three threads ，the same remote directory，sometimes：
com.enterprisedt.net.ftp.FTPException: 550 HW20090709.LF.gz.binew: No such file or directory.
at com.enterprisedt.net.ftp.FTPControlSocket.validateReply(FTPControlSocket.java:1127)
at com.enterprisedt.net.ftp.FTPClient.dir(FTPClient.java:3379)
at com.enterprisedt.net.ftp.FileTransferClient.directoryNameList(FileTransferClient.java:543)
at com.enterprisedt.net.ftp.FileTransferClient.directoryNameList(FileTransferClient.java:527)
why?
public synchronized String[] directoryNameList()
throws FTPException, IOException {
return directoryNameList("", false);
}