The default type resolver could not find a suitable Java type for GraphQL type `ArrayList. Provide a @DgsTypeResolver.`

Question

I have a java - Spring Boot project using graphql and DGS from netflix. I have the following query:

type Query {
ticketingHistory(ticketId: String): TicketingHistory
}

interface TicketingHistory {
ticketId: String
createdOn: String
}
type TicketingEmail implements TicketingHistory {
ticketId: String
createdOn: String
emailId: String
processInstanceId: String
createdBy: EmailCreatedBy
mailFrom: String
to: String
cc: String
bcc: String
messageId: String
messageInReplyTo: String
messageReferences: String
subject: String
messagePlain: String
messageHtml: String
}
type TicketingComment implements TicketingHistory {
ticketId: String
commentId: String
userId: String
createdOn: String
taskId: String
message: String
}

and the data fetcher:

@DgsComponent
@RequiredArgsConstructor
@Slf4j
public class TicketingHistoryDataFetcher {

    private final EmailService emailService;
    private final ProcessInstanceService processInstanceService;

    @DgsQuery
    public List<? extends TicketingHistory> ticketingHistory(@InputArgument String ticketId) {
        List<TicketingHistory> ticketingHistories = new ArrayList<>();
        final Set<Email> emails = emailService.findAll(ticketId);
        final List<CommentDTO> comments = processInstanceService.getComments(ticketId);

        emails.stream()
                .filter(Objects::nonNull)
                .forEach(email -> ticketingHistories.add(ticketingEmail(email)));
        comments.stream()
                .filter(Objects::nonNull)
                .forEach(comment -> ticketingHistories.add(ticketingComment(comment, ticketId)));
        ticketingHistories.sort(Comparator.comparing(TicketingHistory::getCreatedOn));

        return ticketingHistories;
    }
}

when calling it from postman and debugging it I can see that this query is called and I am getting the data but at the moment when it tries to return the list it is failing with:

com.netflix.graphql.dgs.exceptions.InvalidTypeResolverException: The default type resolver could not find a suitable Java type for GraphQL type `ArrayList. Provide a @DgsTypeResolver.`

Answer 1

It looks like Resolver is missing. Check if you have TicketingEmail and TicketingComment classes (Should be spelled the same as the type name as per the doc) Or you should have a resolver (refer to below doc for sample resolver). https://netflix.github.io/dgs/advanced/type-resolvers-for-abstract-types/