Java.Servlets.Listeners

🎯 Why Use Listeners?

They let you hook into the lifecycle of:

• The ServletContext (entire app)
• HttpSession (per user)
• ServletRequest (per request)

You use them to:

• Initialize or clean up resources (e.g. DB pools)
• Log application or session events
• Track user sessions
• Inject defaults or track attribute changes
• Implement analytics or custom metrics

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🔧 Types of Listeners (Servlet API)

Listener Interface	Listens To	When to Use
ServletContextListener	App start and stop	Initialize app-wide resources (e.g. DB, caches)
HttpSessionListener	Session create/destroy	Track logged-in users, cleanup session data
ServletRequestListener	Request start/end	Logging, timing, diagnostics
ServletContextAttributeListener	Attribute added/removed in context	Global config changes or plugin loading
HttpSessionAttributeListener	Session attribute change	Track login/logout events, cart changes, etc.
ServletRequestAttributeListener	Request attribute change	For internal middleware logic (rarely used directly)

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🧪 Example: Tracking Active Sessions

@WebListener
public class SessionCounterListener implements HttpSessionListener {

    private static int activeSessions = 0;

    public void sessionCreated(HttpSessionEvent se) {
        activeSessions++;
        System.out.println("Session created. Active sessions: " + activeSessions);
    }

    public void sessionDestroyed(HttpSessionEvent se) {
        activeSessions--;
        System.out.println("Session destroyed. Active sessions: " + activeSessions);
    }
}

🚀 Example: Initializing Resources on App Startup

@WebListener
public class AppStartupListener implements ServletContextListener {

    public void contextInitialized(ServletContextEvent sce) {
        System.out.println("App started");
        // Initialize DB connection, load config, etc.
    }

    public void contextDestroyed(ServletContextEvent sce) {
        System.out.println("App shutting down");
        // Cleanup resources
    }
}

🛠️ Registering Listeners

1. Using Annotations:

@WebListener
public class MyListener implements HttpSessionListener { ... }

2. Using web.xml:

<listener>
  <listener-class>com.myapp.MyListener</listener-class>
</listener>

✅ Summary

Listener Type	Best For
ServletContextListener	App-level setup & teardown
HttpSessionListener	Tracking sessions, auth, cleanup
ServletRequestListener	Request-level timing/logging
AttributeListeners	Reacting to changes in session/context/request