PuTTY-User-Key-File-2: ssh-rsa
Encryption: none
Comment: rsa-key-test
Public-Lines: 12
...
Private-Lines: 28
...

---- BEGIN SSH2 PUBLIC KEY ----
Comment: "rsa-key-test"
...
---- END SSH2 PUBLIC KEY ----

jsch.setKnownHosts(filePath + "known_hosts");

2018-12-07 15:22:14.900 [main] INFO  d.p.o.common.clients.SftpClient - com.jcraft.jsch.JSchException: invalid privatekey: [B@8e0379d

Jsch jsch = new JSch();
Session session = null;

//add known hosts file for authentication
jsch.setKnownHosts(filePath + "known_hosts");

keyPath = filePath + "privatekey.ppk";
jsch.addIdentity(keyPath);
session = jsch.getSession("test", "172.17.30.91", 22);

session.connect();