老师，我的session第一次传递正确，第二次返回时输出为null？怎么解决？代码如下
还是这个错误，但是我的思路时用session存储值，然后返回上一个页面：
org.apache.jasper.JasperException: An exception occurred processing [show_main.jsp] at line [22]
19: String pwd = (String)session.getAttribute("password");
20: System.out.println("pwd "+pwd);
21: Emp emp = new Emp(name,pwd);
22: boolean flag = DBUtil.selectEmpByAccountAndPassword(emp);
23: Map<String,ShowMessage> map1 = DBUtil.map1;
24:
25: if (flag == true) {
Stacktrace:
org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:593)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:482)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:386)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:330)
javax.servlet.http.HttpServlet.service(HttpServlet.java:741)
org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)
Root Cause
java.lang.NullPointerException
com.imooc.db.DBUtil.selectEmpByAccountAndPassword(DBUtil.java:38)
org.apache.jsp.show_005fmain_jsp._jspService(show_005fmain_jsp.java:136)
org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
javax.servlet.http.HttpServlet.service(HttpServlet.java:741)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:444)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:386)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:330)
javax.servlet.http.HttpServlet.service(HttpServlet.java:741)
org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)
Note The full stack trace of the root cause is available in the server logs.
（1）这是公告信息主页面的代码
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<%@ page language="java" contentType="text/html; charset=UTF-8"
pageEncoding="UTF-8" import="com.imooc.db.*, com.imooc.bean.*, java.util.*"%>
<title>公告列表</title>
</head>
<body>
<%
String nickname = request.getParameter("nickname");
System.out.println("nickname "+nickname);
String password = request.getParameter("password");
System.out.println("password "+password);
session.setAttribute("nickname", nickname);
session.setAttribute("password", password);
String name = (String)session.getAttribute("nickname");
System.out.println("name "+name);
String pwd = (String)session.getAttribute("password");
System.out.println("pwd "+pwd);
Emp emp = new Emp(name,pwd);
boolean flag = DBUtil.selectEmpByAccountAndPassword(emp);
Map<String,ShowMessage> map1 = DBUtil.map1;
if (flag == true) {
%>
<h3>公告列表为: </h3>
<hr>
<form action="select.jsp">
<table align="center" border="1" width="500px">
<tr>
<td>
公告编号：
</td>
<td>
<input type="text" name="num" value="" placeholder="请输入编号"/>
</td>
<td>
<input type="submit" value="select"/>
</td>
</tr>
</table>
</form>
<div height="50px"></div>
<form action="">
<table align="center" border="1" width="500px">
<tr>
<td>
编号
</td>
<td>
名称
</td>
<td>
内容
</td>
<td>
删除
</td>
<td>
修改
</td>
</tr>
<%
for (String key : map1.keySet()) {
ShowMessage e = map1.get(key);
%>
<tr>
<td>
<%= e.getNum() %>
</td>
<td>
<%= e.getName() %>
</td>
<td>
<%= e.getContent() %>
</td>
<td>
<a href="delete.jsp?num=<%= e.getNum() %>
&name=<%= e.getName() %>&content=<%= e.getContent() %>">删除</a>
</td>
<td>
<a href="update.jsp?num=<%= e.getNum() %>
&name=<%= e.getName() %>&content=<%= e.getContent() %>">修改</a>
</td>
</tr>
<%
}
%>
</table>
</form>
<%
} else {
}
%>
</body>
</html>（2）这是点击select后的界面代码（有back按钮的界面）
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<%@ page language="java" contentType="text/html; charset=UTF-8"
pageEncoding="UTF-8" import="com.imooc.db.*, com.imooc.bean.*, java.util.*"%>
<title>公告列表</title>
</head>
<body>
<h3>公告列表为: </h3>
<p>用户名：<%=request.getSession().getAttribute("nickname") %>， 密码：<%=request.getSession().getAttribute("password") %></p>
<%
request.setAttribute("nickname", session.getAttribute("nickname"));
request.setAttribute("password", session.getAttribute("password"));
out.println(session.getId());
%>
<hr>
<form action="show_main.jsp">
<table align="center" border="1" width="500px">
<tr>
<td>
公告编号：
</td>
<td>
<input type="text" name="num" value="<%= request.getParameter("num") %>"/>
</td>
<td>
<a href="show_main.jsp">back</a> ">Back</a>
</td>
</tr>
</table>
</form>
<form action="">
<table align="center" border="1" width="500px">
<tr>
<td>
编号
</td>
<td>
名称
</td>
<td>
内容
</td>
<td>
删除
</td>
<td>
修改
</td>
</tr>
<%
Map<String,ShowMessage> map1 = DBUtil.map1;
ShowMessage e1 = map1.get(request.getParameter("num"));
for (String key : map1.keySet()) {
ShowMessage e2 = map1.get(key);
if (e1.getNum().equals(e2.getNum()))
{
%>
<tr>
<td>
<%= e1.getNum() %>
</td>
<td>
<%= e1.getName() %>
</td>
<td>
<%= e1.getContent() %>
</td>
<td>
<a href="delete.jsp?num=<%= e1.getNum() %>
&name=<%= e1.getName() %>&content=<%= e1.getContent() %>">Delete</a>
</td>
<td>
<a href="">Update</a>
</td>
</tr>
<%
}
}
%>
</table>
</form>
</body>
</html>正在回答 回答被采纳积分+1
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