关于验证码==null

package com.imooc.register.verify;

import java.io.IOException;
import java.util.List;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import com.imooc.register.basics.User;
import com.imooc.service.achieve.UserServiceImpl;
import com.imooc.service.impl.UserService;

@WebServlet("/VerifyServlet")
public class VerifyServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;

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

        // 验证验证码：

        // 获取session中保存的验证码信息
        String verifyCode1 = (String) request.getSession().getAttribute(com.google.code.kaptcha.Constants.KAPTCHA_SESSION_KEY);
        // 接收前台表单提交的验证码信息
        String verifyCode2 = request.getParameter("verifyCode");
        if (verifyCode2.equals("")) {
            request.setAttribute("faile", "请输入验证码！");
            request.getRequestDispatcher("/login.jsp").forward(request, response);
        } else if (!verifyCode2.equalsIgnoreCase(verifyCode1)) {
            request.setAttribute("faile", "验证码输入不正确！");
            request.getRequestDispatcher("/login.jsp").forward(request, response);
        } else {
            // 验证账号密码：

            // 获得参数username和password
            String username = request.getParameter("username");
            String password = request.getParameter("password");
            // 进行封装
            User user = new User(username, password);
            // 获得context属性中的userList
            List<User> userList = (List<User>) getServletContext().getAttribute("userList");
            // 初始化接口
            UserService userService = new UserServiceImpl();
            // 调用接口验证username和password
            User existUser = userService.login(userList, user);
            if (existUser == null) {
                // 登录失败
                request.setAttribute("faile", "用户名或密码错误！");
//                response.sendRedirect(request.getContextPath()+"/login.jsp");
                request.getRequestDispatcher("/login.jsp").forward(request, response);
            } else {
                // 登录成功
                request.getSession().setAttribute("existUser", existUser);
//                request.getRequestDispatcher("/categoryList.jsp").forward(request, response);
                response.sendRedirect(request.getContextPath() + "/categoryList.jsp");
            }

        }

    }

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

        doGet(request, response);
    }

}