AJAX 脚本不向指定地址发出请求

Spring Boot 控制器方法：

    @PostMapping("/bills/add")
    @ResponseBody
    public String createBill(@RequestParam String currency) {
        
        String phone = authentication().getName();
        String answer = null;
        if(currency.isEmpty()) {
            answer = "Please choose currency type";
            return answer;
        }
        accountService.addBill(phone, currency);
        answer = "Success!!!";
        return answer;
    }

百里香叶：

<div>
<form id="currency_form">
        <select class="form-control" th:id="currency" th:name="currency" style="width:auto">
            <option value="">Select currency</option>
            <option th:each="currency : ${T(com.github.irybov.bankdemoboot.Currency).values()}"
                    th:value="${currency.name()}"
                    th:text="${currency.name()}">
            </option>
        </select>
    <button type="submit" class="btn btn-success add_bill">
        Add new bill</button>

    <p th:text="${message}" th:value="${#strings.isEmpty('currency')}" class="error"/>
</form>
</div>

jQuery：

<script>
    $(document).ready(function(){
        $('.add_bill').click(function(){
            var type = ('#currency').val();
            $('#currency_form').submit(function (ev) {
                ev.preventDefault();
                $.ajax({
                    type: 'POST',
                    url: 'http://localhost:8080/bankdemo/bills/add?currency='+type,
                    success: function(msg) {
                        alert(msg);
                    }
                    error: function (xhr, statusText, err) {
                      alert(xhr.status);
                    }
                });
            });
        });
    });
</script>

HTML 最初生成于

http://localhost:8080/bankdemo/accounts/show

    @GetMapping("/accounts/show")
    public String getAccount(ModelMap modelMap) {

        String current = authentication().getName();        
        AccountResponseDTO account = accountService.getAccountDTO(current);
        modelMap.addAttribute("account", account);
        modelMap.addAttribute("currencies", currencies);

        return "/account/private";
    }

所以问题是由于某种原因，在做 AJAX 时，url 变成了浏览器行

http://localhost:8080/bankdemo/accounts/show?currency=USD

而不是沿着脚本中指定的路径执行请求。
如果您通过 Postman 手动输入正确的 url，则控制器方法工作正常，返回响应字符串和状态 200。