Calculating Compressed Filesize from GZIPOutputStream

Question

I am compressing File and writing it into outputstream, How to get the compressed file size to put in response header

    os = response.getOutputStream();
    gzos = new GZIPOutputStream(os);
    fin = new FileInputStream(file);
    in = new BufferedInputStream(fin);

    byte[] buffer = new byte[1024];
    int i;
    while ((i = in.read(buffer)) >= 0) {
           gzos.write(buffer, 0, i);
     }
    gzos.flush();

Answer 1

you don't. Reason:

response will be committed once response buffer is full, that is to say, if you are gzipping a 20MB large file, the header was already sent to client before gzip completes, and you can not modify the committed header when gzip finishes.