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Servlet Sessions, and variable scope.
Tomcat - Configuring Web Applications example from a Well House Consultants training course
More on Tomcat - Configuring 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 Books covering this topic
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Other Examples
This example comes from our "Tomcat - Configuring Web Applications" training module. You'll find a description of the topic and some
other closely related examples on the "Tomcat - Configuring Web Applications" module index page.
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