How to set JSON Object in request.JSON for Integration testing

I have to perform Integration test on a Controller, where an action signIn() defines a JSON Object as:

<< def jsonObj = request.JSON; >>

How do I pass a JSON Object in request.JSON from my controllerTests file?

I tried the following but it ain't working:

...
JSONObject jsonObj = new JSONObject();
jsonObj.put("user", createUserJsonObj());
controller.request.parameters['user'] = jsonObj
controller.signIn()
...

private JSONObject createUserJsonObj() {
JSONObject json = new JSONObject();
json.put("id", "id01");
json.put("displayName", "Test Display Name..");
return json;
}