Guava EventBus

Guava is Google’s utilities, collections and helpers library for Java. Similar in spirit to the Apache Commons set of libraries, but possibly a bit more pragmatic. Google’s engineers use the Guava components in their daily work, and have fine-tuned the APIs to boost their own productivity. The current release is version 21.

In this article, we’ll look at the Guava EventBus class, for publishing and subscribing to application wide events of all kinds. It strikes a good balance between convenience and type-safety, and is much more lightweight than rolling your own Event, Handler and Fire classes. See also the Guava documentations for further details.

To have an example to work with, the following Player skeleton class is implemented. It comes with three separate Event classes, which are very minimal in this contrived example. In a real application, they might inherit from a super-Event class, and possibly carry some content, at least the source of the event or similar. Alternatively, they could have been made simpler, by being elements of an enumeration, however, then we’d have to do manual matching and routing on the receiving end.

Notice, that the receiving part of the EventBus events is already in place, in the form of the @Subscribe annotations.

  class Player {
    boolean playing;
    boolean paused;

    @Subscribe
    void play(PlayEvent e) {
      System.out.println("play");
      playing = true;
      paused = false;
    }

    @Subscribe
    void pause(PauseEvent e) {
      System.out.println("pause");
      playing = false;
      paused = true;
    }

    @Subscribe
    void stop(StopEvent e) {
      System.out.println("stop");
      playing = false;
      paused = false;
    }
  }

  class PlayEvent {}

  class PauseEvent {}

  class StopEvent {}

In the following code, the EventBus is created, and the Player is registered as a receiver. To publish events, all that is required is to pass objects of the same types to the EventBus.post() method. The internals of the EventBus will invoke all methods which are marked with the @Subscribe annotation and match the exact type. Event types are not inherited, so a catch-all subscriber is not possible, which is probably a good design choice.

The EventBus also makes unit testing easier, since sender and receiver can be decoupled without extensive mocking. In fact, we now have a choice of testing the methods of the Player class through the Eventbus, or by invoking its methods directly. Either might be fine when writing unit tests. Although, for component level test, sticking with the EventBus is probably better, as it will be closer to the live application.

  private Player player;
  private EventBus bus;

  @Before
  public void setup() {
    player = new Player();

    bus = new EventBus();
    bus.register(player);
  }

  @Test
  public void testPlay() {
    bus.post(new PlayEvent());

    assertTrue(player.playing);
    assertFalse(player.paused);
  }

To include the Guava library from the Maven Gradle repository, this will suffice.

repositories {
  mavenCentral()
}

dependencies {
  compile 'com.google.guava:guava:21.0'
}

Here is the full code listing.

/* Copyright rememberjava.com. Licensed under GPL 3. See http://rememberjava.com/license */
package com.rememberjava.guava;

import static org.junit.Assert.assertFalse;
import static org.junit.Assert.assertTrue;

import org.junit.Before;
import org.junit.Test;

import com.google.common.eventbus.EventBus;
import com.google.common.eventbus.Subscribe;

public class EventBusTest {

  class Player {
    boolean playing;
    boolean paused;

    @Subscribe
    void play(PlayEvent e) {
      System.out.println("play");
      playing = true;
      paused = false;
    }

    @Subscribe
    void pause(PauseEvent e) {
      System.out.println("pause");
      playing = false;
      paused = true;
    }

    @Subscribe
    void stop(StopEvent e) {
      System.out.println("stop");
      playing = false;
      paused = false;
    }
  }

  class PlayEvent {}

  class PauseEvent {}

  class StopEvent {}

  private Player player;
  private EventBus bus;

  @Before
  public void setup() {
    player = new Player();

    bus = new EventBus();
    bus.register(player);
  }

  @Test
  public void testPlay() {
    bus.post(new PlayEvent());

    assertTrue(player.playing);
    assertFalse(player.paused);
  }

  @Test
  public void testAll() {
    bus.post(new Object());

    assertFalse(player.playing);
    assertFalse(player.paused);
  }
}