Dagger2 Module not getting injected in other module or is deprecated

Question

I am trying to learn Dagger2 but module is coming as deprecated. I am following tutorial from http://www.vogella.com/tutorials/Dagger/article.html Here is my code as follow.

public class TestApp {

    @Inject
    BackendService backendService;

    private void testServer() {
        BackendComponent backendComponent = DaggerBackendComponent.builder()
                .build();
        backendComponent.inject(this);
        backendService.call();
    }

    public static void main(String[] args) {
        new TestApp().testServer();
    }
}

UserModule whose injection is not working

@Module
public class UserModule {

    @Provides
    @Singleton
    User providesUser() {
        return new User("John", "Doe");
    }
}

BackendService

public class BackendService {

    @Inject
    public User user;

    private String serverUrl;

    @Inject
    public BackendService(@Named("serverUrl") String serverUrl) {
        this.serverUrl = serverUrl;
    }

    public boolean call() {
        if (user != null && serverUrl != null && serverUrl.length() > 0) {
            System.out.println("User: " + user.toString());
            return true;
        } else {
            System.err.println("User: " + user);
            System.err.println("ServerUrl: " + serverUrl);
            return false;
        }
    }

}

BackendServiceModule

@Module
public class BackendServiceModule {

    @Provides
    @Singleton
    BackendService proviedBackendServiceModule(@Named("serverUrl") String serverUrl) {
        return new BackendService(serverUrl);
    }

    @Provides
    @Named("testUrl")
    String provideAnotherUrl() {
        return "http://www.facebook.com";
    }

    @Provides
    @Named("serverUrl")
    String provideServerUrl() {
        return "http://www.google.com";
    }

}

BackendComponent

@Singleton
@Component(modules = {UserModule.class, BackendServiceModule.class})
public interface BackendComponent {


    BackendService proviedBackendServiceModule();

    void inject(TestApp app);

}

I am getting output as follows after running:

User: null
ServerUrl: http://www.google.com

If i try following code its working

public class TestApp {

    @Inject
    User user;
    @Inject
    BackendService backendService;

    private void testServer() {
        BackendComponent backendComponent = DaggerBackendComponent.builder()
                .build();
        backendComponent.inject(this);
        backendService.call();
        System.out.println("User: " + user);
    }


    public static void main(String[] args) {
        new TestApp().testServer();
    }
}

User: null
ServerUrl: http://www.google.com
User: User [firstName=John, lastName=Doe]

Answer 1

The reason for the your problem is the way dagger works. It only injects fields annotated with @Inject on the class you specified as parameter of the inject method. Therefore nothing inside your BackendService annotated with @Inject will be injected.

public class TestApp {

    @Inject <-- this is injected
    User user;
    @Inject <-- this is injected
    BackendService backendService; <-- Nothing inside this class will be injected

I would recommend to pass your user object to your BackendService inside the constructor. Just do something like the this:

 @Provides
    @Singleton
    BackendService proviedBackendServiceModule(@Named("serverUrl") String serverUrl, User user) {
        return new BackendService(serverUrl, user);
    }

Another solution would be to define an inject method for the BackendService inside your component, but it will get messy passing around the component object.