muti-threaded test throw exception:Unexpected null reply

Question

java.io.IOException: Unexpected null reply received
at com.enterprisedt.net.ftp.FTPControlSocket.readReply(FTPControlSocket.java:585)
at com.enterprisedt.net.ftp.FTPControlSocket.validateConnection(FTPControlSocket.java:213)
at com.enterprisedt.net.ftp.FTPControlSocket.<init>(FTPControlSocket.java:203)
at com.enterprisedt.net.ftp.FTPControlSocket.<init>(FTPControlSocket.java:179)
at com.enterprisedt.net.ftp.FTPClient.<init>(FTPClient.java:453)
at com.enterprisedt.net.ftp.FTPClient.<init>(FTPClient.java:390)
at com.enterprisedt.net.ftp.FTPClient.<init>(FTPClient.java:374)

Answer 1

I use 5 thread to test edtftpj.Each loop login,dir and quit.About running 5 minutes,it throw the exception occasionally.
My ftp server is serv-u 4.0

Answer 2

Are you continually making requests? It could be that the server is running out of connections. Have a look at the server logs.

I use 5 thread to test edtftpj.Each loop login,dir and quit.About running 5 minutes,it throw the exception occasionally.
My ftp server is serv-u 4.0