HTTP断点续传原理
一般通过Http下载的时候，我们发送一个request，服务器返回一个response，我们可以读取里面InputStream流，然后保存为文件。
比如我们请求一个下载地址：
发送的HTTP Request Headers
:authority: storage.360buyimg.com
:method: GET
:path: /jdmobile/JDMALL-PC2.apk
:scheme: https
accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8
accept-encoding: gzip, deflate, br
accept-language: zh-CN,zh;q=0.9
Cache-Control : no-cache
pragma: no-cache
upgrade-insecure-requests: 1
user-agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/71.0.3578.98 Safari/537.36
返回的Http Response Headers:
accept- RANGE s: bytes access-control-allow-origin: * age: 1045106 cache-control: max-age=315360000 content-length: 87819000 content-type: application/octet-stream date: Sun, 19 May 2019 10:43:56 GMT ETag : "98ccc64969643f58484cff68bad440c2" expires : Thu, 31 Dec 2037 23:55:55 GMT last-modified: Tue, 07 May 2019 08:25:23 GMT server: JDWS/2.0 status: 200 via: http/1.1 BJ-Y-JCS-203 ( [cHs f ]), http/1.1 HB-CM-1-PIX-144 ( [cHs f ]) x-jss-request-id: BCEACE72D5E2E54B
从上面的response headers可以获取到，服务器接收断点续传（accept-ranges: bytes
），并且获取到了文件的大小87819000 bytes（content-length: 87819000）。
如果服务器返回response headers 中包含accept-ranges: bytes，说明服务器支持断点续传的。
所谓断点续传，也就是要从文件已经下载的地方开始继续下载。怎么添加开始的地方？
其实在request headers添加以下header即可：
RANGE: bytes=10000000- //或者 //RANGE: bytes=10000000- 20000000
服务器返回的大小是从开始的位置的大小，77819000 bytes（content-length: 77819000）。 返回的状态码不是200，而是206。
HTTP/1.1 206 Partial Content Server:JDWS/2.0 Access-Control-Allow-Origin:* Content-Range:bytes 17563800-87818999/87819000 x-jss-request-id:BCEACE72D5E2E54B Connection:keep-alive Last-Modified:Tue, 07 May 2019 08:25:23 GMT Date:Sun, 19 May 2019 10:27:08 GMT Via:http/1.1 BJ-Y-JCS-203 ( cHs f ), http/1.1 HB-CM-1-PIX-144 ( cHs f ) Accept-Ranges:bytes Cache-Control:max-age=315360000 ETag:"98ccc64969643f58484cff68bad440c2" Expires:Thu, 31 Dec 2037 23:55:55 GMT Content-Length:70255200 Age:1044098 Content-Type:application/octet-stream
返回的Content-Range就是这次request的开始和结束位置（Content-Range:bytes 17563800-87818999/87819000）。
这样就可以实现断点续传了，后面会为大家实现一个多线程的断点续传。
刚开始写作，欢迎关注，一起进步，谢谢！
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