OkHttp是什么，为什么要用OkHttp

OkHttp是一个Http-client，它的主要优势：
·HTTP/2 支持允许所有访问同一主机的请求共享一个socket
·利用连接池减少请求延迟（如果HTTP/2不可用）
·支持GZIP压缩
·响应缓存减少重复请求

最低版本要求：
Android 2.3
Java1.7

官方说明

Overview
HTTP is the way modern applications network. It’s how we exchange data & media.
Doing HTTP efficiently makes your stuff load faster and saves bandwidth.

OkHttp is an HTTP client that’s efficient by default:

· HTTP/2 support allows all requests to the same host to share a socket.
· Connection pooling reduces request latency (if HTTP/2 isn’t available).
· Transparent GZIP shrinks download sizes.
· Response caching avoids the network completely for repeat requests.
OkHttp perseveres when the network is troublesome: it will silently recover from 
common connection problems. If your service has multiple IP addresses OkHttp will 
attempt alternate addresses if the first connect fails. This is necessary for IPv4+IPv6 
and for services hosted in redundant data centers. OkHttp initiates new connections 
with modern TLS features (SNI, ALPN), and falls back to TLS 1.0 if the handshake fails.

Using OkHttp is easy. Its request/response API is designed with fluent builders and
immutability. It supports both synchronous blocking calls and async calls with callbacks.

OkHttp supports Android 2.3 and above. For Java, the minimum requirement is 1.7.