$.getJSON not working on rhomobile

I have following code in javascript callback function of body load:

alert("calling get json");
$.getJSON("http://localhost:3000/books/show.json", {book: 3},
         alert("data received");
After getting alert of 'calling get json' being called,the server return
status 200 ok. still control is not passing to function(data) and alert
of 'data received' is not called.
so,what is wrong?