🧠 Main Differences Between Filter and Listener

Feature	Filter	Listener
What it is	Interceptor of requests and responses	Event handler for lifecycle events
When it runs	Every time a matching request comes in	When a specific event happens (like app start, session create, request initialize)
Main purpose	Pre-processing and post-processing of requests/responses	Observing and reacting to events
Example use	Authentication, logging, compression, modifying requests or responses	Resource cleanup, monitoring session creation, app startup tasks
Works with	ServletRequest, ServletResponse, FilterChain	ServletContext, HttpSession, ServletRequest events
Configured by	@WebFilter annotation or web.xml	@WebListener annotation or web.xml
Flow control?	Yes (you can stop the request or modify it!)	No (just reacts, can’t control flow)

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🎯 In simple words:

• Filter = Interfere with request/response processing.
• Listener = React to lifecycle events (e.g., “Hey, someone created a session!”)

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🔥 Tiny Examples:

✅ Filter example — Block requests if user is not logged in:

@WebFilter("/secure/*")
public class AuthFilter implements Filter {
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
            throws IOException, ServletException {
        HttpServletRequest req = (HttpServletRequest) request;
        HttpSession session = req.getSession(false);

        if (session == null || session.getAttribute("user") == null) {
            ((HttpServletResponse) response).sendRedirect("/login.jsp");
        } else {
            chain.doFilter(request, response); // continue
        }
    }
}

✅ Listener example — Log when a new session is created:

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

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

🛠️ Quick Memory Trick:

Term	Think
Filter	Like a security gate ➔ can allow, modify, or block
Listener	Like a security camera ➔ just observes and logs

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⚡ Bonus: Types of Listeners

There are many listeners depending on what event you want to catch:

• ServletContextListener ➔ app startup/shutdown
• HttpSessionListener ➔ session creation/destruction
• ServletRequestListener ➔ request creation/destruction
• Plus attribute listeners: ServletContextAttributeListener, HttpSessionAttributeListener, etc.