There exists a large class of IETF protocols that work by sending an ASCII text command and arguments to a server, and then receiving an ASCII text reply. For debugging and other purposes, it is extremely useful to log or keep track of the contents of the protocol messages. The ProtocolCommandListener interface coupled with the ProtocolCommandEvent class facilitate this process.

To receive ProtocolCommandEvents, you merely implement the ProtocolCommandListener interface and register the class as a listener with a ProtocolCommandEvent source such as FTPClient .