How do you implement JWT authentication in Spring Boot? स्प्रिंग बूट में JWT ऑथेंटिकेशन कैसे इम्प्लीमेंट करें?

JWT (JSON Web Token) authentication is stateless: after a user logs in with credentials, the server generates a signed token containing claims (user id, roles, expiry) and returns it to the client, which sends it in the Authorization: Bearer <token> header on every subsequent request.

A custom OncePerRequestFilter is added to the filter chain to extract and validate the token on each request, parse its claims, and populate the SecurityContext if valid, without needing a server-side session — making JWT well suited for stateless REST APIs and microservices.

public class JwtAuthFilter extends OncePerRequestFilter {
    protected void doFilterInternal(HttpServletRequest req, HttpServletResponse res, FilterChain chain) {
        String token = extractToken(req);
        if (token != null && jwtUtil.validateToken(token)) {
            UsernamePasswordAuthenticationToken auth = jwtUtil.getAuthentication(token);
            SecurityContextHolder.getContext().setAuthentication(auth);
        }
        chain.doFilter(req, res);
    }
}