1. Resources
2. Representations
   1. A "representation" is information that is intended to reflect a past, current, or desired state of a given resource, in a format that can be readily communicated via the protocol. A representation consists of a set of representation metadata and a potentially unbounded stream of representation data
3. Connections, Clients, and Servers
4. Messages
5. User Agents
6. Origin Server
   1. The term "origin server" refers to a program that can originate authoritative responses for a given target resource.
7. Intermediaries
8. Caches

RFC对于http报文格式的定义 HTTP-message = start-line CRLF *( field-line CRLF ) CRLF [ message-body ]

start-line = request-line / status-line field-line = field-name ":" OWS field-value OWS message-body = *OCTET

request-line = method SP request-target SP HTTP-version method = token HTTP-version = HTTP-name "/" DIGIT "." DIGIT request-target = origin-form / absolute-form / authority-form / asterisk-form origin-form = absolute-path [ "?" query ]

status-line = HTTP-version SP status-code SP [ reason-phrase ]
reason-phrase = 1*( HTAB / SP / VCHAR / obs-text )

先理解RFC的定义,有助于理解httpclient的类

HttpEntity是对RFC中的message-body的封装和加强

https://hc.apache.org/httpcomponents-client-4.5.x/examples.html