Adding fields to Jackson's JsonNode

Question

I have a Java object represented as JSON and I want to add additional fields to the JSON. I don't want to add these fields to the Java model, I just need to add them to the JSON before it is processed.

Here is a simplified version of what I have at the moment:

public void addAdditionalFields(JsonNode myObjectAsJsonNode) {

    JsonFactory jsonFactory = new JsonFactory();
    JsonGenerator jgen = jsonFactory.createJsonGenerator(new StringWriter());
    jgen.writeStartObject();
    jgen.writeObjectField("test", "myFieldsValue");
    jgen.writeEndObject();

    jacksonObjectMapper.writeTree(jgen, myObjectAsJsonNode);
}

The new field isn't added to the myObjectAsJsonNode object.