HTTP protocol

1. Overview of HTTP protocol

　1. What is HTTP? Hypertext Transfer Protocol.
　2. Describe the data standards on the client and server sides. This protocol is maintained and managed by W3C.
　 Version:
　 HTTP1.0: A network connection needs to be established every time a request is made
　 HTTP1.1: (Mainstream) Multiple requests are made and multiple responses are obtained on one network connection. A few more heads.

　3. Clarity: The browser will automatically issue a request when it encounters the following tags

　　

　　
　　

2. The composition of the HTTP protocol

1. The request part

2. Response part

3. Detailed explanation of the request part

　1. Request line:
　　GET /app1/1.html HTTP/1.1
　　GET: Request method.
　　Common request methods: GET (default), POST, HEAD, OPTIONS, etc.
　　GET: Default
　　http://localhost:8080/app1/1.html?username=abc&password=123
　　The submitted data is displayed , relatively unsafe, the first line of the protocol has a length limit, 　　 (Recommended when requesting data) POST: You can pass

　　username=abc&password=123
　　The submitted data is in the request body, relatively safe, length no limit.

　　/app1/1.html: The requested resource address. (URI)
　 URL: protocol + host: port + resource address
　 http://localhost:8080/app1/1.html

　 HTTP/1.1: The version of the protocol used by the client browser.

　 2. Request message header: (take notes and remember commonly used headers)
　　 Function: transfer additional information (cryptographic instructions) to the server
　　 Accept: inform the server of the data types (MIME types) that the client can accept
File system: distinguishes different files by their file extensions. txt jpeg
　 MIME type: large type/small type. txt--->text/plain html---->text/html js---->text/javascript (specific correspondence: Tomcatconfweb.xml)
　　Accept-Encoding: Inform the server of the compression encoding that the client can accept . For example, gzip
　Accept-Language: Inform the server of the language supported by the client.
　Referer: Tell the server which page it came from.

Function: Statistics of advertising effectiveness; prevent hot links.
　Content-Type: Inform the server of the MIME type of the request body
　　Default type: application/x-www-form-urlencoded (the default value of the form enctype attribute)
　　Specific embodiment: username=abc&password=123
　　Other types: multipart/ form-data (used when uploading files)
　　If-Modified-Since: Inform the server of the resource currently being accessed and the last modification time of the file in the cache.
　　User-Agent: Inform the server of the type of browser
　　Content-Length: The data length of the request body
　　Cookie: (*****Important) Session management related

　3. Request body:
　　The data can be submitted to the server, The input field in the form must have a name attribute value
　　The POST request has the text
　　username=abc&password=123

IV. Detailed explanation of the response part

　　1. Detailed explanation of the response part
　　Response line:
　　　HTTP/1.1 200　OK
　HTTP / 1.1: Indicate the protocol version used on the server side
　　　200：Response status code
　　1xx 2xx 3xx 4xx 5xx
　　2xx: Processed
　　4xx: Request error
　　5xx: Server error
　　Remember the commonly used response codes:
200: Everything Normal
　　 302/307: Request redirection
　　304: The resource on the server has not changed
　　404: The accessed address does not exist
　　500: Server-side error
　　OK: Response? Description

　 2. Response header:
　　 Function: Additional information (secret command) passed from the server to the client
　　 Location: Inform the client of the address you are visiting.
　　 Implement request redirection with 302/307
　　Content-Encoding: Inform the client of the compression encoding (gzip) used in the response body
　　Content-Length: Inform the client of the length of the response body
　　Content-Type: Inform the client of the response body The MIME type of the text. Default text/html
　　Refresh: Notify the client to refresh regularly
　　Content-Disposition: Notify the client to open it by downloading
　　　Attachment; filename=23.jpg
　　Set-Cookie: (*****) Session related
Expires: -1

　　　　　 Cache-Control: no-cache (1.1)
　　　 Pragma: no-cache (1.0)
　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　ccogene in in]: no-cache (1.0)

　　　 Use three heads together to tell the browser not to cache.

　　 3. Response text:

　　 The text content parsed by the browser is the same as right-clicking to view the source code.

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