Following the update, I would suggest passing the JSON as a String into your Java Action. You would then need to use a StringReader and the readObject method to turn it into a JSONObject as this isn’t a Mendix class.
https://docs.oracle.com/javaee/7/api/javax/json/JsonReader.html
Hope this helps.
Either use an export mapping to create a JSON string that you can pass to the Java action and then create a JSON object again from that string or just pass a root object to the Java and then in Java retrieve all the attached objects over the references to that root object.
Regards,
Ronald