在 Servlet 中多次重新加载 Web 应用程序时无法加载数据

问：

我遇到了一个问题，当我多次重新加载我的 Web 应用程序时，它无法调用 Servlet 来加载数据

这是我使用 AJAX 从 DB 加载数据以调用 Servlet 的函数：

function loadData(type) {
    $.post('../../loadData?type=' + type, function (data) {
        if (type == "accounts") {
            $('#accounts > tbody').empty();
            $('h2.data-number.data-account').html(data.length);

            $.each(data, function (i, val) {
                const row = $('<tr>').append($('<th scope="row">').text(i + 1))
                        .append($('<td>').text(val.accountID))
                        .append($('<td>').text(val.username))
                        .append($('<td>').text(val.name))
                        .append($('<td>').text(val.email))
                        .append($('<td>').text(val.phone))
                        .append($('<td>').text((val.role == 1) ? 'Doctor' : 'Patient'));

                $('#accounts > tbody').append(row);
            });
        } else if (type == "doctors") {
            $.post('../../loadData?type=' + type, function (data) {
                var aEdit = $('<a class="editDoc" data-bs-toggle="modal" data-bs-target="#form-editDoc" href=""><i class="fa-solid fa-pen text-dark me-3"></i></a>');
                var aDeleteDoc = $('<a class="deleteDoc" href=""><i class="fa-solid fa-trash-can text-danger"></i></a>');
                $('#doctors > tbody').empty();
                $('h2.data-number.data-doctor').html(data.length);

                $.each(data, function (i, val) {
                    const row = $('<tr>').append($('<th scope="row">').text(i + 1))
                            .append($('<td>').text(val.username))
                            .append($('<td>').text(val.name))
                            .append($('<td>').text(val.email))
                            .append($('<td>').text(val.phone))
                            .append($('<td class="action">'));

                    row.find('td.action')
                            .append(aEdit.clone().attr({'href': '../../loadData?username=' + val.username}))
                            .append(aDeleteDoc.clone().attr({'href': '../../manageDoctor?username=' + val.username + '&type=delete'}));

                    $('#doctors > tbody').append(row);
                });
            });
        } else if (type == "patients") {
            $.post('../../loadData?type=' + type, function (data) {
                var aView = $('<a class="viewPat" data-bs-toggle="modal" data-bs-target="#form-viewPat" href=""><i class="fa-solid fa-eye text-primary me-3"></i></a>');
                var aDeletePat = $('<a class="deletePat" href=""><i class="fa-solid fa-trash-can text-danger"></i></a>');

                $('#patients > tbody').empty();
                $('h2.data-number.data-patient').html(data.length);

                $.each(data, function (i, val) {
                    const row = $('<tr>').append($('<th scope="row">').text(i + 1))
                            .append($('<td>').text(val.username))
                            .append($('<td>').text(val.name))
                            .append($('<td>').text(val.email))
                            .append($('<td>').text(val.phone))
                            .append($('<td class="action">'));

                    row.find('td.action')
                            .append(aView.clone().attr({'href': '../../loadData?username=' + val.username}))
                            .append(aDeletePat.clone().attr({'href': '../../managePatient?username=' + val.username + '&type=delete'}));

                    $('#patients > tbody').append(row);
                })
            })
        } else if (type == "blogs") {
            $.post('../../loadData?type=' + type, function (data) {
                var aEdit = $('<a class="editBlog" data-bs-toggle="modal" data-bs-target="#form-editBlog" href=""><i class="fa-solid fa-pen text-dark me-3"></i></a>');
                var aDeleteBlog = $('<a class="deleteBlog" href=""><i class="fa-solid fa-trash-can text-danger"></i></a>');

                $('#blogs > tbody').empty();
                $('h2.data-number.data-blog').html(data.length);

                $.each(data, function (i, val) {
                    const row = $('<tr>')
                            .append($('<th scope="row">').text(i + 1))
                            .append($('<td>').text(val.title))
                            .append($('<td>').text(val.categoryName))
                            .append($('<td>').text(val.authorName))
                            .append($('<td>').text(val.createdDate))
                            .append($('<td>').html((val.status == 1 ? 'Approved<i class="fa-solid fa-circle-check text-success ms-2"></i>' : 'Pending<i class="fa-solid fa-circle-pause text-danger ms-2"></i>')))
                            .append($('<td>').text(val.publishedDate))
                            .append($('<td class="action">'));

                    row.find('td.action')
                            .append(aEdit.clone().attr({'href': '../../loadData?blogID=' + val.blogID}))
                            .append(aDeleteBlog.clone().attr({'href': '../../manageBlog?blogID=' + val.blogID + '&type=delete'}))

                    $('#blogs > tbody').append(row);
                })
            })
        } else if (type == "categories") {
            $.post('../../loadData?type=' + type, function (data) {
                $('#categoryLabel').empty();
                $('#categoryBlogLabel').empty();

                $.each(data, function (i, val) {
                    $('#categoryLabel').append($('<option value="' + (i + 1) + '">').text(val.categoryName))
                    $('#categoryBlogLabel').append($('<option value="' + (i + 1) + '">').text(val.categoryName))
                })
            })
        } else if (type == "services") {
            $.post('../../loadData?type=' + type, function (data) {
                var aEdit = $('<a class="editService" data-bs-toggle="modal" data-bs-target="#form-editService" href=""><i class="fa-solid fa-pen text-dark me-3"></i></a>');

                $('#services > tbody').empty();

                $.each(data, function (i, val) {
                    const row = $('<tr>').append($('<th scope="row">').text(i + 1))
                            .append($('<td>').text(val.name))
                            .append($('<td>').text(val.price))
                            .append($('<td>').html((val.status == 1 ? 'Available<i class="fa-solid fa-circle-check text-success ms-2"></i>' : 'Disabled<i class="fa-solid fa-ban text-danger ms-2"></i>')))
                            .append($('<td class="action">'));

                    row.find('td.action')
                            .append(aEdit.clone().attr({'href': '../../loadData?serviceID=' + val.serviceID}))

                    $('#services > tbody').append(row);
                })

            })
        } else if (type == "medicines") {
            $.post('../../loadData?type=' + type, function (data) {
                var aEdit = $('<a class="editMedicine" data-bs-toggle="modal" data-bs-target="#form-editMedicine" href=""><i class="fa-solid fa-pen text-dark me-3"></i></a>');

                $('#medicines > tbody').empty();

                $.each(data, function (i, val) {
                    const row = $('<tr>').append($('<th scope="row">').text(i + 1))
                            .append($('<td>').text(val.name))
                            .append($('<td>').text(val.brand))
                            .append($('<td class="action">'));

                    row.find('td.action')
                            .append(aEdit.clone().attr({'href': '../../loadData?medicineID=' + val.medicineID}))

                    $('#medicines > tbody').append(row);
                })

            })
        }
    });
}

我的 Servlet：

AccountDAO accountDAO = new AccountDAO();
        BlogDAO blogDAO = new BlogDAO();
        AppointmentDAO appointmentDAO = new AppointmentDAO();
        ServiceDAO serviceDAO = new ServiceDAO();
        MedicineDAO medicineDAO = new MedicineDAO();
        ExaminationDAO examinationDAO = new ExaminationDAO();
        PrescriptionDAO prescriptionDAO = new PrescriptionDAO();

        String type = request.getParameter("type");
        String username = request.getParameter("username");
        String blogID = request.getParameter("blogID");
        String appointmentID = request.getParameter("appointmentID");
        String serviceID = request.getParameter("serviceID");
        String medicineID = request.getParameter("medicineID");
        String examinationID = request.getParameter("examinationID");
        String prescriptionID = request.getParameter("prescriptionID");

        if (type != null) {
            switch (type) {
                case "accounts": {
                    List<Account> accounts = accountDAO.getAccounts();
                    response.setContentType("application/json");
                    response.setCharacterEncoding("utf-8");
                    response.getWriter().write(new Gson().toJson(accounts));
                    break;
                }
                case "doctors": {
                    List<Account> doctors = accountDAO.getAccountsByRole(1);
                    response.setContentType("application/json");
                    response.setCharacterEncoding("utf-8");
                    response.getWriter().write(new Gson().toJson(doctors));
                    break;
                }
                case "patients": {
                    List<Account> patients = accountDAO.getAccountsByRole(2);
                    response.setContentType("application/json");
                    response.setCharacterEncoding("utf-8");
                    response.getWriter().write(new Gson().toJson(patients));
                    break;
                }
                case "blogs": {
                    List<Blog> blogs = blogDAO.getBlogs();
                    response.setContentType("application/json");
                    response.setCharacterEncoding("utf-8");
                    response.getWriter().write(new Gson().toJson(blogs));
                    break;
                }
                case "categories": {
                    List<Category> categories = blogDAO.getCategories();
                    response.setContentType("application/json");
                    response.setCharacterEncoding("utf-8");
                    response.getWriter().write(new Gson().toJson(categories));
                    break;
                }
                case "appointments": {
                    HttpSession session = request.getSession();
                    Account currentAccount = (Account) session.getAttribute("currentAccount");
                    if (currentAccount.getRole() == 2) {
                        List<Appointment> appointments = appointmentDAO.getAppointmentsForPat(currentAccount.getAccountID());
                        response.setContentType("application/json");
                        response.setCharacterEncoding("utf-8");
                        response.getWriter().write(new Gson().toJson(appointments));
                    } else if (currentAccount.getRole() == 1) {
                        List<Appointment> appointments = appointmentDAO.getAppointmentsForDoc(currentAccount.getAccountID());
                        response.setContentType("application/json");
                        response.setCharacterEncoding("utf-8");
                        response.getWriter().write(new Gson().toJson(appointments));
                    }
                    break;
                }
                case "slots": {
                    String bookedDate_raw = request.getParameter("bookedDate");
                    String doctorID = request.getParameter("doctorID");

                    List<Integer> slots = appointmentDAO.busySlot(Date.valueOf(bookedDate_raw), Integer.parseInt(doctorID));
                    response.setContentType("application/json");
                    response.setCharacterEncoding("utf-8");
                    response.getWriter().write(new Gson().toJson(slots));
                    break;
                }
                case "services": {
                    List<Service> services = serviceDAO.getServices();
                    response.setContentType("application/json");
                    response.setCharacterEncoding("utf-8");
                    response.getWriter().write(new Gson().toJson(services));
                    break;
                }
                case "medicines": {
                    List<Medicine> medicines = medicineDAO.getMedicines();
                    response.setContentType("application/json");
                    response.setCharacterEncoding("utf-8");
                    response.getWriter().write(new Gson().toJson(medicines));
                    break;
                }
                case "examinations": {
                    HttpSession session = request.getSession();
                    Account currentAccount = (Account) session.getAttribute("currentAccount");
                    if (currentAccount.getRole() == 2) {
                        List<Examination> examinations = examinationDAO.getExaminationsForPat(currentAccount.getAccountID());
                        response.setContentType("application/json");
                        response.setCharacterEncoding("utf-8");
                        response.getWriter().write(new Gson().toJson(examinations));
                    } else if (currentAccount.getRole() == 1) {
                        List<Examination> examinations = examinationDAO.getExaminationsForDoc(currentAccount.getAccountID());
                        response.setContentType("application/json");
                        response.setCharacterEncoding("utf-8");
                        response.getWriter().write(new Gson().toJson(examinations));
                    }
                    break;
                }
                case "prescriptions": {
                    HttpSession session = request.getSession();
                    Account currentAccount = (Account) session.getAttribute("currentAccount");
                    if (currentAccount.getRole() == 2) {
                        List<Prescription> prescriptions = prescriptionDAO.getPrescriptionsForPat(currentAccount.getAccountID());
                        response.setContentType("application/json");
                        response.setCharacterEncoding("utf-8");
                        response.getWriter().write(new Gson().toJson(prescriptions));
                    } else if (currentAccount.getRole() == 1) {
                        List<Prescription> prescriptions = prescriptionDAO.getPrescriptionsForDoc(currentAccount.getAccountID());
                        response.setContentType("application/json");
                        response.setCharacterEncoding("utf-8");
                        response.getWriter().write(new Gson().toJson(prescriptions));
                    }
                    break;
                }
                default:
                    break;
            }
        } else if (username != null) {
            Account account = accountDAO.getAccount(username);
            GsonBuilder gsonBuilder = new GsonBuilder();
            gsonBuilder.setDateFormat("yyyy-MM-dd");
            String accountJson = gsonBuilder.create().toJson(account);
            response.setContentType("application/json");
            response.setCharacterEncoding("utf-8");
            response.getWriter().write(accountJson);
        } else if (blogID != null) {
            Blog blog = blogDAO.getBlog(Integer.parseInt(blogID));
            GsonBuilder gsonBuilder = new GsonBuilder();
            gsonBuilder.setDateFormat("yyyy-MM-dd");
            String blogJson = gsonBuilder.create().toJson(blog);
            response.setContentType("application/json");
            response.setCharacterEncoding("utf-8");
            response.getWriter().write(blogJson);
        } else if (appointmentID != null) {
            Appointment appointment = appointmentDAO.getAppointment(Integer.parseInt(appointmentID));
            GsonBuilder gsonBuilder = new GsonBuilder();
            gsonBuilder.setDateFormat("yyyy-MM-dd");
            String appointmentJson = gsonBuilder.create().toJson(appointment);
            response.setContentType("application/json");
            response.setCharacterEncoding("utf-8");
            response.getWriter().write(appointmentJson);
        } else if (serviceID != null) {
            Service service = serviceDAO.getService(Integer.parseInt(serviceID));
            response.setContentType("application/json");
            response.setCharacterEncoding("utf-8");
            response.getWriter().write(new Gson().toJson(service));
        } else if (medicineID != null) {
            Medicine medicine = medicineDAO.getMedicine(Integer.parseInt(medicineID));
            response.setContentType("application/json");
            response.setCharacterEncoding("utf-8");
            response.getWriter().write(new Gson().toJson(medicine));
        } else if (examinationID != null) {
            Examination examination = examinationDAO.getExamination(Integer.parseInt(examinationID));
            GsonBuilder gsonBuilder = new GsonBuilder();
            gsonBuilder.setDateFormat("yyyy-MM-dd");
            String examinationJson = gsonBuilder.create().toJson(examination);
            response.setContentType("application/json");
            response.setCharacterEncoding("utf-8");
            response.getWriter().write(examinationJson);
        } else if (prescriptionID != null) {
            Prescription prescription = prescriptionDAO.getPrescription(Integer.parseInt(prescriptionID));
            GsonBuilder gsonBuilder = new GsonBuilder();
            gsonBuilder.setDateFormat("yyyy-MM-dd");
            String prescriptionJson = gsonBuilder.create().toJson(prescription);
            response.setContentType("application/json");
            response.setCharacterEncoding("utf-8");
            response.getWriter().write(prescriptionJson);
        }
    }

和错误消息：

我不知道我面临什么问题，请帮助我或告诉我我有什么问题。