In the previous article, we explored Method-Level Security using @PreAuthorize, @PostAuthorize, and @Secured. While these features allow us to secure business logic effectively, they still rely on an authenticated user.
In modern web applications, authentication is typically implemented using JSON Web Tokens (JWT). Unlike traditional session-based authentication, JWT enables stateless communication between clients and servers, making it ideal for REST APIs, microservices, and cloud-native applications.
In this article, we will build a complete JWT-based authentication system using Spring Security 7, Spring Boot 4, and Java 21. We will understand how JWT works internally, implement login and registration APIs, generate and validate tokens, create a custom authentication filter, and secure
What is JWT?
Explain:
Definition
Stateless Authentication
Why JWT exists
Difference between Session and JWT
JWT Structure
Explain:
Header
Payload
Signature
Example token
eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9
.
eyJzdWIiOiJqb2huIiwiZXhwIjoxNzM4NzI2MDAwfQ
.
SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5cExplain every section.
Why JWT?
Discuss advantages:
Stateless
Scalable
Faster
Mobile friendly
Microservices
Cloud Native
Real-world example.
JWT Authentication Flow
Client
↓
Login
↓
Spring Security
↓
AuthenticationManager
↓
UserDetailsService
↓
Database
↓
Generate JWT
↓
Return Token
↓
Client Stores Token
↓
Bearer Token
↓
JWT Filter
↓
Validate Token
↓
Security Context
↓
ControllerExplain every step.
Adding Dependencies
Explain Maven dependencies.
Spring Security
Spring Web
Spring Data JPA
JWT Library
Explain why each dependency is required.
Project Structure
config/
controller/
dto/
entity/
exception/
filter/
repository/
security/
service/
util/Explain why every package exists.
Creating User Entity
Complete explanation.
Creating UserRepository
Explain Optional, queries, and authentication.
Registration API
Explain the registration flow.
Receive request
Encode password
Save user
Return response
Login API
Explain login flow.
Username
Password
AuthenticationManager
JWT generation
AuthenticationManager
Explain
authenticate()Internally.
JWT Service
Explain
Generate Token
Extract Username
Validate Token
Expiration
JWT Filter
Explain
OncePerRequestFilterLifecycle.
Explain every overridden method.
Security Configuration
Explain
SecurityFilterChainDisable session
SessionCreationPolicy.STATELESSRegister JWT Filter
AuthenticationProvider
PasswordEncoder
AuthenticationManager
Testing APIs
Register
Login
Receive JWT
Call Protected API
Invalid Token
Expired Token
Missing Token
Expected Responses.
Refresh Tokens
Explain
Why refresh tokens exist
Access Token
Refresh Token
Expiration
Architecture.
Common JWT Mistakes
Storing secret in source code
Long-lived tokens
No expiration
Missing HTTPS
No refresh tokens
Weak secret key
Production Best Practices
Discuss
HTTPS
Strong Secret Keys
Environment Variables
Key Rotation
Refresh Tokens
Token Blacklisting
Short Access Token Lifetime
Role Claims
Exception Handling
Logging
Audit Trail
Real-World Example
Design an e-commerce authentication system.
Customer
↓
Login
↓
JWT
↓
Product APIs
↓
Order APIs
↓
Payment APIs
↓
Admin APIsExplain how JWT secures every request.
Summary
Cover:
JWT Basics
Stateless Authentication
JWT Structure
Authentication Flow
AuthenticationManager
JWT Filter
SecurityContext
Security Configuration
Registration
Login
Production Best Practices
