Your client simply have no know host key from the server side. There are two things that can be done: Either you tell JSch not to check host key at all or supply a valid key.
Not checking HostKeys
In this case you have to supply a configuration with option StrictHostKeyChecking set to no:
...
JSch jsch = new JSch();
Session session = jsch.getSession(user, host, port);
Hashtable<String, String> config = new Hashtable<String, String>();
config.put("StrictHostKeyChecking", "no");
session.setConfig(config);
session.connect();
...
Supply HostKeys
In this case you supply a file with your host keys. The file is a standard OpenSSH known hosts key file and can be created and updated with the standard ssh command on Linux or cygwin.
...
JSch jsch = new JSch();
jsch.setKnownHosts(knownHostsFilename);
Session session = jsch.getSession(user, host, port);
Hashtable<String, String> config = new Hashtable<String, String>();
config.put("StrictHostKeyChecking", "yes");
session.setConfig(config);
session.connect();
...
My personal advice is to use the second way, as known host keys are an additional security feature of ssh you shouldn't abandon if not necessary.
PS: You shouldn't use only docs that are over 6 years old.
