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Spring Security + JWT Explained for Java Developers — The Complete Interview Guide
What every senior Java developer should know but most don’t explain clearly
I’ve been a Java Technical Lead for 11 years. Spring Security comes up in almost every senior interview — and most candidates explain it at surface level. They say “Spring validates the token” and stop there. Interviewers want to know how. They want the filter chain, the proxy mechanism, the cryptographic validation, and the production gotchas.
This guide covers everything that matters: filter chain internals, JWT build and validation, complete Spring Boot implementation, CORS, CSRF, and @PreAuthorize. I'll tell you exactly what to say in an interview and what mistakes to avoid.
The Mental Model First
Before anything else — Spring Security is not inside Spring MVC. It sits completely outside your controllers as a chain of servlet filters. Every HTTP request passes through this chain before it ever reaches DispatcherServlet.
