    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 &mdash; 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    }