Dagger 2 - Singleton class created twice

Question

I have singleton class that 2 other classes depend on: another singleton and a service. The problem is that in practice each of them receives a different instance of what's supposed to be a singleton. I'm trying to figure out why. Here's the gist of it:

The singleton dependency class:

@Singleton
public class SingletonDependency {
    @Inject
    public SingletonDependency() {
        // ...
    }
}

The other singleton which depends on it:

@Singleton
public class DependentClass {
    @Inject
    public DependentClass(SingletonDependency singletonDependency) {
        // ...
    }
}

The service which depends on it:

public class DependentService extends Service {
    @Inject SingletonDependency mSingletonDependency;

    @Override
    public void onCreate() {
        CoreInjector.INSTANCE.getCoreComponent().inject(this);
    }
}

I do not provide SingletonDependency explicitly since it has a @Inject constructor and Dagger2 takes care of it for me. Is this the root of it?

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Edit:

Here's my injector:

public enum CoreInjector {

    INSTANCE;

    private CoreComponent mCoreComponent;

    public void init(Context context) { // called by the Application's onCreate()
        mCoreComponent = DaggerCoreComponent.builder()
                .coreModule(new CoreModule(context))
                .build();
    }

    public CoreComponent getCoreComponent() {
        return mCoreComponent;
    }
}

The Component itself:

@Singleton
@Component(modules = {CoreModule.class})
public interface CoreComponent {

    void inject(DependentService dependentService);

}

The Module itself doesn't provide the dependency explicitly, but once I do, everything works as expected - same instance.

---

Edit 2

Conclusions posted as an answer.

Answer 1

I figured it out. My singleton class implements an interface, which is what my dependent classes depends on - not the implementation. I guess Dagger2 can't make that leap even though the implementation has a injected constructor.

Explicit @Provides method in a module was the only way to go.