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The following examples show the newer session tracking API in use within Servlets.

The first time a user runs the “Barman” servlet, it sets up a session for him and prompts for his name. On subsequent visits, it addresses him by name straight away, and also keeps a tally of how many times he’s visited.

The “Landlord” servlet does the same as the Barman, but in addition it maintains a static vector of sessions so that it can (and does ; – ) ) report on everyone in the bar and how much they’ve had to drink.

Note – “Barman” keeps the users apart, which is typically what you would do with a shopping cart … “Landlord” provides a management information service too ….

if (count == null) {
count = new Integer(0);
out.print(” < h1 >Welcome. Please enter your name< / h1 >“);
out.print(” < form action=/nan/servlet/Barman > “);
out.print(” < input name=whoyouare > “);
out.print(” < / form > “);
} else {
String wanted = request.getParameter(“whoyouare”);
if (wanted != null) {
session.setAttribute(“who”,wanted);
} else {
wanted = (String)session.getAttribute(“who”);
}
count = new Integer(count.intValue() + 1);
out.print(” < h1 > Welcome back “+wanted+”< / h1 > “);
out.println(“This is your visit no. “+count+”< BR >“);
}
session.setAttribute(“mycounter”,count);
out.println(” < / body > “);
out.println(” < / html > “);
}
}

——————————————————————————————-

for (int i=0; i HttpSession present = (HttpSession)(People.elementAt(i));
String ere = (String)present.getAttribute("who");
out.print(ere+ " is here and has drunk ");
int ii = ((Integer)present.getAttribute("mycounter")).intValue();
out.print("" + ii + " previously< BR >“);
}
}
session.setAttribute(“mycounter”,count);
out.println(” < /body > “);
out.println(” < / html > “);
}
}

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/*
* Copyright 2004 The Apache Software Foundation
*
* Licensed under the Apache License, Version 2.0 (the “License”);
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an “AS IS” BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
/* $Id: SessionExample.java,v 1.4 2004/03/18 16:40:33 jfarcand Exp $
*
*/

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

/**
* Example servlet showing request headers
*
* @author James Duncan Davidson
*/

public class SessionExample extends HttpServlet {

ResourceBundle rb = ResourceBundle.getBundle(“LocalStrings”);

public void doGet(HttpServletRequest request, HttpServletResponse response)
throws IOException, ServletException {
response.setContentType(“text/html”);

PrintWriter out = response.getWriter();
out.println(” < html > “);
out.println(” < body bgcolor=\"white\" > “);
out.println(“< head >“);

String title = rb.getString(“sessions.title”);
out.println(” < title > ” + title + “< /title >“);
out.println(” < / head > “);
out.println(” < / body > “);

// img stuff not req’d for source code html showing
// relative links everywhere!

// XXX
// making these absolute till we work out the
// addition of a PathInfo issue

out.println(“< a href=\"../sessions.html\" >“);
out.println(” < img src=" ../images/code.gif\" height=24 "
+ "width=24 align=right border=0 alt=\"view code\ " > < / a >“);
out.println(“< a href=\"../index.html\" >“);
out.println(“< img src=\"../images/return.gif\" height=24 "
+ "width=24 align=right border=0 alt=\"return\" >< / a >“);

out.println(“< h3 >” + title + “< / h3 > “);

HttpSession session = request.getSession(true);
out.println(rb.getString(“sessions.id”) + ” ” + session.getId());
out.println(“
“);
out.println(rb.getString(“sessions.created”) + ” “);
out.println(new Date(session.getCreationTime()) + “
“);
out.println(rb.getString(“sessions.lastaccessed”) + ” “);
out.println(new Date(session.getLastAccessedTime()));

String dataName = request.getParameter(“dataname”);
String dataValue = request.getParameter(“datavalue”);
if (dataName != null && dataValue != null) {
session.setAttribute(dataName, dataValue);
}

out.println(“

“);
out.println(rb.getString(“sessions.data”) + “
“);
Enumeration names = session.getAttributeNames();
while (names.hasMoreElements()) {
String name = (String) names.nextElement();
String value = session.getAttribute(name).toString();
out.println(HTMLFilter.filter(name) + ” = ”
+ HTMLFilter.filter(value) + “
“);
}

out.println(“

“);

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The Perfect Example
The most ubiquitous stateless environment is the World Wide Web. The HTTP protocol, which is the communications vehicle for Web transactions, is stateless. After a Web page is delivered to the user, the connection is closed. Counter measures, such as the use of cookies, have been developed to maintain the state of a user moving from page to page on a Web site. Contrast with “stateful,” which means that continuity is maintained from session to session. See HTTP and cookie.

Related Post

• March 20, 2009 -- Life Cycle of stateless session bean (0)

A) Servlets maintain session in ServleContext and EJB’s in EJBContext. Stateful session beans remembers the previous request and responses. But stateless beans do not. stateful does not have pooling concept, whereas the stateless bean instances are pooled.

Session Bean Developer programs three classes:

–> Home interface, contains methods for creating (and locating for entity beans) bean instances.

–>Remote interface, contains business methods the bean offers. –Bean class, contains the business logic of the enterprise bean.

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EJB containers pools stateless session beans and reuses them to service many clients. Stateful session beans can be passivated and reused for other clients. But this involves I/O bottlenecks. Because a stateful session bean caches client conversation in memory, a bean failure may result in loosing the entire client conversation. Therefore, while writing a stateful session bean the bean developer has to keep the bean failure and client conversation loss in mind.

In case of stateless session beans, client specific data has to be pushed to the bean for each method invocation which will result in increase in the network traffic. This can be avoided in a number of ways like persisting the client specific data in database or in JNDI. But this also results in I/O performance bottlenecks.

If the business process spans multiple invocations thereby requiring a conversation then stateful session bean will be the ideal choice. On the other hand, if business process lasts only for a single method call, stateless session bean model suits.

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