java - Reading the first part of a file using HTTP

Question

I would like to determine the type of a file (generally UTF-8) by reading the first part of the file and analysing the content. (The type is specific to my community but not under my control and not covered by MIME/MediaType which is normally TEXT_PLAIN). I am using the 'org.restlet' library on the client to analyse the header with

Request request = new Request(Method.HEAD, url);

so I know the content-length and can (if necessary and possible) estimate how many bytes I should download for the analysis

CLARIFICATION: I cannot use the MediaType. From answer 1 seems like I have to GET the content. A revised question would therefore be:

"Can I GET part of a file using Restlet?"

ANSWER: The following code does what I want. I have credited @BalusC for showing the way. Please comment if I have missed anything:

public String readFirstChunk(String urlString, int byteCount) {
    String text = null;
    if (urlString != null) {
        org.restlet.Client restletClient = new org.restlet.Client(Protocol.HTTP);
        Request request = new Request(Method.GET, urlString);
        List<Range> ranges = Collections.singletonList(new Range(0, byteCount));
        request.setRanges(ranges);
        Response response = restletClient.handle(request);
        if (Status.SUCCESS_OK.equals(response.getStatus())) {
            text = processSuccessfulChunkRequest(response);
        } else if (Status.SUCCESS_PARTIAL_CONTENT .equals(response.getStatus())) {
            text = processSuccessfulChunkRequest(response);
        } else {
            System.err.println("FAILED "+response.getStatus());
        }
    }
    return text;
}

private String processSuccessfulChunkRequest(Response response) {
    String text = null;
    try {
        text = response.getEntity().getText();
    } catch (IOException e) {
        throw new RuntimeException("Cannot download chunk", e);
    }
    return text;
}

Answer 1

That's only possible if the server has sent the Accept-Ranges and Content-Range headers along with ETag or Last-Modified. E.g.

Accept-Ranges: bytes
Content-Range: bytes 0-1233/1234
ETag: file.ext_1234_1234567890

The Accept-Ranges: bytes indicates that the server supports requests returning partial content in a specified byte range. The Content-Range header informs about the length. The ETag and Last-Modified indicate the unique file idenfier or the last modified timestamp on the resource behind the request URI.

If those headers are present in the response, then you can request a part of the resource using If-Range and Range request headers with respectively the unique file identifier or the last modified timestamp and the desired byte range.

If-Range: file.ext_1234_1234567890
Range: bytes=0-99

The above example returns the first 100 bytes of the file.