Home > Articles

The Servlet Container

This chapter is from the book

Using Listeners

You also must know how to use the servlet context attribute listener and the session attribute listener. You can write code that responds to servlet events by defining listener objects. These objects have methods that the container invokes when life-cycle events occur. The event is triggered by a change in the application. For example, the session listener is invoked by the container when a session attribute changes. Also, this happens if an attribute in the application object changes and when the context or session is created and destroyed. You take advantage of this event model by defining a listener class that implements a listener interface. The container invokes the listener method and passes it information about that event. Notice that the methods in the HttpSessionListener interface are passed an HttpSessionEvent.

You can expect to see four listener interfaces on the exam. The following two tables describe them. Table 3.1 lists the context (Web application) events and the interface/method that your class must implement to respond to the event.

Table 3.1 Servlet Context Events

Event

Interface

Method

Servlet context has just been created

javax.servlet.ServletContextListener

contextInitialized()

Just before killing servlet context

ServletContextListener

contextDestroyed()

Adding an attribute

javax.servlet.ServletContextAttributesListener

attributeAdded()

Removing an attribute

ServletContextAttributesListener

attributeRemoved()

Replacing an attribute

ServletContextAttributesListener

attributeReplaced()


Table 3.2 lists the session events and the interface/method that your class must implement to respond to the event.

Table 3.2 Servlet Session Events

Event

Interface

Method

Session has just been created

javax.servlet.http sessionCreated()

HttpSessionListener

Just before passivating session

HttpSessionListener

sessionDestroyed()

Adding an attribute

javax.servlet.http.HttpSessionAttributesListener

attributeAdded()

Removing an attribute

HttpSessionAttributesListener

attributeRemoved()

Replacing an attribute

HttpSessionAttributesListener

attributeReplaced()


The following is a short commentary on the methods mentioned previously:

  • When a servlet is initialized or destroyed, you listen with javax.servlet.ServletContextListener:

    • contextDestroyed(ServletContextEvent sce)—Notification that the servlet context is about to be shut down

    • contextInitialized(ServletContextEvent sce)—Notification that the Web application is ready to process requests

  • When a session is initialized or destroyed, you listen with javax.servlet.http.HttpSessionListener:

    • sessionCreated(HttpSessionEvent se)—Notification that a session was created

    • sessionDestroyed(HttpSessionEvent se)—Notification that a session became invalid or timed out

  • When a context attribute is added, removed, or replaced you listen with javax.servlet.ServletContextAttributeListener:

    • attributeAdded(ServletContextAttributeEvent scab)—Notifica-tion that a new attribute was added to the servlet context

    • attributeRemoved(ServletContextAttributeEvent scab)—Notification that an existing attribute was removed from the servlet context

    • attributeReplaced(ServletContextAttributeEvent scab)—Notification that an attribute on the servlet context was replaced

  • When a session attribute is added, removed, or replaced you listen with HttpSessionAttributeListener:

    • attributeAdded(HttpSessionBindingEvent se)—Notification that an attribute was added to a session

    • attributeRemoved(HttpSessionBindingEvent se)—Notification that an attribute was removed from a session

    • attributeReplaced(HttpSessionBindingEvent se)—Notification that an attribute was replaced in a session

Obvious similarities exist between context and session interfaces and methods. Use javax.servlet.ServletContextAttributeListener when a context attribute is added, removed, or replaced. On the other hand, use HttpSessionAttributeListener when the same is done in a session. Both of these are public interfaces (they extend java.util.EventListener) and have the same method names but different parameters.

Suppose that you wanted to mark the times when your Web application started and ended. The following code snippet shows how you could use the initialization and destruction events to do that:

public final class ContextListener
  implements ServletContextListener 
{
  public void contextInitialized(
                ServletContextEvent event) 
  {
   ServletContext context = event.getServletContext();
   String IP = "209.83.3.142";
   context.setAttribute("DefaultAddress", IP);
  }
   
  public void contextDestroyed(ServletContextEvent event) 
  {
   ServletContext context = event.getServletContext();
   String IP = context.getAttribute("DefaultAddress");
   //do something with IP
   //context.removeAttribute("DefaultAddress");
  }
}

The attribute DefaultAddress is set when the container initializes the application. Of course, you could dynamically get the IP. Then when the application quits, the same attribute is retrieved. When you have this IP, you could log it and then delete it, for example. For an excellent article that provides an overview of application life-cycle events, see "Servlet App Event Listeners," by Stephanie Fesler (April 12, 2001, http://www.onjava.com/pub/a/onjava/2001/04/12/listeners.html).

For a better example, Listing 3.2 demonstrates a simplified approach to how you could listen to an application and record what is going on.

Listing 3.2 Listening to Context and Session Events

package com.companyname.listening;

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

/**
 * A custom listener for session events. 
 * All events that occur are monitored.
 */

public class MySessionListener
  implements HttpSessionListener, HttpSessionAttributeListener 
{
 private StringBuffer log = new StringBuffer("MySessionListener log\n");
  
 public void attributeAdded(HttpSessionBindingEvent event)
 {
   log.append(event.getName() + "," + event.getValue() +
        "," + new Date() + "\n");
 }

 public void attributeRemoved(HttpSessionBindingEvent event)
 {
   log.append(event.getName() + "," + event.getValue() +
        "," + new Date() + "\n");
 }

 public void attributeReplaced(HttpSessionBindingEvent event) 
 {
   log.append(event.getName() + "," + event.getValue() +
        "," + new Date() + "\n");
 }

 public void sessionCreated(HttpSessionEvent event) 
 {
   log.append("sessionCreated: " + new Date() + "\n");
 }

 public void sessionDestroyed(HttpSessionEvent event) 
 {
   HttpSession session = event.getSession();
   if (session!=null) 
   {
    log.append("Session Id:" + session.getId());
    log.append("Current Time: " + new Date());
    log.append("Created Time: " + 
          session.getCreationTime());
    log.append("Last Accessed: " + 
          session.getLastAccessedTime());
   }

   //permanently record the events; something like:      
   //myLogger.record( log.toString() );
 }
}

TIP

The listener architecture in servlets is based on the event model. The container automatically calls certain methods when an event occurs. For example, the container calls attributeRemoved in a servlet that implements HttpSessionAttributeListener when an attribute is removed from a session object.

Before you can listen to events, you must configure your Web application to have an event listener. Most containers have default listeners already configured, so most likely you don't have to change the configuration. However, you should understand the steps involved. You would edit the web.xml (located in the WEB-INF) deployment descriptor of the Web application for which you are creating an event listener; you would add the <listener> element. Among containers, the required order of top-level elements varies. I recommend placing the <listener> element directly after the <filter> and <filter-mapping> elements and before the <servlet> element. You can specify many listeners, and usually the container invokes the event listeners in the order in which they appear in the deployment descriptor. Conversely, most containers invoke these events in the reverse order during shutdown. The following snippet is an example of two listener declarations:

<listener>
 <listener-class>myContextListener</listener-class>
</listener>
<listener>
 <listener-class>
    mySessionAttributeListener
 </listener-class>
</listener>

Remember that the listener element indicates the deployment properties for a Web application listener bean. The listener-class element declares that a class in the application must be registered as a Web application listener bean. The value is the fully qualified class name of the listener class.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020