What does @Module means in dagger for android?

Question

I have read many blogs but still i am not able to figure out @Module annotation functioning in dagger. @Inject i got that it provides dependency injection at runtime. But what does @Module does. since the object graph is also built on module.

For ex i.e i have this snippet of code from https://github.com/AndroidBootstrap/android-bootstrap.

@Module(
    complete = false,

    injects = {
        BootstrapApplication.class,
        BootstrapAuthenticatorActivity.class,
        MainActivity.class,
        BootstrapTimerActivity.class,  
    }
)
public class BootstrapModule {
}

so what does it basically does. since i am also trying to build one application using dagger as dependency injection for android.But since I am not able to get @Module concept clearly I am just stuck.

Can anyone please help me out with some basic example or concept. I think this will be helpful for all who is using dagger.

Answer 1

If you have a look to the docs for the annotation, a @Module annotated class defines a class that contributes to the dagger object graph. In the Spring framework for example, the equivalent would be the @Configuration anntotation. It defines a configuration point for your object graph, where you declare which objects you want to be available for injection and their scopes.

As a simple example, let's say we want a singleton object to be used by any Activity in the app. It has to be created in the module:

@dagger.Module(injects = {MyActivity.class})
public class Module {

    @Provides
    @Singleton
    public MySinletonBean provideMySingleton() {
        return new MySinletonBean();
    }

}

This will create a MySingleton object which can be injected in MyActivity. This is a very basic example, but we can perform other actions in the graph there, like using dependencies in the constructors:

@dagger.Module(injects = {MyActivity.class})
public class Module {

    private DependencyBean dependency = new DependencyBean();

    @Provides
    @Singleton
    public MySinletonBean provideMySingleton() {
        return new MySinletonBean(dependency);
    }

    @Provides
    @Singleton
    public MySinletonBean provideMyOtherSingleton() {
        return new MyOtherSinletonBean(dependency);
    }

}

Then, in MyActivity we need to access the graph for the application in the onCreate method:

@Inject
MySingletonBean singleton;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.my_activity);
    ((MyApplication) getApplication()).getGraph().inject(this);
}

So, who does create the object graph here? The MyApplication class does it when your application starts (don't forger to add it in your androidManifest.xml):

public class MyApplication extends Application {

    private ObjectGraph graph;

    public ObjectGraph getGraph() {
        return graph;
    }

    @Override
    public void onCreate() {
        super.onCreate();
        graph = ObjectGraph.create(new Module(this));
        graph.inject(this);
    }

}

So the execution flow in a dagger app would be:

1. The android app starts and the MyApplication class builds the graph, parsing the @Module annotated classes and keeping an instance of it.

2. Then, the classes declared in the module can access its objects just injecting themselves in the object graph. Gradle then will evaluate their @Inject annotations and perform the dependency injections.