Dagger 2:Cannot be provided without an @Inject constructor or from an @Provides-annotated method

Question

I have tried dagger2 in my sample application after completion i try to build my code it is showing this exception.

code:

    @Singleton


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

        BackendServiceModule provideBackendService();

        // allow to inject into our Main class
        // method name not important
        void inject(MainActivity mainActivity);
    }

    public class AppGlobal extends Application
    {
        AppComponent appComponent;
        public static AppGlobal global;

        @Override
        public void onCreate() {
            super.onCreate();
            global = this;
            initAppComponent();

        }

        private void initAppComponent() {

        }
        public AppComponent getDataComponent() {
            return appComponent;
        }
    }

    public class BackendService {


        @Inject
        public User user;

        private String serverUrl;

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

        public boolean callServer() {
            if (user !=null && serverUrl!=null && serverUrl.length()>0) {
                System.out.println("User: " + user + " ServerUrl: "  + serverUrl);
                return true;
            }
            return false;
        }
    }
    @Module
    public class BackendServiceModule
    {

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

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

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

    public class MainActivity extends AppCompatActivity {

        @Inject
        BackendService service;

        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);
            AppGlobal.global.getDataComponent().inject(this);
            TextView result = (TextView) findViewById(R.id.resulttxt);
            result.setText(service.callServer()+"");
        }
    }
    public class User {

        private String firstName;
        private String lastName;

        public User(String firstName, String lastName) {
            this.firstName = firstName;
            this.lastName = lastName;
        }

        @Override
        public String toString() {
            return "User [firstName=" + firstName + ", lastName=" + lastName + "]";
        }
    @Module
    public class UserModule
    {
        @Singleton
        @Provides
        User providesUser()
        {
            return new User("syam","jjjj");
        }

    }

exception: Error:(16, 26) error: com.example.intrio.daggersample2.BackendServiceModule cannot be provided without an @Inject constructor or from an @Provides-annotated method. com.example.intrio.daggersample2.BackendServiceModule is provided at com.example.intrio.daggersample2.AppComponent.provideBackendService()

Jeff Bowman Jeff Bowman · Accepted Answer · 2017-11-10T17:57:09

You've swapped your BackendServiceModule and BackendService: Modules are annotated with @Module and used for binding/configuring the Dagger graph, which are then used to assemble the objects you make available by putting a getter ("provision method") on your Component.

@Singleton
@Component(modules = {UserModule.class,BackendServiceModule.class})  // <-- Modules
public interface AppComponent {

    BackendService provideBackendService();  // <-- Getter ("provision method")

    // allow to inject into our Main class
    // method name not important
    void inject(MainActivity mainActivity);  // <-- Members injection method
}

See the documentation for @Component for more details.

Dagger 2:Cannot be provided without an @Inject constructor or from an @Provides-annotated method

2 Answers