How Spring Security Works Internally with JWT

Spring Security + JWT what happens behind the scenes. Let me break it down in simple words- When you log in: Your username & password go to the server - Spring checks your password using BCrypt (never stores plain text) - if correct, it creates a JWT token and sends it back to you. When you make any request after that: You send that token in every request - Spring reads it, checks the signature, checks if it's expired - if everything is fine, it lets you in - if not, straight up 401 error. The magic part? The server never saves your session anywhere. It just trusts the token it signed. That's why JWT scales so well - no DB hit on every request. Think of it like this: JWT is like a stamp on your hand at a club entrance. You show the stamp - they let you in. No need to check the guest list every time.  3 things Java devs often miss: 1) OncePerRequestFilter - runs your JWT check exactly once per request, no duplicates 2) SecurityContextHolder - this is where Spring stores "who is logged in" for that request thread 3) UserDetailsService - this is your code. Spring calls it, you decide how to load the user. #Java #SpringBoot #JWT #JavaDeveloper

Authentication and authorization used to feel like the same thing to me — until I started working with Spring Security and JWT. While revisiting JWT authentication, I was reminded of 3 important things: 1. Authentication verifies who the user is. 2. Authorization decides what the user can access. 3. JWT helps build stateless and secure APIs when implemented correctly. The more I learn backend development, the more I realize security is not a feature you add later — it has to be part of the design from the start. Still learning and improving every day. How are you securing APIs in your projects? #Java #SpringBoot #SpringSecurity #JWT #BackendDevelopment #Microservices #SoftwareEngineering

🔐 Just wrapped up a solid JWT Authentication System using Spring Boot! This project focuses on implementing secure, stateless authentication for REST APIs using JWT and Spring Security. It gave me hands-on experience with how modern applications handle authentication and authorization efficiently If you're looking to understand JWT authentication from basics to advanced level (including best practices and common pitfalls), feel free to check it out: 📖 Full Guide: https://lnkd.in/gBNKjPiM 💻 GitHub Repository: https://lnkd.in/gFkSADPe #Java #SpringBoot #JWT #BackendDevelopment

Understanding JWT( JSON Web Token) Authentication in Spring Boot Most of the time, I was just using JWT in projects without really knowing what was happening behind the scenes. So I spent some time digging into how it actually works. 🔐 What happens during login? * Backend verifies credentials * Generates a JWT token * Sends it to the client After that, the client sends this token with every request instead of credentials. 🧩 The interesting part is Spring Security: Every request passes through a filter chain before reaching the controller. 👉 I implemented a custom JWT filter: * Extract token from header * Validate signature & expiry * If valid → set authentication in Security Context Only then the request is processed. 💡 Took me time to understand this flow — especially tracing how requests move through the filter chain. * JWT = stateless (no session stored on server) * Authentication = who you are * Authorization = what you can access * Everything is decided before controller logic runs 🛠 Still figuring out: * Exact execution point of the filter * Role of SecurityContextHolder * Debugging filter chain flow 📌 Earlier JWT felt like just a library feature. Now it feels like a request validation mechanism at the filter level. Still learning, but this changed how I see backend security. #Java #SpringBoot #SpringSecurity #JWT #BackendDevelopment

🔐 Building Secure Authentication with JWT in Spring Boot Currently working on implementing JWT-based authentication in a Spring Boot application, focusing on handling both access tokens and refresh tokens for a more secure and scalable system. Access tokens are short-lived and used for every request, while refresh tokens help generate new access tokens without forcing users to log in again — improving both security and user experience. Along the way, I’ve been learning key concepts like: • Designing a stateless authentication flow • Proper token lifecycle management (generation, validation, expiration, rotation) • Securing endpoints using Spring Security filters • Handling edge cases like token expiry and invalidation • Structuring clean and maintainable authentication logic This hands-on implementation is helping me better understand how real-world backend systems handle security at scale. 🚀 #SpringBoot #Java #BackendDevelopment #JWT #Authentication #Security #DeveloperLife #LearningJourney

𝟕,𝟎𝟎𝟎+ 𝗺𝗮𝗹𝗶𝗰𝗶𝗼𝘂𝘀 𝗽𝗮𝗰𝗸𝗮𝗴𝗲𝘀 removed from npm in 2025. The average Node.js project pulls 300-800 transitive dependencies. That attack surface is wild. I've shipped production in both ecosystems for 19 years: - npm's "install and hope" culture means one typosquatted package can own your CI - 𝗠𝗮𝘃𝗲𝗻 𝗖𝗲𝗻𝘁𝗿𝗮𝗹 has stricter publishing and far fewer supply chain incidents - Java had Log4Shell. One major breach in 25+ years is not the same as thousands yearly. - npm lets anyone publish anything under any name. That's a design flaw. I still use Node.js. But I stopped trusting the ecosystem blindly years ago. What's your dependency audit process? #NodeJS #Security #Java #Axios #npm

For a long time, I treated Spring Security as “just configuration.” That was a mistake. The real shift happened when I understood it as a filter chain. Every request goes through multiple filters before it even reaches your controller. Which means: Authentication is already decided early Authorization failures (403) often have nothing to do with your business logic In one case, I was debugging a 403 for hours… Turned out the request was getting rejected in the security filter itself—not the API. Since then, I always think in terms of request flow, not annotations. 👉 If you're working with Spring Boot, understanding this will save you a lot of time. Curious—do you actually understand Spring Security, or just make it work #Java #SpringBoot #Microservices #BackendDevelopment #Kafka

(Part 3/5) The real idea behind this system wasn’t JWT. . . . . . . . . It wasn’t Spring Boot either. It was multi-tenancy. That’s what made everything scalable. Here’s how I approached it: 👉 Every request carries an application_id 👉 Every user is mapped to a specific application 👉 Every token is generated with that context So even though I have: One authentication service One codebase It behaves like multiple isolated systems internally. Each application: Has its own users Own access control Own authentication flow But all powered by the same service. This solved a major problem: 👉 Centralization without losing isolation And that’s where things started to feel like real system design — not just implementation. In the next post, I’ll break down how JWT actually works in this setup. #SystemDesign #Java #Microservices #BackendDevelopment #LearningJourney

I wanted to move beyond memorizing Spring Security concepts and actually build the flow end-to-end, so I started with an auth service in Spring Boot. On Day 1, I implemented registration, login, JWT access tokens, refresh tokens, logout, and a protected admin endpoint with role-based access. One thing I wanted to understand properly was where authentication really happens, so instead of treating security config as boilerplate, I spent time breaking down the SecurityFilterChain, JwtDecoder, claim mapping, and refresh token lifecycle. A few tradeoffs became obvious while building it. JWT access tokens are great for stateless authentication, but once you need logout, revocation, and session continuity, refresh tokens bring back server-side state and complexity. H2 was useful for getting the flow working fast, but it also made it obvious why local convenience and deployment correctness are two different things. The biggest takeaway for me was that auth gets much less confusing when it is treated as a sequence of responsibilities rather than one “security setup” problem. Token generation, validation, authorization, and refresh handling all solve different problems, and understanding that separation made the system much easier to reason about. #Java #SpringBoot #JWT #SpringSecurity #BackendEngineering #Microservices

I've been going through Spring Security in Action - 2nd Edition for some time now. It's been a solid read for getting into the fundamentals of how the framework works under the hood. Application security has become a critical area especially today, where systems are heavily exposed through APIs and AI-driven applications are becoming more common. The book covers key concepts that play a major role in building secure applications. #Java #SpringBoot #SpringSecurity #ApplicationSecurity