Java Notes

JWT Authentication — Java Programming

Implementing JWT authentication in Spring Boot - token generation, validation, refresh tokens, security filter, and complete authentication flow.

What is JWT?

JWT (JSON Web Token) is a compact, URL-safe token format used for stateless authentication. It eliminates the need for server-side session storage.

JWT Structure

Example

Header.Payload.Signature eyJhbGciOiJIUzI1NiJ9.eyJzdWIiOiJqb2huQGV4YW1wbGUuY29tIiwiaWF0IjoxNjE1MTYzODAwfQ.SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c

Part	Content
Header	Algorithm (HS256, RS256) + token type
Payload	Claims (sub, exp, iat, roles, custom data)
Signature	HMAC or RSA signature for verification

Setup

xml example

<dependency>
    <groupId>io.jsonwebtoken</groupId>
    <artifactId>jjwt-api</artifactId>
    <version>0.12.5</version>
</dependency>
<dependency>
    <groupId>io.jsonwebtoken</groupId>
    <artifactId>jjwt-impl</artifactId>
    <version>0.12.5</version>
    <scope>runtime</scope>
</dependency>
<dependency>
    <groupId>io.jsonwebtoken</groupId>
    <artifactId>jjwt-jackson</artifactId>
    <version>0.12.5</version>
    <scope>runtime</scope>
</dependency>

JWT Service

java example

@Service
public class JwtService {

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

    @Value("${jwt.expiration}")
    private long accessTokenExpiration; // e.g., 900000 (15 min)

    @Value("${jwt.refresh-expiration}")
    private long refreshTokenExpiration; // e.g., 604800000 (7 days)

    public String generateAccessToken(UserDetails userDetails) {
        Map<String, Object> claims = new HashMap<>();
        claims.put("roles", userDetails.getAuthorities().stream()
                .map(GrantedAuthority::getAuthority).toList());
        return generateToken(claims, userDetails.getUsername(), accessTokenExpiration);
    }

    public String generateRefreshToken(UserDetails userDetails) {
        return generateToken(new HashMap<>(), userDetails.getUsername(), refreshTokenExpiration);
    }

    private String generateToken(Map<String, Object> claims, String subject, long expiration) {
        return Jwts.builder()
                .claims(claims)
                .subject(subject)
                .issuedAt(new Date())
                .expiration(new Date(System.currentTimeMillis() + expiration))
                .signWith(getSigningKey())
                .compact();
    }

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

    public Date extractExpiration(String token) {
        return extractClaim(token, Claims::getExpiration);
    }

    public <T> T extractClaim(String token, Function<Claims, T> claimsResolver) {
        Claims claims = extractAllClaims(token);
        return claimsResolver.apply(claims);
    }

    private Claims extractAllClaims(String token) {
        return Jwts.parser()
                .verifyWith(getSigningKey())
                .build()
                .parseSignedClaims(token)
                .getPayload();
    }

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

    private boolean isTokenExpired(String token) {
        return extractExpiration(token).before(new Date());
    }

    private SecretKey getSigningKey() {
        byte[] keyBytes = Decoders.BASE64.decode(secretKey);
        return Keys.hmacShaKeyFor(keyBytes);
    }
}

JWT Authentication Filter

Example

@Component @RequiredArgsConstructor public class JwtAuthenticationFilter extends OncePerRequestFilter { private final JwtService jwtService; private final UserDetailsService userDetailsService; @Override protected void doFilterInternal(HttpServletRequest request, HttpServletResponse response, FilterChain filterChain) throws ServletException, IOException { // Extract Authorization header String authHeader = request.getHeader("Authorization"); if (authHeader == null || !authHeader.startsWith("Bearer ")) { filterChain.doFilter(request, response); return; } String jwt = authHeader.substring(7); try { String username = jwtService.extractUsername(jwt); if (username != null && SecurityContextHolder.getContext().getAuthentication() == null) { UserDetails userDetails = userDetailsService.loadUserByUsername(username); if (jwtService.isTokenValid(jwt, userDetails)) { UsernamePasswordAuthenticationToken authToken = new UsernamePasswordAuthenticationToken( userDetails, null, userDetails.getAuthorities()); authToken.setDetails(new WebAuthenticationDetailsSource().buildDetails(request)); SecurityContextHolder.getContext().setAuthentication(authToken); } } } catch (ExpiredJwtException e) { response.setStatus(HttpServletResponse.SC_UNAUTHORIZED); response.getWriter().write("Token expired"); return; } catch (JwtException e) { response.setStatus(HttpServletResponse.SC_UNAUTHORIZED); response.getWriter().write("Invalid token"); return; } filterChain.doFilter(request, response); } }

Security Configuration with JWT

java example

@Configuration
@EnableWebSecurity
@RequiredArgsConstructor
public class SecurityConfig {

    private final JwtAuthenticationFilter jwtFilter;
    private final UserDetailsService userDetailsService;

    @Bean
    public SecurityFilterChain securityFilterChain(HttpSecurity http) throws Exception {
        http
            .csrf(csrf -> csrf.disable())
            .sessionManagement(session -> session
                    .sessionCreationPolicy(SessionCreationPolicy.STATELESS))
            .authorizeHttpRequests(auth -> auth
                    .requestMatchers("/api/auth/**").permitAll()
                    .requestMatchers("/api/admin/**").hasRole("ADMIN")
                    .anyRequest().authenticated())
            .addFilterBefore(jwtFilter, UsernamePasswordAuthenticationFilter.class);

        return http.build();
    }

    @Bean
    public AuthenticationManager authenticationManager(AuthenticationConfiguration config) throws Exception {
        return config.getAuthenticationManager();
    }

    @Bean
    public AuthenticationProvider authenticationProvider() {
        DaoAuthenticationProvider provider = new DaoAuthenticationProvider();
        provider.setUserDetailsService(userDetailsService);
        provider.setPasswordEncoder(passwordEncoder());
        return provider;
    }

    @Bean
    public PasswordEncoder passwordEncoder() {
        return new BCryptPasswordEncoder(12);
    }
}

Auth Controller

java example

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

    private final AuthenticationManager authManager;
    private final JwtService jwtService;
    private final UserDetailsService userDetailsService;
    private final UserRepository userRepository;
    private final PasswordEncoder passwordEncoder;

    @PostMapping("/register")
    public ResponseEntity<?> register(@RequestBody @Valid RegisterRequest request) {
        if (userRepository.existsByEmail(request.getEmail())) {
            return ResponseEntity.badRequest().body(Map.of("error", "Email already exists"));
        }

        User user = User.builder()
                .name(request.getName())
                .email(request.getEmail())
                .password(passwordEncoder.encode(request.getPassword()))
                .role(Role.USER)
                .build();
        userRepository.save(user);

        UserDetails userDetails = userDetailsService.loadUserByUsername(user.getEmail());
        String accessToken = jwtService.generateAccessToken(userDetails);
        String refreshToken = jwtService.generateRefreshToken(userDetails);

        return ResponseEntity.status(HttpStatus.CREATED).body(Map.of(
                "accessToken", accessToken,
                "refreshToken", refreshToken,
                "message", "Registration successful"
        ));
    }

    @PostMapping("/login")
    public ResponseEntity<?> login(@RequestBody LoginRequest request) {
        try {
            authManager.authenticate(
                    new UsernamePasswordAuthenticationToken(request.getEmail(), request.getPassword()));

            UserDetails userDetails = userDetailsService.loadUserByUsername(request.getEmail());
            String accessToken = jwtService.generateAccessToken(userDetails);
            String refreshToken = jwtService.generateRefreshToken(userDetails);

            return ResponseEntity.ok(Map.of(
                    "accessToken", accessToken,
                    "refreshToken", refreshToken
            ));
        } catch (AuthenticationException e) {
            return ResponseEntity.status(HttpStatus.UNAUTHORIZED)
                    .body(Map.of("error", "Invalid email or password"));
        }
    }

    @PostMapping("/refresh")
    public ResponseEntity<?> refreshToken(@RequestBody Map<String, String> request) {
        String refreshToken = request.get("refreshToken");
        String username = jwtService.extractUsername(refreshToken);
        UserDetails userDetails = userDetailsService.loadUserByUsername(username);

        if (jwtService.isTokenValid(refreshToken, userDetails)) {
            String newAccessToken = jwtService.generateAccessToken(userDetails);
            return ResponseEntity.ok(Map.of("accessToken", newAccessToken));
        }
        return ResponseEntity.status(HttpStatus.UNAUTHORIZED)
                .body(Map.of("error", "Invalid refresh token"));
    }
}

application.properties

properties example

jwt.secret=dGhpcyBpcyBhIHZlcnkgc2VjdXJlIHNlY3JldCBrZXkgZm9yIGp3dA==
jwt.expiration=900000
jwt.refresh-expiration=604800000

Interview Key Points

JWT = Header.Payload.Signature (Base64URL encoded)
JWT is stateless — no server-side session storage needed
Access tokens are short-lived (15 min); refresh tokens are long-lived (7 days)
OncePerRequestFilter ensures JWT filter runs once per request
JWT filter extracts token → validates → sets SecurityContext
SecurityContextHolder holds current authenticated user
Never store sensitive data in JWT payload (it's Base64 encoded, not encrypted)
Use HTTPS always — JWT can be stolen if transmitted over HTTP

Summary

In this chapter, we learned about JWT Authentication in detail. Here are the key points:

Basic introduction to the concept and why it is needed
Syntax and structure with complete examples
Internal working and best practices
Real-world applications and use cases
Common mistakes and how to avoid them
How this topic is asked in interviews

To practice these concepts, write and run the code in your IDE and verify the output. Modify each example and experiment so that you deeply understand the concept.

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for JWT Authentication — Java Programming.

Interview Use

Prepare one clear explanation, one practical example, and one common mistake for this Java Master Course topic.

Search Terms

java-master-course, java master course, java, master, course, spring, framework, boot

Continue learning this concept

Spring FrameworkCRUD ApplicationBuild a complete CRUD application with Spring Boot - project setup, layered architecture, full CRUD operations, testing, and deployment.Spring FrameworkException Handling in Spring BootComplete guide to exception handling in Spring Boot - @ExceptionHandler, @ControllerAdvice, custom exceptions, error responses, and validation error handling.Spring FrameworkREST API with Spring BootBuilding RESTful APIs with Spring Boot - REST principles, controller annotations, request/response handling, pagination, HATEOAS, and API versioning.Spring FrameworkSpring SecurityComplete guide to Spring Security - authentication, authorization, security filter chain, role-based access control, password encoding, and security configuration.Spring FrameworkSpring Boot IntroductionComplete introduction to Spring Boot - auto-configuration, starter dependencies, embedded servers, creating your first Spring Boot application, and project structure.Spring FrameworkSpring Data JPAComplete guide to Spring Data JPA - entity mapping, repository interfaces, query methods, JPQL, native queries, relationships, and database operations.

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Java Notes

JWT Authentication — Java Programming

Implementing JWT authentication in Spring Boot - token generation, validation, refresh tokens, security filter, and complete authentication flow.

What is JWT?

JWT (JSON Web Token) is a compact, URL-safe token format used for stateless authentication. It eliminates the need for server-side session storage.

JWT Structure

Example

Header.Payload.Signature eyJhbGciOiJIUzI1NiJ9.eyJzdWIiOiJqb2huQGV4YW1wbGUuY29tIiwiaWF0IjoxNjE1MTYzODAwfQ.SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c

Part	Content
Header	Algorithm (HS256, RS256) + token type
Payload	Claims (sub, exp, iat, roles, custom data)
Signature	HMAC or RSA signature for verification

Setup

xml example

<dependency>
    <groupId>io.jsonwebtoken</groupId>
    <artifactId>jjwt-api</artifactId>
    <version>0.12.5</version>
</dependency>
<dependency>
    <groupId>io.jsonwebtoken</groupId>
    <artifactId>jjwt-impl</artifactId>
    <version>0.12.5</version>
    <scope>runtime</scope>
</dependency>
<dependency>
    <groupId>io.jsonwebtoken</groupId>
    <artifactId>jjwt-jackson</artifactId>
    <version>0.12.5</version>
    <scope>runtime</scope>
</dependency>

JWT Service

java example

@Service
public class JwtService {

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

    @Value("${jwt.expiration}")
    private long accessTokenExpiration; // e.g., 900000 (15 min)

    @Value("${jwt.refresh-expiration}")
    private long refreshTokenExpiration; // e.g., 604800000 (7 days)

    public String generateAccessToken(UserDetails userDetails) {
        Map<String, Object> claims = new HashMap<>();
        claims.put("roles", userDetails.getAuthorities().stream()
                .map(GrantedAuthority::getAuthority).toList());
        return generateToken(claims, userDetails.getUsername(), accessTokenExpiration);
    }

    public String generateRefreshToken(UserDetails userDetails) {
        return generateToken(new HashMap<>(), userDetails.getUsername(), refreshTokenExpiration);
    }

    private String generateToken(Map<String, Object> claims, String subject, long expiration) {
        return Jwts.builder()
                .claims(claims)
                .subject(subject)
                .issuedAt(new Date())
                .expiration(new Date(System.currentTimeMillis() + expiration))
                .signWith(getSigningKey())
                .compact();
    }

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

    public Date extractExpiration(String token) {
        return extractClaim(token, Claims::getExpiration);
    }

    public <T> T extractClaim(String token, Function<Claims, T> claimsResolver) {
        Claims claims = extractAllClaims(token);
        return claimsResolver.apply(claims);
    }

    private Claims extractAllClaims(String token) {
        return Jwts.parser()
                .verifyWith(getSigningKey())
                .build()
                .parseSignedClaims(token)
                .getPayload();
    }

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

    private boolean isTokenExpired(String token) {
        return extractExpiration(token).before(new Date());
    }

    private SecretKey getSigningKey() {
        byte[] keyBytes = Decoders.BASE64.decode(secretKey);
        return Keys.hmacShaKeyFor(keyBytes);
    }
}

JWT Authentication Filter

Example

Security Configuration with JWT

java example

@Configuration
@EnableWebSecurity
@RequiredArgsConstructor
public class SecurityConfig {

    private final JwtAuthenticationFilter jwtFilter;
    private final UserDetailsService userDetailsService;

    @Bean
    public SecurityFilterChain securityFilterChain(HttpSecurity http) throws Exception {
        http
            .csrf(csrf -> csrf.disable())
            .sessionManagement(session -> session
                    .sessionCreationPolicy(SessionCreationPolicy.STATELESS))
            .authorizeHttpRequests(auth -> auth
                    .requestMatchers("/api/auth/**").permitAll()
                    .requestMatchers("/api/admin/**").hasRole("ADMIN")
                    .anyRequest().authenticated())
            .addFilterBefore(jwtFilter, UsernamePasswordAuthenticationFilter.class);

        return http.build();
    }

    @Bean
    public AuthenticationManager authenticationManager(AuthenticationConfiguration config) throws Exception {
        return config.getAuthenticationManager();
    }

    @Bean
    public AuthenticationProvider authenticationProvider() {
        DaoAuthenticationProvider provider = new DaoAuthenticationProvider();
        provider.setUserDetailsService(userDetailsService);
        provider.setPasswordEncoder(passwordEncoder());
        return provider;
    }

    @Bean
    public PasswordEncoder passwordEncoder() {
        return new BCryptPasswordEncoder(12);
    }
}

Auth Controller

java example

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

    private final AuthenticationManager authManager;
    private final JwtService jwtService;
    private final UserDetailsService userDetailsService;
    private final UserRepository userRepository;
    private final PasswordEncoder passwordEncoder;

    @PostMapping("/register")
    public ResponseEntity<?> register(@RequestBody @Valid RegisterRequest request) {
        if (userRepository.existsByEmail(request.getEmail())) {
            return ResponseEntity.badRequest().body(Map.of("error", "Email already exists"));
        }

        User user = User.builder()
                .name(request.getName())
                .email(request.getEmail())
                .password(passwordEncoder.encode(request.getPassword()))
                .role(Role.USER)
                .build();
        userRepository.save(user);

        UserDetails userDetails = userDetailsService.loadUserByUsername(user.getEmail());
        String accessToken = jwtService.generateAccessToken(userDetails);
        String refreshToken = jwtService.generateRefreshToken(userDetails);

        return ResponseEntity.status(HttpStatus.CREATED).body(Map.of(
                "accessToken", accessToken,
                "refreshToken", refreshToken,
                "message", "Registration successful"
        ));
    }

    @PostMapping("/login")
    public ResponseEntity<?> login(@RequestBody LoginRequest request) {
        try {
            authManager.authenticate(
                    new UsernamePasswordAuthenticationToken(request.getEmail(), request.getPassword()));

            UserDetails userDetails = userDetailsService.loadUserByUsername(request.getEmail());
            String accessToken = jwtService.generateAccessToken(userDetails);
            String refreshToken = jwtService.generateRefreshToken(userDetails);

            return ResponseEntity.ok(Map.of(
                    "accessToken", accessToken,
                    "refreshToken", refreshToken
            ));
        } catch (AuthenticationException e) {
            return ResponseEntity.status(HttpStatus.UNAUTHORIZED)
                    .body(Map.of("error", "Invalid email or password"));
        }
    }

    @PostMapping("/refresh")
    public ResponseEntity<?> refreshToken(@RequestBody Map<String, String> request) {
        String refreshToken = request.get("refreshToken");
        String username = jwtService.extractUsername(refreshToken);
        UserDetails userDetails = userDetailsService.loadUserByUsername(username);

        if (jwtService.isTokenValid(refreshToken, userDetails)) {
            String newAccessToken = jwtService.generateAccessToken(userDetails);
            return ResponseEntity.ok(Map.of("accessToken", newAccessToken));
        }
        return ResponseEntity.status(HttpStatus.UNAUTHORIZED)
                .body(Map.of("error", "Invalid refresh token"));
    }
}

application.properties

properties example

jwt.secret=dGhpcyBpcyBhIHZlcnkgc2VjdXJlIHNlY3JldCBrZXkgZm9yIGp3dA==
jwt.expiration=900000
jwt.refresh-expiration=604800000

Interview Key Points

JWT = Header.Payload.Signature (Base64URL encoded)
JWT is stateless — no server-side session storage needed
Access tokens are short-lived (15 min); refresh tokens are long-lived (7 days)
OncePerRequestFilter ensures JWT filter runs once per request
JWT filter extracts token → validates → sets SecurityContext
SecurityContextHolder holds current authenticated user
Never store sensitive data in JWT payload (it's Base64 encoded, not encrypted)
Use HTTPS always — JWT can be stolen if transmitted over HTTP

Summary

In this chapter, we learned about JWT Authentication in detail. Here are the key points:

Basic introduction to the concept and why it is needed
Syntax and structure with complete examples
Internal working and best practices
Real-world applications and use cases
Common mistakes and how to avoid them
How this topic is asked in interviews

To practice these concepts, write and run the code in your IDE and verify the output. Modify each example and experiment so that you deeply understand the concept.

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for JWT Authentication — Java Programming.

Interview Use

Prepare one clear explanation, one practical example, and one common mistake for this Java Master Course topic.

Search Terms

java-master-course, java master course, java, master, course, spring, framework, boot

Continue learning this concept

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