Servlet Sessions, and variable scope.

Tomcat - Configuring Web Applications example from a Well House Consultants training course
More on Tomcat - Configuring Web Applications [link]

This example is described in the following article(s):
• Variable scope in Java Servlets and other web applications - [link]

Source code: Autumn.java Module: A653

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;

public class Autumn extends HttpServlet{

/* For the moment, push the GET and POST requests to the
same internal method */

        public static int peeps = 0; // Visitor count
        public static int latest = 0; // Latest value across threads

        public void doGet (HttpServletRequest inp,
                        HttpServletResponse outp)
                        throws ServletException, IOException {
                dojob(inp,outp);
                }
        public void doPost (HttpServletRequest inp,
                        HttpServletResponse outp)
                        throws ServletException, IOException {
                dojob(inp,outp);
                }

/* This application notes the integer that each user enters of each
page and keeps a separate running total for each of the users via a
session. (Trivial application ... replace "Integer" by "Things added
to shopping cart so far" object later on for a practical program! */

        protected void dojob (HttpServletRequest inp,
                        HttpServletResponse outp)
                        throws ServletException, IOException {

/* Get any data off the incoming form */

                int aok = 0; int dim1 = 0;
                try {
                        dim1 = Integer.parseInt(inp.getParameter("quant"));
                        aok = 1;
                } catch (Exception e) {
                        // Set up a sensible error string here
                        }

                outp.setContentType("text/html");
                PrintWriter out = outp.getWriter();

                int report = latest;

/* Here is the session stuff ... all together for this first demo */
        // Get the current session (i.e. what the visitor left last time)
                HttpSession who = inp.getSession(true);
        // Get the total so far that was saved in previous visits
                Integer runtot = (Integer)who.getAttribute("sofar");
                if (runtot == null) {
                        runtot = new Integer(0);
                        peeps++;
                        }
        // Add in any new value
                if (aok == 1)
                        runtot = new Integer(runtot.intValue() + dim1);
                        latest = dim1;
        // Save back into session
                who.setAttribute("sofar",runtot);
/* That's it for the session stuff */

        out.println("<head><title>Summing across requests</title></head>");
        out.println("<body><h1>Session based adder</h1>");

/* Illustration of scope of variables
Iteration level - for this current server access only
        e.g. temportary variables / echoing back immediate user inputs
        achieved by local variables in doGet or doPost
User/Session level - for this user across all their accesses in this session
        e.g. variables that the user is maintaining state with (shopping cart)
        achieved by session object
Application level - for all users across all session
        Overall variables - e.g. top bid, number of people, latest sale price
        achieved by static variables

NOTE ... need to save away / possibly reload variables if servlet may reload

Also note - to share variables between servlets, probably best to pick them
up from a database OR use multiple service levels with a common worker task
on the back server.
*/

                if (aok > 0) {
                        out.print("The total so far is ");
                        out.print(runtot);
                        out.print(" units [USER/SESSION level]<hr>");

                        out.print("The previous value ANYONE said was ");
                        out.print(report);
                        out.print(" units [APPLICATION level]<hr>");

                        out.print("The number of unique people is ");
                        out.print(peeps);
                        out.print(" people [APPLICATION level]<hr>");

                        out.print("The number this time is ");
                        out.print(dims);
                        out.print(" people [ITERATION level]<hr>");

                } else {
                        // Initial error message is poor ...
                        // More explanation / help needed by user!
                        out.print("Bitova Issue");
                }

        /* Make up a repeat form to try with more data */
                out.print("<br><form method=POST>");
                out.print("Rows <input name=quant><br>");
                out.print("<input type=submit></form>");

                out.println("</body>");
                }

                }

Learn about this subject

This module and example are covered on the following public courses:
* Deploying Apache and Tomcat
* Deploying Java Applications on Linux / Unix
Also available on on site courses for larger groups

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