...
txt = txt + "<td class="pull-right">" + "<form name="form1" method="get" action="SetTrue">" +
    "<input type="hidden" name="id" value="" + nn[0].firstChild.nodeValue + ""/>"
    + "<button type="submit" class="btn btn-link glyphicon glyphicon-star"></button>"
    + "</form>" + "</td>";
...

protected void doGet(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {
    HttpSession session = request.getSession();
    response.setContentType("text/html;");
    PrintWriter out = response.getWriter();
    Integer UserID = (Integer)session.getAttribute("iduser");
    String errorMsg = null;
    String successMsg = null;

    try {
        ConnectionManager conn = new ConnectionManager();
        Connection connection = conn.getConnection();
        PreparedStatement statement = null;

        String string_id = request.getParameter("id");
        int id = Integer.parseInt(string_id);

        boolean set_true = true;

        String update = "UPDATE Table SET Attribute=? WHERE ID=? AND UserID=?";
        statement = connection.prepareStatement(update);

        statement.setBoolean(1, set_true);
        statement.setInt(2, id);
        statement.setInt(3, UserID);

        int num = statement.executeUpdate();
        if (num > 0) {
            successMsg = "OK";
            out.println(successMsg);
            RequestDispatcher reqDisp = getServletContext().getRequestDispatcher("/Welcome.jsp");
            reqDisp.forward(request, response);
        }
        else {
            errorMsg = "Error";
            session.setAttribute("error", errorMsg);
            RequestDispatcher reqDisp = getServletContext().getRequestDispatcher("/Welcome.jsp");
            reqDisp.forward(request, response);
        }
        connection.close();
        statement.close();
    }
    catch (Exception e) {
        e.printStackTrace();
    }
}

...
$("#form1").submit(function(){
      $.ajax({
         type : form.attr('method'),
         url : form.attr('action'),
         success : function(data) {
               alert(data);
         }
      });
...