Request (request object) and response (response object)

1. Overview of requests and responses ：
     The concept of request ： Enter the address in the browser address field , Click enter to request the server , This process is a request process .
     The concept of response ： The server sends the request according to the browser , Return the data to the browser for display on the web page , This process is called response

2.HttpServletResponse object
    HttpServletResponse It's an interface , Its parent interface is ServletResponse, Commonly used in development HttpServletResponse
    
     The response information is divided into three parts ： Response line 、 Response head 、 Response Content （ Response body ）
    1. Related methods of sending status code ：
         >>> Set status code
        setStatus(ing sc): The parameter is the status code .// The default is 200
        setStatus(int sc,String sm)：sc Table status code ,sm Table description of status code
         Common status codes ：
            200： Corresponding success
            302： Redirect
            404： Requested resource not found
            500： An exception occurred in the server code
         >>> Error message method ：
        senError(int sc)： Parameters are also status codes
        senError(int sc,String sm)：sc Table status code ,sm Table description of status code
         such as ：response.sendError(404,"chapter04 The requested resource for the project does not exist ");
    
    1.2 Methods related to sending response message headers ：
         The data format of the response header ：key-value In the form of , There is a key One value, There is a key Multiple value
    
        >>>1. Set the response header ：
            *   One key, Corresponding to one value
                 setHeader(String key,String value);
                such as ：
                setHeader("aa","AA");
                   If ：setHeader("aa","AA");
                setHeader("aa","BB");
                 final result ：aa The corresponding value of this header is BB
            
            *   One key Corresponding multiple value
               addHeader(String key,String value);
                 for example ：addHeader("aa","AA");
                addHeader("aa","BB");
                 final result ：aa The value used for this head pair is AA and BB
             such as ：// Methods related to the response header
            // In case of wrong user name and password , Go back to the login page and let the user restart the data
            //1. Set status code
            response.setStatus(302);
            //2. Set the response header , Complete redirection to the login page
            response.setHeader("Location","chapter04/login.html");
    
        >>>2. Set the length of the response data ：setContentLength(int src) Method
        >>>3. Set the method to deal with garbled code ：
            1.setCharacterEncoding(String a); Parameter is a code table format , such as ：utf-8
            2.setContenType(String a); Parameters represent the type and code table format of response data , such as ："text/thml;charset=utf-8"
    
    1.3 Send methods related to the response body
        1.getOutputStream, adopt response Return pictures to the browser 、 Binary files such as video , Of course, you can also return text data to the browser
         such as ：
            //** byte ** Stream object , Return data to the browser
            ServletOutputStream out=response.getOutputStream();
            String value="itcast";
            out.write(value.getBytes());
        2.getWriter(), Usually response Return text data to the browser , Cannot return pictures to the browser 、 Video and other binary files
         such as ：
            //** character ** Output data to browser
            PrintWriter pw=response.getWriter();
            String value="itcast-----";
            //pw.print(value);
            pw.write(value);    
    1.4 Solve the problem of garbled Chinese output
        1. The byte stream will appear garbled in the browser output first
             reason ： It is caused by the inconsistency between the server-side encoding format and the browser encoding format
    
             such as ：
            // 1. Handle the Chinese garbled code problem of byte stream
            String data=" China ";
            ServletOutputStream out=response.getOutputStream();
            // Deal with the mess ： The short encoding of the server is consistent with that of the browser .
            // Let the browser side coding also utf-8
            // Let the Chinese coding format on the server side be utf-8, Make sure there's consistency
            response.setHeader("Content-Type","text/html;charset=utf-8");
            out.write(data.getBytes());
    
          **** There are two ways to deal with the Chinese garbled code of byte stream
            1.response.setHeader("Content-Type","text/html;charset=utf-8");
            2.response.setContentype("text/html;charset=utf-8");
        
        2. Characters flow to browser output at noon , There will also be garbled code
             Two forms of garbled code ：？？, This question mark indicates that the Chinese coding format is iso8859-1 code , This encoding format does not support Chinese
            
             Handle ： Keep the coding of browser and server consistent
             such as ：
            // 1. Handle the Chinese garbled code problem of character stream
            //2. Handle The encoding of the server , Let this code support Chinese
            response.setCharacterEncoding("utf-8");
            //3. Handle browser side coding
            response.setHeader("Content-Type","text/html;charset=utf-8");
            String data=" China ";
            //4. Get character stream
            PrintWriter pw=response.getWriter();
            pw.print(data);
            
          **** There are two ways to deal with the Chinese garbled code problem of character stream
            1.response.setCharacterEncoding("utf-8");
                response.setHeader("Content-Type","text/html;charset=utf-8");
            2.response.setContentype("text/html;charset=utf-8");
    
         Conclusion ： It is used in dealing with the problem of Chinese garbled code output from character stream and byte stream response.setHeader("Content-Type","text/html;charset=utf-8");
         Be careful ：
            1. Common encoding formats ：GBK、GB18030、GB2312 Both support Chinese coding format , Of course, some other characters are supported
            2.utf-8 Universal code table format , All characters are supported
            3.ISO8859-1： No Chinese support
                
    1.5 Realize the regular jump and refresh of web pages ：
         Timed jump ： Jump from a registration page to a login page , Enter the previously registered user name and password .
             for example ：
            // after 5 Seconds to jump to “login.html”
            response.setHeader("Refresh","5;URL=/chapter04/login.html");
    
         Refresh regularly ： The address on the current page does not change , But the current page refreshes itself regularly , Page data changes .
             for example ：
            // Is it realized 5 Refresh every second
            response.setHeader("Refresh","5");
            // Convenient for observation , Output a time , Find every time 5 Seconds will refresh, and then the time will change
            response.getWriter().print(new Date());
    
    1.6 Implement redirection     
         Implement redirection related methods
        >>>1：302 and Location In combination with , Implement redirection
            response.setStatus(302);
            response.setHeader("Refresh","5;URL=/chapter04/login.html");
        >>>2：sendRediret("/chapter04/login.html");
             Redirection is done through login cases ：
             Demand analysis ： Enter your user name and password on the login page , stay LoginServlet Inside , Determine whether the user and password are correct .
                 If the input is wrong , Will be redirected to the login page , If correct , Jump to the welcome page .
             Get the user name and password submitted on the login page ：request.getParameter() Method ;
             Features of redirection ：1. Browser address bar changes
                      2. Send two requests , Are all requests sent by the browser
             Redirected path ： Because the request is sent by the browser . So you must bring the project name , such as ：/chapter04/welcome.html;        

3.HttpServletRequest Request object
        HttpServletRequest It's an interface , Its parent interface ServletRequest, In development , Commonly used is the request object with protocol
         The request message ： Request line 、 Request header 、 Request parameters
    1. Get the relevant method of the request line message ：
         Common request line methods ：
        >>>1. Get the method of Request Submission ：getMethod();
        >>>2. Get the requested protocol :getProtocol();
        >>>3. Get the project name :getContextPath();        // Get the URL”
        >>>4. obtain servlet route getServletPath();
        >>>5. Get the request path ：getRequsetURI(),getRequestURL();
            uri Corresponding value ：/ Project name /servlet route
            url Corresponding value ：http://localhost:8080/uri
        2. Get the method related to the request header ：
            >>>1. According to the message header , Get value ：String value=getHeader( The name of the message header );
            >>>2. Get all headers ：Enumeration er=getHeaderName();

        3. Get the method related to the request parameters ：
            >>>1. according to name Property value , Get the value entered by the user ：String value=getParameter(name The value of the property );
            >>>2. according to name Property value , Get the value selected by the user ：String[] values=getParameterValues(name The value of the property );
            >>>3. No parameters , Get all values ：Map<String,String[]> map=getParameterMap();
        4. Solve the problem of Chinese garbled code of request parameters ：
             Case one ： Handle get Chinese garbled code submitted by
                 The use of string The construction method of solves Chinese garbled code :new String( Byte array , Coding format );
                 Code : String newvalue = new String(value.getBytes("IS08859-1"), "utf-8");
             The second case ： Handle post Chinese garbled problem of request :
                post Submit , Will first submit the parameters to request Object , The default encoding of the buffer is IS08859-1, No Chinese support
                 processing method ： hold request The encoding of the buffer is set to support Chinese code table format .
                 Code ：request.setCharacterEncoding("utf-8");
        5. adopt Requset Object passing data
             The concept of domain objects ： To a certain extent , You can store values and take values .
            request Domain object ： In a request , You can store values and take values
                >>> Store value :setAttribute(String key,0bject obj);
                >>> Value : getAttribute(string key);
                >>> remove :removeAttribute(String key);
    1.2 RequestDispatcher Interface
         In development, access to a web After resources , The server needs to jump to another resource to process the request , Can pass sendRedirect Redirect
         Method to implement , It's fine too RequestDispatcher The of the forwarding object forward Method to implement .
         Get the forwarding object : RequestDispatcher forwardObj = request.getRequestDispatcher(" The path to another resource ");
    
         Common methods ：
            >>>1. Implement request forwarding :forward0bj.forward(request,response); 
            >>> 2. The implementation request contains :forwardObj.include(request,response);
    
        1． Request forwarding :
             characteristic : Sent a request , Respond once , and request Use with domain objects , Can achieve worth passing .
             Forwarding path : The forwarding action is implemented inside the server , Therefore, the forwarding path should not carry the project name
    
               Ask for redirection ：
             characteristic ： Two requests were sent , Responded twice , Unable to join request Use with domain objects .
             Redirection path ： The redirection action is executed on the browser side , Therefore, the path needs to carry the project name .
    
        2. Request includes
             Request includes ： Done inside the server , The path does not need to carry the project name .
             characteristic : Send a request , Request inclusion is done by the server
             Request contains path : Done inside the server , The path does not need to carry the project name .

The difference between redirection and forwarding ：
    1. Redirection is two requests , Two responses , Forwarding is a request , One response
    2. The first request parameter of redirection will disappear in the second time , Forwarding will not disappear
    3. The address bar will change after the redirection request , The address bar will not change after forwarding the request
    4. Redirection can access the address outside the stack , Forwarding is not allowed
    5. Forwarding first proportion orientation is relatively safe

In forwarding forward and include The difference between ：（ repair ）
    1.forward The browser part of the current page will be ignored later , Then display the browser part of the page after jump
    2.include The browser part of the following current page will be added after the browser part of the page after jump