When should servlet filters be used, and when should…

Servlet filters handle requests and responses between the servlet container and the servlet itself. They can modify requests/responses, interrupt the processing chain, or add logic before, after, and around servlet execution. They are used for authentication, logging, encryption/decryption, data compression.

Listeners respond to lifecycle events of various web application objects: servlet context, HTTP session, servlet request. Each listener type is intended for a specific event. They are used for resource initialization, cleanup, session monitoring, error handling.

Application:

Filters: When you need to influence the request and response flow, modify them, or perform actions before/after servlet execution for a group of servlets.

// Example Filter.java
public class LoggingFilter implements Filter {

    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        // Filter initialization
    }

    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
        // Logic before servlet call
        System.out.println("Request received: " + request.getRemoteAddr());

        chain.doFilter(request, response); // Pass request further down the chain

        // Logic after servlet call
        System.out.println("Response sent.");
    }

    @Override
    public void destroy() {
        // Resource cleanup for filter
    }
}

Listeners: When you need to perform actions in response to changes in the web application's state or its components (creation/deletion of sessions, initialization/termination of context).

// Example ServletContextListener.java
public class AppInitializer implements ServletContextListener {

    @Override
    public void contextInitialized(ServletContextEvent sce) {
        // Logic at web application startup
        System.out.println("Web application started.");
        // Initialization of resources, connection pools, etc.
    }

    @Override
    public void contextDestroyed(ServletContextEvent sce) {
        // Logic at web application shutdown
        System.out.println("Web application stopped.");
        // Resource cleanup
    }
}

Key difference: Filters work around servlet execution, processing data streams. Listeners work in response to changes in the states of web application objects.

When should servlet filters be used, and when should listeners be used?