Application life cycle listeners and events

 

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With application life cycle listeners and events we can notify listeners when servlet contexts and sessions change. For example, we can notify users when attributes change and if sessions or servlet contexts are created or destroyed.

The life cycle listeners give the application developer greater control over interactions with ServletContext and HttpSession objects. Servlet context listeners manage resources at an application level. Session listeners manage resources that are associated with a series of requests from a single client. The listener developer creates a class that implements the javax listener interface, corresponding to the listener functionality that you want.

At application startup time, the container uses introspection to create an instance of the listener class and registers it with the appropriate event generator.

When a servlet context is created, the contextInitialized method of the listener class is invoked, which creates the database connection for the servlets in the application to use if this context is for the application. All servlet context listeners are notified of context initialization before any servlet in the Web app is initialized.

When the servlet context is destroyed, the contextDestroyed method is invoked, which releases the database connection, if this context is for the application. You must destroy all servlets before any servlet context listeners are notified of context destruction.

Notifications to session listeners precede notifications to context listeners.

 

Listener classes for servlet context and session changes

The following methods are defined as part of the javax.servlet.ServletContextListener interface:

• void contextInitialized(ServletContextEvent)

  Notification that the Web app is ready to process requests. Place code in this method to see if the created context is for the Web app and if it is, allocate a database connection and store the connection in the servlet context.

• void contextDestroyed(ServletContextEvent)

  Notification that the servlet context is about to shut down. Place code in this method to see if the created context is for the Web app and if it is, close the database connection stored in the servlet context.

The following methods are defined as part of the javax.servlet.ServletRequestListener interface:

• public void requestInitialized(ServletRequestEvent re)

  • Notification that the request is about to come into scope

    A request is defined as coming into scope when it is about to enter the first filter in the filter chain that processes the request.

• public void requestDestroyed(ServletRequestEvent re)

  • Notification that the request is about to go out of scope

    A request is defined as going out of scope when it exits the last filter in its filter chain.

The following listener interfaces are defined as part of the javax.servlet package:

• ServletContextListener
• ServletContextAttributeListener

The following filter interface is defined as part of the javax.servlet package:

• FilterChain interface - methods: doFilter()

The following event classes are defined as part of the javax.servlet package:

• ServletContextEvent
• ServletContextAttributeEvent

The following interfaces are defined as part of the javax.servlet.http package:

• HttpSessionListener
• HttpSessionAttributeListener
• HttpSessionActivationListener

The following event class is defined as part of the javax.servlet.http package:

• HttpSessionEvent

 

Example

The following example shows how to create a servlet context listener:

 package com.ibm.websphere;
 import java.io.*;
 import javax.servlet.*;
 public class DBConnectionListener implements ServletContextListener 
{
     
// implement the required context init method
    void contextInitialized(ServletContextEvent sce)
    {
    }

     
// implement the required context destroy method
    void contextDestroyed(ServletContextEvent sce)
    {
    }
}

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