Guava EventBus: Concept Overview

The Guava EventBus, part of the Google Guava library for Java, implements a publish-subscribe (pub-sub) messaging pattern. This design pattern decouples the components that send events (publishers) from those that handle them (subscribers).

1. 🔁 Core Concept

The Guava EventBus allows objects to communicate with each other without being tightly coupled. Instead of directly calling methods on other objects, an object can post an event to the EventBus, and any object that is registered as a subscriber to that type of event will automatically receive it.

2. 🧠 Key Features

  • Decoupling: Publishers don’t need to know who the subscribers are.

  • Type-based routing: Events are dispatched to subscribers based on the event’s type.

  • Annotation-driven: Methods that should handle events are annotated with @Subscribe.

  • Thread-safe: Internally synchronized so that multiple threads can safely post events or register subscribers.

  • Synchronous by default: Event handlers are called in the same thread as the caller that posts the event (can be made async with AsyncEventBus).

3. 🧱 Basic Components

3.1. 1. EventBus Instance

EventBus eventBus = new EventBus();

3.2. 2. Event Class

@Builder
@AllArgsConstructor
@Getter
public class UserCreatedEvent {
    private final String username;
}

3.3. 3. Subscriber

public class UserEventListener {
    @Subscribe
    public void handleUserCreated(UserCreatedEvent event) {
        System.out.println("User created: " + event.getUsername());
    }
}

3.4. 4. Posting an Event

eventBus.register(new UserEventListener());
eventBus.post(new UserCreatedEvent("Alice"));

4. ⚡ Use Cases

  • Event-driven applications

  • UI updates in response to model changes

  • Decoupling business logic from controllers or services

  • In-process communication within complex systems