Class EventBus

java.lang.Object
com.google.common.eventbus.EventBus
Direct Known Subclasses:
AsyncEventBus
public class EventBus extends Object
Dispatches events to listeners, and provides ways for listeners to register themselves.

Avoid EventBus

We recommend against using EventBus. It was designed many years ago, and newer libraries offer better ways to decouple components and react to events.

To decouple components, we recommend a dependency-injection framework. For Android code, most apps use Dagger. For server code, common options include Guice and Spring. Frameworks typically offer a way to register multiple listeners independently and then request them together as a set (Dagger, Guice, Spring).

To react to events, we recommend a reactive-streams framework like RxJava (supplemented with its RxAndroid extension if you are building for Android) or Project Reactor. (For the basics of translating code from using an event bus to using a reactive-streams framework, see these two guides: 1, 2.) Some usages of EventBus may be better written using Kotlin coroutines, including Flow and Channels. Yet other usages are better served by individual libraries that provide specialized support for particular use cases.

Disadvantages of EventBus include:

  • It makes the cross-references between producer and subscriber harder to find. This can complicate debugging, lead to unintentional reentrant calls, and force apps to eagerly initialize all possible subscribers at startup time.
  • It uses reflection in ways that break when code is processed by optimizers/minimizers like R8 and Proguard.
  • It doesn't offer a way to wait for multiple events before taking action. For example, it doesn't offer a way to wait for multiple producers to all report that they're "ready," nor does it offer a way to batch multiple events from a single producer together.
  • It doesn't support backpressure and other features needed for resilience.
  • It doesn't provide much control of threading.
  • It doesn't offer much monitoring.
  • It doesn't propagate exceptions, so apps don't have a way to react to them.
  • It doesn't interoperate well with RxJava, coroutines, and other more commonly used alternatives.
  • It imposes requirements on the lifecycle of its subscribers. For example, if an event occurs between when one subscriber is removed and the next subscriber is added, the event is dropped.
  • Its performance is suboptimal, especially under Android.
  • It doesn't support parameterized types.
  • With the introduction of lambdas in Java 8, EventBus went from less verbose than listeners to more verbose.

EventBus Summary

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 subscriber, which accepts a single argument of the type of event desired;
  2. Mark it with a Subscribe annotation;
  3. Pass itself to an EventBus instance's register(Object) method.

Posting Events

To post an event, simply provide the event object to the post(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 subscriber 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 subscribers for an event are run in sequence, so subscribers 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 AsyncEventBus.)

Subscriber Methods

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

Subscribers 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 subscriber method from multiple threads simultaneously, unless the method explicitly allows it by bearing the AllowConcurrentEvents annotation. If this annotation is not present, subscriber methods need not worry about being reentrant, unless also called from outside the EventBus.

Dead Events

If an event is posted, but no registered subscribers 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 DeadEvent and reposted.

If a subscriber 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 Object, a subscriber registered to receive any Object will never receive a DeadEvent.

This class is safe for concurrent use.

See the Guava User Guide article on EventBus.

Since:
10.0
Author:
Cliff Biffle

  • Constructor Details

    • EventBus

      public EventBus()
      Creates a new EventBus named "default".

    • EventBus

      public EventBus(String identifier)
      Creates a new EventBus with the given identifier.
      Parameters:
      identifier - a brief name for this bus, for logging purposes. Should be a valid Java identifier.

    • EventBus

      public EventBus(SubscriberExceptionHandler exceptionHandler)
      Creates a new EventBus with the given SubscriberExceptionHandler.
      Parameters:
      exceptionHandler - Handler for subscriber exceptions.
      Since:
      16.0

  • Method Details

    • identifier

      public final String identifier()
      Returns the identifier for this event bus.
      Since:
      19.0

    • register

      public void register(Object object)
      Registers all subscriber methods on object to receive events.
      Parameters:
      object - object whose subscriber methods should be registered.

    • unregister

      public void unregister(Object object)
      Unregisters all subscriber methods on a registered object.
      Parameters:
      object - object whose subscriber methods should be unregistered.
      Throws:
      IllegalArgumentException - if the object was not previously registered.

    • post

      public void post(Object event)
      Posts an event to all registered subscribers. This method will return successfully after the event has been posted to all subscribers, and regardless of any exceptions thrown by subscribers.

      If no subscribers have been subscribed for event's class, and event is not already a DeadEvent, it will be wrapped in a DeadEvent and reposted.

      Parameters:
      event - event to post.

    • toString

      public String toString()
      Overrides:
      toString in class Object