Guide to Guava’s EventBus

<dependency>
    <groupId>com.google.guava</groupId>
    <artifactId>guava</artifactId>
    <version>21.0</version>
</dependency>

EventBus eventBus = new EventBus();

We create a listener class that has handler methods to receive specific events. We annotate the handler methods with @Subscribe. The method accepts as an argument an object of the same type as the event being posted:

public class EventListener {

    private static int eventsHandled;

    @Subscribe
    public void stringEvent(String event) {
        eventsHandled++;
    }
}

We can subscribe to an event by registering our EventListener class on the EventBus:

EventListener listener = new EventListener();
eventBus.register(listener);

eventBus.unregister(listener);

We can post events as well with the EventBus:

@Test
public void givenStringEvent_whenEventHandled_thenSuccess() {
    eventBus.post("String Event");
    assertEquals(1, listener.getEventsHandled());
}

 We can also specify a custom event class and post that event. We start by creating a custom event:

public class CustomEvent {
    private String action;

    // standard getters/setters and constructors
}

Adding a handler method in the EventListener class for that event:

@Subscribe
public void someCustomEvent(CustomEvent customEvent) {
    eventsHandled++;
}

We can now post our custom event:

@Test
public void givenCustomEvent_whenEventHandled_thenSuccess() {
    CustomEvent customEvent = new CustomEvent("Custom Event");
    eventBus.post(customEvent);

    assertEquals(1, listener.getEventsHandled());
}

3.7. Handling an Unsubscribed Event

We are provided with a DeadEvent class that allows us to handle any events that have no listeners. We can add a method to handle the DeadEvent class:

@Subscribe
public void handleDeadEvent(DeadEvent deadEvent) {
    eventsHandled++;
}

4. Conclusion

In this tutorial, we used a simple example as a guide on how to use the Guava EventBus.

You can find the complete source code and all code snippets for this article over on GitHub.