Why @Requestparam returns null, but value is filled?

Question

I have Java with Spring

    public String addStudent(@RequestParam(value = "selStud") Long idS,
                             @RequestParam(required = false, value = "selDisc") Long idD,
                             ModelMap modelMap){
        long selectedId = idS;
        long selDiscipline = idD;
        return "redirect:/";
    }

and this HTML:

<form action="#" th:action="@{/add-student}" class="new-student-form" method="post" >
<ul name="selDisc" type="none" th:each="discipline : ${disciplineList.disciplines}" th:value="${discipline.id}">
                <div th:onclick="|javascript:showDiv('${discipline.id}');|"
                   th:text="${discipline.name}"
                     th:value="${discipline.id}"
                   class="discipline-name" >..
                </div>
                <div th:id="${discipline.id}">
                    <ul  class="discipline-info"
                        th:unless="${#lists.isEmpty(discipline.students)}">
                        <li th:each="student : ${discipline.students}"
                            th:text="${student.student}" class="studName" name="studName">
                            </li>
                        </li>
                    </ul>
                    <div name="studOption" th:id="${discipline.id}">
                        <select name="selStud" required >
                            <option th:each="student : ${students}"
                                    th:value="${student.id}"
                                    th:id="${student.id}"
                                    name="studentOption"
                                    th:text="${student.student}">Select</option>
                        </select>
                        <input type="submit" class="add-student-to-disc" value="Добавить в дисциплину">
                    </div>
                </div>
            </li>
        </ul>
</form>

With @RequestParam(value = "selStud") Long idS I get th:value="${student.id}" from Option, from Select with name "selStud". Example selectedId = 5 .

But with @RequestParam(required = false, value = "selDisc") Long idD I get NULL from <ul name="selDisc"> . And @RequestParam(value = "selDisc") Long idD does not work, because selDisc all time return null. I have an error "Required Long parameter is not present"

I want to get discipline id, what am I doing wrong?

Answer 1

I put <input name="selDisc" th:value="${discipline.id}" type="hidden"> after the first div.

But it works by chance.