Thanks to @Martin Prikryl answer, solved as following:
// Get sftp private/public key for JSch identity
FSDataInputStream fis = fs.open(privateKeyPath);
byte[] privateKeyBytes = IOUtils.toByteArray(fis);
fis = fs.open(publicKeyPath);
byte[] publicKeyBytes = IOUtils.toByteArray(fis);
fis.close();
JSch jsch = new JSch();
String idName = "ksftp";
byte[] passphrase = null;  
jsch.addIdentity(idName, privateKeyBytes, publicKeyBytes, passphrase);