I have a small command-line program that I wrote for re-downloading files from a server in bulk. It works flawlessly in Windows, but doesn't seem to work in macOS.

Here is the error I receive:

Download failed: java.io.IOException: Server returned HTTP response code: 403 for URL:
https://cdn.modrinth.com/data/P7dR8mSH/versions/0.56.3 1.19/fabric-api-0.56.3+1.19.jar
    at sun.net.www.protocol.http.HttpURLConnection.getInputStream0(HttpURLConnection.java:1914)
    at sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1512)
    at sun.net.www.protocol.https.HttpsURLConnectionImpl.getInputStream(HttpsURLConnectionImpl.java:268)
    at java.net.URL.openStream(URL.java:1092)
    at main.downloadUsingNIO(main.java:37)
    at main.main(main.java:20)

Relevant code:

   private static void downloadUsingNIO(String urlStr, String file) throws IOException {
       URL url = new URL(urlStr);
       ReadableByteChannel rbc = Channels.newChannel(url.openStream());
       FileOutputStream fos = new FileOutputStream(file);
       fos.getChannel().transferFrom(rbc, 0, Long.MAX_VALUE);
       fos.close();
       rbc.close();
   }

Usage:

downloadUsingNIO(URLOFFILE, WHERETOSAVEANDTOWHATFILE);

Other information:

1. Both windows and macos accounts used are the primary administrator accounts.
2. running the jar with elevated permissions through "sudo java -jar " does not fix it.
3. The file is downloadable in safari with the same link.
4. The download works with "curl -o thing.jar "
5. Tried Apache Commons.IO, didn't do anything as it uses the same code underneath (the stacktrace was the same)

Thank you for the answers, this is the code that I used to fix the issue:

private static void downloadUsingNIO(String Strurl, String path) throws IOException {

        final URLConnection connection = new URL(Strurl).openConnection();
        connection.addRequestProperty("User-Agent",
                "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:102.0) Gecko/20100101 Firefox/102.0");

        final int contentLength = connection.getContentLength();
        final File end = new File(path);

        if (end.exists()) {
            final URLConnection savedFileConnection = end.toURI().toURL().openConnection();
            if (savedFileConnection.getContentLength() == contentLength) {
                return;
            }
        } else {
            final File dir = end.getParentFile();
            if (!dir.exists())
                dir.mkdirs();
        }

        final ReadableByteChannel rbc = Channels.newChannel(connection.getInputStream());
        final FileOutputStream fos = new FileOutputStream(end);
        fos.getChannel().transferFrom(rbc, 0, Long.MAX_VALUE);
        fos.close();
        
        return;
    }

CodePudding user response：

With high probably, the HTTP server doesn't like the Java default user agent header, which is being sent - or it's absence; use another client. This can easily be validated by comparing the server logs. https://datatracker.ietf.org/doc/html/rfc7231#section-6.5.3

The point is, that neither NIO Channel nor URL permit setting that header.