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Guava is a suite of core and expanded libraries that include utility classes, google's collections, io classes, and much much more. Guava has only one code dependency - javax.annotation, per the JSR-305 spec.

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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 java.util.concurrent.ConcurrentLinkedQueue;
import java.util.concurrent.Executor;

/**
 * An {@link EventBus} that takes the Executor of your choice and uses it to
 * dispatch events, allowing dispatch to occur asynchronously.
 *
 * @author Cliff Biffle
 * @since 10.0
 */
@Beta
public class AsyncEventBus extends EventBus {
  private final Executor executor;

  /** the queue of events is shared across all threads */
  private final ConcurrentLinkedQueue eventsToDispatch =
      new ConcurrentLinkedQueue();

  /**
   * Creates a new AsyncEventBus that will use {@code executor} to dispatch
   * events.  Assigns {@code identifier} as the bus's name for logging purposes.
   *
   * @param identifier short name for the bus, for logging purposes.
   * @param executor   Executor to use to dispatch events. It is the caller's
   *        responsibility to shut down the executor after the last event has
   *        been posted to this event bus.
   */
  public AsyncEventBus(String identifier, Executor executor) {
    super(identifier);
    this.executor = checkNotNull(executor);
  }

  /**
   * Creates a new AsyncEventBus that will use {@code executor} to dispatch
   * events.
   *
   * @param executor Executor to use to dispatch events. It is the caller's
   *        responsibility to shut down the executor after the last event has
   *        been posted to this event bus.
   * @param subscriberExceptionHandler Handler used to handle exceptions thrown from subscribers.
   *    See {@link SubscriberExceptionHandler} for more information.
   * @since 16.0
   */
  public AsyncEventBus(Executor executor, SubscriberExceptionHandler subscriberExceptionHandler) {
    super(subscriberExceptionHandler);
    this.executor = checkNotNull(executor);
  }

  /**
   * Creates a new AsyncEventBus that will use {@code executor} to dispatch
   * events.
   *
   * @param executor Executor to use to dispatch events. It is the caller's
   *        responsibility to shut down the executor after the last event has
   *        been posted to this event bus.
   */
  public AsyncEventBus(Executor executor) {
    super("default");
    this.executor = checkNotNull(executor);
  }

  @Override
  void enqueueEvent(Object event, EventSubscriber subscriber) {
    eventsToDispatch.offer(new EventWithSubscriber(event, subscriber));
  }

  /**
   * Dispatch {@code events} in the order they were posted, regardless of
   * the posting thread.
   */
  @SuppressWarnings("deprecation") // only deprecated for external subclasses
  @Override
  protected void dispatchQueuedEvents() {
    while (true) {
      EventWithSubscriber eventWithSubscriber = eventsToDispatch.poll();
      if (eventWithSubscriber == null) {
        break;
      }

      dispatch(eventWithSubscriber.event, eventWithSubscriber.subscriber);
    }
  }

  /**
   * Calls the {@link #executor} to dispatch {@code event} to {@code subscriber}.
   */
  @Override
  void dispatch(final Object event, final EventSubscriber subscriber) {
    checkNotNull(event);
    checkNotNull(subscriber);
    executor.execute(
        new Runnable() {
          @Override
          public void run() {
            AsyncEventBus.super.dispatch(event, subscriber);
          }
        });
  }
}