In Java, Servlet filters are objects that intercept and modify incoming requests and outgoing responses to a web application. They can be used to implement common functionalities like authentication, logging, encryption, compression, and URL rewriting. Servlet filters work by sitting between the client and the server, processing requests before they reach the Servlet and responses before they are sent back to the client.

Servlet filters implement the javax.servlet.Filter interface and are configured in the deployment descriptor (web.xml) of a web application. The javax.servlet.Filter interface defines three methods that are called by the Servlet container when the filter is initialized, a request is processed, and the filter is destroyed:

• void init(FilterConfig filterConfig): Called by the Servlet container when the filter is first loaded.
• void doFilter(ServletRequest request, ServletResponse response, FilterChain filterChain): Called for every request and response that passes through the filter. This method can modify the request or response or call the doFilter() method of the FilterChain object to pass the request to the next filter in the chain.
• void destroy(): Called by the Servlet container when the filter is unloaded.

Here's an example of a simple Servlet filter that logs the details of every incoming request:

import javax.servlet.*;
import javax.servlet.http.HttpServletRequest;
import java.io.IOException;

public class RequestLoggingFilter implements Filter {
    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        // Initialize the filter
    }

    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
        HttpServletRequest httpRequest = (HttpServletRequest) request;
        System.out.println("Received request: " + httpRequest.getMethod() + " " + httpRequest.getRequestURI());
        chain.doFilter(request, response);
    }

    @Override
    public void destroy() {
        // Cleanup the filter
    }
}

In this example, we create a new Servlet filter by implementing the javax.servlet.Filter interface. The doFilter() method is called for every incoming request and response. We use it to log the details of the request and then call chain.doFilter() to pass the request to the next filter in the chain or to the Servlet if there are no more filters.

To use this filter in a web application, you need to add it to the web.xml file:

<filter>
    <filter-name>requestLoggingFilter</filter-name>
    <filter-class>com.example.RequestLoggingFilter</filter-class>
</filter>
<filter-mapping>
    <filter-name>requestLoggingFilter</filter-name>
    <url-pattern>/*</url-pattern>
</filter-mapping>

In this example, we define a new filter with the name requestLoggingFilter and the class com.example.RequestLoggingFilter. We then add a filter mapping that associates the filter with all requests by specifying a URL pattern of /*.

Servlet filters are a powerful tool for implementing common functionalities in a web application. By implementing the javax.servlet.Filter interface and configuring the filter in the web.xml file, you can intercept and modify incoming requests and outgoing responses in a flexible and modular way.