common/eventbus/EventBus.java

/*
 * Copyright (C) 2007 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.common.eventbus;

import com.google.common.annotations.Beta;
import com.google.common.annotations.VisibleForTesting;
import com.google.common.base.Supplier;
import com.google.common.base.Throwables;
import com.google.common.cache.CacheBuilder;
import com.google.common.cache.CacheLoader;
import com.google.common.cache.LoadingCache;
import com.google.common.collect.Lists;
import com.google.common.collect.Multimap;
import com.google.common.collect.Multimaps;
import com.google.common.collect.SetMultimap;
import com.google.common.collect.Sets;

import java.lang.reflect.InvocationTargetException;
import java.util.Collection;
import java.util.List;
import java.util.Map.Entry;
import java.util.Set;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentLinkedQueue;
import java.util.concurrent.CopyOnWriteArraySet;
import java.util.concurrent.ExecutionException;
import java.util.logging.Level;
import java.util.logging.Logger;

/**
 * Dispatches events to listeners, and provides ways for listeners to register
 * themselves.
 *
 * <p>The EventBus allows publish-subscribe-style communication between
 * components without requiring the components to explicitly register with one
 * another (and thus be aware of each other).  It is designed exclusively to
 * replace traditional Java in-process event distribution using explicit
 * registration. It is <em>not</em> a general-purpose publish-subscribe system,
 * nor is it intended for interprocess communication.
 *
 * <h2>Receiving Events</h2>
 * To receive events, an object should:<ol>
 * <li>Expose a public method, known as the <i>event handler</i>, which accepts
 *     a single argument of the type of event desired;</li>
 * <li>Mark it with a {@link Subscribe} annotation;</li>
 * <li>Pass itself to an EventBus instance's {@link #register(Object)} method.
 *     </li>
 * </ol>
 *
 * <h2>Posting Events</h2>
 * To post an event, simply provide the event object to the
 * {@link #post(Object)} method.  The EventBus instance will determine the type
 * of event and route it to all registered listeners.
 *
 * <p>Events are routed based on their type &mdash; an event will be delivered
 * to any handler for any type to which the event is <em>assignable.</em>  This
 * includes implemented interfaces, all superclasses, and all interfaces
 * implemented by superclasses.
 *
 * <p>When {@code post} is called, all registered handlers for an event are run
 * in sequence, so handlers should be reasonably quick.  If an event may trigger
 * an extended process (such as a database load), spawn a thread or queue it for
 * later.  (For a convenient way to do this, use an {@link AsyncEventBus}.)
 *
 * <h2>Handler Methods</h2>
 * Event handler methods must accept only one argument: the event.
 *
 * <p>Handlers should not, in general, throw.  If they do, the EventBus will
 * catch and log the exception.  This is rarely the right solution for error
 * handling and should not be relied upon; it is intended solely to help find
 * problems during development.
 *
 * <p>The EventBus guarantees that it will not call a handler method from
 * multiple threads simultaneously, unless the method explicitly allows it by
 * bearing the {@link AllowConcurrentEvents} annotation.  If this annotation is
 * not present, handler methods need not worry about being reentrant, unless
 * also called from outside the EventBus.
 *
 * <h2>Dead Events</h2>
 * If an event is posted, but no registered handlers can accept it, it is
 * considered "dead."  To give the system a second chance to handle dead events,
 * they are wrapped in an instance of {@link DeadEvent} and reposted.
 *
 * <p>If a handler for a supertype of all events (such as Object) is registered,
 * no event will ever be considered dead, and no DeadEvents will be generated.
 * Accordingly, while DeadEvent extends {@link Object}, a handler registered to
 * receive any Object will never receive a DeadEvent.
 *
 * <p>This class is safe for concurrent use.
 *
 * @author Cliff Biffle
 * @since 10.0
 */
@Beta
public class EventBus {

  /**
   * All registered event handlers, indexed by event type.
   */
  private final SetMultimap<Class<?>, EventHandler> handlersByType =
      Multimaps.newSetMultimap(new ConcurrentHashMap<Class<?>, Collection<EventHandler>>(),
          new Supplier<Set<EventHandler>>() {
            @Override
            public Set<EventHandler> get() {
              return new CopyOnWriteArraySet<EventHandler>();
            }
          });

  /**
   * Logger for event dispatch failures.  Named by the fully-qualified name of
   * this class, followed by the identifier provided at construction.
   */
  private final Logger logger;

  /**
   * Strategy for finding handler methods in registered objects.  Currently,
   * only the {@link AnnotatedHandlerFinder} is supported, but this is
   * encapsulated for future expansion.
   */
  private final HandlerFindingStrategy finder = new AnnotatedHandlerFinder();

  /** queues of events for the current thread to dispatch */
  private final ThreadLocal<ConcurrentLinkedQueue<EventWithHandler>>
      eventsToDispatch =
      new ThreadLocal<ConcurrentLinkedQueue<EventWithHandler>>() {
    @Override protected ConcurrentLinkedQueue<EventWithHandler> initialValue() {
      return new ConcurrentLinkedQueue<EventWithHandler>();
    }
  };

  /** true if the current thread is currently dispatching an event */
  private final ThreadLocal<Boolean> isDispatching =
      new ThreadLocal<Boolean>() {
    @Override protected Boolean initialValue() {
      return false;
    }
  };

  /**
   * A thread-safe cache for flattenHierarch(). The Class class is immutable.
   */
  private LoadingCache<Class<?>, Set<Class<?>>> flattenHierarchyCache =
      CacheBuilder.newBuilder()
          .weakKeys()
          .build(new CacheLoader<Class<?>, Set<Class<?>>>() {
            @Override
            public Set<Class<?>> load(Class<?> concreteClass) throws Exception {
              List<Class<?>> parents = Lists.newLinkedList();
              Set<Class<?>> classes = Sets.newHashSet();

              parents.add(concreteClass);

              while (!parents.isEmpty()) {
                Class<?> clazz = parents.remove(0);
                classes.add(clazz);

                Class<?> parent = clazz.getSuperclass();
                if (parent != null) {
                  parents.add(parent);
                }

                for (Class<?> iface : clazz.getInterfaces()) {
                  parents.add(iface);
                }
              }

              return classes;
            }
          });

  /**
   * Creates a new EventBus named "default".
   */
  public EventBus() {
    this("default");
  }

  /**
   * Creates a new EventBus with the given {@code identifier}.
   *
   * @param identifier  a brief name for this bus, for logging purposes.  Should
   *                    be a valid Java identifier.
   */
  public EventBus(String identifier) {
    logger = Logger.getLogger(EventBus.class.getName() + "." + identifier);
  }

  /**
   * Registers all handler methods on {@code object} to receive events.
   * Handler methods are selected and classified using this EventBus's
   * {@link HandlerFindingStrategy}; the default strategy is the
   * {@link AnnotatedHandlerFinder}.
   *
   * @param object  object whose handler methods should be registered.
   */
  public void register(Object object) {
    handlersByType.putAll(finder.findAllHandlers(object));
  }

  /**
   * Unregisters all handler methods on a registered {@code object}.
   *
   * @param object  object whose handler methods should be unregistered.
   * @throws IllegalArgumentException if the object was not previously registered.
   */
  public void unregister(Object object) {
    Multimap<Class<?>, EventHandler> methodsInListener = finder.findAllHandlers(object);
    for (Entry<Class<?>, Collection<EventHandler>> entry : methodsInListener.asMap().entrySet()) {
      Set<EventHandler> currentHandlers = getHandlersForEventType(entry.getKey());
      Collection<EventHandler> eventMethodsInListener = entry.getValue();

      if (currentHandlers == null || !currentHandlers.containsAll(entry.getValue())) {
        throw new IllegalArgumentException(
            "missing event handler for an annotated method. Is " + object + " registered?");
      }
      currentHandlers.removeAll(eventMethodsInListener);
    }
  }

  /**
   * Posts an event to all registered handlers.  This method will return
   * successfully after the event has been posted to all handlers, and
   * regardless of any exceptions thrown by handlers.
   *
   * <p>If no handlers have been subscribed for {@code event}'s class, and
   * {@code event} is not already a {@link DeadEvent}, it will be wrapped in a
   * DeadEvent and reposted.
   *
   * @param event  event to post.
   */
  public void post(Object event) {
    Set<Class<?>> dispatchTypes = flattenHierarchy(event.getClass());

    boolean dispatched = false;
    for (Class<?> eventType : dispatchTypes) {
      Set<EventHandler> wrappers = getHandlersForEventType(eventType);

      if (wrappers != null && !wrappers.isEmpty()) {
        dispatched = true;
        for (EventHandler wrapper : wrappers) {
          enqueueEvent(event, wrapper);
        }
      }
    }

    if (!dispatched && !(event instanceof DeadEvent)) {
      post(new DeadEvent(this, event));
    }

    dispatchQueuedEvents();
  }

  /**
   * Queue the {@code event} for dispatch during
   * {@link #dispatchQueuedEvents()}. Events are queued in-order of occurrence
   * so they can be dispatched in the same order.
   */
  protected void enqueueEvent(Object event, EventHandler handler) {
    eventsToDispatch.get().offer(new EventWithHandler(event, handler));
  }

  /**
   * Drain the queue of events to be dispatched. As the queue is being drained,
   * new events may be posted to the end of the queue.
   */
  protected void dispatchQueuedEvents() {
    // don't dispatch if we're already dispatching, that would allow reentrancy
    // and out-of-order events. Instead, leave the events to be dispatched
    // after the in-progress dispatch is complete.
    if (isDispatching.get()) {
      return;
    }

    isDispatching.set(true);
    try {
      while (true) {
        EventWithHandler eventWithHandler = eventsToDispatch.get().poll();
        if (eventWithHandler == null) {
          break;
        }

        dispatch(eventWithHandler.event, eventWithHandler.handler);
      }
    } finally {
      isDispatching.set(false);
    }
  }

  /**
   * Dispatches {@code event} to the handler in {@code wrapper}.  This method
   * is an appropriate override point for subclasses that wish to make
   * event delivery asynchronous.
   *
   * @param event  event to dispatch.
   * @param wrapper  wrapper that will call the handler.
   */
  protected void dispatch(Object event, EventHandler wrapper) {
    try {
      wrapper.handleEvent(event);
    } catch (InvocationTargetException e) {
      logger.log(Level.SEVERE,
          "Could not dispatch event: " + event + " to handler " + wrapper, e);
    }
  }

  /**
   * Retrieves a mutable set of the currently registered handlers for
   * {@code type}.  If no handlers are currently registered for {@code type},
   * this method may either return {@code null} or an empty set.
   *
   * @param type  type of handlers to retrieve.
   * @return currently registered handlers, or {@code null}.
   */
  Set<EventHandler> getHandlersForEventType(Class<?> type) {
    return handlersByType.get(type);
  }

  /**
   * Creates a new Set for insertion into the handler map.  This is provided
   * as an override point for subclasses. The returned set should support
   * concurrent access.
   *
   * @return a new, mutable set for handlers.
   */
  protected Set<EventHandler> newHandlerSet() {
    return new CopyOnWriteArraySet<EventHandler>();
  }

  /**
   * Flattens a class's type hierarchy into a set of Class objects.  The set
   * will include all superclasses (transitively), and all interfaces
   * implemented by these superclasses.
   *
   * @param concreteClass  class whose type hierarchy will be retrieved.
   * @return {@code clazz}'s complete type hierarchy, flattened and uniqued.
   */
  @VisibleForTesting
  Set<Class<?>> flattenHierarchy(Class<?> concreteClass) {
    try {
      return flattenHierarchyCache.get(concreteClass);
    } catch (ExecutionException e) {
      throw Throwables.propagate(e.getCause());
    }
  }

  /** simple struct representing an event and it's handler */
  static class EventWithHandler {
    final Object event;
    final EventHandler handler;
    public EventWithHandler(Object event, EventHandler handler) {
      this.event = event;
      this.handler = handler;
    }
  }
}