Resume a Java FTP file download

Question

I'm developing, in Java, an application that has to download from a server to client some very large files. So far I'm using the apache commons-net:

FileOutputStream out = new FileOutputStream(file);
client.retrieveFile(filename, out);

The connection commonly fails before the client finishes downloading the file. I need a way to resume the download of the file from the point where the connection failed, without downloading the whole file again, is it possible?

Answer 1

Things to know:

FileOutputStream has an append parameter, from doc;

@param append if true, then bytes will be written to the end of the file rather than the beginning

FileClient has setRestartOffset which takes offset as parameter, from doc;

@param offset The offset into the remote file at which to start the next file transfer. This must be a value greater than or equal to zero.

We need to combine these two;

boolean downloadFile(String remoteFilePath, String localFilePath) {
  try {
    File localFile = new File(localFilePath);
    if (localFile.exists()) {
      // If file exist set append=true, set ofset localFile size and resume
      OutputStream fos = new FileOutputStream(localFile, true);
      ftp.setRestartOffset(localFile.length());
      ftp.retrieveFile(remoteFilePath, fos);
    } else {
      // Create file with directories if necessary(safer) and start download
      localFile.getParentFile().mkdirs();
      localFile.createNewFile();
      val fos = new FileOutputStream(localFile);
      ftp.retrieveFile(remoteFilePath, fos);
    }
  } catch (Exception ex) {
    System.out.println("Could not download file " + ex.getMessage());
    return false;
  }
}

Answer 2

Commons-net FTPClient supports restarting transfers from a specific offset. You'll have to keep track of what you've successfully retrieved, send the correct offset, and manage appending to the existing file. Assuming, of course, that the FTP server you're connecting to supports the REST (restart) command.