Can there be more than one instance of a servlet at one time ?

Question

public class ServletClassName extends HttpServlet {
//declare instance variables for the servlet
int  instanceVar ; // not thread−safe
// overriding the Generic Servlet.init ( )
@Override
public void init ( ) throws ServletException {
// initialize instance variables, etc.
instanceVar = 0 ;
}
. . .
}

Per each page request, in the service() method (which eventually will be either doGet(), doPost(), or whatever),
you can manipulate instance variables.

Write a Servlet that counts the total number of visits (how many times it's being visited from all clients). The Servlet should display the following information

• Total Page Visits:
• Client Remote Address:
• Client Host Address: <client_fully_qualied_url__or__client_IP_address>

The last two can be obtained from the HttpServletRequest, Refer to the Java EE API

then how can re-do these using the Java API for HttpSession.

Answer 1

It is important to note that there can be more than one instance of a given Servlet class in the servlet container. For example, this can occur where there was more than one servlet definition that utilized a specific servlet class with different initialization parameters. This can also occur when a servlet implements the SingleThreadModel interface and the container creates a pool of servlet instances to use.