doFilter

doFilter is a Java Servlet API interface that allows for the interception and processing of requests and responses before they reach their ultimate destination or after they have been generated. It is a fundamental component of the Java EE (now Jakarta EE) web tier, enabling developers to implement cross-cutting concerns such as authentication, logging, character encoding, and data transformation.

A doFilter method is the core of a Filter implementation. This method receives a ServletRequest, a ServletResponse, and a FilterChain object. The ServletRequest and ServletResponse represent the current request and response being processed, while the FilterChain represents the remaining filters and the target servlet in the processing chain. Inside the doFilter method, a filter can examine the request and response objects, modify them, or perform actions before or after passing the request to the next element in the chain by calling the doFilter method on the FilterChain.

Filters are configured in the deployment descriptor (web.xml) or through annotations. They can be mapped to specific URL patterns or servlets. Multiple filters can be chained together, forming a pipeline where each filter processes the request and response sequentially. The order of filters in the chain is crucial, as it determines the order in which their logic is applied. doFilter is a powerful mechanism for modularizing and enhancing web application functionality without modifying the core business logic of servlets.