Question

in this

code in z.JSP I want to create the object y only if it does not exists. I made a research but couldn’t find it how.

Answer 1

Do you mean:

if(request.getSession().getAttribute("yy") == null) {
  usecase y = new y();
  request.getSession().setAttribute("yy", y);
}

which can be further simplified to:

if(session.getAttribute("yy") == null) {
  usecase y = new y();
  session.setAttribute("yy", y);
}

BTW read about Java naming conventions.