Checking if a stream is a zip file

Question

We have a requirement to determine whether an incoming InputStream is a reference to an zip file or zip data. We do not have reference to the underlying source of the stream. We aim to copy the contents of this stream into an OutputStream directed at an alternate location.

I tried reading the stream using ZipInputStream and extracting a ZipEntry. The ZipEntry is null if the stream is a regular file - as expected - however, in checking for a ZipEntry I loose the initial couple of bytes from the stream. Hence, by the time I know that the stream is a regular stream, I have already lost initial data from the stream.

Any thoughts around how to check if the InputStream is an archive without data loss would be helpful.

Thanks.

Answer 1

Assuming your original inputstream is not buffered, I would try wrapping the original stream in a BufferedInputStream, before wrapping that in a ZipInputStream to check. You can use "mark" and "reset" in the BufferedInputStream to return to the initial position in the stream, after your check.

Answer 2

This is how I did it.

Using mark/reset to restore the stream if the GZIPInputStream detects incorrect zip format (throws the ZipException).

/**
 * Wraps the input stream with GZIPInputStream if needed. 
 * @param inputStream
 * @return
 * @throws IOException
 */
private InputStream wrapIfZip(InputStream inputStream) throws IOException {
    if (!inputStream.markSupported()) {
        inputStream = new BufferedInputStream(inputStream);
    }
    inputStream.mark(1000);
    try {
        return new GZIPInputStream(inputStream);
    } catch (ZipException e) {
        inputStream.reset();
        return inputStream;
    }
}