Conversational Technology

1. Session: multiple requests and responses in one session
   • One session: the browser sends a request to the server for the first time, and the session is established until one party disconnects
2. Function: share data among multiple requests within the scope of one session
3. Method:
   1. Client session Technology: Cookie
   2. Session: session

Cookie

1. Concept: client session technology, saving data to client

2. Quick start:

   • Use steps:
     1. Create Cookie object and bind data
        • new Cookie(String name，String value)
     2. Send Cookie object
        • response.addCookie(Cookie cookie)
     3. Get cookies, get data
        • Cookie[] request.get

3. Implementation principle

   • Implementation based on response header set cookie and request header cookie

4. Cookie details

   1. Can I send multiple cookies at a time

      • Sure
      • You can create multiple cookie objects and use response to call addCookie method multiple times to send cookies

   2. How long does the Cookie stay in the browser

      • Default: Cookie data is destroyed when the browser is closed
      • Persistent storage
        • setMaxAge(int seconds)
          1. Positive: writes cookie data to a file on the hard disk. Persistent storage. Cookie lifetime
          2. Negative: default
          3. Zero: delete Cookie information

   3. Can cookies be saved in Chinese

      • Not supported before tomcat8
      • Support after tomcat8

   4. What is the scope of Cookie acquisition?

      1. If there are multiple web projects deployed in a tomcat server, can cookies be shared among these web projects
         • Cookie s cannot be shared by default
         • setPath (String path): set the scope of cookie acquisition. By default, sets the current virtual directory
           • If you want to share, you can set path to "/"
      2. Different Cookie sharing problems of tomcat server
         • setDomain(String path): if the primary domain name is the same, cookie s between multiple servers can be shared
           • setDomain (". Baidu. Com"), then cookies in tieba.baidu.com and news.baidu.com can be shared

   5. Characteristics and functions of cookies

      • Characteristic

        1. Cookies store data in client browser
        2. The browser has a limit on the size of a single cookie (4kb) and the total number of cookies under the same domain name (20)

      • Effect

        1. Cookies are generally used to store a small amount of less sensitive data
        2. Complete the server's identification of the client without logging in

   6. Case: remember last visit time

      1. Requirements:

         1. Visit a Servlet. If it's the first time, you will be prompted: Hello, welcome to visit for the first time
         2. If it is not the first visit, prompt: Welcome back, the last time you visited is: display time string

      2. Analysis

         1. Can be done with cookies
         2. The Servlet in the server determines whether there is a cookie named lastTime
            1. Yes: not the first visit
            2. No: first visit
               1. Response data: Hello, welcome to visit for the first time
               2. Write back cookie: lastTime=xxxxx

      3. Code

         public class CookieTest extends HttpServlet {
             @Override
             protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
                 //Set the data format and encoding of the message body of the response
                 resp.setContentType("text/html;charset=utf-8");
                 //1. Get all cookie s
                 Cookie[] cookies = req.getCookies();
                 boolean flag = false;//No cookie is lastTime
                 //2. Traverse the cookies array
                 if (cookies != null && cookies.length > 0){
                     for (Cookie cookie : cookies) {
                         //3. Get the name of the cookie
                         String name = cookie.getName();
                         //4. Judge whether the name is lastTime
                         if ("lastTime".equals(name)){
                             //This cookie is available, not the first time
                             flag = true; //Cookies with lastTime
                             //Set the value of the Cookie
                             //Get the string of the current time, reset the value of the cookie, and resend the cookie
                             Date date = new Date();
                             SimpleDateFormat sdf = new SimpleDateFormat("yyyy year mm month dd day HH:MM:SS");
                             String str_date = sdf.format(date);
                             System.out.println("Before coding"+str_date);
                             //URL encoding
                             str_date = URLEncoder.encode(str_date,"utf-8");
                             System.out.println("After coding"+str_date);
                             //Set cookie lifetime
                             cookie.setMaxAge(60*60*24*30);//one month
                             resp.addCookie(cookie);
                             //Response data
                             //Get the value of the cookie
                             String value = cookie.getValue();
                             System.out.println("Before decoding"+value);
                             value = URLDecoder.decode(value,"utf-8");
                             System.out.println("After decoding"+value);
                             resp.getWriter().write("<h1>Welcome back. Ning's last visit was:"+value+"</h1>");
                             break;
                         }
                     }
                 }
         
                 if(cookies == null || cookies.length == 0 || flag == false){
                     //No cookie s, first visit
                     //Set the value of the Cookie
                     //Get the string of the current time, reset the value of the cookie, and resend the cookie
                     Date date = new Date();
                     SimpleDateFormat sdf = new SimpleDateFormat("yyyy year mm month dd day HH:MM:SS");
                     String str_date = sdf.format(date);
         
                     System.out.println("Before coding"+str_date);
                     //URL encoding
                     str_date = URLEncoder.encode(str_date,"utf-8");
                     System.out.println("After coding"+str_date);
         
                     Cookie cookie = new Cookie("lastTime",str_date);
                     //Set cookie lifetime
                     cookie.setMaxAge(60*60*24*30);//one month
                     resp.addCookie(cookie);
                     resp.getWriter().write("<h1>Hello, welcome to your first visit</h1>");
                 }
             }
         
             @Override
             protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
                 this.doPost(req,resp);
             }
         }