Why my request getRequestDispatcher() is going into ThreadPoolExceutor.java (1150)?

Question

I am trying to redirecting request from Servlet to JSP. and my request is going to threadpoolexecutor. Here is my servlet code.I am jsut writing the doPost method for time saveing purpose.

protected void doPost(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {

    processRequest(request, response);
    String action=request.getParameter("action");
    HttpSession session=request.getSession();

    if(action==null){
        String user=request.getParameter("username");
        String pass=request.getParameter("password");
        boolean remember=request.getParameter("remember")!=null;
        ConnectionTest cn=new ConnectionTest();
        conn=cn.getConn();

        try{
            psmt=conn.prepareStatement(qry);
            psmt.setString(1, user);
            psmt.setString(2, pass);
            rs=psmt.executeQuery();

            if(rs.next()){
                session.setAttribute("username", user);
                // response.sendRedirect("/main.jsp");
                if(remember){
                    Cookie cku=new Cookie("username", user);
                    cku.setMaxAge(3600);
                    response.addCookie(cku);
                    Cookie ckp=new Cookie("password", pass);
                    ckp.setMaxAge(3600);
                    response.addCookie(ckp);
                    JOptionPane.showMessageDialog(null, user+" : "+pass);
                }
                request.getRequestDispatcher("main.jsp").forward(request, response);
                //response.sendRedirect("/main.jsp");
            }
            else{
                session.setAttribute("error", error);
                request.getRequestDispatcher("login.jsp").forward(request, response);
                //response.sendRedirect("/login.jsp");
            }
        }
        catch(Exception e){
            JOptionPane.showMessageDialog(null, e);
            System.out.println(e);
        }
    }
}

Here is The Error I am getting

java.lang.IllegalStateException: Cannot forward after response has been committed