Passing an array as a parameter in $.getJSON() method

Question

Hi I am trying to pass an array to Struts2 action class using jquery's getJSON() method but everytime its passing null elements.

var userIds=[];    

Then populating UserIds array with push() method:

$.getJSON('abc.action',{userArray:userIds}, function(json) {
                  for(var i=0;i<json.returnMessages.length;i++){ 
                  alert(json.returnMessages[i]);
                  }
              });

Also , in the action class should i use array or arraylist will work as receiver variable. Please suggest ! Help is appreciated.

Server-Side code:

import java.util.ArrayList;
import java.util.List;


import com.opensymphony.xwork2.Action;

public class UserDetails {

    private static final long serialVersionUID = 1L;
    String[] userArray = new String[2];



    private List<String> returnMessages = new ArrayList<String>();



    public String[] getuserArray() {
        return userArray;
    }
    public void setuserArray(String[] userArray) {
        this.userArray = userArray;
    }

    public void setReturnMessages(List<String> returnMessages) {
        this.returnMessages = returnMessages;
    }
    public List<String> getReturnMessages() {
        return returnMessages;
    }

    public String execute() {
        try
        {  



            for (int i = 0; i < userArray.length; i++) {

                System.out.println("Success");
                returnMessages.add("Success");




            }


         }
         catch (Exception e)
         {
            e.printStackTrace();
         }




        return Action.SUCCESS;
    }


}