Getting null values in servlet through ajax call why?

Question

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();