JSP页面跳转出现关于传值错误

Question

关于JSP页面跳转传值出现问题
Servlet将用户的信息转发到xiaofei.jsp页面

                    request.setAttribute("user", user);
                    request.getRequestDispatcher("/xiaofei.jsp")
                            .forward(request,response);

然后xiaofei.jsp页面接收代码是这样的

16: <%
17:     User user = (User) request.getAttribute("user");
18:     String username = user.getUsername();
19:     System.out.println(username);
20: %>
欢迎您：
<%
    out.print(user.getUsername());
%>

xiaofei.jsp页面链接数据库，根据用户名输出用户详细信息

String sql="select * 
from user u join xuexi x on u.id=x.id join yinshi y on y.hao=x.hao join youxi o on o.day=y.day"+" 
where username='"+user.getUsername()+"'";

xiaofei.jsp页面有添加个人信息的按钮

<td><a href="add.jsp">添加</a></th>

点击添加，跳转到add.jsp页面，在添加完成后，重新跳转到xiaofei.jsp页面

 try{
            Class.forName("com.mysql.jdbc.Driver");
            Connection con=DriverManager.getConnection("jdbc:mysql://localhost:3306/user","root","root");
            Statement stmt=con.createStatement();
            String sql="insert into xuexi(id,hao,day,book,price1) values('"+id+"','"+hao+"','"+day+"','"+name+"','"+price+"')";
            int i=stmt.executeUpdate(sql);
            if(i==1){
                out.print("<script>alert('添加成功');</script>");
                response.setHeader("refresh", "1;url=xiaofei.jsp");
            }else{
                out.println("<script>alert('添加失败');</script>");
                response.setHeader("refresh","1;url=add.jsp");
            }
            con.close();
            stmt.close();
        }catch(Exception e){
            out.println("<script>alert('失败,请重试');</script>");
            response.setHeader("refresh","1;url=add.jsp");
        }

此时问题出现了，跳转失败了，org.apache.jasper.JasperException: 在 [18] 行处理 [/xiaofei.jsp] 时发生异常
但是数据已经成功插入，再次登录能把新添加的信息显示出来

org.apache.jasper.JasperException: 在 [18] 行处理 [/xiaofei.jsp] 时发生异常

15: <br>
16: <%
17:     User user = (User) request.getAttribute("user");
18:     String username = user.getUsername();
19:     System.out.println(username);
20: %>
欢迎您：
<%
    out.print(user.getUsername());
%>


Stacktrace:
    org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:605)
    org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:498)
    org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:383)
    org.apache.jasper.servlet.JspServlet.service(JspServlet.java:331)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:764)
    org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
根本原因。

java.lang.NullPointerException
    org.apache.jsp.xiaofei_jsp._jspService(xiaofei_jsp.java:124)
    org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:764)
    org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:465)
    org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:383)
    org.apache.jasper.servlet.JspServlet.service(JspServlet.java:331)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:764)
    org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
):注意 主要问题的全部 stack 信息可以在 server logs 里查看

该怎么办，查找好多方法还是没有用，我的是Tomacat8.5，JDK是1.8，已经导入Jstl.jar包和standard.jar包了，

Answer 1

你这里你得再经过这个servler啊，你这里用的是请i求转发，你前后的request不是一个，根本getAttribute("user")这个没有，望采纳，谢谢