Serializing Java objects for REST API Requests in Open Event Organizer App

Open Event Organizer App is a client side application which uses REST API for network requests. The server supports sending and receiving of data only in JSONAPI spec, so, we needed to serialize java models into JSON objects and deserialize JSON data into java models following JSONAPI spec. To achieve this we followed the following steps.

Specifications

We will be using jasminb/jsonapi-converter which handles request/response parsing of models following JSONAPI Spec and Retrofit plugin of jackson converter to serializing JSON to Java Models and vice versa.

Let’s create a java model. We are using some annotations provided by Lombok library to avoid writing boilerplate code. @JsonNaming annotation is used to apply KebabCaseStrategy while serializing fields

@Data
@Type(“order”)
@AllArgsConstructor
@JsonNaming(PropertyNamingStrategy.KebabCaseStrategy.class)
@Table(database = OrgaDatabase.class, allFields = true)
public class Order {

@PrimaryKey
@Id(LongIdHandler.class)
public Long id;

public float amount;
public String completedAt;
public String identifier;
public String paidVia;
public String paymentMode;
public String status;

@Relationship(“event”)
@ForeignKey(stubbedRelationship = true, onDelete = ForeignKeyAction.CASCADE)
public Event event;

public Order() { }
}

In the NetworkModule class, there is a method providesMappedClasses() containing a list of classes that needs to be serialized/deserialized. We need to add the above model in the list. Then, this list is provided to Singleton instance of JSONAPIConvertorFactory through Dagger. JSONAPIConvertorFactory uses the Retrofit ObjectMapper and maps the classes that are handled by this instance.

@Provides
Class[] providesMappedClasses() {
return new Class[]{Event.class, Attendee.class, Ticket.class, Order.class};
}

Further, various serialization properties can be used while building Singleton ObjectMapper instance. Adding any properties here ensures that these are applied to all the mapped classes by JSONAPIConvertorFactory. For eg, we are using the serialization property to throw an exception and fail whenever empty beans are encountered.

@Provides
@Singleton
ObjectMapper providesObjectMapper() {
return new ObjectMapper()
.disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES)
.disable(SerializationFeature.FAIL_ON_EMPTY_BEANS)
// Handle constant breaking changes in API by not including null fields
// TODO: Remove when API stabilizes and/or need to include null values is there
.setSerializationInclusion(JsonInclude.Include.NON_ABSENT);
}

Resources

  1. Github Repository for jsonapi-converter https://github.com/jasminb/jsonapi-converter
  2. Github repository for Jackson Retrofit Plugin https://github.com/square/retrofit/tree/master/retrofit-converters/jackson
  3. Official Website for Project Lombok https://projectlombok.org/

Github Repository for Open-Event-Orga-App https://github.com/fossasia/open-event-orga-app

Create Session in Open Event Android Organizer Application

Open Event Android Organizer Application offered variety of features to Organizers from all over the world to help them host their Events globally but it didn’t had the functionality to create Sessions in the app itself and associate it to Tracks. This feature addition was crucial since it enables Organizer to create Sessions which every common person enquires about before attending and event. In this Blog Post we will see how we added this functionality in the app.

Open Event Android Organizer Application is a client for Open Event Server which provides the REST API.

Problem

There can be various Sessions associated with Tracks for an Event. Open Event API had the endpoint to implement Creating Session but the Orga app didn’t, so we worked on creating a Session in the app.

The UI for creating a Session is shown above. User can fill in the necessary details and click on the green Floating Action Button to create a Session.

How to implement functionality?

We will follow MVP Architecture and use Retrofit 2.x, RxJava, Dagger, Jackson, Data Binding and other industry standard libraries to implement this functionality.

Firstly, let’s create Session model class specifying the attributes and relationships to set up in database using RazizLabs DbFlow library. The POJO will be serialized into JSON by Jackson library to be passed on as a part of RequestBody to server.

Now we will create SessionApi class that will contain the request details to be passed to Retrofit. @POST denotes a POST request and @Body denotes the requestBody of the request which is a Session object.

public interface SessionApi {
@POST(“sessions”)
Observable<Session> postSession(@Body Session session);
}

This is the CreateSessionFragment class that contains the code binding model to the view. The Fragment class implements the CreateSessionView class overriding the method declarations present there. The @Inject annotation of Dagger is used to load singleton presenter instance lazily to improve app’s performance.

Event-Id and Track-Id’s are retrieved from Bundle from Fragment Transaction. These are then passed on to presenter when Create Session button is pressed. There are other methods to show binding progressbar, snackbar and other UI components to show progress of the background request to server and database.

public class CreateSessionFragment extends BaseFragment<CreateSessionPresenter> implements CreateSessionView {

@Override
public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup

container, @Nullable Bundle savedInstanceState) {
binding = DataBindingUtil.inflate(inflater,

R.layout.session_create_layout, container, false);
validator = new Validator(binding.form);

binding.sessionCreate.setOnClickListener(view -> {
if (validator.validate()) {
getPresenter().createSession(trackId, eventId);
}
});

return binding.getRoot();
}

@Override
public void onStart() {
super.onStart();
getPresenter().attach(this);
binding.setSession(getPresenter().getSession());
}
}

In the Presenter createSession method is called when create button is pressed in UI. The method attaches track-id and event-id to Session object. This is necessary for Relationship constraints on Session Model. Then after binding all the data to Session object, we pass it on to SessionRepository. The success response is provided to user by passing success response in getView().onSuccess() method.

public class CreateSessionPresenter extends

AbstractBasePresenter<CreateSessionView> {

public Session getSession() {
return session;
}

public void createSession(long trackId, long eventId) {

Track track = new Track();
Event event = new Event();

track.setId(trackId);
event.setId(eventId);
session.setTrack(track);
session.setEvent(event);

sessionRepository
.createSession(session)
.compose(dispose(getDisposable()))
.compose(progressiveErroneous(getView()))
.subscribe(createdSession ->

getView().onSuccess(“Session Created”), Logger::logError);
}
}

The SessionRepository uses RxJava to make asynchronous Retrofit Call to Server. We throw a Network Error to user if the device does not have Internet Connectivity.

The session object accepted as a parameter in createSession method is passed on to sessionApi. It will return Observable<Session> Response which we will process in doOnNext() method. Then the Session object along with required foreign key relationships with Track and Event is saved in database for offline use.

@Override
public Observable<Session> createSession(Session session) {
if (!repository.isConnected()) {
return Observable.error(new Throwable(Constants.NO_NETWORK));
}return sessionApi
.postSession(session)
.doOnNext(created -> {
created.setTrack(session.getTrack());
created.setEvent(session.getEvent());
repository
.save(Session.class, created)
.subscribe();
})
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread());
}

The above code snippets are from Open Event Orga Application. For exploring the entire codebase please refer here. For details about the REST API used by the app please visit here.

References

  1. Official RxJava Project on Github by ReactiveX https://github.com/ReactiveX/RxJava.
  2. Official Retrofit Project on Github by Square Inc https://github.com/square/retrofit.
  3. Official Open Event Organizer App on Github by FOSSASIA https://github.com/fossasia/open-event-orga-app.
  4. Documentation for REST API of Open Event Server on Heroku by FOSSASIA https://open-event-api-dev.herokuapp.com/.