How to ignore the root JSON node in struts2 action

I added this method in my struts action,

public String execute() {
    long start = System.currentTimeMillis();
    simDetails = new SIMDetails();
    return GET_SIM_DETAILS;
}

and added below action in struts config file,

<result type="json" name="getSIMDetails">
    <param name="noCache">true</param>
    <param name="includeProperties">simDetails.*</param>
</result>

Then i got below JSON response

{
    "simDetails": {
        "void": null,
        "ban": null,
        "currentTariff": null,
        "currentTariffDescription": null,
        "defaultTariff": null,
        "defaultTariffDescription": null,
        "imsi": null,
        "packageItemId": null,
        "simSerialNumber": null,
        "simStatus": null,
        "simStatusCC": null,
        "status": null,
        "subscriberNumber": null,
        "subsidaryCode": null
    }
}

but I need this response instead of the above,

{
    "void": null,
    "ban": null,
    "currentTariff": null,
    "currentTariffDescription": null,
    "defaultTariff": null,
    "defaultTariffDescription": null,
    "imsi": null,
    "packageItemId": null,
    "simSerialNumber": null,
    "simStatus": null,
    "simStatusCC": null,
    "status": null,
    "subscriberNumber": null,
    "subsidaryCode": null
}

Any idea to get the required response with out add the above field to my action class.

You can use the root attribute as specified in the Root Object section of the documentation:

Use the "root" attribute(OGNL expression) to specify the root object to be serialized.

In your case:

<result type="json" name="getSIMDetails">
    <param name="noCache">true</param>
    <param name="root">simDetails</param>
</result>

P.S: this answer might be worthy of a read. And in the other answer to that question you can also see the Stream technique suggested by @IntelliData.