Spring Security Filter Chain: A Deep Dive for Java Backend Engineers If you've ever wondered what actually happens between the moment an HTTP request hits your Spring Boot application and the moment your controller method executes — this post is for you. The answer is the Security Filter Chain , and understanding it is the difference between cargo-culting security configs and actually knowing what you're doing. This is the foundation that every other authentication and authorization mechanism in Spring Security builds on. What Is the Security Filter Chain? Spring Security is built on a chain of servlet filters. When a request arrives, it passes through a series of filters in order before reaching your controller. Each filter can inspect, modify, reject, or pass through the request and response. Think of it like airport security — each checkpoint (filter) does a specific check. If you fail at any checkpoint, you're rejected. Pass all checkpoints and you board the plane.…