public void execPROT(String prot) throws SSLException, IOException {
if (prot == null) prot = DEFAULT_PROT;
if (!checkPROTValue(prot)) throw new IllegalArgumentException();
if (FTPReply.COMMAND_OK != sendCommand(
FTPSCommand._commands[FTPSCommand.PROT], prot))
throw new SSLException(getReplyString());
if (DEFAULT_PROT.equals(prot)) {
setSocketFactory(null);
setServerSocketFactory(null);
} else {
setSocketFactory(new FTPSSocketFactory(context));