官方用一句话进行了介绍：

An HTTP & HTTP/2 client for Android and Java applications.

一、概述

HTTP 是互联网上应用最为广泛的一种网络协议，可用于传输数据和媒体
高效使用HTTP不仅让内容加载更快，且节省带宽

对HTTP 不熟可以看之前的一篇文章：HTTP，请求消息，响应消息，Cookie（详细总结）

• OkHttp是从设计之初就是一款高效的 HTTP client

• HTTP/2 允许同一 host 的所有请求共享 socket

• 连接池降低了请求延迟 (if HTTP/2 isn’t available)

• GZIP 缩减了下载的大小

• 响应缓存完全避免网络重复请求

• OkHttp 会确保网络的连接，它会悄悄地恢复一些常见的连接问题

• 如果你的 service 有多个IP地址且第一个连接失败，OkHttp将尝试备用地址。

• 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.

• OkHttp 的易用性

• Its request/response API is designed with fluent builders and immutability.

• 它同时支持同步阻塞调用和回调的异步调用

OkHttp支持Android 2.3及以上。对于Java，最低要求是1.7

二、使用例子

1. GET A URL

下面这个程序下载一个URL，并以字符串打印出其内容：

// 关键代码
OkHttpClient client = new OkHttpClient();

String run(String url) throws IOException { 
    Request request = new Request.Builder() 
          .url(url) 
          .build(); 

    Response response = client.newCall(request).execute(); 
    return response.body().string();
}

// 完整代码
package okhttp3.guide;

import java.io.IOException;
import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.Response;

public class GetExample {
  OkHttpClient client = new OkHttpClient();

  String run(String url) throws IOException {
    Request request = new Request.Builder()
        .url(url)
        .build();

    Response response = client.newCall(request).execute();
    return response.body().string();
  }

  public static void main(String[] args) throws IOException {
    GetExample example = new GetExample();
    String response = example.run("https://raw.github.com/square/okhttp/master/README.md");
    System.out.println(response);
  }
}

2. POST TO A SERVER

提交数据到服务端

// 关键代码
public static final MediaType JSON
    = MediaType.parse("application/json; charset=utf-8");

OkHttpClient client = new OkHttpClient();

String post(String url, String json) throws IOException {
  RequestBody body = RequestBody.create(JSON, json);
  Request request = new Request.Builder()
      .url(url)
      .post(body)
      .build();
  Response response = client.newCall(request).execute();
  return response.body().string();
}

// 完整代码
package okhttp3.guide;

import java.io.IOException;
import okhttp3.MediaType;
import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.RequestBody;
import okhttp3.Response;

public class PostExample {
  public static final MediaType JSON
      = MediaType.parse("application/json; charset=utf-8");

  OkHttpClient client = new OkHttpClient();

  String post(String url, String json) throws IOException {
    RequestBody body = RequestBody.create(JSON, json);
    Request request = new Request.Builder()
        .url(url)
        .post(body)
        .build();
    Response response = client.newCall(request).execute();
    return response.body().string();
  }

  String bowlingJson(String player1, String player2) {
    return "{'winCondition':'HIGH_SCORE',"
        + "'name':'Bowling',"
        + "'round':4,"
        + "'lastSaved':1367702411696,"
        + "'dateStarted':1367702378785,"
        + "'players':["
        + "{'name':'" + player1 + "','history':[10,8,6,7,8],'color':-13388315,'total':39},"
        + "{'name':'" + player2 + "','history':[6,10,5,10,10],'color':-48060,'total':41}"
        + "]}";
  }

  public static void main(String[] args) throws IOException {
    PostExample example = new PostExample();
    String json = example.bowlingJson("Jesse", "Jake");
    String response = example.post("http://www.roundsapp.com/post", json);
    System.out.println(response);
  }
}

三、ja包下载或引用

• 下载okhttp的最新jar包
• okhttp的源码，samples 在 github 上都可以找到
• MAVEN

<dependency>
    <groupId>com.squareup.okhttp3</groupId>
    <artifactId>okhttp</artifactId>
    <version>3.2.0</version>
</dependency>

• GRADLE

    compile 'com.squareup.okhttp3:okhttp:3.2.0'

参考文章：
[1] 官方原文：OkHttp