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   * Copyright (c) 2007-2012 Sonatype, Inc. All rights reserved.
   * This program is licensed to you under the Apache License Version 2.0,
   * and you may not use this file except in compliance with the Apache License Version 2.0.
   * You may obtain a copy of the Apache License Version 2.0 at
   * Unless required by applicable law or agreed to in writing,
   * software distributed under the Apache License Version 2.0 is distributed on an
  * See the Apache License Version 2.0 for the specific language governing permissions and limitations there under.
 package org.sonatype.sisu.goodies.eventbus;

Dispatches events to listeners, and provides ways for listeners to register themselves.

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 not a general-purpose publish-subscribe system, nor is it intended for interprocess communication.

Receiving Events

To receive events, an object should:
  1. Expose a public method, known as the event handler, which accepts a single argument of the type of event desired;
  2. Mark it with a annotation;
  3. Pass itself to an EventBus instance's register(java.lang.Object) method.

Posting Events

To post an event, simply provide the event object to the post(java.lang.Object) method. The EventBus instance will determine the type of event and route it to all registered listeners.

Events are routed based on their type — an event will be delivered to any handler for any type to which the event is assignable. This includes implemented interfaces, all superclasses, and all interfaces implemented by superclasses.

When 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 org.sonatype.sisu.goodies.eventbus.internal.guava.AsyncEventBus.)

Handler Methods

Event handler methods must accept only one argument: the event.

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.

The EventBus guarantees that it will not call a handler method from multiple threads simultaneously, unless the method explicitly allows it by bearing the annotation. If this annotation is not present, handler methods need not worry about being reentrant, unless also called from outside the EventBus.

Dead Events

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 org.sonatype.sisu.goodies.eventbus.internal.guava.DeadEvent and reposted.

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 java.lang.Object, a handler registered to receive any Object will never receive a DeadEvent.

This class is safe for concurrent use.

 public interface EventBus

Registers an event handler with this event bus.

handler to be registered
itself, for fluent api usage
   EventBus register(Object handler);

Unregisters an event handler from this event bus.

handler to be registered
itself, for fluent api usage
   EventBus unregister(Object handler);

Posts an event. Event bus will notify all previously registered handlers about this event.

event an event
itself, for fluent api usage
  EventBus post(Object event);
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