Dagger 2，在模块中提供应用程序上下文

@Module
public class AppModule {
    private Context context;

    public AppModule(Context context) {
        this.context = context;
    }

    @Provides
    @Singleton
    Context provideApplicationContext() {
        return context.getApplicationContext();
    }
}

@Component(modules = {AppModule.class})
@Singleton
public interface AppComponent {
    void inject(MainActivity activity);
    // ...
}

public class MyApp extends Application {
    private AppComponent appComponent;

    @Override
    public void onCreate() {
        super.onCreate();
        appComponent = DaggerAppComponent.builder()
                .appModule(new AppModule(this))
                .build();
    }

    public AppComponent getAppComponent() {
        return appComponent;
    }
}

public class MainActivity extends AppCompatActivity {
    @Inject
    Context applicationContext;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        ((MyApp) getApplicationContext()).getAppComponent().inject(this);
        // ...
    }
}