🧠 Full List of Important Servlet Listeners + Examples

Listener Interface	Purpose	Example Event It Catches
ServletContextListener	App startup and shutdown	Server starts your app, or app is undeployed
HttpSessionListener	Session creation and destruction	User opens a session (new visit) or session expires
ServletRequestListener	Request creation and destruction	Each new HTTP request starts or ends
ServletContextAttributeListener	Attribute added/removed/replaced in context	Someone puts something into ServletContext
HttpSessionAttributeListener	Attribute added/removed/replaced in session	User sets attribute into session
ServletRequestAttributeListener	Attribute added/removed/replaced in request	Developer adds data into request object

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🎯 Examples for Each Listener

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1. ServletContextListener

✅ Do something when the application starts or shuts down.

@WebListener
public class AppStartupShutdownListener implements ServletContextListener {
    @Override
    public void contextInitialized(ServletContextEvent sce) {
        System.out.println("App started at: " + new java.util.Date());
    }

    @Override
    public void contextDestroyed(ServletContextEvent sce) {
        System.out.println("App is shutting down...");
    }
}

Use case:

• Initialize DB connections at app start.
• Clean up threads, close connections at app shutdown.

2. HttpSessionListener

✅ Track when user sessions are created or destroyed.

@WebListener
public class MySessionListener implements HttpSessionListener {
    @Override
    public void sessionCreated(HttpSessionEvent se) {
        System.out.println("New session created: " + se.getSession().getId());
    }

    @Override
    public void sessionDestroyed(HttpSessionEvent se) {
        System.out.println("Session destroyed: " + se.getSession().getId());
    }
}

Use case:

• Count online users.
• Cleanup session-related resources.

3. ServletRequestListener

✅ Hook into each HTTP request lifecycle.

@WebListener
public class RequestListenerExample implements ServletRequestListener {
    @Override
    public void requestInitialized(ServletRequestEvent sre) {
        System.out.println("New request started at " + new java.util.Date());
    }

    @Override
    public void requestDestroyed(ServletRequestEvent sre) {
        System.out.println("Request finished.");
    }
}

Use case:

• Start timing requests (for performance monitoring).
• Set up default values for every request.

4. ServletContextAttributeListener

✅ Listen for attributes being added/removed/changed in the application scope.

@WebListener
public class AppAttributeListener implements ServletContextAttributeListener {
    @Override
    public void attributeAdded(ServletContextAttributeEvent event) {
        System.out.println("Attribute added: " + event.getName() + " = " + event.getValue());
    }
}

Use case:

• Track global setting changes.
• Audit what data is being put into ServletContext.

5. HttpSessionAttributeListener

✅ Listen for attributes being added/removed/changed inside a user session.

@WebListener
public class SessionAttributeLogger implements HttpSessionAttributeListener {
    @Override
    public void attributeAdded(HttpSessionBindingEvent event) {
        System.out.println("Session attribute added: " + event.getName());
    }
}

Use case:

• Track login/logout events stored in session.
• Auto-update session statistics.

6. ServletRequestAttributeListener

✅ Watch attributes being added/removed/changed in individual requests.

@WebListener
public class RequestAttributeLogger implements ServletRequestAttributeListener {
    @Override
    public void attributeAdded(ServletRequestAttributeEvent event) {
        System.out.println("Request attribute added: " + event.getName() + " = " + event.getValue());
    }
}

Use case:

• Debugging request data.
• Auditing sensitive request changes.

🛠️ Quick Memory Table

Event	Listener
App starts or stops	ServletContextListener
User session starts or ends	HttpSessionListener
HTTP request begins or ends	ServletRequestListener
Attribute added/removed at app level	ServletContextAttributeListener
Attribute added/removed at session level	HttpSessionAttributeListener
Attribute added/removed at request level	ServletRequestAttributeListener

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🚨 Bonus Tip:

• Listeners do not block or control the flow (unlike filters).
• They are passive observers — they listen and react.