Class EventBus

  • Direct Known Subclasses:

    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.

    Cliff Biffle
    • Constructor Detail

      • EventBus

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

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

      • identifier

        public final String identifier()
        Returns the identifier for this event bus.
      • register

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

        public void unregister​(Object object)
        Unregisters all subscriber methods on a registered object.
        object - object whose subscriber methods should be unregistered.
        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.

        event - event to post.
      • toString

        public String toString()
        Description copied from class: java.lang.Object
        Returns a string representation of the object. In general, the toString method returns a string that "textually represents" this object. The result should be a concise but informative representation that is easy for a person to read. It is recommended that all subclasses override this method.

        The toString method for class Object returns a string consisting of the name of the class of which the object is an instance, the at-sign character `@', and the unsigned hexadecimal representation of the hash code of the object. In other words, this method returns a string equal to the value of:

         getClass().getName() + '@' + Integer.toHexString(hashCode())
        toString in class Object
        a string representation of the object.