session.setAttribute only updates after page reload - ajax

Question

I have an ajax request which invokes GetTierNamesServlet:
$('#application').change(function() {
$.ajax({
url : 'GetTierNamesServlet',
data : {
name : $('#application').find(":selected").text()
},
type : 'get',
cache : false,
success : function(data) {
},
error : function() {
alert('error');
}
}).done(function() {
var test = '<c:out value="${tiers}" />';
alert(test)
})
});
GetTierNamesServlet saves 'tiers' to a session attribute as follows and forwards back to the same page (index.html).
HttpSession session = request.getSession(false);
session.setAttribute("tiers", tiers);
getServletContext().getRequestDispatcher("/index.jsp").forward(request, response);
When alert(test) is called, it alerts the selected tiers from the previous time the ajax request was processed.
The session attribute 'tiers' always seems to "lag" one refresh behind.
What am I doing incorrectly here? I would expect that by placing the alert within the .done portion of the ajax request it would wait the asynchronous call to return before doing something.

Answer 1

This fragment of JavaScript:
var test = '<c:out value="${tiers}" />';
is rendered with the value of ${tiers} before your servlet is called. If you inspect the HTML on the page you will likely find something like:
...
}).done(function() {
var test = 'null'; // or some other "old" value
alert(test)
})
the JSP content is translated to HTML and sent to the browser (page 1)
some event on page 1 causes the JavaScript to be executed
the AJAX call results in the page being rendered again and returned to the browser (page 2)
the JavaScript in page 1 finishes executing via the .done(...) function.
You AJAX call is returning a page when it should probably return a JSON fragment containing your tiers content which will then be consumed by the .done function.