How to check username availability using Ajax in Struts 2

I am trying to check username available or not when a user tries to create his account. when a user types it's username there should be an instant check that username is available or not and it will show a message in just below the username box.

I was tried to achieve it by calling an Ajax but not able to understand what to return basically and how will it work, actually I am very new in Struts 2 , I am able to check the username but did not under what to return.

My Ajax Call

<script>
            $(document).ready(function () {
                $("input").blur(function () {
                    var input = $(this).val();
                    alert(input);
                    $.ajax({
                        url:'checkUsername',
                        method:"POST",
                        data:{username:input},
                        success:function(data)
                        {
                            if(data!='0'){
                                $('#availability').html('<span>not available</span>')
                                $('#update').attr("disabled",true);
                            }
                            else{
                                $('#availability').html('<span>available</span>')
                                $('#update').attr("disabled",false);
                            }
                        }
                    })
                });
            });
        </script>

checkUsername Action

public String checkUsername() {
        try {
            setCtr(admin.checkUsername(username));
            if (ctr > 0) {
                System.out.println(ctr);
                setNoData(false);
            } else {
                setNoData(true);
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
        return "CHECKUSER";
    }

method to check username in dao

public int checkUsername(String username) throws Exception {
        ResultSet rs = null;
        Connection con = null;
        try {
            con = ConnectionManager.getConnection();
            System.out.println(username);
            String sql = "SELECT * FROM userinfo WHERE username =?";
            PreparedStatement ps = con.prepareStatement(sql);
            ps.setString(1, username);
            rs = ps.executeQuery();
            if (rs.next()) {
                return 1;
            }

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

        } finally {
            if (con != null) {
                con.close();
            }
        }
        return 0;
    }
}

This method in dao is able to check the username but what to return in that ajax data which i am trying to check that the if rows > 0 it should print not available. How to return and how to check?
struts.xml

 <action name="checkUsername" class="com.redress.actions.AdminAction" method = "checkUsername"> </action>

Can anyone please correct me, how to achieve this?

Simply return a json result. You should define it in the action config. I've already explained how you can return a json result without struts2-json-plugin. Now you will use it to return an JSON object to the success callback function via jQuery ajax.

First you need to add it to the project build path. Then make package of your action config to extend json-default package. It will add JSON result type to the Struts config.

Make the property noData (whatever) you want to access having getter and setter.

Add result of type JSON to the action config.

 <action name="checkUsername" class="com.redress.actions.AdminAction" method = "checkUsername">
    <result type="json">
        <param name="root">action</param>
    </result>
</action>

The action method should return ActionSupport.SUCCESS result code.

In the success callback function you should get JSON object which you can check

success:function(data){
  if(!data.noData){                        
    $('#availability').html('<span>not available</span>');        
    $('#update').attr("disabled",true);
   }
   else{   
     $('#availability').html('<span>available</span>');
     $('#update').attr("disabled",false);
  }
}