Servlets + injection -multithreading issues

Question

So for a school project we created a site where a user could submit a report on underwater life etc. We used simple dependency injection (javax.inject) and an error checking pattern as follows :

ReportService.java

public interface ReportService {
    public static enum ReportServiceErrorsENUM {
        DB_FAILURE, WRONG_COORD // etc
    }
    public Set<ReportServiceErrorsENUM> getLastErrors();
    public int addNewReport(Report report);
}

ReportServiceImpl.java

public class ReportServiceImpl implements ReportService {

    private Set<ReportServiceErrorsENUM> lastErrors;
    private @Inject ReportDAO reportDAO;

    @Override
    public Set<ReportServiceErrorsENUM> getLastErrors() {
        return this.lastErrors;
    }

    @Override
    public int addNewReport(Report report) {
        lastErrors= new HashSet<ReportServiceErrorsENUM>();//throw away previous errors
        UserInput input = report.getUserInput();
        if (input.getLatitude() == null) {
            addError(ReportServiceErrorsENUM.WRONG_COORD);
        }
        // etc etc
        if (reportDAO.insertReport(report) != 0) {
            // failure inserting the report in the DB
            addError(ReportServiceErrorsENUM.DB_ERROR);
        }
        if (lastErrors.isEmpty()) // if there were no errors
            return EXIT_SUCCESS; // 0
        return EXIT_FAILURE;  // 1
    }
}

SubmitReportController.java

@WebServlet("/submitreport")
public class SubmitReportController extends HttpServlet {
    private static final long serialVersionUID = 1L;
    private @Inject ReportService reportService;

    @Override
    protected void doPost(HttpServletRequest request,
    HttpServletResponse response) throws ServletException, IOException {
        Report report = new Report();
        // set the report's fields from the HttpServletRequest attributes
        if(reportService.addNewReport(report) == ReportService.EXIT_FAILURE) {
            for(ReportServiceErrorsENUM error : reportService.getLastErrors())
                // display the errors etc
        } else {
            // display confirmation
        }
    }
}

The idea is that the Servlet controller calls the service (which is injected) then checks the services' return value and calls getLastErrors() on the service if there was an error - to inform the user what went wrong etc. Now I just came to realize that this is not thread safe - the @Inject'ed ReportService (reportService) will be shared by all threads using the servlet

1. Is it (crosses fingers) ?
2. How could one improve on this error mechanism ?

Thanks

Answer 1

typically for servlets you'd want to keep those variables (generally called "state") in some container-managed context. i'd move these errors to the request scope - that way they're stored on the request object (conceptually) and any servlet/jsp/whatever working on that same request can see/edit them. different requests mean different data storage.

example code for using the request scope from a servlet can be found here: http://www.exampledepot.com/egs/javax.servlet/State.html