Question

I can’t find a simple way to add a custom field during serialization in Gson and I was hoping someone else may be able to help.

Here is a sample class to show my issue:

public class A {
  String id;
  String name;
  ...
}

When I serialize class A I would like to return something like:

{ "id":"123", "name":"John Doe", "url_to_user":"http://www.example.com/123" }

where url_to_user is not stored in my instance of class A, but can be generated with data in the instance of class A.

Is there a simple way of doing this? I would prefer to avoid writing an entire serializer just to add one field.

Answer 1

Use Gson.toJsonTree to get a JsonElement, with which you can interact dynamically.

A a = getYourAInstanceHere();
Gson gson = new Gson();
JsonElement jsonElement = gson.toJsonTree(a);
jsonElement.getAsJsonObject().addProperty("url_to_user", url);
return gson.toJson(jsonElement);