Handling Servlet Lifecycle Events

You can monitor and react to events in a servlet’s lifecycle by defining listener objects whose methods get invoked when lifecycle events occur. To use these listener objects, you must define and specify the listener class.

Defining the Listener Class

You define a listener class as an implementation of a listener interface. Table 10–1 lists the events that can be monitored and the corresponding interface that must be implemented. When a listener method is invoked, it is passed an event that contains information appropriate to the event. For example, the methods in the HttpSessionListener interface are passed an HttpSessionEvent, which contains an HttpSession.

Use the @WebListener annotation to define a listener to get events for various operations on the particular web application context. Classes annotated with @WebListener must implement one of the following interfaces:

javax.servlet.ServletContextListener
javax.servlet.ServletContextAttributeListener
javax.servlet.ServletRequestListener
javax.servlet.ServletRequestAttributeListener
javax.servlet..http.HttpSessionListener
javax.servlet..http.HttpSessionAttributeListener

For example, the following code snippet defines a listener that implements two of these interfaces:

import javax.servlet.ServletContextAttributeListener;
import javax.servlet.ServletContextListener;
import javax.servlet.annotation.WebListener;

@WebListener()
public class SimpleServletListener implements ServletContextListener,
        ServletContextAttributeListener {
    ...