public class StudentController extends HttpServlet {
    private static final long serialVersionUID = 1L;

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    System.out.println("inside servlet");

     dao dao=new daoimpl();
    String action=(request.getParameter("action"));
    if(action.equalsIgnoreCase("login")){
        System.out.println("inside login");
        Student student=new Student();
        System.out.println(request.getParameter("username"));


        student.setUsername(request.getParameter("username"));
        student.setPassword(request.getParameter("password"));
        ArrayList<Student>al=dao.getAllStudent(student);
        Gson gson=new Gson();
        //PrintWriter pw=response.getWriter();
            String json = gson.toJson(al);
            response.setContentType("application/json");
            response.setCharacterEncoding("UTF-8");
            response.getWriter().write(json);
    }

<script type="text/javascript">
$(document).ready(
  function() {
      $('form').submit(function(event) {
        event.preventDefault();

         var uname = $('#username').val();
            var pass = $('#pass').val();
            alert(uname+"-----"+pass);
            alert("on submit");


            $.ajax(
              {
                url:"Controller?action=login",
                data: {username : uname,password : pass}
                type:'post',
                cache:false,
                timeout:8000
              }
            ).done(function(data){
                alert("sucess");
                var json=$.parseJSON(data);
                alert(json);

            }).fail(function(jqxhr, textStatus, error){
              alert(textStatus);
              alert(error);
            });
      });
    }
  );
</script>