An Introduction to Dagger 2 (Android DI) – Part 2

In the previous article, we have a quick introduction to Dagger 2 with a simple introductory example. In this article, I will show you how to use Android Build variants with Dagger 2 to have different implementations (for the service interface) that are switched automatically when the app is in the debug mode or in the release mode.

First of all, under the release directory, let’s create a new component DaggerGraphComponent which extends from DaggerGraph as follows.

@Component(modules = {MainModule.class, ServiceModule.class})
public interface DaggerGraphComponent extends DaggerGraph {

    static final class Initializer {
        private Initializer() {

        public static DaggerGraph init(DaggerApplication app) {
            return DaggerDaggerGraphComponent.builder()
                                             .mainModule(new MainModule(app))

Secondly, create a service module which will be responsible for creating the instances of HelloService implementations (in debug and release modes).

For the release mode, ServiceModule provides only an instance of the HelloServiceReleaseManager class which implements HelloService.

public class ServiceModule {

    HelloService provideHelloService() {
        return new HelloServiceReleaseManager();

Below is HelloServiceReleaseManager simple implementation.

public class HelloServiceReleaseManager implements HelloService {

    public String greet(String userName) {
        return "Hello " + userName + "! [Release]";

Apply the same steps under the debug directory, in debug case, you can have a HelloServiceDebugManager class which implements HelloService interface as shown below.

public class HelloServiceDebugManager implements HelloService {

    public String greet(String userName) {
        return "Hello " + userName + "! [Debug]";

Make sure to have the following code hierarchy in release and debug.
Release Directory

Debug Screenshot

Running the project

Now, you can run the app in debug mode, and then enter some name and then click “Greet!” button, you will find the debug service response as follows.
Debug Mode

And then change the build variant to run the app in the release mode and do the same previous steps to see the results as shown below.
Release Mode

Checkout the code

Check the sample app code in GitHub:

What is next?

So in the next article, I will discuss how can we apply unit testing techniques for Dagger 2 applications. Stay tuned!

This entry was posted in Android and tagged , , , , , by Hazem Saleh. Bookmark the permalink.

About Hazem Saleh

Hazem Saleh has more than eleven years of experience in Cloud, Mobile and Open Source technologies. He worked as a software engineer, technical leader, application architect, and technical consultant for many clients around the world. He is an Apache PMC (Project Management Committee) member and a person who spent many years of his life writing open source software. Beside being the author of the "JavaScript Unit Testing" book, "JavaScript Mobile Application Development" book, "Pro JSF and HTML5" book and the co-author of the "Definitive guide to Apache MyFaces" book, Hazem is also an author of many technical articles, a developerWorks contributing author and a technical speaker in both local and international conferences such as ApacheCon North America, Geecon, JavaLand, JSFDays, CON-FESS Vienna and JavaOne. Hazem is an XIBMer, he worked in IBM for ten years. Now, He is working for Nickelodeon New York as a Mobile Architect. He is also an OpenGroup Master Certified Specialist.