Now the question is;
Every time I run client, it just stop at "Naming.lookup(url)" and said
"java.rmi.UnexpectedException,nested exception is:null ".

1,server and client are running on the same machine;
2,all the classes are in same path;
3,the same program ran successfully under win98/2000;
4,the rmiregistry and server ran successfully too under RedHat 7.1