Unable to get call back in firebase FirebaseInstanceIdService to get token

Question

this is my class to get Token:->

  public class MyFirebaseInstanceIDService extends FirebaseInstanceIdService {
            private static final String TAG = MyFirebaseInstanceIDService.class.getSimpleName();
            @Override
            public void onTokenRefresh() {
                super.onTokenRefresh();
                String refreshedToken = FirebaseInstanceId.getInstance().getToken();

        }
      }

this is my manifest file :->

 <service
            android:name=".service.MyFirebaseInstanceIDService"
            android:exported="false">
            <intent-filter>
                <action android:name="com.google.firebase.INSTANCE_ID_EVENT" />
            </intent-filter>
        </service>

i have written this code to get device token but i am not getting call back in this class so that i can get device token can you please tell me where i am doing wrong i have given internet permission also i have puted google.json file in app folder can any one please suggest or help me in this issue .