Google Guava in version number 10 introduced new package eventbus with a few very interesting classes to deal with listener (or publisher – subscriber) use case. Below I present my short introduction to EventBus class and its family.

Basics

To listen to some events we need a listener class. Such class created in google-guava-way doesn’t have to implement any particular interface or extend any specified class. It can be any class with just one required element: a method marked with @Subscribe annotation:

public class EventListener {

    public int lastMessage = 0;

    @Subscribe
    public void listen(OurTestEvent event) {
        lastMessage = event.getMessage();
    }

    public int getLastMessage() {
        return lastMessage;
    }
}

lastMessage property is used in tests below to check if events were received successfully.

And of course we need an event class to send it around:

public class OurTestEvent {

    private final int message;

    public OurTestEvent(int message) {
        this.message = message;
    }

    public int getMessage() {
        return message;
    }
}

How it works

The best way to show something in action is to write some tests, so let’s see how simple usage of EventBus looks like:

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