1/*
2 * Copyright (C) 2007 The Guava Authors
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 * http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17package com.google.common.eventbus;
18
19import static com.google.common.base.Preconditions.checkNotNull;
20
21import com.google.common.annotations.Beta;
22
23import java.util.concurrent.ConcurrentLinkedQueue;
24import java.util.concurrent.Executor;
25
26/**
27 * An {@link EventBus} that takes the Executor of your choice and uses it to
28 * dispatch events, allowing dispatch to occur asynchronously.
29 *
30 * @author Cliff Biffle
31 * @since 10.0
32 */
33@Beta
34public class AsyncEventBus extends EventBus {
35  private final Executor executor;
36
37  /** the queue of events is shared across all threads */
38  private final ConcurrentLinkedQueue<EventWithSubscriber> eventsToDispatch =
39      new ConcurrentLinkedQueue<EventWithSubscriber>();
40
41  /**
42   * Creates a new AsyncEventBus that will use {@code executor} to dispatch
43   * events.  Assigns {@code identifier} as the bus's name for logging purposes.
44   *
45   * @param identifier short name for the bus, for logging purposes.
46   * @param executor   Executor to use to dispatch events. It is the caller's
47   *        responsibility to shut down the executor after the last event has
48   *        been posted to this event bus.
49   */
50  public AsyncEventBus(String identifier, Executor executor) {
51    super(identifier);
52    this.executor = checkNotNull(executor);
53  }
54
55  /**
56   * Creates a new AsyncEventBus that will use {@code executor} to dispatch
57   * events.
58   *
59   * @param executor Executor to use to dispatch events. It is the caller's
60   *        responsibility to shut down the executor after the last event has
61   *        been posted to this event bus.
62   * @param subscriberExceptionHandler Handler used to handle exceptions thrown from subscribers.
63   *    See {@link SubscriberExceptionHandler} for more information.
64   * @since 16.0
65   */
66  public AsyncEventBus(Executor executor, SubscriberExceptionHandler subscriberExceptionHandler) {
67    super(subscriberExceptionHandler);
68    this.executor = checkNotNull(executor);
69  }
70
71  /**
72   * Creates a new AsyncEventBus that will use {@code executor} to dispatch
73   * events.
74   *
75   * @param executor Executor to use to dispatch events. It is the caller's
76   *        responsibility to shut down the executor after the last event has
77   *        been posted to this event bus.
78   */
79  public AsyncEventBus(Executor executor) {
80    super("default");
81    this.executor = checkNotNull(executor);
82  }
83
84  @Override
85  void enqueueEvent(Object event, EventSubscriber subscriber) {
86    eventsToDispatch.offer(new EventWithSubscriber(event, subscriber));
87  }
88
89  /**
90   * Dispatch {@code events} in the order they were posted, regardless of
91   * the posting thread.
92   */
93  @SuppressWarnings("deprecation") // only deprecated for external subclasses
94  @Override
95  protected void dispatchQueuedEvents() {
96    while (true) {
97      EventWithSubscriber eventWithSubscriber = eventsToDispatch.poll();
98      if (eventWithSubscriber == null) {
99        break;
100      }
101
102      dispatch(eventWithSubscriber.event, eventWithSubscriber.subscriber);
103    }
104  }
105
106  /**
107   * Calls the {@link #executor} to dispatch {@code event} to {@code subscriber}.
108   */
109  @Override
110  void dispatch(final Object event, final EventSubscriber subscriber) {
111    checkNotNull(event);
112    checkNotNull(subscriber);
113    executor.execute(
114        new Runnable() {
115          @Override
116          public void run() {
117            AsyncEventBus.super.dispatch(event, subscriber);
118          }
119        });
120  }
121}
122