com.google.common.eventbus
Date: 2016-05-24

# Package com.google.common.eventbus

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).

See: Description

- Interface Summary
  Interface | Description
  --- | ---
  SubscriberExceptionHandler | Handler for exceptions thrown by event subscribers.

- Class Summary
  Class | Description
  --- | ---
  AsyncEventBus | An `EventBus` that takes the Executor of your choice and uses it to dispatch events, allowing dispatch to occur asynchronously.
  DeadEvent | Wraps an event that was posted, but which had no subscribers and thus could not be delivered.
  EventBus | Dispatches events to listeners, and provides ways for listeners to register themselves.
  SubscriberExceptionContext | Context for an exception thrown by a subscriber.

- Annotation Types Summary
  Annotation Type | Description
  --- | ---
  AllowConcurrentEvents | Marks an event subscriber method as being thread-safe.
  Subscribe | Marks a method as an event subscriber.

## Package com.google.common.eventbus Description

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.

See the Guava User Guide article on `EventBus`.

## One-Minute Guide

Converting an existing EventListener-based system to use the EventBus is easy.

### For Listeners

To listen for a specific flavor of event (say, a CustomerChangeEvent)...

- ...in traditional Java events: implement an interface defined with the event — such as CustomerChangeEventListener.
- ...with EventBus: create a method that accepts CustomerChangeEvent as its sole argument, and mark it with the `Subscribe` annotation.

To register your listener methods with the event producers...

- ...in traditional Java events: pass your object to each producer's `registerCustomerChangeEventListener` method. These methods are rarely defined in common interfaces, so in addition to knowing every possible producer, you must also know its type.
- ...with EventBus: pass your object to the `EventBus.register(Object)` method on an EventBus. You'll need to make sure that your object shares an EventBus instance with the event producers.

To listen for a common event supertype (such as EventObject or Object)...

- ...in traditional Java events: not easy.
- ...with EventBus: events are automatically dispatched to listeners of any supertype, allowing listeners for interface types or "wildcard listeners" for Object.

To listen for and detect events that were dispatched without listeners...

- ...in traditional Java events: add code to each event-dispatching method (perhaps using AOP).
- ...with EventBus: subscribe to `DeadEvent`. The EventBus will notify you of any events that were posted but not delivered. (Handy for debugging.)

### For Producers

To keep track of listeners to your events...

- ...in traditional Java events: write code to manage a list of listeners to your object, including synchronization, or use a utility class like EventListenerList.
- ...with EventBus: EventBus does this for you.

To dispatch an event to listeners...

- ...in traditional Java events: write a method to dispatch events to each event listener, including error isolation and (if desired) asynchronicity.
- ...with EventBus: pass the event object to an EventBus's `EventBus.post(Object)` method.

## Glossary

The EventBus system and code use the following terms to discuss event distribution:

Event
  Any object that may be posted to a bus.
Subscribing
  The act of registering a listener with an EventBus, so that its subscriber methods will receive events.
Listener
  An object that wishes to receive events, by exposing subscriber methods.
Subscriber method
  A public method that the EventBus should use to deliver posted events. Subscriber methods are marked by the `Subscribe` annotation.
Posting an event
  Making the event available to any listeners through the EventBus.

## FAQ

### Why must I create my own Event Bus, rather than using a singleton?

The Event Bus doesn't specify how you use it; there's nothing stopping your application from having separate EventBus instances for each component, or using separate instances to separate events by context or topic. This also makes it trivial to set up and tear down EventBus objects in your tests.

Of course, if you'd like to have a process-wide EventBus singleton, there's nothing stopping you from doing it that way. Simply have your container (such as Guice) create the EventBus as a singleton at global scope (or stash it in a static field, if you're into that sort of thing).

In short, the EventBus is not a singleton because we'd rather not make that decision for you. Use it how you like.

### Why use an annotation to mark subscriber methods, rather than requiring the listener to implement an interface?

We feel that the Event Bus's `@Subscribe` annotation conveys your intentions just as explicitly as implementing an interface (or perhaps more so), while leaving you free to place event subscriber methods wherever you wish and give them intention-revealing names.

Traditional Java Events use a listener interface which typically sports only a handful of methods -- typically one. This has a number of disadvantages:

- Any one class can only implement a single response to a given event.
- Listener interface methods may conflict.
- The method must be named after the event (e.g. `handleChangeEvent`), rather than its purpose (e.g. `recordChangeInJournal`).
- Each event usually has its own interface, without a common parent interface for a family of events (e.g. all UI events).

The difficulties in implementing this cleanly has given rise to a pattern, particularly common in Swing apps, of using tiny anonymous classes to implement event listener interfaces.

Compare these two cases:

       class ChangeRecorder {
         void setCustomer(Customer cust) {
           cust.addChangeListener(new ChangeListener() {
             void customerChanged(ChangeEvent e) {
               recordChange(e.getChange());
             }
           };
         }
       }

       // Class is typically registered by the container.
       class EventBusChangeRecorder {
         @Subscribe void recordCustomerChange(ChangeEvent e) {
           recordChange(e.getChange());
         }
       }

The intent is actually clearer in the second case: there's less noise code, and the event subscriber has a clear and meaningful name.

### What about a generic `Subscriber<T>` interface?

Some have proposed a generic `Subscriber<T>` interface for EventBus listeners. This runs into issues with Java's use of type erasure, not to mention problems in usability.

Let's say the interface looked something like the following:

       interface Subscriber<T> {
         void handleEvent(T event);
       }

Due to erasure, no single class can implement a generic interface more than once with different type parameters. This is a giant step backwards from traditional Java Events, where even if `actionPerformed` and `keyPressed` aren't very meaningful names, at least you can implement both methods!

### Doesn't EventBus destroy static typing and eliminate automated refactoring support?

Some have freaked out about EventBus's `register(Object)` and `post(Object)` methods' use of the `Object` type.

`Object` is used here for a good reason: the Event Bus library places no restrictions on the types of either your event listeners (as in `register(Object)`) or the events themselves (in `post(Object)`).

Event subscriber methods, on the other hand, must explicitly declare their argument type -- the type of event desired (or one of its supertypes). Thus, searching for references to an event class will instantly find all subscriber methods for that event, and renaming the type will affect all subscriber methods within view of your IDE (and any code that creates the event).

It's true that you can rename your `@Subscribed` event subscriber methods at will; Event Bus will not stop this or do anything to propagate the rename because, to Event Bus, the names of your subscriber methods are irrelevant. Test code that calls the methods directly, of course, will be affected by your renaming -- but that's what your refactoring tools are for.

### What happens if I `register` a listener without any subscriber methods?

Nothing at all.

The Event Bus was designed to integrate with containers and module systems, with Guice as the prototypical example. In these cases, it's convenient to have the container/factory/environment pass every created object to an EventBus's `register(Object)` method.

This way, any object created by the container/factory/environment can hook into the system's event model simply by exposing subscriber methods.

### What Event Bus problems can be detected at compile time?

Any problem that can be unambiguously detected by Java's type system. For example, defining a subscriber method for a nonexistent event type.

### What Event Bus problems can be detected immediately at registration?

Immediately upon invoking `register(Object)` , the listener being registered is checked for the well-formedness of its subscriber methods. Specifically, any methods marked with `@Subscribe` must take only a single argument.

Any violations of this rule will cause an `IllegalArgumentException` to be thrown.

(This check could be moved to compile-time using APT, a solution we're researching.)

### What Event Bus problems may only be detected later, at runtime?

If a component posts events with no registered listeners, it may indicate an error (typically an indication that you missed a `@Subscribe` annotation, or that the listening component is not loaded).

(Note that this is not necessarily indicative of a problem. There are many cases where an application will deliberately ignore a posted event, particularly if the event is coming from code you don't control.)

To handle such events, register a subscriber method for the `DeadEvent` class. Whenever EventBus receives an event with no registered subscribers, it will turn it into a `DeadEvent` and pass it your way -- allowing you to log it or otherwise recover.

### How do I test event listeners and their subscriber methods?

Because subscriber methods on your listener classes are normal methods, you can simply call them from your test code to simulate the EventBus.