Class EventBus
- Direct Known Subclasses:
AsyncEventBus
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:
- Expose a public method, known as the event subscriber, which accepts a single argument of the type of event desired;
- Mark it with a
Subscribe
annotation; - 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
-
Constructor Summary
ConstructorsConstructorDescriptionEventBus()
Creates a new EventBus named "default".EventBus
(SubscriberExceptionHandler exceptionHandler) Creates a new EventBus with the givenSubscriberExceptionHandler
.Creates a new EventBus with the givenidentifier
. -
Method Summary
Modifier and TypeMethodDescriptionfinal String
Returns the identifier for this event bus.void
Posts an event to all registered subscribers.void
Registers all subscriber methods onobject
to receive events.toString()
void
unregister
(Object object) Unregisters all subscriber methods on a registeredobject
.
-
Constructor Details
-
EventBus
public EventBus()Creates a new EventBus named "default". -
EventBus
Creates a new EventBus with the givenidentifier
.- Parameters:
identifier
- a brief name for this bus, for logging purposes. Should be a valid Java identifier.
-
EventBus
Creates a new EventBus with the givenSubscriberExceptionHandler
.- Parameters:
exceptionHandler
- Handler for subscriber exceptions.- Since:
- 16.0
-
-
Method Details
-
identifier
Returns the identifier for this event bus.- Since:
- 19.0
-
register
Registers all subscriber methods onobject
to receive events.- Parameters:
object
- object whose subscriber methods should be registered.
-
unregister
Unregisters all subscriber methods on a registeredobject
.- Parameters:
object
- object whose subscriber methods should be unregistered.- Throws:
IllegalArgumentException
- if the object was not previously registered.
-
post
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, andevent
is not already aDeadEvent
, it will be wrapped in a DeadEvent and reposted.- Parameters:
event
- event to post.
-
toString
-