Getting null values in servlet through ajax call why? [duplicate] - ajax

Question

This question already has answers here:
How should I use servlets and Ajax?
(7 answers)
Closed 6 years ago.
I am very new to ajax and I am getting null values with ajax call to servlet,don't know why?any idea?here below i mentioned my code,For any help thanks in advance.
function addSubjectMarks(){
var postData = $(this).serializeArray();
var formURL = $(this).attr("action");
var selectedStudent=document.getElementById("selectedStudent").value;
var selectedSubject=document.getElementById("selectedSubject").value;
var marks=document.getElementById("stMarks").value;
$.ajax({
type: "POST",
url: "WeeklyMarks",
data:postData,
success: function(data,status){
//var res = data.split(',');
}
});
}
//servlet code is:
protected void doPost(HttpServletRequest request,
HttpServletResponse response) throws ServletException, IOException {
String selectedSubject=request.getParameter("stSubjectMarks");
System.out.println("selectedSubject :"+selectedSubject);//here getting null value
}
my html form is:
form id="ajaxform" action="weeklyMarks" method="post">
<table>
<td>
<label id="stForm" for="Marks">Marks Obtained:</label>
<input type="text" id="stMarks" name="stMarks" style="margin-left: 33px; width: 150px;" />
<label id="stMarksError" style="display: none; color: red;">*Please enter marks</label>
<br>
<br>
</td>
</tr>
<tr>
<td>
<input type="button" value="Add" id="addButton" class="btn-warning" style="margin-left: 200px;"" onclick="addSubjectMarks() ">
</td>
</tr>
</table>
</div>
</div>

Answer 1

postData = $('#ajaxform').serializeArray();