Testing with Dagger

When using Dagger inside the application, testing the app requires a testcomponent to be present. This component contains all the dependencies the main module provides, but modified in a way that doesn’t require a connection for example.

The component itself can just extend the main component and then use modules to override certain dependencies. Inside the modules, Mockito methods such as spy() and mock() can be used to create the required instances. If specific behaviour is required, the instances can also be created manually.

@Component(modules = {MainModule.class, TestModule.class})
@Singleton
public interface TestComponent extends MainComponent {

    @Component.Builder
    interface Builder extends MainComponent.Builder {
        TestComponent build();
    }
}

@Module
public class TestModule {

    @Provides
    GameService gameService() {
        return new GameService(new Random(42));
    }

}

Now that the component and modules exist, we have to create a way of setting the component our app uses. This step however is dependent on how the application is structured. The easiest way is to create a setter method and call it, before the app starts.

// ...
protected TestComponent testComponent;

@Override
public void start(Stage stage) throws Exception {
    super.start(stage);
    this.testComponent = (TestComponent) DaggerTestComponent.builder().mainApp(app).build();
    app.setComponent(testComponent);
    app.start(stage);
    stage.requestFocus();
}

// ...

The component instance makes it possible to inject services from test classes e.g. AppTest to redefine their behavior.

public class AppTest extends ControllerTest {
    // ...
    
    @BeforeEach
    void setup() {
        final AuthApiService authApiService = testComponent.authApiService();
        // ...
    }
    
    // ...
}

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⬅ Testing SubComponents

Overview