Download via servlet

File file = new File("nomedoarquivo");                
                    
if (file.exists())
{
    response.setHeader("Content-type", "application/octet-stream");
    response.setHeader("Content-Disposition","attachment;filename="" + file.getName() + """);

    FileInputStream in = new FileInputStream(file);
    OutputStream out = response.getOutputStream();

    int buflen = 1024;
    byte[] buffer = new byte[buflen];

    int read;
    int length = 0;

    while ((read = in.read(buffer, 0, buflen)) > 0) {
        length += read;
        out.write(buffer, 0, read);
    }

    response.setContentLength(length);

    out.flush();
    out.close();

    in.close();
}