I have a web app, when I sent parameters from the view to the Servlet it is added to a list. But as the list belongs to the Visitor class I need that the object visitor will not be created every time I that I call the Servlet.
So instead of creating a new object every time you need to check whether there is one in the session already. If there is, use that. If there isn't, create a new one and put it in the session.
By the way, the "processRequest" method name leads me to suspect that you're treating GET and POST identically, by having both doGet and doPost call processRequest. That is something you should never do, as GET and POST do different things.
From the code excerpts you posted it's not possible to tell whether using isNew is the right thing to do. The important point is whether the object already exists in the session.