Enable logging in Apache Commons Net for FTP protocol

Question

Apache Commons Net library does not seem to send anything to any "logger".

Can I somehow obtain a log file from an (FTP) session, for debugging purposes? For example raw FTP commands and responses from the server, like this:

220 Welcome
USER *******
331 Password required for ...
PASS *******
230 Logged on
TYPE I
200 Type set to I
QUIT
221 Goodbye

Answer 1

All protocol implementations in Apache Commons Net, including FTPClient, derive from SocketClient, which has a method addProtocolCommandListener. You can pass it an implementation of ProtocolCommandListener to implement logging.

There's a ready-made implementation PrintCommandListener, which prints the protocol log to provided PrintStream.

With a code like this:

ftpClient.addProtocolCommandListener(
    new PrintCommandListener(
        new PrintWriter(new OutputStreamWriter(System.out, "UTF-8")), true));

..., you will get exactly the output that you have asked for.