写servlet中出现了500错误，各位大神能不能帮忙看看

Question

LoginServlet.java
package servlets;

import java.io.*;

import javax.servlet.*;
import javax.servlet.http.*;

import service.AdminService;
import service.CommonService;
public class LoginServlet extends HttpServlet {

public void doGet(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {

    doPost(request,response);
}

public void doPost(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {

    request.setCharacterEncoding("GBK");
    String username=request.getParameter("username");
    String password=request.getParameter("password");
    String loginIdentity=request.getParameter("loginIdentity");
    RequestDispatcher rd=null;
    if(loginIdentity!=null){
        if(loginIdentity.equals("admin")){
        AdminService as=new AdminService();
        //验证管理员用户名密码是否正确
        if(as.checkLogin(username, password)){
            request.getSession().setAttribute("login", "ok");
            rd=request.getRequestDispatcher("/adminheader.jsp");
            rd.forward(request, response);
            return;
            }
        }
        else {
                CommonService cs=new CommonService();
                if(loginIdentity.equals("worker"))
                {//验证复印店用户名密码是否正确
                    if(cs.shopCheckLogin(username, password)){
                        request.getSession().setAttribute("login", "ok");
                        rd=request.getRequestDispatcher("/shopHeader.jsp");
                        rd.forward(request, response);
                        return;
                        }
                }
                else{
                    if(cs.studentCheckLogin(username, password)){
                        //验证学生的用户名密码是否正确
                        request.getSession().setAttribute("login", "ok");
                        rd=request.getRequestDispatcher("/studentHeader.jsp");
                        rd.forward(request, response);
                        return;
                        }
                }
        }
    }
    //当用户名或密码为null，或者用户名密码不正确时，返回登录页面，并给出提示信息。
    rd=request.getRequestDispatcher("/admin/login.jsp?error=yes");
    rd.forward(request, response);
}

}
/////////////////////////////////
login.jsp
<%@ page pageEncoding="GBK" %>
<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core"%>

function check(){ username=document.f.username.value; password=document.f.password.value; if(username==""){ alert("用户名不能为空！"); return false; }else if(password==""){ alert("密码不能为空！"); return false; }else{ return true; } }

欢迎使用复印店管理系统

用户名
密码
学生 店主 管理员

---

用户名或密码不正确！
/c:if

Answer 1

servlet在web.xml配置了吗