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Re: Range requests (information only)

From: Matthew Kerwin <matthew@kerwin.net.au>
Date: Tue, 22 Dec 2015 20:42:29 +1000
Message-ID: <CACweHND_O9RKkBH9Lzk_SA8XuagUExocPLe8jLX7NKNNh9OAtg@mail.gmail.com>
To: Dave Wain <dave.wain@ntlworld.com>, "ietf-http-wg@w3.org" <ietf-http-wg@w3.org>
Ugh, that's why we can't have nice things... This is using
Transfer-Encoding, but is calling it Content-Encoding. It will probably
work in the wild because you'll either get a 200 (complete response) with
Content-Encoding:gzip, *or* a 206 (partial response) without. If you
happened on both at once, and the range didn't start at zero, your
`in.read(...)` would fail with a ZipException because of the missing gzip
header.

I'm assuming differentiation of 200/206 and Content-Range are elided for
brevity; but that 'clear and close' line makes me nervous.



On 22 December 2015 at 20:10, Dave Wain <dave.wain@ntlworld.com> wrote:

> A typical Java get with a range request looks like this:
>
>         /**
>          * Retrieves the part of the file required via HTTP.
>          */
>         private byte[] getHttpBytes(HeaderAndData hnd, long index, int
> nbytes)
>         {
>                 // Allocate a new buffer
>                 final byte[] bytes = new byte[nbytes];
>
>                 try {
>                         // Create a header to read the correct data
>                         final URLConnection uc = hnd.data.openConnection();
>                         final long last = index + nbytes - 1;
>
> uc.setRequestProperty("Range","bytes="+index+"-"+last);
>                         uc.setRequestProperty("Accept-Encoding","gzip,
> deflate");
>                         uc.setUseCaches(false); //BUG applets
>
>                         // Do the request
>                         final InputStream raw = uc.getInputStream();
>                         final String encoding =
> uc.getHeaderField("Content-Encoding");
>                         final InputStream in;
>                         if ("gzip".equals(encoding)) {
>                                 in = new GZIPInputStream(raw);
>                         } else if ("deflate".equals(encoding)) {
>                                 in = new InflaterInputStream(raw);
>                         } else {
>                                 in = raw;
>                         }
>
>                         // Read the required data
>                         int nread = 0;
>                         for (int result; nread<nbytes; nread+=result) {
>                                 result = in.read(bytes, nread,
> nbytes-nread);
>                                 if (result < 0) break;
>                         }
>
>                         // Check for incomplete reads
>                         if (nread < nbytes) {
>
> DEMViewer.log("Incomplete,nbytes="+nbytes+",nread="+nread);
>                         }
>
>                         // Clear and close (implies Stay-Alive)
>                         while (raw.available() > 0) raw.read();
>                         raw.close();
>                 }
>                 catch(IOException ex) {
>                         DEMViewer.log(ex+",index="+index+",nbytes"+nbytes);
>                 }
>
>                 return bytes;
>         }
>
> Hope this helps :)
>
> DW
>
> Note: This message is private and confidential and hence
> must be received without interception or distortion by the
> intended recipients only. Permission to use the information
> explicitly must come from the sender (and recipients).
>
> My personal web site and alternate contact details are at:
> http://homepage.ntlworld.com/dave.wain/.
>
>
>
>
>


-- 
  Matthew Kerwin
  http://matthew.kerwin.net.au/
Received on Tuesday, 22 December 2015 10:42:58 UTC

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