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 public Set<EventHandler> get() {
120 return newHandlerSet();
121 }
122 });
123
124 /**
125 * Logger for event dispatch failures. Named by the fully-qualified name of
126 * this class, followed by the identifier provided at construction.
127 */
128 private final Logger logger;
129
130 /**
131 * Strategy for finding handler methods in registered objects. Currently,
132 * only the {@link AnnotatedHandlerFinder} is supported, but this is
133 * encapsulated for future expansion.
134 */
135 private final HandlerFindingStrategy finder = new AnnotatedHandlerFinder();
136
137 /** queues of events for the current thread to dispatch */
138 private final ThreadLocal<ConcurrentLinkedQueue<EventWithHandler>>
139 eventsToDispatch =
140 new ThreadLocal<ConcurrentLinkedQueue<EventWithHandler>>() {
141
142 @Override
143 protected ConcurrentLinkedQueue<EventWithHandler> initialValue() {
144 return new ConcurrentLinkedQueue<EventWithHandler>();
145 }
146 };
147
148 /** true if the current thread is currently dispatching an event */
149 private final ThreadLocal<Boolean> isDispatching =
150 new ThreadLocal<Boolean>() {
151
152 @Override
153 protected Boolean initialValue() {
154 return false;
155 }
156 };
157
158 /**
159 * A thread-safe cache for flattenHierarchy(). The Class class is immutable.
160 */
161 private final LoadingCache<Class<?>, Set<Class<?>>> flattenHierarchyCache =
162 CacheBuilder.newBuilder()
163 .weakKeys()
164 .build(new CacheLoader<Class<?>, Set<Class<?>>>() {
165 @Override
166 @SuppressWarnings({"unchecked", "rawtypes"}) // safe cast
167
168 public Set<Class<?>> load(Class<?> concreteClass) throws Exception {
169 return (Set) TypeToken.of(concreteClass).getTypes().rawTypes();
170 }
171 });
172
173 /**
174 * Creates a new EventBus named "default".
175 */
176 public EventBus() {
177 this("default");
178 }
179
180 /**
181 * Creates a new EventBus with the given {@code identifier}.
182 *
183 * @param identifier a brief name for this bus, for logging purposes. Should
184 * be a valid Java identifier.
185 */
186 public EventBus(String identifier) {
187 logger = Logger.getLogger(EventBus.class.getName() + "." + identifier);
188 }
189
190 /**
191 * Registers all handler methods on {@code object} to receive events.
192 * Handler methods are selected and classified using this EventBus's
193 * {@link HandlerFindingStrategy}; the default strategy is the
194 * {@link AnnotatedHandlerFinder}.
195 *
196 * @param object object whose handler methods should be registered.
197 */
198 public void register(Object object) {
199 handlersByType.putAll(finder.findAllHandlers(object));
200 }
201
202 /**
203 * Unregisters all handler methods on a registered {@code object}.
204 *
205 * @param object object whose handler methods should be unregistered.
206 * @throws IllegalArgumentException if the object was not previously registered.
207 */
208 public void unregister(Object object) {
209 Multimap<Class<?>, EventHandler> methodsInListener = finder.findAllHandlers(object);
210 for (Entry<Class<?>, Collection<EventHandler>> entry : methodsInListener.asMap().entrySet()) {
211 Set<EventHandler> currentHandlers = getHandlersForEventType(entry.getKey());
212 Collection<EventHandler> eventMethodsInListener = entry.getValue();
213
214 if (currentHandlers == null || !currentHandlers.containsAll(entry.getValue())) {
215 throw new IllegalArgumentException(
216 "missing event handler for an annotated method. Is " + object + " registered?");
217 }
218 currentHandlers.removeAll(eventMethodsInListener);
219 }
220 }
221
222 /**
223 * Posts an event to all registered handlers. This method will return
224 * successfully after the event has been posted to all handlers, and
225 * regardless of any exceptions thrown by handlers.
226 *
227 * <p>If no handlers have been subscribed for {@code event}'s class, and
228 * {@code event} is not already a {@link DeadEvent}, it will be wrapped in a
229 * DeadEvent and reposted.
230 *
231 * @param event event to post.
232 */
233 @SuppressWarnings("deprecation") // only deprecated for external subclasses
234 public void post(Object event) {
235 Set<Class<?>> dispatchTypes = flattenHierarchy(event.getClass());
236
237 boolean dispatched = false;
238 for (Class<?> eventType : dispatchTypes) {
239 Set<EventHandler> wrappers = getHandlersForEventType(eventType);
240
241 if (wrappers != null && !wrappers.isEmpty()) {
242 dispatched = true;
243 for (EventHandler wrapper : wrappers) {
244 enqueueEvent(event, wrapper);
245 }
246 }
247 }
248
249 if (!dispatched && !(event instanceof DeadEvent)) {
250 post(new DeadEvent(this, event));
251 }
252
253 dispatchQueuedEvents();
254 }
255
256 /**
257 * Queue the {@code event} for dispatch during
258 * {@link #dispatchQueuedEvents()}. Events are queued in-order of occurrence
259 * so they can be dispatched in the same order.
260 */
261 void enqueueEvent(Object event, EventHandler handler) {
262 eventsToDispatch.get().offer(new EventWithHandler(event, handler));
263 }
264
265 /**
266 * Drain the queue of events to be dispatched. As the queue is being drained,
267 * new events may be posted to the end of the queue.
268 *
269 * @deprecated This method should not be overridden outside of the eventbus package. It is
270 * scheduled for removal in Guava 14.0.
271 */
272 @Deprecated
273 protected void dispatchQueuedEvents() {
274 // don't dispatch if we're already dispatching, that would allow reentrancy
275 // and out-of-order events. Instead, leave the events to be dispatched
276 // after the in-progress dispatch is complete.
277 if (isDispatching.get()) {
278 return;
279 }
280
281 isDispatching.set(true);
282 try {
283 while (true) {
284 EventWithHandler eventWithHandler = eventsToDispatch.get().poll();
285 if (eventWithHandler == null) {
286 break;
287 }
288
289 dispatch(eventWithHandler.event, eventWithHandler.handler);
290 }
291 } finally {
292 isDispatching.set(false);
293 }
294 }
295
296 /**
297 * Dispatches {@code event} to the handler in {@code wrapper}. This method
298 * is an appropriate override point for subclasses that wish to make
299 * event delivery asynchronous.
300 *
301 * @param event event to dispatch.
302 * @param wrapper wrapper that will call the handler.
303 */
304 void dispatch(Object event, EventHandler wrapper) {
305 try {
306 wrapper.handleEvent(event);
307 } catch (InvocationTargetException e) {
308 logger.log(Level.SEVERE,
309 "Could not dispatch event: " + event + " to handler " + wrapper, e);
310 }
311 }
312
313 /**
314 * Retrieves a mutable set of the currently registered handlers for
315 * {@code type}. If no handlers are currently registered for {@code type},
316 * this method may either return {@code null} or an empty set.
317 *
318 * @param type type of handlers to retrieve.
319 * @return currently registered handlers, or {@code null}.
320 */
321 Set<EventHandler> getHandlersForEventType(Class<?> type) {
322 return handlersByType.get(type);
323 }
324
325 /**
326 * Creates a new Set for insertion into the handler map. This is provided
327 * as an override point for subclasses. The returned set should support
328 * concurrent access.
329 *
330 * @return a new, mutable set for handlers.
331 */
332 Set<EventHandler> newHandlerSet() {
333 return new CopyOnWriteArraySet<EventHandler>();
334 }
335
336 /**
337 * Flattens a class's type hierarchy into a set of Class objects. The set
338 * will include all superclasses (transitively), and all interfaces
339 * implemented by these superclasses.
340 *
341 * @param concreteClass class whose type hierarchy will be retrieved.
342 * @return {@code clazz}'s complete type hierarchy, flattened and uniqued.
343 */
344 @VisibleForTesting
345 Set<Class<?>> flattenHierarchy(Class<?> concreteClass) {
346 try {
347 return flattenHierarchyCache.get(concreteClass);
348 } catch (ExecutionException e) {
349 throw Throwables.propagate(e.getCause());
350 }
351 }
352
353 /** simple struct representing an event and it's handler */
354 static class EventWithHandler {
355 final Object event;
356 final EventHandler handler;
357 public EventWithHandler(Object event, EventHandler handler) {
358 this.event = event;
359 this.handler = handler;
360 }
361 }
362 }