Detect Session In JSP page of java web application

At the beginning of each page you can do this:

String username = (String)session.getAttribute("USER");
if(username==null) { // session expired
// FORWARD WITH ANY WAY YOU WANT

// I usually use the Request Dispatcher class.
RequestDispatcher rd = context.getRequestDispatcher("/demo/inner.jsp");
rd.forward(request,response);
}