You seem to have some misconceptions.
Once a JSP is sent to the browser, it is no longer a JSP -- it's a simple HTML page like any other. So there is no way for an Ajax request (or anything else) to return a
Java construct that the JSP page can operate upon.
An Ajax request returns a response like any other request. It can be an XML document, an HTML fragment, a JSON construct, or any other text value. But it is a text value.
If you need to return a list of values, you could do so in XML or JSON, and have JavaScript deal with the list, but there's no way for JSP on the page to get "re-activated".