Class 1:
public class Class1 implements ConfigurationSerializable {

    Class2 attribute;

    @Override
    public Map<String, Object> serialize() {
        Map<String, Object> result = new LinkedHashMap<>();
        result.put("atr",attribute);
        args.get("atr");//this is not null
        return result;
    }

    public static Class1 deserialize(Map<String, Object> args) {
        Class1 result = new Class1();
         args.get("atr"); //this is null
        result.attribute = (Class2) args.get("atr");
        return result;
    }

}

Class 2:
public class Class2 implements ConfigurationSerializable {

    String one,two,three;

    @Override
    public Map<String, Object> serialize() {
        Map<String, Object> result = new LinkedHashMap<>();
        result.put("one",one);
        result.put("two",two);
        result.put("three",three);
        return result;
    }

    public static Class2 deserialize(Map<String, Object> args) {
        Class2 result = new Class2();
        result.one = (String) args.get("one");
        result.two = (String) args.get("two");
        result.three = (String) args.get("three");
        return result;
    }

}