用servlet更新mysql数据不成功，没有报错，只是无法更新~求大神指点

mysqlservlet

这是更新的函数，在productdaoimpl类里：
public int updateProduct(Product product){
int flag = 0;
DBConnect conn = null;
PreparedStatement pstmt = null;

//update statement    String sql = "update productinfo set productnumber=?,productvalue=?  where productname=?";    try {//connect DB    conn = new DBConnect() ;    pstmt = conn.getConnection().prepareStatement(sql) ;    pstmt.setString(1, product.getProductnumber());    pstmt.setString(2, product.getProductvalue());    pstmt.setString(3, product.getProductname());//update    pstmt.executeUpdate();    flag = 1;    pstmt.close();    } catch (Exception e) {    e.printStackTrace();    } finally {    conn.close();    }    return flag;

}

updateServlet的代码：
package jspservlet.servlet;

import java.io.IOException;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

import jspservlet.dao.ProductDAO;
import jspservlet.dao.impl.ProductDAOImpl;
import jspservlet.vo.Product;

public class ProductUpdateServlet extends HttpServlet {

 public void doGet(HttpServletRequest req, HttpServletResponse res)    throws IOException, ServletException{     this.doPost(req, res); } public void doPost(HttpServletRequest req, HttpServletResponse res)            throws IOException, ServletException{        Product product = new Product();        product.setProductname(req.getParameter("productname"));        ProductDAO dao=new ProductDAOImpl();        int flag = 0;        int flag2 = 1;        try {            flag2 = dao.queryByProductName(product);        } catch (Exception e) {            // TODO Auto-generated catch block            e.printStackTrace();        }        if (flag2 == 1) {            res.sendRedirect("./errorRegister.jsp");        } else {        try {            flag = dao.updateProduct(product);        } catch (Exception e) {            // TODO Auto-generated catch block            e.printStackTrace();        }        if( flag == 1) {            //session是干嘛的？            HttpSession session = req.getSession();            session.setAttribute("productname", product.getProductname());            res.sendRedirect("./success.jsp");        } else {            res.sendRedirect("./error.jsp");        }         } } 

}

JSP页面：

Delete Product

the name of the product you want to update:

    
         productname : 
the number and price of the product you want to change to:
        productnumber : 
        productvalue :