1.Servlet

Server applet: a small program running on the server side

public class ServletDemo implements Servlet {
    @Override
    public void init(ServletConfig servletConfig) throws ServletException {
        
    }

    @Override
    public ServletConfig getServletConfig() {
        return null;
    }

    @Override
    public void service(ServletRequest servletRequest, ServletResponse servletResponse) throws ServletException, IOException {

    }

    @Override
    public String getServletInfo() {
        return null;
    }

    @Override
    public void destroy() {

    }
}

1.1 Servlet life cycle

• init

  • Initialization method: executed when the Servlet is created, only once

• service

  • Execute multiple times each time the Servlet is accessed

• destory

  • Execute once when the server shuts down normally

• getServletConfig

  • Gets the configuration object of the Servlet

• getServletInfo

  • Get some information about the Servlet, version, author, etc

1.2 Servlet architecture

@WebServlet("/httpServletTest")
public class HttpServletDemo extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        super.doGet(req, resp);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        super.doPost(req, resp);
    }
}

2 Hypertext Transfer Protocol Http

2.1 introduction

• Transmission protocol: defines the format of sending data when the client and server communicate

• characteristic

  • Advanced protocol based on TCP/IP

  • The default port number is 80

  • Based on request / correspondence model: one request corresponds to one correspondence

  • Stateless: each request is independent of each other and cannot interact with data

• Historical version

  • 1.0: each request response will establish a new connection

  • 1.1: multiplex connection

2.2 request message data format

2.2.1 request line

Request mode	Request url	Request protocol / version
Get	Baidu once, you know	HTTP/1.1

• Request method:

  • There are seven request modes in HTTP protocol, two of which are commonly used

    • GET :

      • The request parameters are in the request line, after the url (? Name = aoligei & age = 22)

      • The length of the requested url is limited

      • Not very safe

    • POST :

      • The request parameter is in the request body

      • There is no limit on the length of the requested url

      • Relative safety

• Results after advanced browser parsing:

2.2.2 request header

Request header: (some information that the client browser tells the server itself)

• Results after advanced browser parsing

User agent: the browser tells the server that I can access the version information of the browser you use

• The header information can be obtained on the server side to solve the compatibility problem of the browser

• Accept: tell the server, as a browser, what kind of corresponding information format can I accept from you

• Accept language: tell the server the locale I support as a browser

• Accept encoding: tell the server that as a browser, I support compression

• Referer: tells the server where the current request comes from (where i comes)

  • effect

    • Anti theft chain

statistical work

• 

• Connection: this link can be reused

2.2.3 request blank line

Blank line: separates the request header from the request body

2.2.4 request body (text)

• The of the request body that encapsulates the POST request

• The get method has no request body

• The original format of the request message

2.3 Request

• Principle of request object and response object

  • The request and respnse objects are created by the server, and we will use them

  • The request object is used to get the request message, and the respise object is used to set the corresponding message

2.3.1 request object inheritance architecture

ServletRequest -- Interface

| extends

HttpServletRequest -- Interface

| implements

org.apache.catalina.connector.RequestFacade class (tomcat Implementation)

2.3.2 functions

2.3.2.1 get request line data

• Get request line data    : GET /day14/demo1?name=zhangsan HTTP/1.1

  • GET request method: GET

    • request.getMethod();

  • Get virtual directory:  / day14

    • request.getContextPath();

  • Get Servlet path: / demo1

    • request.getServletPath();

  • Get get mode request parameters:   name=zhangsan

    • request.getQueryString();

  • Get request URI /day14/demo1

    • request.getRequestURI();

    • request.getRequestURL();

      • http://localhost/day/demo1

  • Get protocol and version: HTTP/1.1

    • request.getProtocol();

  • Get the IP address of the client:

    • request.getRemoteAddr();

2.3.2.2 get request header data  

• Get request header data  

  • Get the value of the request header by the name of the request header

    • request.getHeader(String name)

    • request.getHeader("referer");// If you input directly through the browser address bar, you can't get the referer header information (the same as other methods, such as hyperlinks)

  • Gets the names of all request headers

    • Enumeration<String> request.getHeaderNames

2.3.2.3 obtaining request body data

• Get request body data

  • Only the POST request mode has a request body, which encapsulates the request parameters of the POST request

  • step

    • Get stream object

      • BufferedReader getReader(): gets the character input stream. Only character data can be manipulated

      • ServletInputStream getInputStream(): get byte input stream, which can operate on all types of data

    • Then get the data from the stream object

    @PostMapping("/test")
    public String test(HttpServletRequest request) throws IOException {
        
        //1. Get character stream
        BufferedReader br = request.getReader();

        //2. Read data
        String line = null;
        while((line = br.readLine()) != null){
            System.out.println(line);
        }
        return "Project start test";
    }

2.3.2.4 general method for obtaining request parameters

• General method for obtaining request parameters

  • Get parameter value according to parameter name

    • request.getParameter(String name)

  • Get the array of parameter values according to the parameter name (such as? Hobby = swimming & Hobby = basketball)

    • String[ ] request.getParameterValues(String name)

  • Gets the parameter names for all requests

    • Enumeration<String> getParameterNames():

  • Gets the map collection of all parameters

    • Map<String,String[ ]> getParamterMap()

@PostMapping("/test")
    public String test(HttpServletRequest request) throws IOException {

        //Get parameter age
        String age = request.getParameter("age");
        System.out.println("age===>"+age);
        
        //Get the hobby parameter array
        String[] bobbies = request.getParameterValues("bobby");
        for (String bobby : bobbies) {
            System.out.println("hobby===>"+bobby);
        }

        //Get all parameter names
        Enumeration<String> parameterNames = request.getParameterNames();
        while (parameterNames.hasMoreElements()) {
            System.out.println("The parameter name is"+parameterNames.nextElement());
        }
        
        //Get parameter key value pair
        Map<String, String[]> parameterMap = request.getParameterMap();
        String[] ages = parameterMap.get("age");
        for (String s : ages) {
            System.out.println(s);
        }
        String[] bobbies1 = parameterMap.get("bobby");
        for (String s : bobbies1) {
            System.out.println(s);
        }
        return "Project start test";
   
    }

POST request mode garbled

• Sets the character set of the stream

request.setCharacterEncoding("utf-8");

2.3.2.5 request forwarding

• Request forwarding: a resource jump mode within the server

  • step

    • Get the request forwarder object through the request object: RequestDispatcher getRequestDispatcher(String path)

    • Use the RequestDispatcher object to forward: forward (ServletRequest, servletresponse, response)

  • characteristic

    • The browser address bar path does not change

    • It can only be forwarded to internal resources of the current server

    • Forwarding is a request

@RestController
public class TestController {

    @PostMapping("/test")
    public void testController(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        System.out.println("test");
        //Go to another URL
        //RequestDispatcher requestDispatcher = request.getRequestDispatcher("toDestination");
        //requestDispatcher.forward(request, response);
        request.getRequestDispatcher("toDestination").forward(request, response);
    }


    @PostMapping("/toDestination")
    public void toDestination(){
        System.out.println("redirect to  ReachDestination");
    }
    
}

2.3.2.6 shared data

• shared data

  • Domain object: an object with scope that can share data within the scope

  • Request field: represents the scope of a request. It is generally used to share data among multiple resources forwarded by a request

method:

• void setAttribute(String name,Object obj): stores data

• Object getAttribute(String name): get the value through the key

• void removeAttribute(String name): remove key value pairs through keys

@RestController
public class TestController {

    @PostMapping("/test")
    public void testController(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        System.out.println("test");
        //Go to another URL
        //RequestDispatcher requestDispatcher = request.getRequestDispatcher("toDestination");
        //requestDispatcher.forward(request, response);
        request.setAttribute("messageKey", "messageValue");
        request.getRequestDispatcher("toDestination").forward(request, response);
    }


    @PostMapping("/toDestination")
    public void toDestination(HttpServletRequest request){
        System.out.println("redirect to  ReachDestination");
        System.out.println(request.getAttribute("messageKey"));
    }
}

2.3.2.7 get ServletContext

2.3.2.7 get ServletContext

• Get ServletContext

@RestController
public class TestController {

    @PostMapping("/test")
    public void testController(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        ServletContext servletContext = request.getServletContext();
        System.out.println(servletContext);

    }
}

2.4 Response

• data format

  • Corresponding line

  • Response header

  • Response blank line

  • Responder

• Response string format

Response line

• Composition: Protocol / version response status code status code description

  • Status code: all three digits

  • Classification:

    • 1xx: the server receives the client message, but does not accept it. After waiting for a period of time, it sends 1xx and other status codes

    • 2xx: success. Representative: 200

    • 3xx: redirection. Representative: 302 (redirection): the client accesses A. A says I can't do it. Go to C. 304: access the cache. (picture)

• 4xx: client error. Typical: 404 request path has no corresponding resource: 405 request mode mismatch

• 5xx: server side error typical: 500

Response header

• Format: header name: value

• Common response headers:

  • Content type: the server tells the client the data format and encoding format of this response body

  • Content disposition: the server tells the client in what format to open the response body data. The values are as follows

    • In line: the default value, which is opened on the current page

    • attachment;filename=xxx: open the response body as an attachment. File download

  • Content - Length: page character length

Response body: transmitted data

2.4.1 function of response object

2.4.1.1 set corresponding row

• Set corresponding row

  • Set status code: void setStatus(int StatusCode)

2.4.1.2 set response header

• Set response header

  • void setHeadr(String ,name,String value)

2.4.1.3 setting the responder

• Use steps

  • Get output stream

    • Character output stream

      • PrintWriter getWriter();

    • Byte output stream

      • ServletOutPutStream getOutputStream();

  • Use output stream

    • Export data to client browser

2.4.2 redirection cases

• Redirection: how resources jump

  • When accessing A resource, A tells the browser to redirect, and the status code is 302

  • Tell the browser the path of the B resource, and the response header location: the path of the B resource

@RestController
public class TestController {

    @GetMapping("/test")
    public void testController(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        System.out.println("servlet A");
        //Set status code 302
        response.setStatus(302);
        //2. Set response header
        response.setHeader("location", "/redirect");

    }


    @GetMapping("/redirect")
    public void test2(HttpServletRequest request, HttpServletResponse response){
        System.out.println("servlet B");
    }
} 

Simplify response.sendRedirect(String url);

@RestController
public class TestController {

    @GetMapping("/test")
    public void testController(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        System.out.println("servlet A");   
        //simplify
        response.sendRedirect("/redirect");
    }

    @GetMapping("/redirect")
    public void test2(HttpServletRequest request, HttpServletResponse response){
        System.out.println("servlet B");
    }
}

• Characteristics of redirection

  • Address bar send change

  • Redirect resources that can access other sites (servers)

  • Redirection is a two-time request. Data cannot be shared using the requse object t

• route

  • classification

    • Relative path: unique resources cannot be determined through relative path

    • Absolute path: unique resources can be determined by absolute path

      • For example, htttp:// www.baidu.com /day15/responseDemo2

      • Get virtual directory dynamically: request.getContextPath();

2.4.3 case of server outputting byte data to browser

    @GetMapping("/test")
    public void testController(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //1. Get character output stream
        PrintWriter writer = response.getWriter();
        writer.write("<h1>hello print writer</h1>");
    }

Chinese garbled code problem

• Cause: the character sets used for encoding and decoding are inconsistent

• Solution: set the browser encoding format and tell the browser to use this encoding rule

  @GetMapping("/test")
    public void testController(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

        //Before obtaining the stream object, set the default encoding of the stream: ISO-8859-1 GBK
        //response.setCharacterEncoding("utf-8");
        //Tell the browser the encoding of the message body data sent by the server. It is recommended that the browser use this encoding for decoding
        response.setHeader("content-type", "text/html;charset=utf-8");
        //1. Get character output stream
        PrintWriter writer = response.getWriter();
        writer.write("<h1>Hello print writer</h1>");
    }

Simplification: use the response method setContentType

   @GetMapping("/test")
    public void testController(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.setContentType("text/html;charset=utf-8");
        //1. Get character output stream
        PrintWriter writer = response.getWriter();
        writer.write("<h1>Hello print writer</h1>");

    }

2.4.4 case of server outputting byte data to browser

• step

  • Get byte output stream

  • output data

  @GetMapping("/test")
    public void testController(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.setContentType("text/html;charset=utf-8");
        ServletOutputStream outputStream = response.getOutputStream();
        outputStream.write("You also".getBytes());
    }

2.4.5 verification code

• Create an object, a picture in memory (verification code picture object)

• Beautify pictures

• Output picture to page

   @GetMapping("/test")
    public void testController(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

        int width = 200;
        int height = 100;

        //Create an object, a picture in memory (verification code picture object)
        BufferedImage image = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);

        //Output pictures to page display
        ImageIO.write(image, "jpg", response.getOutputStream());
    }

2.5 ServletContext

• Concept: it represents the whole web application and can communicate with the container (server) of the program

• obtain

  • Get through the request object: request.getServletContext();

  • Get through HttpServlet: this.getServletContext();

• function

  • Get MIME type:

  • Domain objects: sharing data

  • Gets the real (server) path of the file

2.5.1 get MIME type

• Get MIME type

  • MIME type: a file data type defined in the process of Internet communication

    • Format large type / small type text/html image/jpeg

  • Get: getMimeType(String file)

    @GetMapping("/test")
    public void testController(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //obtain
        ServletContext servletContext = request.getServletContext();

        //Definition file name
        String filename = "a.jpg";

        //Get MIME type
        String mimeType = servletContext.getMimeType(filename);
        System.out.println(mimeType); //image/jpeg
    }

2.5.2 domain object: shared data (use with caution)

Domain object: shared data (use with caution)

• ServletContext object scope: data requested by all users

servletContext.setAttribute("messageKey", "messageValue");

Object messageKey = servletContext.getAttribute("messageKey");

servletContext.removeAttribute("messageKey");

@RestController
public class TestController {

    @GetMapping("/test")
    public void testController(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //obtain
        ServletContext servletContext = request.getServletContext();
        servletContext.setAttribute("messageKey", "messageValue");
    }


    @GetMapping("/redirect")
    public void test2(HttpServletRequest request, HttpServletResponse response){
        ServletContext servletContext = request.getServletContext();
        Object messageKey = servletContext.getAttribute("messageKey");
        System.out.println(messageKey);
    }
}

2.5.3 get the real (server) path of the file

• Gets the real (server) path of the file

  • servletContext.getRealPath(String path);

   @GetMapping("/test")
    public void testController(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //obtain
        ServletContext servletContext = request.getServletContext();
        //Get real path (tomcat)
        String realPath = servletContext.getRealPath("src\\main\\resources\\application.properties");
        System.out.println(realPath);
    }

2.6 file download cases

• File download requirements

  • Page display hyperlink

  • Click the hyperlink to pop up the download prompt box

  • Complete picture file download

• analysis

  • If the resource pointed to by the hyperlink can be parsed by the browser, it will be displayed in the browser. If it cannot be parsed, a download prompt box will pop up. It does not meet the requirements

  • The download prompt box must pop up for any resource

  • Use the response header to set how resources are opened:

    • content-disposition : attachment;filename = xxx

• step

  • Define the page, edit the href attribute of the hyperlink, point to the Servlet, and pass the resource name filename

  • Define Servlet

    • Get file name

    • Load file into memory using byte input stream

    • Specify the response header of reply: content disposition: attach; filename=xxx

    • Write data out to the response output stream

@GetMapping("/downServlet")
    public void test2(HttpServletRequest request, HttpServletResponse response) throws IOException {
        //Get parameters
        String fileName = request.getParameter("fileName");

        //Get the real path of the server and load the file into memory using the byte input stream
        ServletContext servletContext = request.getServletContext();
        String realPath = servletContext.getRealPath("src\\main\\resources\\img\\"+fileName);
        FileInputStream fileInputStream = new FileInputStream(realPath);

        //Set the response header, download the file as an attachment and MIME type
        response.setHeader("content-type",servletContext.getMimeType(fileName));
        response.setHeader("content-disposition", "attachment;filename="+fileName);

        //Write input stream to output stream
        ServletOutputStream servletOutputStream = response.getOutputStream();
        byte[] buffer = new byte[1024];
        int len = 0;
        while((len=fileInputStream.read(buffer)) != -1){
            servletOutputStream.write(buffer,0,len);
        }
    }

Chinese file name problem

• Tool class

public class DownUtils {
    public static String getFileName(String agent,String filename) throws UnsupportedEncodingException {

        if(agent.contains("MSIE")){
            //IE
            filename = URLEncoder.encode(filename, "utf-8");
            filename = filename.replace("+", " ");
        }else if(agent.contains("Firefox")){
            //Firefox
            BASE64Encoder base64Encoder = new BASE64Encoder();
            filename = "?utf-8?B?" +base64Encoder.encode(filename.getBytes("utf-8"))+"?=";
        }else {
            //Other browsers
            filename = URLEncoder.encode(filename, "utf-8");
        }
        return filename;
    }
}

step

• Get user agent request header

• Use tool classes to encode file names according to different browsers

@GetMapping("/downServlet")
    public void test2(HttpServletRequest request, HttpServletResponse response) throws IOException {
        //Get parameters
        String fileName = request.getParameter("fileName");

        //Get the real path of the server and load the file into memory using the byte input stream
        ServletContext servletContext = request.getServletContext();
        String realPath = servletContext.getRealPath("src\\main\\resources\\img\\"+fileName);
        FileInputStream fileInputStream = new FileInputStream(realPath);

        //Set the response header, download the file as an attachment and MIME type
        response.setHeader("content-type",servletContext.getMimeType(fileName));

        //Solve the problem of Chinese file name
        //Get user agent request header
        String agent = request.getHeader("user-agent");
        //Encode file names according to different browsers
        fileName = DownUtils.getFileName(agent, fileName);
        response.setHeader("content-disposition", "attachment;filename="+fileName);

        //Write input stream to output stream
        ServletOutputStream servletOutputStream = response.getOutputStream();
        byte[] buffer = new byte[1024];
        int len = 0;
        while((len=fileInputStream.read(buffer)) != -1){
            servletOutputStream.write(buffer,0,len);
        }
    }

3. Session Technology: Cookie

• Session: a session contains multiple requests and responses

  • One session: the browser sends a request to the server resource for the first time, and the session is established until one party is disconnected (the browser is closed, or the server is closed, etc.)

  • Function: share data among multiple requests within the scope of a session

  • Method:

    • Client session Technology: cookies

    • Server side Session Technology: Session

3.1 concept

• Concept: client session technology, which saves data to the client

  • Use steps

    • Create Cookie object and bind data

      • new Cookie(String name,String value);

    • Send Cookie object

      • response.addCookie(Cookie cookie);

    • Get cookies and get data

      • Cookie[] request.getCookie();

@RestController
public class TestController {

    @GetMapping("/cookieTest")
    public void testController(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //Create Cookie object
        Cookie cookie = new Cookie("cookieKey", "cookieValue");
        //Send cookies to client browsers
        response.addCookie(cookie);
    }


    @GetMapping("/getCookie")
    public void test2(HttpServletRequest request, HttpServletResponse response) throws IOException {
        //Get Cookie
        Cookie[] cookies = request.getCookies();

        //Get data and traverse cookies
        if(cookies != null){
            for (Cookie cookie : cookies) {
                String cookieKey = cookie.getValue();
                System.out.println(cookieKey);
            }
        }

    }
}

3.2 sending multiple cookies at a time+

• Send multiple cookies at a time

  @GetMapping("/cookieTest")
    public void testController(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //Create Cookie object
        Cookie cookie = new Cookie("cookieKey", "cookieValue");
        Cookie cookie1 = new Cookie("cookieKey2", "cookieValue2");
        //Send cookies to client browsers
        response.addCookie(cookie);
        response.addCookie(cookie1);
    }

3.3 Cookie lifetime

• By default, when the browser is closed, the Cookie data is destroyed

• Persistent storage:

  • setMaxAge(int seconds)

    • Positive number: write cookie data to the file on the hard disk. Persistent storage. Cookie lifetime. (start timing after the browser is closed)

    • Negative: default

    • Zero: delete cookie information

3.4 Cookie storage Chinese

• The key name of the cookie cannot be Chinese

• Before Tomcat 8, Chinese data could not be stored directly in cookie s

  • Chinese (special character) data needs to be transcoded --- generally URL encoding ()

3.5 cookie acquisition scope

• cookie sharing problem

  • Suppose multiple web applications are deployed in a tomcat server, can cookie s be shared in these web projects?

    • Cookies cannot be shared by default

    • cookie.setPaht(String path): sets the cookie acquisition range. By default, the current virtual directory is set

      • If you want all projects on the current server to be shared, you can set the path to "/"

 @GetMapping("/cookieTest")
    public void testController(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //Create Cookie object
        Cookie cookie = new Cookie("cookieKey", "cookie value");
        
        //Set shared scope server scope
        cookie.setPath("/");
        
        //Send cookies to client browsers
        response.addCookie(cookie);
    }

• See the cookie sharing problem for different tomcat servers?

  • cookie.setDomain(String path): if the primary domain name is set to be the same, cookies can share data before multiple servers

  • cookie.setDomain(".baidu.com"), then cookies in tieba.baidu.com and news.baidu.com can be shared

  • Mostly used for distributed

3.6 characteristics and functions of cookies

• characteristic:

  • Cookies store data in the client browser (relatively low security)

  • The browser also limits the size of a single cookie (4kb) and the total number of cookies under the same domain name (20)

• effect:

  • Cookies are generally used to store a small amount of less sensitive data

  • Complete the identification of the client by the server without logging in

3.7 cases

• demand

  • Access a Servlet. If it is the first time, you will be prompted: Hello, welcome to visit for the first time

  • If it is not your first visit, you will be prompted: Welcome back. Your last visit time is: display the time string

• analysis:

  • This can be done using cookies

  • The Servlet in the server determines whether there is a Cookie named lastTime

    • Yes: not the first visit

      • Response data: Welcome back. Your last visit was at 11:50:20, June 10, 2018

      • Write back Cookie: lasttime = 11:50:20, June 12, 2018

    • No,

      • Response data: Hello, welcome to visit for the first time

      • Write back Cookie: lasttime = 11:50:20, June 12, 2018

@RestController
public class TestController {


    public static SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
    public static Date now = new Date();

    @GetMapping("/cookieLastTime")
    public String testController(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        Boolean flag = false;

        //1. Get all cookies
        Cookie[] cookies = request.getCookies();

        //Traversal cookie array
        if (cookies != null  || cookies.length != 0){
            for (Cookie cookie : cookies) {
                String cookieName = cookie.getName();
                if("lastTime".equals(cookieName)){        //There is a lastTime Cookie
                    flag = true;
                    //set
                    String format = sdf.format(now);

                    System.out.println("Before coding:"+format);
                    //URL encoding
                    format = URLEncoder.encode(format, "utf-8");
                    cookie.setValue(format);
                    cookie.setMaxAge(60*60*24*30); // one month
                    response.addCookie(cookie);

                    //get
                    String lastTimeValue = cookie.getValue();
                    //decode
                    System.out.println("Before decoding:"+lastTimeValue);
                    //URL decoding
                    lastTimeValue = URLDecoder.decode(lastTimeValue, "utf-8");
                    System.out.println("After decoding:"+lastTimeValue);
                    return "Your last visit was"+lastTimeValue;
                }
            }
        }


        if(cookies == null || cookies.length == 0 || flag ==false){
            //No, first visit
            String format = URLEncoder.encode(sdf.format(now), "utf-8");
            Cookie cookie = new Cookie("lastTime", format);
            cookie.setMaxAge(60*60*24*30);
            response.addCookie(cookie);
            return "Hello!,Welcome to your first visit";
         }


        return null;
    }
}

4. Session Technology: session

• Concept: server-side session technology, which shares data between multiple requests of a session and saves the data in the server-side object. HttpSession

• Quick start:

  • Get HttpSession object

  • Using the HttpSession object

  • HttpSession object:

    • Object getAttribute(String name)

    • void setAttribute(String name,Object value)

    • void removeAttribute(String name)

@RestController
public class TestController {
    
    @GetMapping("/sessionSet")
    public void testController(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

        //Get Session object
        HttpSession session = request.getSession();
        //set
        session.setAttribute("sessionKey","sessionValue");
    }


    @GetMapping("/sessionGet")
    public void test2(HttpServletRequest request, HttpServletResponse response) throws IOException {
        HttpSession session = request.getSession();
        //get
        Object sessionValue = session.getAttribute("sessionKey");
        System.out.println(sessionValue);
    }
}

4.1 Session principle

• How does the server ensure that the Session object you get multiple times is the same in a Session scope?

• Session depends on cookies!

4.2 Session uniformity X

• When the client is shut down, the server is not shut down. Is the Session obtained twice one?

  • By default, it is not. Because the last session has ended

  • If you need the same, you can create a cookie, key JSESSIONID, set the maximum survival time, and make the cookie persistent

Cookie c = new Cookie("JSESSIONID",session.getId());
c.setMaxAge(60*60);
response.addCookie(c);

4.3 Session uniformity Ω

• When the server is shut down, the Session is not unique

  • Although the Session object is not the same, make sure that the data is not lost

    • Passivation of Session:

      • Serialize the session object to the hard disk before the server shuts down normally

    • Activation of Session:

      • After the server starts, convert the session file into a session object in memory

4.4 Session aging

• When was the Session destroyed

  • Server shutdown

  • The session object calls invalidate();

  • The default session expiration time is 30min (there is no operation when the page is opened)

4.5 characteristics of session

• Session is used to store the data of multiple requests of a session, which is stored on the server side

• session can store any type of data and any size of data