Find the size of the file inside a GZIP file

Question

Is there a way to find out the size of the original file which is inside a GZIP file in java?

As in, I have a file a.txt of 15 MB which has been GZipped to a.gz of s

Answer 1

Below is one approach for this problem - certainly not the best approach, however since Java doesn't provide an API method for this (unlike that when dealing with Zip files), it's the only way I could think of, apart from one of the comments above, which talked about reading in the last 4 bytes (assuming the file is under 2Gb in size).

GZIPInputStream zis = new GZIPInputStream(new FileInputStream(new File("myFile.gz")));
long size = 0;

while (zis.available() > 0)
{
  byte[] buf = new byte[1024];
  int read = zis.read(buf);
  if (read > 0) size += read;
}

System.out.println("File Size: " + size + "bytes");
zis.close();

As you can see, the gzip file is read in, and the number of bytes read in is totalled indicating the uncompressed file size.

While this method does work, I really cannot recommend using it for very large files, as this may take several seconds. (unless time is not really too much of a constraint)