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Developing Web Applications for WebLogic Server

# Application Events and Event Listener Classes

The following sections discuss application events and event listener classes:

- Overview of Application Event Listener Classes
- Servlet Context Events
- HTTP Session Events
- Configuring an Event Listener Class
- Writing an Event Listener Class
- Templates for Event Listener Classes
- Additional Resources

# Overview of Application Event Listener Classes

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 runtime, the servlet container invokes on the instance of the 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

# Servlet Context Events

The following table lists 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.

Table 5-1

Type of Event | Interface | Method
--- | --- | ---
Servlet context is created. | `javax.servlet.ServletContextListener` | `contextInitialized()`
Servlet context is about to be shut down. | `javax.servlet.ServletContextListener` | `contextDestroyed()`
An attribute is added. | `javax.servlet. ServletContextAttributesListener` | `attributeAdded()`
An attribute is removed. | `javax.servlet. ServletContextAttributesListener` | `attributeRemoved()`
An attribute is replaced. | `javax.servlet. ServletContextAttributesListener` | `attributeReplaced()`

Servlet Context Events

# HTTP Session Events

The following table lists the types of HTTP session events your event listener class must implement to respond to the HTTP session events and the methods invoked when the HTTP session events occur.

Table 5-2 HTTP Session Events

Type of Event | Interface | Method
--- | --- | ---
An HTTP session is activated. | `javax.servlet.http. HttpSessionListener` | `sessionCreated()`
An HTTP session is about to be passivated. | `javax.servlet.http. HttpSessionListener` | `sessionDestroyed()`
An attribute is added. | `javax.servlet.http. HttpSessionAttributeListener` | `attributeAdded()`
An attribute is removed. | `javax.servlet.http. HttpSessionAttributeListener` | `attributeRemoved()`
An attribute is replaced. | `javax.servlet.http. HttpSessionAttributeListener` | `attributeReplaced()`

Note: The Servlet 2.3 specification also contains the `javax.servlet.http.HttpSessionBindingListener` and the `javax.servlet.http.HttpSessionActivationListener` interfaces. These interfaces are implemented by objects that are stored as session attributes and do not require registration of an event listener in `web.xml`. For more information, see the Javadocs for these interfaces.

# Configuring an Event Listener Class

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. 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.

# Writing an Event Listener Class

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 J2EE API Reference (Javadocs) 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

# Templates for Event Listener Classes

The following examples provide some basic templates for event listener classes.

## Servlet Context 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 when the servlet context is
>              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.
>           */

>         }
>     }

## HTTP Session Attribute Event Listener Class Example

>     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.
>           */
>         }
>     }

# Additional Resources

- Servlet 2.3 Specification from Sun Microsystems
- J2EE API Reference (Javadocs)
- The J2EE Tutorial from Sun Microsystems: