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.Cache;
024 import com.google.common.cache.CacheBuilder;
025 import com.google.common.cache.CacheLoader;
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 * @author Cliff Biffle
106 * @since 10.0
107 */
108 @Beta
109 public class EventBus {
110
111 /**
112 * All registered event handlers, indexed by event type.
113 */
114 private final SetMultimap<Class<?>, EventHandler> handlersByType =
115 Multimaps.newSetMultimap(new ConcurrentHashMap<Class<?>, Collection<EventHandler>>(),
116 new Supplier<Set<EventHandler>>() {
117 @Override
118 public Set<EventHandler> get() {
119 return new CopyOnWriteArraySet<EventHandler>();
120 }
121 });
122
123 /**
124 * Logger for event dispatch failures. Named by the fully-qualified name of
125 * this class, followed by the identifier provided at construction.
126 */
127 private final Logger logger;
128
129 /**
130 * Strategy for finding handler methods in registered objects. Currently,
131 * only the {@link AnnotatedHandlerFinder} is supported, but this is
132 * encapsulated for future expansion.
133 */
134 private final HandlerFindingStrategy finder = new AnnotatedHandlerFinder();
135
136 /** queues of events for the current thread to dispatch */
137 private final ThreadLocal<ConcurrentLinkedQueue<EventWithHandler>>
138 eventsToDispatch =
139 new ThreadLocal<ConcurrentLinkedQueue<EventWithHandler>>() {
140 @Override protected ConcurrentLinkedQueue<EventWithHandler> initialValue() {
141 return new ConcurrentLinkedQueue<EventWithHandler>();
142 }
143 };
144
145 /** true if the current thread is currently dispatching an event */
146 private final ThreadLocal<Boolean> isDispatching =
147 new ThreadLocal<Boolean>() {
148 @Override protected Boolean initialValue() {
149 return false;
150 }
151 };
152
153 /**
154 * A thread-safe cache for flattenHierarch(). The Class class is immutable.
155 */
156 private Cache<Class<?>, Set<Class<?>>> flattenHierarchyCache =
157 CacheBuilder.newBuilder()
158 .weakKeys()
159 .build(new CacheLoader<Class<?>, Set<Class<?>>>() {
160 @Override
161 public Set<Class<?>> load(Class<?> concreteClass) throws Exception {
162 List<Class<?>> parents = Lists.newLinkedList();
163 Set<Class<?>> classes = Sets.newHashSet();
164
165 parents.add(concreteClass);
166
167 while (!parents.isEmpty()) {
168 Class<?> clazz = parents.remove(0);
169 classes.add(clazz);
170
171 Class<?> parent = clazz.getSuperclass();
172 if (parent != null) {
173 parents.add(parent);
174 }
175
176 for (Class<?> iface : clazz.getInterfaces()) {
177 parents.add(iface);
178 }
179 }
180
181 return classes;
182 }
183 });
184
185 /**
186 * Creates a new EventBus named "default".
187 */
188 public EventBus() {
189 this("default");
190 }
191
192 /**
193 * Creates a new EventBus with the given {@code identifier}.
194 *
195 * @param identifier a brief name for this bus, for logging purposes. Should
196 * be a valid Java identifier.
197 */
198 public EventBus(String identifier) {
199 logger = Logger.getLogger(EventBus.class.getName() + "." + identifier);
200 }
201
202 /**
203 * Registers all handler methods on {@code object} to receive events.
204 * Handler methods are selected and classified using this EventBus's
205 * {@link HandlerFindingStrategy}; the default strategy is the
206 * {@link AnnotatedHandlerFinder}.
207 *
208 * @param object object whose handler methods should be registered.
209 */
210 public void register(Object object) {
211 handlersByType.putAll(finder.findAllHandlers(object));
212 }
213
214 /**
215 * Unregisters all handler methods on a registered {@code object}.
216 *
217 * @param object object whose handler methods should be unregistered.
218 * @throws IllegalArgumentException if the object was not previously registered.
219 */
220 public void unregister(Object object) {
221 Multimap<Class<?>, EventHandler> methodsInListener = finder.findAllHandlers(object);
222 for (Entry<Class<?>, Collection<EventHandler>> entry : methodsInListener.asMap().entrySet()) {
223 Set<EventHandler> currentHandlers = getHandlersForEventType(entry.getKey());
224 Collection<EventHandler> eventMethodsInListener = entry.getValue();
225
226 if (currentHandlers == null || !currentHandlers.containsAll(entry.getValue())) {
227 throw new IllegalArgumentException(
228 "missing event handler for an annotated method. Is " + object + " registered?");
229 }
230 currentHandlers.removeAll(eventMethodsInListener);
231 }
232 }
233
234 /**
235 * Posts an event to all registered handlers. This method will return
236 * successfully after the event has been posted to all handlers, and
237 * regardless of any exceptions thrown by handlers.
238 *
239 * <p>If no handlers have been subscribed for {@code event}'s class, and
240 * {@code event} is not already a {@link DeadEvent}, it will be wrapped in a
241 * DeadEvent and reposted.
242 *
243 * @param event event to post.
244 */
245 public void post(Object event) {
246 Set<Class<?>> dispatchTypes = flattenHierarchy(event.getClass());
247
248 boolean dispatched = false;
249 for (Class<?> eventType : dispatchTypes) {
250 Set<EventHandler> wrappers = getHandlersForEventType(eventType);
251
252 if (wrappers != null && !wrappers.isEmpty()) {
253 dispatched = true;
254 for (EventHandler wrapper : wrappers) {
255 enqueueEvent(event, wrapper);
256 }
257 }
258 }
259
260 if (!dispatched && !(event instanceof DeadEvent)) {
261 post(new DeadEvent(this, event));
262 }
263
264 dispatchQueuedEvents();
265 }
266
267 /**
268 * Queue the {@code event} for dispatch during
269 * {@link #dispatchQueuedEvents()}. Events are queued in-order of occurrence
270 * so they can be dispatched in the same order.
271 */
272 protected void enqueueEvent(Object event, EventHandler handler) {
273 eventsToDispatch.get().offer(new EventWithHandler(event, handler));
274 }
275
276 /**
277 * Drain the queue of events to be dispatched. As the queue is being drained,
278 * new events may be posted to the end of the queue.
279 */
280 protected void dispatchQueuedEvents() {
281 // don't dispatch if we're already dispatching, that would allow reentrancy
282 // and out-of-order events. Instead, leave the events to be dispatched
283 // after the in-progress dispatch is complete.
284 if (isDispatching.get()) {
285 return;
286 }
287
288 isDispatching.set(true);
289 try {
290 while (true) {
291 EventWithHandler eventWithHandler = eventsToDispatch.get().poll();
292 if (eventWithHandler == null) {
293 break;
294 }
295
296 dispatch(eventWithHandler.event, eventWithHandler.handler);
297 }
298 } finally {
299 isDispatching.set(false);
300 }
301 }
302
303 /**
304 * Dispatches {@code event} to the handler in {@code wrapper}. This method
305 * is an appropriate override point for subclasses that wish to make
306 * event delivery asynchronous.
307 *
308 * @param event event to dispatch.
309 * @param wrapper wrapper that will call the handler.
310 */
311 protected void dispatch(Object event, EventHandler wrapper) {
312 try {
313 wrapper.handleEvent(event);
314 } catch (InvocationTargetException e) {
315 logger.log(Level.SEVERE,
316 "Could not dispatch event: " + event + " to handler " + wrapper, e);
317 }
318 }
319
320 /**
321 * Retrieves a mutable set of the currently registered handlers for
322 * {@code type}. If no handlers are currently registered for {@code type},
323 * this method may either return {@code null} or an empty set.
324 *
325 * @param type type of handlers to retrieve.
326 * @return currently registered handlers, or {@code null}.
327 */
328 Set<EventHandler> getHandlersForEventType(Class<?> type) {
329 return handlersByType.get(type);
330 }
331
332 /**
333 * Creates a new Set for insertion into the handler map. This is provided
334 * as an override point for subclasses. The returned set should support
335 * concurrent access.
336 *
337 * @return a new, mutable set for handlers.
338 */
339 protected Set<EventHandler> newHandlerSet() {
340 return new CopyOnWriteArraySet<EventHandler>();
341 }
342
343 /**
344 * Flattens a class's type hierarchy into a set of Class objects. The set
345 * will include all superclasses (transitively), and all interfaces
346 * implemented by these superclasses.
347 *
348 * @param concreteClass class whose type hierarchy will be retrieved.
349 * @return {@code clazz}'s complete type hierarchy, flattened and uniqued.
350 */
351 @VisibleForTesting
352 Set<Class<?>> flattenHierarchy(Class<?> concreteClass) {
353 try {
354 return flattenHierarchyCache.get(concreteClass);
355 } catch (ExecutionException e) {
356 throw Throwables.propagate(e.getCause());
357 }
358 }
359
360 /** simple struct representing an event and it's handler */
361 static class EventWithHandler {
362 final Object event;
363 final EventHandler handler;
364 public EventWithHandler(Object event, EventHandler handler) {
365 this.event = event;
366 this.handler = handler;
367 }
368 }
369 }