select标签获取不到值

我在categoryServlet中将所有categoryName放到一个集合里，把集合放到request中，到addBook.jsp中用c:foreach遍历却得不到里面的值，请老师看一下怎么回事。

package com.imooc.servlet;
/**
 * 图书分类的servlet
 */

import com.imooc.dao.LibDaoImpl;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;

public class CategoryServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        request.setCharacterEncoding("UTF-8");

        //接收来自category.jsp的数据
        Long id = Long.valueOf(request.getParameter("id"));
        String categoryName = request.getParameter("categoryName");
        String desc = request.getParameter("desc");

        LibDaoImpl libDao = new LibDaoImpl();
        List<Map<String,Object>> categoryDb = libDao.getCategoryDb();
        List<String> categoryNameList = libDao.getCategoryNameList();

        for(Map<String,Object> map:categoryDb){
            if(map.get("id").equals(id) | map.get("categoryName").equals(categoryName)){
                request.setAttribute("msg","id或类名已存在，请重新输入！");
                request.getRequestDispatcher("/category.jsp").forward(request,response);
                return;
            }
        }
        libDao.addBookCategory(id,categoryName,desc);

        //将categoryName存放到categoryNameList集合中，以便在addBook.jsp的<c：foreach>中获取
        categoryNameList.add(categoryName);

        System.out.println(categoryNameList);

        request.setAttribute("categoryNameList",categoryNameList);
        return;

    }

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        this.doGet(request,response);
    }
}

<tr>
   <td>图书分类</td>
   <td>
      <select name="categoryName" >
         <c:forEach var="categoryName" items="${requestScope.categoryNameList}" >
         
            <option value="${categoryName}">${categoryName}</option>

         </c:forEach>
      </select>
   </td>
</tr>