Dispatcher.java
/*
* Copyright (C) 2014 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.collect.Queues;
import java.util.Iterator;
import java.util.Queue;
import java.util.concurrent.ConcurrentLinkedQueue;
/**
* Handler for dispatching events to subscribers, providing different event ordering guarantees that
* make sense for different situations.
*
* <p><b>Note:</b> The dispatcher is orthogonal to the subscriber's {@code Executor}. The dispatcher
* controls the order in which events are dispatched, while the executor controls how (i.e. on which
* thread) the subscriber is actually called when an event is dispatched to it.
*
* @author Colin Decker
*/
@ElementTypesAreNonnullByDefault
abstract class Dispatcher {
/**
* Returns a dispatcher that queues events that are posted reentrantly on a thread that is already
* dispatching an event, guaranteeing that all events posted on a single thread are dispatched to
* all subscribers in the order they are posted.
*
* <p>When all subscribers are dispatched to using a <i>direct</i> executor (which dispatches on
* the same thread that posts the event), this yields a breadth-first dispatch order on each
* thread. That is, all subscribers to a single event A will be called before any subscribers to
* any events B and C that are posted to the event bus by the subscribers to A.
*/
static Dispatcher perThreadDispatchQueue() {
return new PerThreadQueuedDispatcher();
}
/**
* Returns a dispatcher that queues events that are posted in a single global queue. This behavior
* matches the original behavior of AsyncEventBus exactly, but is otherwise not especially useful.
* For async dispatch, an {@linkplain #immediate() immediate} dispatcher should generally be
* preferable.
*/
static Dispatcher legacyAsync() {
return new LegacyAsyncDispatcher();
}
/**
* Returns a dispatcher that dispatches events to subscribers immediately as they're posted
* without using an intermediate queue to change the dispatch order. This is effectively a
* depth-first dispatch order, vs. breadth-first when using a queue.
*/
static Dispatcher immediate() {
return ImmediateDispatcher.INSTANCE;
}
/** Dispatches the given {@code event} to the given {@code subscribers}. */
abstract void dispatch(Object event, Iterator<Subscriber> subscribers);
/** Implementation of a {@link #perThreadDispatchQueue()} dispatcher. */
private static final class PerThreadQueuedDispatcher extends Dispatcher {
// This dispatcher matches the original dispatch behavior of EventBus.
/** Per-thread queue of events to dispatch. */
private final ThreadLocal<Queue<Event>> queue =
new ThreadLocal<Queue<Event>>() {
@Override
protected Queue<Event> initialValue() {
return Queues.newArrayDeque();
}
};
/** Per-thread dispatch state, used to avoid reentrant event dispatching. */
private final ThreadLocal<Boolean> dispatching =
new ThreadLocal<Boolean>() {
@Override
protected Boolean initialValue() {
return false;
}
};
@Override
void dispatch(Object event, Iterator<Subscriber> subscribers) {
checkNotNull(event);
checkNotNull(subscribers);
Queue<Event> queueForThread = queue.get();
queueForThread.offer(new Event(event, subscribers));
if (!dispatching.get()) {
dispatching.set(true);
try {
Event nextEvent;
while ((nextEvent = queueForThread.poll()) != null) {
while (nextEvent.subscribers.hasNext()) {
nextEvent.subscribers.next().dispatchEvent(nextEvent.event);
}
}
} finally {
dispatching.remove();
queue.remove();
}
}
}
private static final class Event {
private final Object event;
private final Iterator<Subscriber> subscribers;
private Event(Object event, Iterator<Subscriber> subscribers) {
this.event = event;
this.subscribers = subscribers;
}
}
}
/** Implementation of a {@link #legacyAsync()} dispatcher. */
private static final class LegacyAsyncDispatcher extends Dispatcher {
// This dispatcher matches the original dispatch behavior of AsyncEventBus.
//
// We can't really make any guarantees about the overall dispatch order for this dispatcher in
// a multithreaded environment for a couple reasons:
//
// 1. Subscribers to events posted on different threads can be interleaved with each other
// freely. (A event on one thread, B event on another could yield any of
// [a1, a2, a3, b1, b2], [a1, b2, a2, a3, b2], [a1, b2, b3, a2, a3], etc.)
// 2. It's possible for subscribers to actually be dispatched to in a different order than they
// were added to the queue. It's easily possible for one thread to take the head of the
// queue, immediately followed by another thread taking the next element in the queue. That
// second thread can then dispatch to the subscriber it took before the first thread does.
//
// All this makes me really wonder if there's any value in queueing here at all. A dispatcher
// that simply loops through the subscribers and dispatches the event to each would actually
// probably provide a stronger order guarantee, though that order would obviously be different
// in some cases.
/** Global event queue. */
private final ConcurrentLinkedQueue<EventWithSubscriber> queue =
Queues.newConcurrentLinkedQueue();
@Override
void dispatch(Object event, Iterator<Subscriber> subscribers) {
checkNotNull(event);
while (subscribers.hasNext()) {
queue.add(new EventWithSubscriber(event, subscribers.next()));
}
EventWithSubscriber e;
while ((e = queue.poll()) != null) {
e.subscriber.dispatchEvent(e.event);
}
}
private static final class EventWithSubscriber {
private final Object event;
private final Subscriber subscriber;
private EventWithSubscriber(Object event, Subscriber subscriber) {
this.event = event;
this.subscriber = subscriber;
}
}
}
/** Implementation of {@link #immediate()}. */
private static final class ImmediateDispatcher extends Dispatcher {
private static final ImmediateDispatcher INSTANCE = new ImmediateDispatcher();
@Override
void dispatch(Object event, Iterator<Subscriber> subscribers) {
checkNotNull(event);
while (subscribers.hasNext()) {
subscribers.next().dispatchEvent(event);
}
}
}
}