@Component
public class JwtAuthFilter extends OncePerRequestFilter {
  private final JwtService jwtService;
 
  public JwtAuthFilter(JwtService jwtService) {
    this.jwtService = jwtService;
  }
 
  @Override
  protected void doFilterInternal(HttpServletRequest request, HttpServletResponse response,
      FilterChain chain) throws ServletException, IOException {
    String header = request.getHeader(HttpHeaders.AUTHORIZATION);
    if (header != null && header.startsWith("Bearer ")) {
      String token = header.substring(7);
      Authentication auth = jwtService.authenticate(token);
      SecurityContextHolder.getContext().setAuthentication(auth);
    }
    chain.doFilter(request, response);
  }
}

@RestController
@RequestMapping("/api/admin")
public class AdminController {
 
  @PreAuthorize("hasRole('ADMIN')")
  @GetMapping("/users")
  public List<UserDto> listUsers() {
    return userService.findAll();
  }
 
  @PreAuthorize("@authz.isOwner(#id, authentication)")
  @DeleteMapping("/users/{id}")
  public ResponseEntity<Void> deleteUser(@PathVariable Long id) {
    userService.delete(id);
    return ResponseEntity.noContent().build();
  }
}

// BAD: No validation
@PostMapping("/users")
public User createUser(@RequestBody UserDto dto) {
  return userService.create(dto);
}
 
// GOOD: Validated DTO
public record CreateUserDto(
    @NotBlank @Size(max = 100) String name,
    @NotBlank @Email String email,
    @NotNull @Min(0) @Max(150) Integer age
) {}
 
@PostMapping("/users")
public ResponseEntity<UserDto> createUser(@Valid @RequestBody CreateUserDto dto) {
  return ResponseEntity.status(HttpStatus.CREATED)
      .body(userService.create(dto));
}

// BAD: String concatenation in native query
@Query(value = "SELECT * FROM users WHERE name = '" + name + "'", nativeQuery = true)
 
// GOOD: Parameterized native query
@Query(value = "SELECT * FROM users WHERE name = :name", nativeQuery = true)
List<User> findByName(@Param("name") String name);
 
// GOOD: Spring Data derived query (auto-parameterized)
List<User> findByEmailAndActiveTrue(String email);

@Bean
public PasswordEncoder passwordEncoder() {
  return new BCryptPasswordEncoder(12); // cost factor 12
}
 
// In service
public User register(CreateUserDto dto) {
  String hashedPassword = passwordEncoder.encode(dto.password());
  return userRepository.save(new User(dto.email(), hashedPassword));
}

http
  .csrf(csrf -> csrf.disable())
  .sessionManagement(sm -> sm.sessionCreationPolicy(SessionCreationPolicy.STATELESS));

# BAD: Hardcoded in application.yml
spring:
  datasource:
    password: mySecretPassword123
 
# GOOD: Environment variable placeholder
spring:
  datasource:
    password: ${DB_PASSWORD}
 
# GOOD: Spring Cloud Vault integration
spring:
  cloud:
    vault:
      uri: https://vault.example.com
      token: ${VAULT_TOKEN}

http
  .headers(headers -> headers
    .contentSecurityPolicy(csp -> csp
      .policyDirectives("default-src 'self'"))
    .frameOptions(HeadersConfigurer.FrameOptionsConfig::sameOrigin)
    .xssProtection(Customizer.withDefaults())
    .referrerPolicy(rp -> rp.policy(ReferrerPolicyHeaderWriter.ReferrerPolicy.NO_REFERRER)));

@Bean
public CorsConfigurationSource corsConfigurationSource() {
  CorsConfiguration config = new CorsConfiguration();
  config.setAllowedOrigins(List.of("https://app.example.com"));
  config.setAllowedMethods(List.of("GET", "POST", "PUT", "DELETE"));
  config.setAllowedHeaders(List.of("Authorization", "Content-Type"));
  config.setAllowCredentials(true);
  config.setMaxAge(3600L);
 
  UrlBasedCorsConfigurationSource source = new UrlBasedCorsConfigurationSource();
  source.registerCorsConfiguration("/api/**", config);
  return source;
}
 
// In SecurityFilterChain:
http.cors(cors -> cors.configurationSource(corsConfigurationSource()));

// Using Bucket4j for per-endpoint rate limiting
@Component
public class RateLimitFilter extends OncePerRequestFilter {
  private final Map<String, Bucket> buckets = new ConcurrentHashMap<>();
 
  private Bucket createBucket() {
    return Bucket.builder()
        .addLimit(Bandwidth.classic(100, Refill.intervally(100, Duration.ofMinutes(1))))
        .build();
  }
 
  @Override
  protected void doFilterInternal(HttpServletRequest request, HttpServletResponse response,
      FilterChain chain) throws ServletException, IOException {
    String clientIp = request.getRemoteAddr();
    Bucket bucket = buckets.computeIfAbsent(clientIp, k -> createBucket());
 
    if (bucket.tryConsume(1)) {
      chain.doFilter(request, response);
    } else {
      response.setStatus(HttpStatus.TOO_MANY_REQUESTS.value());
      response.getWriter().write("{\"error\": \"Rate limit exceeded\"}");
    }
  }
}