Application events provide notifications of a change in state of the servlet context (each Web application uses its own servlet context) or of an HTTP session object. You write event listener classes that respond to these changes in state, and you configure and deploy them in a Web application. The servlet container generates events that cause the event listener classes to do something. In other words, the servlet container calls the methods on a user's event listener class.

The following is an overview of this process:

1. The user creates an event listener class that implements one of the listener interfaces.

2. This implementation is registered in the deployment descriptor.

3. At deployment time, the servlet container constructs an instance of the event listener class. (This is why the public constructor must exist, as discussed in Writing an Event Listener Class.)

4. At run time, the servlet container invokes on the instance of the listener class.

For servlet context events, the event listener classes can receive notification when the Web application is deployed or undeployed (or when WebLogic Server shuts down), and when attributes are added, removed, or replaced.

For HTTP session events, the event listener classes can receive notification when an HTTP session is activated or is about to be passivated, and when an HTTP session attribute is added, removed, or replaced.

Use Web application event listener classes to:

• Manage database connections when a Web application is deployed or shuts down

• Create standard counter utilities

• Monitor the state of HTTP sessions and their attributes

Examine a listing of the types of Servlet context events, the interface your event listener class must implement to respond to each Servlet context event, and the methods invoked when the Servlet context event occurs.

The HTTP Session Events contains a list of event types, interfaces and methods that are used to indicate the activation and deactivation of a HTTP session along with the addition and removal of attributes during a HTTP session.

Examine a listing of the types of servlet request events, the interface your event listener class must implement to manage state across the life cycle of servlet requests and the methods invoked when the request events occur.

Learn how to configure an event listener class.

1. Open the web.xml deployment descriptor of the Web application for which you are creating an event listener class in a text editor. The web.xml file is located in the WEB-INF directory of your Web application.
2. Add an event declaration using the listener element of the web.xml deployment descriptor. The event declaration defines the event listener class that is invoked when the event occurs. The listener element must directly follow the filter and filter-mapping elements and directly precede the servlet element. You can specify more than one event listener class for each type of event. WebLogic Server invokes the event listener classes in the order that they appear in the deployment descriptor (except for shutdown events, which are invoked in the reverse order). For example:

   <listener>
     <listener-class>myApp.MyContextListenerClass</listener-class>
   </listener>
   <listener>
     <listener-class>myApp.MySessionAttributeListenerClass</listener-class>
   </listener>
   

3. Write and deploy the event listener class. For details, see the section, Writing an Event Listener Class.

Learn how to write an event listener class.

1. Create a new event listener class that implements the appropriate interface for the type of event to which your class responds. For a list of these interfaces, see Servlet Context Events or HTTP Session Events. See Templates for Event Listener Classes for sample templates you can use to get started.
2. Create a public constructor that takes no arguments. For example:

   public class MyListener { 
   // public constructor 
   public MyListener() { /* ... */ } 
   }

3. Implement the required methods of the interface. See the Jakarta EE API Reference (Javadocs) at https://docs.oracle.com/en/java/javase/17/index.html for more information.
4. Copy the compiled event listener classes into the WEB-INF/classes directory of the Web application, or package them into a JAR file and copy the JAR file into the WEB-INF/lib directory of the Web application.

   The following useful classes are passed into the methods in an event listener class:

   • javax.servlet.http.HttpSessionEvent—provides access to the HTTP session object

   • javax.servlet.ServletContextEvent—provides access to the servlet context object.

   • javax.servlet.ServletContextAttributeEvent—provides access to servlet context and its attributes

   • javax.servlet.http.HttpSessionBindingEvent—provides access to an HTTP session and its attributes

Examine examples that provide some basic templates for event listener classes.

• Servlet Context Event Listener Class Example
  
• HTTP Session Attribute Event Listener Class Example
  

package myApp;
import javax.servlet.http.*;
public final class MyContextListenerClass implements
   ServletContextListener {
    public void contextInitialized(ServletContextEvent event) {

      /* This method is called prior to the servlet context being
         initialized (when the Web application is deployed). 
         You can initialize servlet context related data here.
      */ 

    }
    public void contextDestroyed(ServletContextEvent event) {

      /* This method is invoked when the Servlet Context 
         (the Web application) is undeployed or 
         WebLogic Server shuts down.
      */    

    }
}

package myApp;
import javax.servlet.*;

public final class MySessionAttributeListenerClass implements
   HttpSessionAttributeListener {

    public void attributeAdded(HttpSessionBindingEvent sbe) {
      /* This method is called when an attribute 
         is added to a session.
      */
    }
    public void attributeRemoved(HttpSessionBindingEvent sbe) {
      /* This method is called when an attribute
         is removed from a session.
      */
    }
    public void attributeReplaced(HttpSessionBindingEvent sbe) {
      /* This method is invoked when an attibute
         is replaced in a session.
      */
    }
}

• Servlet 4.0 specification at https://jcp.org/en/jsr/detail?id=369

• The Java EE tutorial at https://javaee.github.io/tutorial/toc.html