HTTP protocol

1. What is the HTTP protocol

What is an agreement

Agreement refers to the rules agreed by both parties or multiple parties and everyone should abide by

The so-called HTTP protocol refers to the rules that the data sent needs to comply with when communicating between the client and the server

The data in HTTP protocol is also called message

2. HTTP protocol format of the request

The client sends a data call request to the server

The server sends back data to the client to call the response

Requests are divided into GET requests and POST requests

1) GET request

1. Request line
   • The method of the request is GET
   • Requested resource path [+? + request parameters]
   • Requested protocol and version number HTTP/1.1
2. Request header
   • It is composed of key: value. Different key value pairs represent different meanings

The following is the request sent when you click the submit button in useServlet.html

GET /myfirst/contextservlet HTTP/1.1
Host: localhost:8080
Connection: keep-alive
sec-ch-ua: "Microsoft Edge";v="95", "Chromium";v="95", ";Not A Brand";v="99"
sec-ch-ua-mobile: ?0
sec-ch-ua-platform: "Windows"
Upgrade-Insecure-Requests: 1
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/95.0.4638.54 Safari/537.36 Edg/95.0.1020.30
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9
Sec-Fetch-Site: same-origin
Sec-Fetch-Mode: navigate
Sec-Fetch-User: ?1
Sec-Fetch-Dest: document
Referer: http://localhost:8080/myfirst/useServlet.html
Accept-Encoding: gzip, deflate, br
Accept-Language: zh-CN,zh;q=0.9,en;q=0.8,en-GB;q=0.7,en-US;q=0.6
Cookie: JSESSIONID=AAF5BDFD5ADADA3F3BC4FD7B938BA2D1; Idea-862f61e3=6af5da74-a9f1-4dc4-af02-e10aeb57dad4

• Host: indicates the ip address and port number of the requested server
• Connection: tells the server how to handle the connection request
  • Keep alive: tell the server not to close the data returned immediately and keep the connection for a short period of time
  • Closed: close now
• User agent: user agent, that is, the information of the browser
• Accept: tells the server the type of data the client can receive
• Referer: indicates the address of the source page of the current request page
• Accept encoding: tells the server the data encoding (compression) format that the client can receive
• Accept language: tells the server the type of language the client can receive
  • ZH CN: Chinese
  • En US: English American

2) POST request

1. Request line
   • Request mode POST
   • Requested resource path [+? + request parameters]
   • Requested protocol and version number HTTP/1.1
2. Request header
   • It is composed of key: value. Different key value pairs represent different meanings
3. Blank line
   • Used to split the request header and the request body
4. Request body
   • Is the data sent to the server

POST /myfirst/hello HTTP/1.1
Host: localhost:8080
Connection: keep-alive
Content-Length: 26
Cache-Control: max-age=0
sec-ch-ua: "Microsoft Edge";v="95", "Chromium";v="95", ";Not A Brand";v="99"
sec-ch-ua-mobile: ?0
sec-ch-ua-platform: "Windows"
Upgrade-Insecure-Requests: 1
Origin: http://localhost:8080
Content-Type: application/x-www-form-urlencoded
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/95.0.4638.54 Safari/537.36 Edg/95.0.1020.30
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9
Sec-Fetch-Site: same-origin
Sec-Fetch-Mode: navigate
Sec-Fetch-User: ?1
Sec-Fetch-Dest: document
Referer: http://localhost:8080/myfirst/useServlet.html
Accept-Encoding: gzip, deflate, br
Accept-Language: zh-CN,zh;q=0.9,en;q=0.8,en-GB;q=0.7,en-US;q=0.6
Cookie: JSESSIONID=EB29494F56A4D56E2B59C8177383B578; Idea-862f61e3=6af5da74-a9f1-4dc4-af02-e10aeb57dad4

• Accept: tells the server the type of data the client can receive

• Accept language: tells the server the type of language the client can receive

• Referer: indicates the address of the source page of the current request page

• User agent: represents the information of the browser

• Content type: indicates the type of data sent

  • application/x-www-form-urlencoded
    • Indicates that the submitted data format is: name = value & name = value, and then url encodes it
    • url encoding is to convert non English content into:% xx%xx
  • multipart/form-data
    • It means submitting data to the server in the form of multiple segments (submitting in the form of stream for uploading)

• Content lengh: indicates the length of the sent data

• Cache control: indicates how to control the cache

  • No cache: no cache

3) Description of common request headers

4) Which are GET requests and which are POST requests

What are the GET requests:

1. form tag method=get
2. a label
3. link tag introduces css
4. script tag import js file
5. img tag import picture
6. iframe introduces html pages
7. Enter the address in the browser address bar and press enter

What are the POST requests:

1. form label method=post

3. The HTTP protocol format of the response

1. Response line
   • The protocol and version number of the response
   • Response status code
   • Response state descriptor
2. Response header
   • It is composed of key: value. Different key value pairs represent different meanings
3. Blank line
   • Used to split the request header and the request body
4. Responder
   • Is the data returned to the client

visit http://localhost:8080/myfirst/useServlet.html?a=123 , that is, this page is requested, so the response body returns this page

HTTP/1.1 200
Accept-Ranges: bytes
ETag: W/"367-1635527591401"
Last-Modified: Fri, 29 Oct 2021 17:13:11 GMT
Content-Type: text/html
Content-Length: 367
Date: Fri, 29 Oct 2021 17:17:56 GMT
Keep-Alive: timeout=20
Connection: keep-alive

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
    <form action="http://localhost:8080/myfirst/hello" method="get">
        <input type="hidden" name="action" value="login" />
        <input type="hidden" name="username" value="root" />
        <input type="submit">
    </form>
</body>
</html>

• Server: information representing the server
• Content type: indicates the data type of the response body
• Content length: the length of the response body
• Date: time of request response (Greenwich mean time, with a difference of eight hours)

Description of common response codes

• 200: indicates that the request was successful
• 302: indicates request redirection
• 404: indicates that the request server has received it, but the data you want does not exist (the request address is wrong)
• 500: indicates that the request has been received by the server, but the server has an internal error (code error)

4. MIME type description

MIME is a data type in the HTTP protocol

The full name of MIME is "Multipurpose Internet Mail Extensions"

The format of MIME type is "large type / small type" and corresponds to the extension of a file

file	MIME type
Hypertext markup language text	.html, .htm – txt/html
Plain text	.txt – text/plain
Rich text	.rtf – application/rtf
GIF graphics	.gif – image/gif
JPEG graphics	.jpeg, .jpg – image/jpeg
au sound file	.au – audio/basic
MIDI music files	.mid, .midi – audio/midi
RealAudio music files	.ra, .ram – audio/x-pn-realaudio
MPEG file	.mpg, .mpeg – video/mpeg
AVI file	.avi – video/x-msvideo
GZIP file	.gz – applicaiton/x-gzip
TAR file	.tar – application/x-tar