Question

In my application I upload a byte[] (serialized Object) to my FTP server, which is working perfectly. However when I try to download it only the first part (like 3000 bytes) of the array are correct, the rest is filled with zeros.

I can’t seem to figure out what is wrong, any help would be appreciated.
I am using the package
org.apache.commons.net.*

public static byte[] downloadBoard( String host, int port, String usr, String pwd) throws IOException {
  FTPClient ftpClient = new FTPClient();
  byte[] buf = new byte[20000];

  try {
    ftpClient.connect( host, port );
    ftpClient.login( usr, pwd );

    ftpClient.setFileType(FTP.BINARY_FILE_TYPE);
    InputStream is = ftpClient.retrieveFileStream("asdf.board");
    is.read(buf);
    is.close();

    ftpClient.completePendingCommand();
    ftpClient.logout();
  } finally {
    ftpClient.disconnect();
  }
  return buf;
}

Answer 1

is.read() may not return the full content. You’ll need to put read() into a loop similar to this:

int pos = 0;
while (true) {
  int count = is.read(buf, pos, buf.length - pos);
  if (count <= 0) {
    break;
  }
  pos += count;
}

P.S.:

If you know the size of the file, you can use a DataInputStream to read the buffer without a loop:

byte[] buf = new byte[exactFileSize];
DataInputStream dis = new DataInputStream(is);
dis.readFully(buf);