JSP 网上商城为什么修改不了产品的信息？？
<% request.setCharacterEncoding("GBK"); int id = Integer.parseInt(request.getParameter("id")); Product p = ProductMgr.getInstance().loadById(id); String action = request.getParameter("add"); if (action != null && action.trim().equals("add")) { String name = request.getParameter("name"); double normalPrice = Double.parseDouble(request.getParameter("normalPrice")); double memberPrice = Double.parseDouble(request.getParameter("memberPrice")); int categoryId = Integer.parseInt(request.getParameter("categoryId")); String descr = request.getParameter("descr"); p.setName(name); p.setNormalPrice(normalPrice); p.setMemberPrice(memberPrice); p.setDescr(descr); p.setCategoryId(categoryId); p.setPdate(new Date()); ProductMgr.getInstance().update(p); out.print("修改成功"); }%><!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"><html> <head> <title>产品信息修改</title> </head> <body> <form action="ProductModify.jsp" name="productmodify" method="post"> <table border="1" align="center"> <input type="hidden" name="action" value="add" /> <input type="hidden" name="id" value="<%=id %>"/> <tr> <td> 产品名称: </td> <td> <input name="name" size="25" maxlength="25" type="text" value="<%=p.getName()%>"> </tr> <tr> <td> 市场价格: </td> <td> <input name="normalPrice" size="25" type="text" value="<%=p.getNormalPrice()%>"> </td> </tr> <tr> <td> 会员价格: </td> <td> <input name="memberPrice" size="25" type="text" value="<%=p.getMemberPrice()%>"> </td> </tr> <tr> <td> 所属类别 </td> <td> <select name="categoryId"> <% List<Category> categories = CategoryService.getInstance() .getCategories(); for (Iterator<Category> it = categories.iterator(); it.hasNext();) { Category c = it.next(); String selected = ""; if (c.getId() == p.getCategoryId()) selected = "selected"; String preStr = ""; for (int i = 1; i < c.getGrade(); i++) preStr += "--"; %> <option value="<%=c.getId()%>" <%=selected%>><%=preStr + c.getName()%></option> <% } %> </select> </td> </tr> <tr> <td valign="top"> 产品描述: </td> <td> <textarea name="descr" cols="60" rows="5" id="descr"><%=p.getDescr()%></textarea> </td> </tr> </table> <center> <input type="submit" name="submit" value="提 交"> <input type="reset" name="reset" value="重 置"> <br> <br> <a href="ProductList.jsp">返回列表</a> </center> </form> </body></html>