One of the most exciting features of the new Java Servlet 2.3 specification is filtering. At first sight, Servlet 2.3 filtering is deceptively similar to existing legacy filters in Apache, IIS, Netscape Web servers and others. In reality, Servlet 2.3 filtering is a completely different design architecturally -- leveraging the object-oriented nature of the Java platform to provide a new level of performance. This article introduces you to filtering in Tomcat 4 and shows you how to make productive use of filters in your projects.