Implementing Service layer

Request handlers in the Controller layer call upon the methods defined in the Service layer to perform business logic on the RequestData and prepare the Response to be returned back to the client.

For this guide, to create the service layer -

Create a new folder within digit folder called service.
Create a new class in this folder by the name VoterRegistrationService.
Annotate the class with @Service annotation.
Add the following content to the class -

@Service
public class VoterRegistrationService {

    @Autowired
    private VoterApplicationValidator validator;

    @Autowired
    private VoterApplicationEnrichment enrichmentUtil;

    @Autowired
    private UserService userService;

    @Autowired
    private WorkflowService workflowService;

    @Autowired
    private VoterRegistrationRepository voterRegistrationRepository;

    @Autowired
    private Producer producer;

    public List<VoterRegistrationApplication> registerVtRequest(VoterRegistrationRequest voterRegistrationRequest) {
        // Validate applications
        validator.validateVoterApplication(voterRegistrationRequest);

        // Enrich applications
        enrichmentUtil.enrichVoterApplication(voterRegistrationRequest);

        // Enrich/Upsert user in upon voter registration
        userService.callUserService(voterRegistrationRequest);

        // Initiate workflow for the new application
        workflowService.updateWorkflowStatus(voterRegistrationRequest);

        // Push the application to the topic for persister to listen and persist
        producer.push("save-vt-application", voterRegistrationRequest);

        // Return the response back to user
        return voterRegistrationRequest.getVoterRegistrationApplications();
    }

    public List<VoterRegistrationApplication> searchVtApplications(RequestInfo requestInfo, VoterApplicationSearchCriteria voterApplicationSearchCriteria) {
        // Fetch applications from database according to the given search criteria
        List<VoterRegistrationApplication> applications = voterRegistrationRepository.getApplications(voterApplicationSearchCriteria);

        // If no applications are found matching the given criteria, return an empty list
        if(CollectionUtils.isEmpty(applications))
            return new ArrayList<>();

        // Otherwise return the found applications
        return applications;
    }

    public VoterRegistrationApplication updateVtApplication(VoterRegistrationRequest voterRegistrationRequest) {
        // Validate whether the application that is being requested for update indeed exists
        VoterRegistrationApplication existingApplication = validator.validateApplicationExistence(voterRegistrationRequest.getVoterRegistrationApplications().get(0));
        existingApplication.setWorkflow(voterRegistrationRequest.getVoterRegistrationApplications().get(0).getWorkflow());
        voterRegistrationRequest.setVoterRegistrationApplications(Collections.singletonList(existingApplication));
        
        // Enrich application upon update
        enrichmentUtil.enrichVoterApplicationUponUpdate(voterRegistrationRequest);

        workflowService.updateWorkflowStatus(voterRegistrationRequest);

        // Just like create request, update request will be handled asynchronously by the persister
        producer.push("update-vt-application", voterRegistrationRequest.getVoterRegistrationApplications().get(0));

        return voterRegistrationRequest.getVoterRegistrationApplications().get(0);
    }
}

*** NOTE: At this point, your IDE must be showing a lot of errors but do not worry we will add all dependent layers as we progress through this guide and the errors will go away.