- Direct Known Subclasses:
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:
- 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.
postis 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 bearing the
AllowConcurrentEventsannotation. 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
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
- Cliff Biffle
Constructors Constructor Description
EventBus()Creates a new EventBus named "default".
EventBus(SubscriberExceptionHandler exceptionHandler)Creates a new EventBus with the given
EventBus(String identifier)Creates a new EventBus with the given
All Methods Instance Methods Concrete Methods Modifier and Type Method Description
identifier()Returns the identifier for this event bus.
post(Object event)Posts an event to all registered subscribers.
register(Object object)Registers all subscriber methods on
objectto receive events.
toString()Returns a string representation of the object.
unregister(Object object)Unregisters all subscriber methods on a registered
public EventBus()Creates a new EventBus named "default".
EventBusCreates a new EventBus with the given
identifier- a brief name for this bus, for logging purposes. Should be a valid Java identifier.
registerRegisters all subscriber methods on
objectto receive events.
object- object whose subscriber methods should be registered.
public void unregister(Object object)Unregisters all subscriber methods on a registered
object- object whose subscriber methods should be unregistered.
IllegalArgumentException- if the object was not previously registered.
postPosts 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
eventis not already a
DeadEvent, it will be wrapped in a DeadEvent and reposted.
event- event to post.
toStringDescription copied from class:
java.lang.ObjectReturns a string representation of the object. In general, the
toStringmethod 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.
toStringmethod for class
Objectreturns 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())