url override and HttpSession implements session for form

url 重写结合HttpSession实现会话管理之 form 提交
package com.test;

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 javax.servlet.http.HttpSession;
import java.io.IOException;
import java.io.PrintWriter;

@WebServlet("/questionnaire")
public class Questionnaire extends HttpServlet {
    protected void processRequest(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        request.setCharacterEncoding("UTF-8");
        response.setContentType("text/html;charset=UTF-8");

        PrintWriter out = response.getWriter();
        out.println("<!DOCTYPE html>");
        out.println("<html>");
        out.println("<head>");
        out.println("<meta charset='UTF-8'>");
        out.println("<title>问卷调查</title>");
        out.println("</head>");
        out.println("<body>");
        String page = request.getParameter("page");
        if(page == null) {                   // 第一頁問卷
            out.println("<form action='questionnaire' method='get'>");
            out.println("問題一：<input type='text' name='p1q1'><br>");
            out.println("問題二：<input type='text' name='p1q2'><br>");
            out.println("<input type='submit' name='page' value='page2'>");
            out.println("</form>");
        }
        else if("page2".equals(page)) {    // 第二頁問卷
            /**
             * 下面这三行代码的顺序不可变
        　　*/
            HttpSession session = request.getSession();
            String url = response.encodeURL("questionnaire");
            out.println("<form action="" + url + "" method="get">");
            session.setAttribute("p1q1", request.getParameter("p1q1"));
            session.setAttribute("p1q2", request.getParameter("p1q2"));
            out.println("問題三：<input type='text' name='p2q1'> <br>");
            out.println("<input type='submit' name='page' value='finish'>");
            out.println("</form>");
        }
        else if("finish".equals(page)) {    // 最後答案收集
            HttpSession session = request.getSession();
            out.println(session.getAttribute("p1q1") + "<br>");
            out.println(session.getAttribute("p1q2") + "<br>");
            out.println(request.getParameter("p2q1") + "<br>");
        }
        out.println("</body>");
        out.println("</html>");
        out.close();
    }

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

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