Скачать или смотреть JSP Tutorial: Servlets-Application Event Listener Classes in JSp-Servlets. Video Part 25.

Application Event Listener Classes:
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: The user creates an event listener class that implements one of the listener interfaces.

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:
The user creates an event listener class that implements one of the listener interfaces. This implementation is registered in the deployment descriptor.
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.)
At runtime, 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