    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 &mdash; 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    }