What is a Servlet Listener?

A Servlet Listener is a special class in the Servlet API that listens to specific events in a web application. These events include initialization, destruction, session creation, attribute changes, and more.

Types of Servlet Listeners

• ServletContextListener: Listens to lifecycle events of the ServletContext (application startup and shutdown).
• HttpSessionListener: Listens to lifecycle events of HttpSession objects (creation and destruction).
• ServletRequestListener: Listens to lifecycle events of ServletRequest (creation and destruction).
• Attribute Listeners: Listens to events when attributes are added, removed, or replaced:
  • ServletContextAttributeListener
  • HttpSessionAttributeListener
  • ServletRequestAttributeListener

Example: ServletContextListener

import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.annotation.WebListener;

@WebListener
public class MyServletContextListener implements ServletContextListener {
    @Override
    public void contextInitialized(ServletContextEvent event) {
        System.out.println("Application started!");
    }

    @Override
    public void contextDestroyed(ServletContextEvent event) {
        System.out.println("Application stopped!");
    }
}
      

What is a Filter?

A Filter is an object that intercepts requests and responses in a web application. Filters are used for pre-processing or post-processing requests and responses.

Uses of Filters

• Authentication and authorization.
• Logging and auditing request data.
• Data compression or decompression.
• Adding or modifying HTTP headers.
• Input validation and sanitation.

Example: Filter for Logging

import java.io.IOException;
import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.annotation.WebFilter;

@WebFilter("/*")
public class LoggingFilter implements Filter {
    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        System.out.println("Filter initialized.");
    }

    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
            throws IOException, ServletException {
        System.out.println("Request received at: " + System.currentTimeMillis());
        chain.doFilter(request, response); // Pass request to next filter or servlet
    }

    @Override
    public void destroy() {
        System.out.println("Filter destroyed.");
    }
}
      

Differences Between Listeners and Filters