GraphQLRuleService.java

@Service
public class GraphQLRuleService {
	// fetches messages from appropriate .prop file
	@Autowired
	private final MessagesService messagesService;

	public RuleSingle stringNonBlank(final String argumentName, final String fieldName) {
		return new RuleSingle() {
				@Override
				public Optional<GraphQLError> applySingle(FieldAndArguments fieldAndArguments, FieldValidationEnvironment environment)  {
				LinkedHashMap<String, String> arg = fieldAndArguments.getArgumentValue(argumentName);
				String fieldValue = arg.get(fieldName);
				if (StringUtils.isBlank(fieldValue))
					// the string is blank, so create an error
					return Optional.of(environment.mkError(messagesService.getErrorBlankField(fieldName), fieldAndArguments));
				else
					return Optional.empty();
				}
		};
	}
}