我的Servlet代码是
package DBCon;
import java.io.*;
import java.net.*;
import java.util.ArrayList;
import javax.servlet.*;
import javax.servlet.http.*;
import java.sql.*;
/**
*
* @author Nayan
*/
public class loadCourseId extends HttpServlet {
/**
* Processes requests for both HTTP <code>GET</code> and <code>POST</code> methods.
* @param request servlet request
* @param response servlet response
*/
protected void processRequest(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html;charset=UTF-8");
PrintWriter out = response.getWriter();
ArrayList ar1=new ArrayList();
ArrayList ar2=new ArrayList();
int i;
i=0;
try
{
Class.forName("com.mysql.jdbc.Driver").newInstance();
Connection con=DriverManager.getConnection("jdbc:mysql://localhost/online_exam?"+"user=root&password=pass");
Statement stmt=con.createStatement();
ResultSet rs=stmt.executeQuery("select * from course");
while(rs.next())
{
ar1.add(rs.getString(1));
ar2.add(rs.getString(2));
}
request.getSession().setAttribute("CourseID", ar1);
request.getSession().setAttribute("CourseName", ar2);
RequestDispatcher requestDispatcher=getServletContext().getRequestDispatcher("http://localhost:8080/ONLINE_EXAM/removeCourse.jsp");
requestDispatcher.forward(request,response);
}
catch(Exception e) {
out.println("<h1>"+e.getStackTrace()+"</h1>");
}
}
// <editor-fold defaultstate="collapsed" desc="HttpServlet methods. Click on the + sign on the left to edit the code.">
/**
* Handles the HTTP <code>GET</code> method.
* @param request servlet request
* @param response servlet response
*/
protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);
}
/**
* Handles the HTTP <code>POST</code> method.
* @param request servlet request
* @param response servlet response
*/
protected void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);
}
/**
* Returns a short description of the servlet.
*/
public String getServletInfo() {
return "Short description";
}
// </editor-fold>
}
Jsp Code是
<%@page contentType="text/html" pageEncoding="UTF-8"%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/html4/loose.dtd">
<%@page import="javax.servlet.*"%>
<%@ page import="java.util.ArrayList.*" %>
<%@ page import="java.sql.*;" %>
<html>
<head>
<script type="text/javascript" language="Javascript" >
window.onload=function LoadCombo()
{
window.action="loadCourseId.do";
ArrayList cd=new ArrayList();
cd.add(request.getSession().getAttribute("CourseID"));
if(cd.isEmpty()==false)
{
for(int i=0;i<cd.size();i++)
{
var newOpt = cid.appendChild(document.createElement('option'));
newOpt.text = cd.get(i);
}
}
else
{
alert("Course table is empty")
}
}
</script>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Remove Course</title>
<style type="text/css">
<!--
body {
background-color: #FFCCFF;
}
.style1 {
color: #0066FF;
font-weight: bold;
}
.style2 {font-size: 18px}
.style17 { font-family: "Monotype Corsiva";
font-size: 24px;
font-weight: bold;
font-style: italic;
color: #6633CC;
}
.style19 {color: #000099}
.style21 {color: #000099; font-weight: bold; }
-->
</style>
</head>
<body>
<jsp:include page="Log_Admin.jsp"/><br/>
<form action="" method="post" name="form1" id="form1" >
<table width="46%" height="43" border="3" bgcolor="##CCCC99" align="center">
<tr>
<td width="85%" align="center" bgcolor="##CCCC99"><label><span class="style17">Course Information</span></label></td>
</tr>
<tr><td>
<table width="666" height="207" border="0" align="center" bordercolor="#F0F0F0" bgcolor="#CCCC99" >
<tr>
<td width="186" height="46" align="left"><div align="left"><span class="style19">
<label><strong>Course ID</strong></label>
</span></div></td>
<td><label>
<select name="cid" size="1" id="cid" align="left">
</select>
</label></td>
</tr>
<tr>
<td height="53" align="left"><div align="left"><label><span class="style21">Course Name</span></label></div></td>
<td align="left"><input name="cname" type="text" id="cname" size="50" maxlength="50" /></td>
</tr>
<tr>
<td> </td>
<td> <input name="save" type="submit" id="save" value="Save" />
<input name="reset" type="reset" id="reset" value="Reset" /></td>
</tr>
</table>
</td></tr>
</table>
</form>
</body>
</html>
通过编写此代码，我无法将项目CourseId添加到组合框cid中。你能说我问题在哪里吗？感谢。
答案 0 :(得分：4)
您有2个同步列表，其项目彼此相关。这不容易维护和遍历。而是将两个列表的值放在Map。
Map<String, String> courses = new LinkedHashMap<String, String>();
// ...
while(resultSet.next()) {
map.put(resultSet.getString(1), resultSet.getString(2));
}
// ...
request.setAttribute("courses", courses);
在JSP中，您可以使用JSTL <c:forEach>标记来迭代List或Map。如果是Map，则每次迭代都会在var属性中为您提供Map.Entry，而getKey()属性又包含getValue()和<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>
...
<select name="cid" size="1" id="cid" align="left">
<c:forEach items="${courses}" var="course">
<option value="${course.key}">${course.value}</option>
</c:forEach>
</select>
方法。所以这应该做：
processRequest()此外，response.setContentType("text/html;charset=UTF-8");
PrintWriter out = response.getWriter();
方法
IllegalStateException应该删除，因为这是JSP的负责任，而不是servlet。否则，这样做会有@page import个错误。
还要删除JSP顶部的{{1}}。它们位于错误的位置，所有相关的代码都属于servlet。
答案 1 :(得分：1)
class MyBean{
String val;
String label;
//+getters setters method
}
的Servlet
//fetching list of MyBean and setting it to request as attribute
request.setAttribute("beanList",beanList);
// forward this request to jsp
JSP
<select>
<c:forEach var="bean" items="${beanList}">
<option value="${bean.value}">${bean.label}</option>
</c:forEach>
</select>