@Beta public class EventBus extends Object
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.
To receive events, an object should:
Subscribe annotation;register(Object) method. 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.)
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.
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.
| Constructor and Description |
|---|
EventBus()
Creates a new EventBus named “default”.
|
EventBus(String identifier)
Creates a new EventBus with the given
identifier. |
EventBus(SubscriberExceptionHandler exceptionHandler)
Creates a new EventBus with the given
SubscriberExceptionHandler. |
| Modifier and Type | Method and Description |
|---|---|
String |
identifier()
Returns the identifier for this event bus.
|
void |
post(Object event)
Posts an event to all registered subscribers.
|
void |
register(Object object)
Registers all subscriber methods on
object to receive events. |
String |
toString() |
void |
unregister(Object object)
Unregisters all subscriber methods on a registered
object. |
public EventBus()
Creates a new EventBus named “default”.
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.public EventBus(SubscriberExceptionHandler exceptionHandler)
Creates a new EventBus with the given SubscriberExceptionHandler.
exceptionHandler - Handler for subscriber exceptions.public final String identifier()
Returns the identifier for this event bus.
public void register(Object object)
Registers all subscriber methods on object to receive events.
object - object whose subscriber methods should be registered.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.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.