Dispatches events to listeners, and provides ways for listeners to register
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.
To receive events, an object should:
- Expose a public method, known as the event subscriber, which accepts
a single argument of the type of event desired;
- Mark it with a
- Pass itself to an EventBus instance's
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.
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
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
AllowConcurrentEvents annotation. If this annotation is
not present, subscriber methods need not worry about being reentrant, unless
also called from outside the EventBus.
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