Thank you for your suggestions!
I also have solved it with 2 possibilites:
1) you can make the field transient:
transient FTPClient() client;
2) Make a Singleton of parent class where FTPClient will be used
public class AnyClass implements Serializable {
private static AnyClass instance;
private AnyClass() {
}
public static AnyClass getInstance() {
if (instance == null) {
instance = new AnyClass();
}
return instance;
}
FTPClient client = new FTPClient();
...
}
3) As suggested by biziclop: Create an own Class only for FTP Communication.