single argument of the type of event desired;</li>
057 * <li>Mark it with a {@link Subscribe} annotation;</li>
058 * <li>Pass itself to an EventBus instance's {@link #register(Object)} method.
059 * </li>
060 * </ol>
061 *
062 * <h2>Posting Events</h2>
063 * To post an event, simply provide the event object to the
064 * {@link #post(Object)} method. The EventBus instance will determine the type
065 * of event and route it to all registered listeners.
066 *
067 * <p>Events are routed based on their type — an event will be delivered
068 * to any handler for any type to which the event is <em>assignable.</em> This
069 * includes implemented interfaces, all superclasses, and all interfaces
070 * implemented by superclasses.
071 *
072 * <p>When {@code post} is called, all registered handlers for an event are run
073 * in sequence, so handlers should be reasonably quick. If an event may trigger
074 * an extended process (such as a database load), spawn a thread or queue it for
075 * later. (For a convenient way to do this, use an {@link AsyncEventBus}.)
076 *
077 * <h2>Handler Methods</h2>
078 * Event handler methods must accept only one argument: the event.
079 *
080 * <p>Handlers should not, in general, throw. If they do, the EventBus will
081 * catch and log the exception. This is rarely the right solution for error
082 * handling and should not be relied upon; it is intended solely to help find
083 * problems during development.
084 *
085 * <p>The EventBus guarantees that it will not call a handler method from
086 * multiple threads simultaneously, unless the method explicitly allows it by
087 * bearing the {@link AllowConcurrentEvents} annotation. If this annotation is
088 * not present, handler methods need not worry about being reentrant, unless
089 * also called from outside the EventBus.
090 *
091 * <h2>Dead Events</h2>
092 * If an event is posted, but no registered handlers can accept it, it is
093 * considered "dead." To give the system a second chance to handle dead events,
094 * they are wrapped in an instance of {@link DeadEvent} and reposted.
095 *
096 * <p>If a handler for a supertype of all events (such as Object) is registered,
097 * no event will ever be considered dead, and no DeadEvents will be generated.
098 * Accordingly, while DeadEvent extends {@link Object}, a handler registered to
099 * receive any Object will never receive a DeadEvent.
100 *
101 * <p>This class is safe for concurrent use.
102 *
103 * <p>See the Guava User Guide article on <a href=
104 * "http://code.google.com/p/guava-libraries/wiki/EventBusExplained">
105 * {@code EventBus}</a>.
106 *
107 * @author Cliff Biffle
108 * @since 10.0
109 */
110 @Beta
111 public class EventBus {
112
113 /**
114 * All registered event handlers, indexed by event type.
115 */
116 private final SetMultimap<Class<?>, EventHandler> handlersByType =
117 Multimaps.newSetMultimap(new ConcurrentHashMap<Class<?>, Collection<EventHandler>>(),
118 new Supplier<Set<EventHandler>>() {
119 @Override
120 public Set<EventHandler> get() {
121 return newHandlerSet();
122 }
123 });
124
125 /**
126 * Logger for event dispatch failures. Named by the fully-qualified name of
127 * this class, followed by the identifier provided at construction.
128 */
129 private final Logger logger;
130
131 /**
132 * Strategy for finding handler methods in registered objects. Currently,
133 * only the {@link AnnotatedHandlerFinder} is supported, but this is
134 * encapsulated for future expansion.
135 */
136 private final HandlerFindingStrategy finder = new AnnotatedHandlerFinder();
137
138 /** queues of events for the current thread to dispatch */
139 private final ThreadLocal<ConcurrentLinkedQueue<EventWithHandler>>
140 eventsToDispatch =
141 new ThreadLocal<ConcurrentLinkedQueue<EventWithHandler>>() {
142 @Override protected ConcurrentLinkedQueue<EventWithHandler> initialValue() {
143 return new ConcurrentLinkedQueue<EventWithHandler>();
144 }
145 };
146
147 /** true if the current thread is currently dispatching an event */
148 private final ThreadLocal<Boolean> isDispatching =
149 new ThreadLocal<Boolean>() {
150 @Override protected Boolean initialValue() {
151 return false;
152 }
153 };
154
155 /**
156 * A thread-safe cache for flattenHierarchy(). The Class class is immutable.
157 */
158 private final LoadingCache<Class<?>, Set<Class<?>>> flattenHierarchyCache =
159 CacheBuilder.newBuilder()
160 .weakKeys()
161 .build(new CacheLoader<Class<?>, Set<Class<?>>>() {
162 @SuppressWarnings({"unchecked", "rawtypes"}) // safe cast
163 @Override
164 public Set<Class<?>> load(Class<?> concreteClass) throws Exception {
165 return (Set) TypeToken.of(concreteClass).getTypes().rawTypes();
166 }
167 });
168
169 /**
170 * Creates a new EventBus named "default".
171 */
172 public EventBus() {
173 this("default");
174 }
175
176 /**
177 * Creates a new EventBus with the given {@code identifier}.
178 *
179 * @param identifier a brief name for this bus, for logging purposes. Should
180 * be a valid Java identifier.
181 */
182 public EventBus(String identifier) {
183 logger = Logger.getLogger(EventBus.class.getName() + "." + identifier);
184 }
185
186 /**
187 * Registers all handler methods on {@code object} to receive events.
188 * Handler methods are selected and classified using this EventBus's
189 * {@link HandlerFindingStrategy}; the default strategy is the
190 * {@link AnnotatedHandlerFinder}.
191 *
192 * @param object object whose handler methods should be registered.
193 */
194 public void register(Object object) {
195 handlersByType.putAll(finder.findAllHandlers(object));
196 }
197
198 /**
199 * Unregisters all handler methods on a registered {@code object}.
200 *
201 * @param object object whose handler methods should be unregistered.
202 * @throws IllegalArgumentException if the object was not previously registered.
203 */
204 public void unregister(Object object) {
205 Multimap<Class<?>, EventHandler> methodsInListener = finder.findAllHandlers(object);
206 for (Entry<Class<?>, Collection<EventHandler>> entry : methodsInListener.asMap().entrySet()) {
207 Set<EventHandler> currentHandlers = getHandlersForEventType(entry.getKey());
208 Collection<EventHandler> eventMethodsInListener = entry.getValue();
209
210 if (currentHandlers == null || !currentHandlers.containsAll(entry.getValue())) {
211 throw new IllegalArgumentException(
212 "missing event handler for an annotated method. Is " + object + " registered?");
213 }
214 currentHandlers.removeAll(eventMethodsInListener);
215 }
216 }
217
218 /**
219 * Posts an event to all registered handlers. This method will return
220 * successfully after the event has been posted to all handlers, and
221 * regardless of any exceptions thrown by handlers.
222 *
223 * <p>If no handlers have been subscribed for {@code event}'s class, and
224 * {@code event} is not already a {@link DeadEvent}, it will be wrapped in a
225 * DeadEvent and reposted.
226 *
227 * @param event event to post.
228 */
229 @SuppressWarnings("deprecation") // only deprecated for external subclasses
230 public void post(Object event) {
231 Set<Class<?>> dispatchTypes = flattenHierarchy(event.getClass());
232
233 boolean dispatched = false;
234 for (Class<?> eventType : dispatchTypes) {
235 Set<EventHandler> wrappers = getHandlersForEventType(eventType);
236
237 if (wrappers != null && !wrappers.isEmpty()) {
238 dispatched = true;
239 for (EventHandler wrapper : wrappers) {
240 enqueueEvent(event, wrapper);
241 }
242 }
243 }
244
245 if (!dispatched && !(event instanceof DeadEvent)) {
246 post(new DeadEvent(this, event));
247 }
248
249 dispatchQueuedEvents();
250 }
251
252 /**
253 * Queue the {@code event} for dispatch during
254 * {@link #dispatchQueuedEvents()}. Events are queued in-order of occurrence
255 * so they can be dispatched in the same order.
256 */
257 void enqueueEvent(Object event, EventHandler handler) {
258 eventsToDispatch.get().offer(new EventWithHandler(event, handler));
259 }
260
261 /**
262 * Drain the queue of events to be dispatched. As the queue is being drained,
263 * new events may be posted to the end of the queue.
264 *
265 * @deprecated This method should not be overridden outside of the eventbus package. It is
266 * scheduled for removal in Guava 14.0.
267 */
268 @Deprecated
269 protected void dispatchQueuedEvents() {
270 // don't dispatch if we're already dispatching, that would allow reentrancy
271 // and out-of-order events. Instead, leave the events to be dispatched
272 // after the in-progress dispatch is complete.
273 if (isDispatching.get()) {
274 return;
275 }
276
277 isDispatching.set(true);
278 try {
279 while (true) {
280 EventWithHandler eventWithHandler = eventsToDispatch.get().poll();
281 if (eventWithHandler == null) {
282 break;
283 }
284
285 dispatch(eventWithHandler.event, eventWithHandler.handler);
286 }
287 } finally {
288 isDispatching.set(false);
289 }
290 }
291
292 /**
293 * Dispatches {@code event} to the handler in {@code wrapper}. This method
294 * is an appropriate override point for subclasses that wish to make
295 * event delivery asynchronous.
296 *
297 * @param event event to dispatch.
298 * @param wrapper wrapper that will call the handler.
299 */
300 void dispatch(Object event, EventHandler wrapper) {
301 try {
302 wrapper.handleEvent(event);
303 } catch (InvocationTargetException e) {
304 logger.log(Level.SEVERE,
305 "Could not dispatch event: " + event + " to handler " + wrapper, e);
306 }
307 }
308
309 /**
310 * Retrieves a mutable set of the currently registered handlers for
311 * {@code type}. If no handlers are currently registered for {@code type},
312 * this method may either return {@code null} or an empty set.
313 *
314 * @param type type of handlers to retrieve.
315 * @return currently registered handlers, or {@code null}.
316 */
317 Set<EventHandler> getHandlersForEventType(Class<?> type) {
318 return handlersByType.get(type);
319 }
320
321 /**
322 * Creates a new Set for insertion into the handler map. This is provided
323 * as an override point for subclasses. The returned set should support
324 * concurrent access.
325 *
326 * @return a new, mutable set for handlers.
327 */
328 Set<EventHandler> newHandlerSet() {
329 return new CopyOnWriteArraySet<EventHandler>();
330 }
331
332 /**
333 * Flattens a class's type hierarchy into a set of Class objects. The set
334 * will include all superclasses (transitively), and all interfaces
335 * implemented by these superclasses.
336 *
337 * @param concreteClass class whose type hierarchy will be retrieved.
338 * @return {@code clazz}'s complete type hierarchy, flattened and uniqued.
339 */
340 @VisibleForTesting
341 Set<Class<?>> flattenHierarchy(Class<?> concreteClass) {
342 try {
343 return flattenHierarchyCache.get(concreteClass);
344 } catch (ExecutionException e) {
345 throw Throwables.propagate(e.getCause());
346 }
347 }
348
349 /** simple struct representing an event and it's handler */
350 static class EventWithHandler {
351 final Object event;
352 final EventHandler handler;
353 public EventWithHandler(Object event, EventHandler handler) {
354 this.event = event;
355 this.handler = handler;
356 }
357 }
358 }