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.
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.
Table 10–1 Servlet Lifecycle Events
Object |
Event |
Listener Interface and Event Class |
---|---|---|
Web context (see Accessing the Web Context) |
Initialization and destruction |
javax.servlet.ServletContextListener and ServletContextEvent |
Attribute added, removed, or replaced |
javax.servlet.ServletContextAttributeListener and ServletContextAttributeEvent |
|
Session (See Maintaining Client State) |
Creation, invalidation, activation, passivation, and timeout |
javax.servlet.http.HttpSessionListener, javax.servlet.http.HttpSessionActivationListener, and HttpSessionEvent |
Attribute added, removed, or replaced |
javax.servlet.http.HttpSessionAttributeListener and HttpSessionBindingEvent |
|
Request |
A servlet request has started being processed by web components |
javax.servlet.ServletRequestListener and ServletRequestEvent |
Attribute added, removed, or replaced |
javax.servlet.ServletRequestAttributeListener and ServletRequestAttributeEvent |
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 { ...