Thansk Piuysh, It looks that I have to read more carefully the javadoc.
Now using the other #addListener() method I managed to retrieve the response.
I suppose that this sentence:
public void addListener(asyncListener)
...
If startAsync(req, res) or startAsync() is called on the
request, the exact same request and response objects are available from the
AsyncEvent when the AsyncListener is notified.
means that AsyncEvent has access to the request and response objects that were used when async process was started but we do not have access to them(especially using getSuppliedRequest/Response() methods).