HTTP request & response

3月 12, 2017

HTTP request & response

一樣是最近一直需要反覆了解的東西, 今天花大半天了解它們,

因為這標題說明的範圍可大可小,

因而此篇只說明自己最近釐清及想了解的地方...

怕思路斷掉, 寫完這篇累死自己... >"<

p.s. 圖感覺太小可點選便會放大

第一件事是說明在HTTP request & response流程中傳遞的到底是什麼?

回答:

- request中傳遞的是一堆binary, 在此可視為是inputStream(代表"資料的來源",可從中讀取資料)數據流 [以此類推: response中傳遞的還是一堆binary, 在此可視為是outputStream(代表是"資料的目的地",可以寫入資料)] --> inputStream會在serverSocket.accept()後封包成socket物件(由此可知socket物件仍是由binary組成的)。

- 當我們想要讀取/寫入binary時, 可以這麼寫: socket.getInputStream() 和 socket.getOutputStream() --> 但這操作會因太原始而難操作, 於是包裝成DataInputStream和DataOutputStream, 可更方便去操作讀取/寫入binary。

- 寫code的手法: 因為其實request和response中傳遞的header fields可以很多, 所以把DataInputStream解析出的各種header fields包起來存在一個Request物件,以方便寫code時操作Request物件就好, 而不用從DataInputStream中解析後再操作。同理可證, DataOutputStream也可解析出各種header fields包起來存在一個Response物件中, 以方便寫code時操作。

第二件事是說明HTTP、client、server定義 & URI的作用?

回答:

- HTTP定義: HTTP is based on the client-server architecture model and a stateless request/response protocol that operates by exchanging messages across a reliable TCP/IP connection.

- client定義: An HTTP "client" is a program (Web browser or any other client) that establishes a connection to a server for the purpose of sending one or more HTTP request messages.

- server定義: An HTTP "server" is a program ( generally a web server like Apache Web Server or Internet Information Services IIS, etc. ) that accepts connections in order to serve HTTP requests by sending HTTP response messages.

- URI的作用: HTTP makes use of the Uniform Resource Identifier (URI) to identify a given resource and to establish a connection.

因為有URI幫我們建立連結, 讓我們可以傳遞HTTP Messages, 所以第三件事是說明HTTP Messages的內容。

第三件事是說明HTTP Messages?

1.1. HTTP requests and HTTP responses use a generic message format(格式)?

回答:
- This generic message format consists of the following 4 items,如下所述:

1. A Start-line: 可分為request的"A Request-line" & response的"A Status-line"。
舉例一下好了:
- GET /hello.htm HTTP/1.1 (This is Request-Line sent by the client)
- HTTP/1.1 200 OK (This is Status-Line sent by the server)

----(Start-line和header間會有一個空行(CRLF))----

2. Zero or more header (General|Request|Entity) fields followed by CRLF

3. An empty line (i.e., a line with nothing preceding the CRLF)
indicating the end of the header fields

p.s. 因爲2和3之間有一個空行, 而3又是一個空行 --> 由此可知, header和body之間總共需要2個空行

4. Optionally a message-body

- 可搭配看下圖:

這是request messages & response messages的架構:

這是request messages & response messages的格式:

加碼看一下request messages介紹: (可以知道某些optional request header ＆有identifies additional data的request body皆是屬於提供additional data)

加碼看一下response messages介紹:(可以知道response header ＆有identifies additional data的response body皆是屬於提供additional data)

1.2. 介紹Header Fields?

- HTTP header fields provide required information about the request or response, or about the object sent in the message body.

- There are 4 types of HTTP message headers:

General-header: These header fields have general applicability for both request and response messages.

Request-header: These header fields have applicability only for request messages.

--> The request-header fields allow the client to pass additional information about the request, and about the client itself, to the server. These fields act as request modifiers.Here is a list of some important Request-header fields that can be used based on the requirement:

a. Accept-Charset

b. Accept-Encoding

c. Accept-Language

d. Authorization

e. Expect

f. From

g. Host

h. If-Match

i. If-Modified-Since

j. If-None-Match

k. If-Range

l. If-Unmodified-Since

m. Max-Forwards

n. Proxy-Authorization

o. Range

p. Referer

q. TE

r. User-Agent

--> You can introduce your custom fields in case you are going to write your own custom Client and Web Server.

Response-header: These header fields have applicability only for response messages.

--> The response-header fields allow the server to pass additional information about the response which cannot be placed in the Status- Line. These header fields give information about the server and about further access to the resource identified by the Request-URI:

a. Accept-Ranges

b. Age

c. ETag

d. Location

e. Proxy-Authenticate

f. Retry-After

g. Server

h. Vary

i. WWW-Authenticate

--> You can introduce your custom fields in case you are going to write your own custom Web Client and Server.

Entity-header: These header fields define meta information about the entity-body or, if no body is present, about the resource identified by the request.

p.s. 上述我想加以說明, 因為會有點易混淆:

a. General-header大多帶的是整個HTTP transaction相關的欄位名稱(field names),所以包含了request和response。下圖從藍色表格可看到這類header會有的欄位名稱:

b. Request-header大多帶的是client本身或request本身的相關的欄位名稱,下圖從藍色表格可看到這類header會有的欄位名稱:

c. Entity-header大多帶的是和"content(內容)"相關的事, 如content-type或content-length, 所以大多會在response中出現,下圖從藍色表格可看到這類header會有的欄位名稱:

1.3. 介紹Message Body?

- The message body part is optional for an HTTP message but if it is available, then it is used to carry the entity-body associated with the request or response. If entity body is associated, then usually Content-Type and Content-Length headers lines specify the nature of the body associated.

- A message body is the one which carries the actual HTTP request data (including form data and uploaded, etc.) and HTTP response data from the server ( including files, images, etc.). Shown below is the simple content of a message body:

<html>

<body>

<h1>Hello, World!</h1>

</body>

</html>

p.s. 下圖這個request(可能是圖片或webfont的request)就未含body部分: