A servlet filter in Java is a server-side component that intercepts and processes requests and responses between the client and the web application. It is a Java class that implements the javax.servlet.Filter interface and is used to modify or enhance the behavior of a servlet or JSP page.

When a client sends a request to a web application, the servlet container invokes the appropriate filter chain before passing the request to the servlet or JSP page. Each filter in the chain has the opportunity to examine and modify the request and response objects, or to perform other processing as needed.

Some common uses of servlet filters include:

• Authentication and authorization - Filters can be used to require authentication for certain pages or to restrict access based on user roles or permissions.

• Logging and auditing - Filters can be used to log request and response data for auditing or debugging purposes.

• Compression and caching - Filters can be used to compress responses to reduce bandwidth usage or to cache frequently accessed content for improved performance.

• Input validation and sanitization - Filters can be used to validate and sanitize input data to prevent security vulnerabilities such as SQL injection or cross-site scripting (XSS).

To create a filter in Java, you typically create a Java class that implements the Filter interface and overrides the init(), doFilter(), and destroy() methods. The init() method is used to perform any initialization tasks that the filter needs to perform, and the doFilter() method is where the main processing logic is implemented. The destroy() method is used to perform any cleanup tasks that the filter needs to perform when it is removed from the filter chain.

Overall, servlet filters provide a powerful and flexible way to modify the behavior of a web application without having to modify the servlet or JSP pages themselves. By intercepting and processing requests and responses, filters can enhance the security, performance, and functionality of a web application in a modular and reusable way.