@MessageMapping("/hello")
        @SendTo("/topic/whoIsOnline")
        public List<User> greeting(String name) throws Exception {
            List<User> list = new ArrayList<User>();
            User user = new User();
            user.setEmail("test@gmail.com");
            list.add(user);
            User user1 = new User();
            user1.setEmail("example@gmail.com");
            list.add(user1);
            System.out.println(list);
            return list;
        }

var stompClient = null;
    $(document).ready(function() {

        var socket = new SockJS('onlineUsers');
        stompClient = Stomp.over(socket);
        stompClient.connect({}, function(frame) {

            console.log('Connected: ' + frame);
            stompClient.subscribe('/topic/whoIsOnline', function(result) {
                var results = result.body;
                for(var i in results){
                      console.log(results[i].email);
                    }

            });
        });
    });

    function sendName() {
        var txtUser = $('#txtUser').val();
        stompClient.send("/app/hello", {}, JSON.stringify(txtUser));
    }

[{"email":"test@gmail.com"},{"email":"example@gmail.com"}]

test@gmail.com
example@gmail.com

for(var i in results)
   {
      console.log(results[i].email);
   }