JWT Authentication

Header.Payload.Signature

eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9   ← Base64(Header)
.eyJzdWIiOiJ1c2VyMTIzIiwiZXhwIjoxNjk5fQ ← Base64(Payload)
.SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c ← HMAC signature

{
  "sub": "user@example.com",
  "iat": 1699000000,
  "exp": 1699086400,
  "roles": ["ROLE_USER"]
}

<dependency>
    <groupId>io.jsonwebtoken</groupId>
    <artifactId>jjwt-api</artifactId>
    <version>0.12.3</version>
</dependency>
<dependency>
    <groupId>io.jsonwebtoken</groupId>
    <artifactId>jjwt-impl</artifactId>
    <version>0.12.3</version>
    <scope>runtime</scope>
</dependency>

@Service
public class JwtService {

    @Value("${app.jwt.secret}")
    private String secret;

    @Value("${app.jwt.expiration-ms:86400000}")
    private long expirationMs;

    private SecretKey key() {
        return Keys.hmacShaKeyFor(Decoders.BASE64.decode(secret));
    }

    public String generateToken(UserDetails userDetails) {
        return Jwts.builder()
            .subject(userDetails.getUsername())
            .issuedAt(new Date())
            .expiration(new Date(System.currentTimeMillis() + expirationMs))
            .claim("roles", userDetails.getAuthorities().stream()
                .map(GrantedAuthority::getAuthority).toList())
            .signWith(key())
            .compact();
    }

    public String extractUsername(String token) {
        return extractClaim(token, Claims::getSubject);
    }

    public boolean isValid(String token, UserDetails userDetails) {
        String username = extractUsername(token);
        return username.equals(userDetails.getUsername())
            && !isExpired(token);
    }

    private boolean isExpired(String token) {
        return extractClaim(token, Claims::getExpiration).before(new Date());
    }

    private <T> T extractClaim(String token, Function<Claims, T> resolver) {
        Claims claims = Jwts.parser()
            .verifyWith(key())
            .build()
            .parseSignedClaims(token)
            .getPayload();
        return resolver.apply(claims);
    }
}

@Component
@RequiredArgsConstructor
public class JwtAuthFilter extends OncePerRequestFilter {

    private final JwtService jwtService;
    private final UserDetailsService userDetailsService;

    @Override
    protected void doFilterInternal(HttpServletRequest request,
            HttpServletResponse response, FilterChain chain)
            throws ServletException, IOException {

        String authHeader = request.getHeader("Authorization");
        if (authHeader == null || !authHeader.startsWith("Bearer ")) {
            chain.doFilter(request, response);
            return;
        }

        String token = authHeader.substring(7);
        String username = jwtService.extractUsername(token);

        if (username != null &&
                SecurityContextHolder.getContext().getAuthentication() == null) {
            UserDetails userDetails = userDetailsService.loadUserByUsername(username);

            if (jwtService.isValid(token, userDetails)) {
                UsernamePasswordAuthenticationToken authToken =
                    new UsernamePasswordAuthenticationToken(
                        userDetails, null, userDetails.getAuthorities());
                authToken.setDetails(
                    new WebAuthenticationDetailsSource().buildDetails(request));
                SecurityContextHolder.getContext().setAuthentication(authToken);
            }
        }
        chain.doFilter(request, response);
    }
}

@RestController
@RequestMapping("/api/v1/auth")
@RequiredArgsConstructor
public class AuthController {

    private final AuthenticationManager authManager;
    private final JwtService jwtService;
    private final UserDetailsService userDetailsService;

    @PostMapping("/login")
    public ResponseEntity<Map<String, String>> login(
            @RequestBody @Valid LoginRequest req) {
        authManager.authenticate(
            new UsernamePasswordAuthenticationToken(req.email(), req.password()));

        UserDetails user = userDetailsService.loadUserByUsername(req.email());
        String token = jwtService.generateToken(user);
        return ResponseEntity.ok(Map.of("token", token));
    }
}