Storing Json object "_class" property missing

I am using java drive 4.4 with ReactiveTemplate.

I have a json which i am trying to store in Mongo Collection. Json looks something like this. When an upsert is called over this json object _class attribute is missing. and this package is a third party package (that is assigned to _class attribute)

Can some one suggest how to retain the _class attribute, I want to store the hierarchy as is. I am stuck on this. In my Java Domain Object I refer to childObject as Map<String, Object>. I am able to retain all values excep _class

“_id”: “123232”,
childObject : {
“_class” : “com.testt.example.Survivor”,
“action” : 2,