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.Lists;
027 import com.google.common.collect.Multimap;
028 import com.google.common.collect.Multimaps;
029 import com.google.common.collect.SetMultimap;
030 import com.google.common.collect.Sets;
031
032 import java.lang.reflect.InvocationTargetException;
033 import java.util.Collection;
034 import java.util.List;
035 import java.util.Map.Entry;
036 import java.util.Set;
037 import java.util.concurrent.ConcurrentHashMap;
038 import java.util.concurrent.ConcurrentLinkedQueue;
039 import java.util.concurrent.CopyOnWriteArraySet;
040 import java.util.concurrent.ExecutionException;
041 import java.util.logging.Level;
042 import java.util.logging.Logger;
043
044 /**
045 * Dispatches events to listeners, and provides ways for listeners to register
046 * themselves.
047 *
048 * <p>The EventBus allows publish-subscribe-style communication between
049 * components without requiring the components to explicitly register with one
050 * another (and thus be aware of each other). It is designed exclusively to
051 * replace traditional Java in-process event distribution using explicit
052 * registration. It is <em>not</em> a general-purpose publish-subscribe system,
053 * nor is it intended for interprocess communication.
054 *
055 * <h2>Receiving Events</h2>
056 * To receive events, an object should:<ol>
057 * <li>Expose a public method, known as the <i>event handler</i>, which accepts
058 * a single argument of the type of event desired;</li>
059 * <li>Mark it with a {@link Subscribe} annotation;</li>
060 * <li>Pass itself to an EventBus instance's {@link #register(Object)} method.
061 * </li>
062 * </ol>
063 *
064 * <h2>Posting Events</h2>
065 * To post an event, simply provide the event object to the
066 * {@link #post(Object)} method. The EventBus instance will determine the type
067 * of event and route it to all registered listeners.
068 *
069 * <p>Events are routed based on their type — an event will be delivered
070 * to any handler for any type to which the event is <em>assignable.</em> This
071 * includes implemented interfaces, all superclasses, and all interfaces
072 * implemented by superclasses.
073 *
074 * <p>When {@code post} is called, all registered handlers for an event are run
075 * in sequence, so handlers should be reasonably quick. If an event may trigger
076 * an extended process (such as a database load), spawn a thread or queue it for
077 * later. (For a convenient way to do this, use an {@link AsyncEventBus}.)
078 *
079 * <h2>Handler Methods</h2>
080 * Event handler methods must accept only one argument: the event.
081 *
082 * <p>Handlers should not, in general, throw. If they do, the EventBus will
083 * catch and log the exception. This is rarely the right solution for error
084 * handling and should not be relied upon; it is intended solely to help find
085 * problems during development.
086 *
087 * <p>The EventBus guarantees that it will not call a handler method from
088 * multiple threads simultaneously, unless the method explicitly allows it by
089 * bearing the {@link AllowConcurrentEvents} annotation. If this annotation is
090 * not present, handler methods need not worry about being reentrant, unless
091 * also called from outside the EventBus.
092 *
093 * <h2>Dead Events</h2>
094 * If an event is posted, but no registered handlers can accept it, it is
095 * considered "dead." To give the system a second chance to handle dead events,
096 * they are wrapped in an instance of {@link DeadEvent} and reposted.
097 *
098 * <p>If a handler for a supertype of all events (such as Object) is registered,
099 * no event will ever be considered dead, and no DeadEvents will be generated.
100 * Accordingly, while DeadEvent extends {@link Object}, a handler registered to
101 * receive any Object will never receive a DeadEvent.
102 *
103 * <p>This class is safe for concurrent use.
104 *
105 * <p>See the Guava User Guide article on <a href=
106 * "http://code.google.com/p/guava-libraries/wiki/EventBusExplained">
107 * {@code EventBus}</a>.
108 *
109 * @author Cliff Biffle
110 * @since 10.0
111 */
112 @Beta
113 public class EventBus {
114
115 /**
116 * All registered event handlers, indexed by event type.
117 */
118 private final SetMultimap<Class<?>, EventHandler> handlersByType =
119 Multimaps.newSetMultimap(new ConcurrentHashMap<Class<?>, Collection<EventHandler>>(),
120 new Supplier<Set<EventHandler>>() {
121 @Override
122 public Set<EventHandler> get() {
123 return new CopyOnWriteArraySet<EventHandler>();
124 }
125 });
126
127 /**
128 * Logger for event dispatch failures. Named by the fully-qualified name of
129 * this class, followed by the identifier provided at construction.
130 */
131 private final Logger logger;
132
133 /**
134 * Strategy for finding handler methods in registered objects. Currently,
135 * only the {@link AnnotatedHandlerFinder} is supported, but this is
136 * encapsulated for future expansion.
137 */
138 private final HandlerFindingStrategy finder = new AnnotatedHandlerFinder();
139
140 /** queues of events for the current thread to dispatch */
141 private final ThreadLocal<ConcurrentLinkedQueue<EventWithHandler>>
142 eventsToDispatch =
143 new ThreadLocal<ConcurrentLinkedQueue<EventWithHandler>>() {
144 @Override protected ConcurrentLinkedQueue<EventWithHandler> initialValue() {
145 return new ConcurrentLinkedQueue<EventWithHandler>();
146 }
147 };
148
149 /** true if the current thread is currently dispatching an event */
150 private final ThreadLocal<Boolean> isDispatching =
151 new ThreadLocal<Boolean>() {
152 @Override protected Boolean initialValue() {
153 return false;
154 }
155 };
156
157 /**
158 * A thread-safe cache for flattenHierarch(). The Class class is immutable.
159 */
160 private LoadingCache<Class<?>, Set<Class<?>>> flattenHierarchyCache =
161 CacheBuilder.newBuilder()
162 .weakKeys()
163 .build(new CacheLoader<Class<?>, Set<Class<?>>>() {
164 @Override
165 public Set<Class<?>> load(Class<?> concreteClass) throws Exception {
166 List<Class<?>> parents = Lists.newLinkedList();
167 Set<Class<?>> classes = Sets.newHashSet();
168
169 parents.add(concreteClass);
170
171 while (!parents.isEmpty()) {
172 Class<?> clazz = parents.remove(0);
173 classes.add(clazz);
174
175 Class<?> parent = clazz.getSuperclass();
176 if (parent != null) {
177 parents.add(parent);
178 }
179
180 for (Class<?> iface : clazz.getInterfaces()) {
181 parents.add(iface);
182 }
183 }
184
185 return classes;
186 }
187 });
188
189 /**
190 * Creates a new EventBus named "default".
191 */
192 public EventBus() {
193 this("default");
194 }
195
196 /**
197 * Creates a new EventBus with the given {@code identifier}.
198 *
199 * @param identifier a brief name for this bus, for logging purposes. Should
200 * be a valid Java identifier.
201 */
202 public EventBus(String identifier) {
203 logger = Logger.getLogger(EventBus.class.getName() + "." + identifier);
204 }
205
206 /**
207 * Registers all handler methods on {@code object} to receive events.
208 * Handler methods are selected and classified using this EventBus's
209 * {@link HandlerFindingStrategy}; the default strategy is the
210 * {@link AnnotatedHandlerFinder}.
211 *
212 * @param object object whose handler methods should be registered.
213 */
214 public void register(Object object) {
215 handlersByType.putAll(finder.findAllHandlers(object));
216 }
217
218 /**
219 * Unregisters all handler methods on a registered {@code object}.
220 *
221 * @param object object whose handler methods should be unregistered.
222 * @throws IllegalArgumentException if the object was not previously registered.
223 */
224 public void unregister(Object object) {
225 Multimap<Class<?>, EventHandler> methodsInListener = finder.findAllHandlers(object);
226 for (Entry<Class<?>, Collection<EventHandler>> entry : methodsInListener.asMap().entrySet()) {
227 Set<EventHandler> currentHandlers = getHandlersForEventType(entry.getKey());
228 Collection<EventHandler> eventMethodsInListener = entry.getValue();
229
230 if (currentHandlers == null || !currentHandlers.containsAll(entry.getValue())) {
231 throw new IllegalArgumentException(
232 "missing event handler for an annotated method. Is " + object + " registered?");
233 }
234 currentHandlers.removeAll(eventMethodsInListener);
235 }
236 }
237
238 /**
239 * Posts an event to all registered handlers. This method will return
240 * successfully after the event has been posted to all handlers, and
241 * regardless of any exceptions thrown by handlers.
242 *
243 * <p>If no handlers have been subscribed for {@code event}'s class, and
244 * {@code event} is not already a {@link DeadEvent}, it will be wrapped in a
245 * DeadEvent and reposted.
246 *
247 * @param event event to post.
248 */
249 public void post(Object event) {
250 Set<Class<?>> dispatchTypes = flattenHierarchy(event.getClass());
251
252 boolean dispatched = false;
253 for (Class<?> eventType : dispatchTypes) {
254 Set<EventHandler> wrappers = getHandlersForEventType(eventType);
255
256 if (wrappers != null && !wrappers.isEmpty()) {
257 dispatched = true;
258 for (EventHandler wrapper : wrappers) {
259 enqueueEvent(event, wrapper);
260 }
261 }
262 }
263
264 if (!dispatched && !(event instanceof DeadEvent)) {
265 post(new DeadEvent(this, event));
266 }
267
268 dispatchQueuedEvents();
269 }
270
271 /**
272 * Queue the {@code event} for dispatch during
273 * {@link #dispatchQueuedEvents()}. Events are queued in-order of occurrence
274 * so they can be dispatched in the same order.
275 */
276 protected void enqueueEvent(Object event, EventHandler handler) {
277 eventsToDispatch.get().offer(new EventWithHandler(event, handler));
278 }
279
280 /**
281 * Drain the queue of events to be dispatched. As the queue is being drained,
282 * new events may be posted to the end of the queue.
283 */
284 protected void dispatchQueuedEvents() {
285 // don't dispatch if we're already dispatching, that would allow reentrancy
286 // and out-of-order events. Instead, leave the events to be dispatched
287 // after the in-progress dispatch is complete.
288 if (isDispatching.get()) {
289 return;
290 }
291
292 isDispatching.set(true);
293 try {
294 while (true) {
295 EventWithHandler eventWithHandler = eventsToDispatch.get().poll();
296 if (eventWithHandler == null) {
297 break;
298 }
299
300 dispatch(eventWithHandler.event, eventWithHandler.handler);
301 }
302 } finally {
303 isDispatching.set(false);
304 }
305 }
306
307 /**
308 * Dispatches {@code event} to the handler in {@code wrapper}. This method
309 * is an appropriate override point for subclasses that wish to make
310 * event delivery asynchronous.
311 *
312 * @param event event to dispatch.
313 * @param wrapper wrapper that will call the handler.
314 */
315 protected void dispatch(Object event, EventHandler wrapper) {
316 try {
317 wrapper.handleEvent(event);
318 } catch (InvocationTargetException e) {
319 logger.log(Level.SEVERE,
320 "Could not dispatch event: " + event + " to handler " + wrapper, e);
321 }
322 }
323
324 /**
325 * Retrieves a mutable set of the currently registered handlers for
326 * {@code type}. If no handlers are currently registered for {@code type},
327 * this method may either return {@code null} or an empty set.
328 *
329 * @param type type of handlers to retrieve.
330 * @return currently registered handlers, or {@code null}.
331 */
332 Set<EventHandler> getHandlersForEventType(Class<?> type) {
333 return handlersByType.get(type);
334 }
335
336 /**
337 * Creates a new Set for insertion into the handler map. This is provided
338 * as an override point for subclasses. The returned set should support
339 * concurrent access.
340 *
341 * @return a new, mutable set for handlers.
342 */
343 protected Set<EventHandler> newHandlerSet() {
344 return new CopyOnWriteArraySet<EventHandler>();
345 }
346
347 /**
348 * Flattens a class's type hierarchy into a set of Class objects. The set
349 * will include all superclasses (transitively), and all interfaces
350 * implemented by these superclasses.
351 *
352 * @param concreteClass class whose type hierarchy will be retrieved.
353 * @return {@code clazz}'s complete type hierarchy, flattened and uniqued.
354 */
355 @VisibleForTesting
356 Set<Class<?>> flattenHierarchy(Class<?> concreteClass) {
357 try {
358 return flattenHierarchyCache.get(concreteClass);
359 } catch (ExecutionException e) {
360 throw Throwables.propagate(e.getCause());
361 }
362 }
363
364 /** simple struct representing an event and it's handler */
365 static class EventWithHandler {
366 final Object event;
367 final EventHandler handler;
368 public EventWithHandler(Object event, EventHandler handler) {
369 this.event = event;
370 this.handler = handler;
371 }
372 }
373 }