Allow nested event dispatch. Yet another thing which I should one day try and PR to Guava

api/src/main/java/com/google/common/eventbus/EventBus.java

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+/*
+ * 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 static com.google.common.base.Preconditions.checkNotNull;
+
+import com.google.common.annotations.Beta;
+import com.google.common.annotations.VisibleForTesting;
+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.HashMultimap;
+import com.google.common.collect.Multimap;
+import com.google.common.collect.SetMultimap;
+import com.google.common.reflect.TypeToken;
+import com.google.common.util.concurrent.UncheckedExecutionException;
+
+import java.lang.reflect.InvocationTargetException;
+import java.util.Collection;
+import java.util.LinkedList;
+import java.util.Map.Entry;
+import java.util.Queue;
+import java.util.Set;
+import java.util.concurrent.locks.ReadWriteLock;
+import java.util.concurrent.locks.ReentrantReadWriteLock;
+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.
+ * 
+ * <p>See the Guava User Guide article on <a href=
+ * "http://code.google.com/p/guava-libraries/wiki/EventBusExplained">
+ * {@code EventBus}</a>.
+ *
+ * @author Cliff Biffle
+ * @since 10.0
+ */
+@Beta
+public class EventBus {
+
+  /**
+   * A thread-safe cache for flattenHierarchy(). The Class class is immutable. This cache is shared
+   * across all EventBus instances, which greatly improves performance if multiple such instances
+   * are created and objects of the same class are posted on all of them.
+   */
+  private static final LoadingCache<Class<?>, Set<Class<?>>> flattenHierarchyCache =
+      CacheBuilder.newBuilder()
+          .weakKeys()
+          .build(new CacheLoader<Class<?>, Set<Class<?>>>() {
+            @SuppressWarnings({"unchecked", "rawtypes"}) // safe cast
+            @Override
+            public Set<Class<?>> load(Class<?> concreteClass) {
+              return (Set) TypeToken.of(concreteClass).getTypes().rawTypes();
+            }
+          });
+
+  /**
+   * All registered event handlers, indexed by event type.
+   *
+   * <p>This SetMultimap is NOT safe for concurrent use; all access should be
+   * made after acquiring a read or write lock via {@link #handlersByTypeLock}.
+   */
+  private final SetMultimap<Class<?>, EventHandler> handlersByType =
+      HashMultimap.create();
+  private final ReadWriteLock handlersByTypeLock = new ReentrantReadWriteLock();
+
+  /**
+   * 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<Queue<EventWithHandler>> eventsToDispatch =
+      new ThreadLocal<Queue<EventWithHandler>>() {
+    @Override protected Queue<EventWithHandler> initialValue() {
+      return new LinkedList<EventWithHandler>();
+    }
+  };
+
+  /**
+   * 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() + "." + checkNotNull(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) {
+    Multimap<Class<?>, EventHandler> methodsInListener =
+        finder.findAllHandlers(object);
+    handlersByTypeLock.writeLock().lock();
+    try {
+      handlersByType.putAll(methodsInListener);
+    } finally {
+      handlersByTypeLock.writeLock().unlock();
+    }
+  }
+
+  /**
+   * 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()) {
+      Class<?> eventType = entry.getKey();
+      Collection<EventHandler> eventMethodsInListener = entry.getValue();
+
+      handlersByTypeLock.writeLock().lock();
+      try {
+        Set<EventHandler> currentHandlers = handlersByType.get(eventType);
+        if (!currentHandlers.containsAll(eventMethodsInListener)) {
+          throw new IllegalArgumentException(
+              "missing event handler for an annotated method. Is " + object + " registered?");
+        }
+        currentHandlers.removeAll(eventMethodsInListener);
+      } finally {
+        handlersByTypeLock.writeLock().unlock();
+      }
+    }
+  }
+
+  /**
+   * 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) {
+      handlersByTypeLock.readLock().lock();
+      try {
+        Set<EventHandler> wrappers = handlersByType.get(eventType);
+
+        if (!wrappers.isEmpty()) {
+          dispatched = true;
+          for (EventHandler wrapper : wrappers) {
+            enqueueEvent(event, wrapper);
+          }
+        }
+      } finally {
+        handlersByTypeLock.readLock().unlock();
+      }
+    }
+
+    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.
+   */
+  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.
+   */
+  void dispatchQueuedEvents() {
+    try {
+      Queue<EventWithHandler> events = eventsToDispatch.get();
+      EventWithHandler eventWithHandler;
+      while ((eventWithHandler = events.poll()) != null) {
+        dispatch(eventWithHandler.event, eventWithHandler.handler);
+      }
+    } finally {
+      eventsToDispatch.remove();
+    }
+  }
+
+  /**
+   * 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.
+   */
+  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);
+    }
+  }
+
+  /**
+   * 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.getUnchecked(concreteClass);
+    } catch (UncheckedExecutionException 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 = checkNotNull(event);
+      this.handler = checkNotNull(handler);
+    }
+  }
+}