spring-boot-expert

Expert guidance for Spring Boot development, Spring Framework, building REST APIs, and microservices architecture.

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Install skill "spring-boot-expert" with this command: npx skills add personamanagmentlayer/pcl/personamanagmentlayer-pcl-spring-boot-expert

Spring Boot Expert

Expert guidance for Spring Boot development, Spring Framework, building REST APIs, and microservices architecture.

Core Concepts

Spring Boot Fundamentals

  • Auto-configuration

  • Dependency injection

  • Spring Boot Starters

  • Application properties

  • Profiles and configuration

  • Spring Boot Actuator

Spring Framework

  • Spring Core (IoC, DI)

  • Spring Data JPA

  • Spring Security

  • Spring Web MVC

  • Spring AOP

  • Spring Transaction Management

Microservices

  • Service discovery

  • API Gateway

  • Circuit breakers

  • Distributed tracing

  • Configuration management

Spring Boot Application

// Main application class @SpringBootApplication @EnableJpaAuditing public class Application { public static void main(String[] args) { SpringApplication.run(Application.class, args); } }

// Entity with JPA @Entity @Table(name = "users") @EntityListeners(AuditingEntityListener.class) public class User { @Id @GeneratedValue(strategy = GenerationType.IDENTITY) private Long id;

@Column(nullable = false, unique = true)
private String email;

@Column(nullable = false)
private String password;

@CreatedDate
@Column(nullable = false, updatable = false)
private LocalDateTime createdAt;

@LastModifiedDate
private LocalDateTime updatedAt;

@OneToMany(mappedBy = "author", cascade = CascadeType.ALL)
private List<Post> posts = new ArrayList<>();

// Getters and setters

}

@Entity @Table(name = "posts") public class Post { @Id @GeneratedValue(strategy = GenerationType.IDENTITY) private Long id;

@Column(nullable = false)
private String title;

@Column(columnDefinition = "TEXT")
private String content;

@ManyToOne(fetch = FetchType.LAZY)
@JoinColumn(name = "user_id", nullable = false)
private User author;

@CreatedDate
private LocalDateTime createdAt;

// Getters and setters

}

REST API Controller

@RestController @RequestMapping("/api/users") @RequiredArgsConstructor public class UserController {

private final UserService userService;

@GetMapping
public ResponseEntity<Page<UserDto>> getUsers(
        @RequestParam(defaultValue = "0") int page,
        @RequestParam(defaultValue = "20") int size,
        @RequestParam(defaultValue = "id") String sortBy,
        @RequestParam(defaultValue = "ASC") Sort.Direction direction
) {
    Pageable pageable = PageRequest.of(page, size, Sort.by(direction, sortBy));
    Page<UserDto> users = userService.findAll(pageable);
    return ResponseEntity.ok(users);
}

@GetMapping("/{id}")
public ResponseEntity<UserDto> getUser(@PathVariable Long id) {
    return userService.findById(id)
            .map(ResponseEntity::ok)
            .orElse(ResponseEntity.notFound().build());
}

@PostMapping
public ResponseEntity<UserDto> createUser(
        @Valid @RequestBody UserCreateDto userDto
) {
    UserDto created = userService.create(userDto);
    URI location = ServletUriComponentsBuilder
            .fromCurrentRequest()
            .path("/{id}")
            .buildAndExpand(created.getId())
            .toUri();

    return ResponseEntity.created(location).body(created);
}

@PutMapping("/{id}")
public ResponseEntity<UserDto> updateUser(
        @PathVariable Long id,
        @Valid @RequestBody UserUpdateDto userDto
) {
    return userService.update(id, userDto)
            .map(ResponseEntity::ok)
            .orElse(ResponseEntity.notFound().build());
}

@DeleteMapping("/{id}")
public ResponseEntity<Void> deleteUser(@PathVariable Long id) {
    if (userService.delete(id)) {
        return ResponseEntity.noContent().build();
    }
    return ResponseEntity.notFound().build();
}

}

// DTOs with validation public record UserDto( Long id, String email, LocalDateTime createdAt ) {}

public record UserCreateDto( @NotBlank @Email String email, @NotBlank @Size(min = 8) String password ) {}

public record UserUpdateDto( @Email String email ) {}

Service Layer

@Service @RequiredArgsConstructor @Transactional(readOnly = true) public class UserService {

private final UserRepository userRepository;
private final PasswordEncoder passwordEncoder;
private final UserMapper userMapper;

public Page<UserDto> findAll(Pageable pageable) {
    return userRepository.findAll(pageable)
            .map(userMapper::toDto);
}

public Optional<UserDto> findById(Long id) {
    return userRepository.findById(id)
            .map(userMapper::toDto);
}

public Optional<UserDto> findByEmail(String email) {
    return userRepository.findByEmail(email)
            .map(userMapper::toDto);
}

@Transactional
public UserDto create(UserCreateDto dto) {
    if (userRepository.existsByEmail(dto.email())) {
        throw new DuplicateEmailException("Email already exists");
    }

    User user = new User();
    user.setEmail(dto.email());
    user.setPassword(passwordEncoder.encode(dto.password()));

    User saved = userRepository.save(user);
    return userMapper.toDto(saved);
}

@Transactional
public Optional<UserDto> update(Long id, UserUpdateDto dto) {
    return userRepository.findById(id)
            .map(user -> {
                if (dto.email() != null) {
                    user.setEmail(dto.email());
                }
                return userMapper.toDto(user);
            });
}

@Transactional
public boolean delete(Long id) {
    if (userRepository.existsById(id)) {
        userRepository.deleteById(id);
        return true;
    }
    return false;
}

}

// Repository @Repository public interface UserRepository extends JpaRepository<User, Long> { Optional<User> findByEmail(String email); boolean existsByEmail(String email);

@Query("SELECT u FROM User u WHERE u.createdAt > :date")
List&#x3C;User> findRecentUsers(@Param("date") LocalDateTime date);

}

// Mapper with MapStruct @Mapper(componentModel = "spring") public interface UserMapper { UserDto toDto(User user); User toEntity(UserCreateDto dto); }

Spring Security with JWT

@Configuration @EnableWebSecurity @EnableMethodSecurity @RequiredArgsConstructor public class SecurityConfig {

private final JwtAuthenticationFilter jwtAuthFilter;
private final AuthenticationProvider authenticationProvider;

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

    return http.build();
}

}

@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 {
    final String authHeader = request.getHeader("Authorization");

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

    final String jwt = authHeader.substring(7);
    final String userEmail = jwtService.extractUsername(jwt);

    if (userEmail != null &#x26;&#x26; SecurityContextHolder.getContext().getAuthentication() == null) {
        UserDetails userDetails = userDetailsService.loadUserByUsername(userEmail);

        if (jwtService.isTokenValid(jwt, userDetails)) {
            UsernamePasswordAuthenticationToken authToken = new UsernamePasswordAuthenticationToken(
                    userDetails,
                    null,
                    userDetails.getAuthorities()
            );
            authToken.setDetails(new WebAuthenticationDetailsSource().buildDetails(request));
            SecurityContextHolder.getContext().setAuthentication(authToken);
        }
    }

    filterChain.doFilter(request, response);
}

}

@Service @RequiredArgsConstructor public class JwtService {

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

@Value("${jwt.expiration}")
private long jwtExpiration;

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

public String generateToken(UserDetails userDetails) {
    return buildToken(new HashMap&#x3C;>(), userDetails, jwtExpiration);
}

public boolean isTokenValid(String token, UserDetails userDetails) {
    final String username = extractUsername(token);
    return username.equals(userDetails.getUsername()) &#x26;&#x26; !isTokenExpired(token);
}

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

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

private String buildToken(
        Map&#x3C;String, Object> extraClaims,
        UserDetails userDetails,
        long expiration
) {
    return Jwts
            .builder()
            .setClaims(extraClaims)
            .setSubject(userDetails.getUsername())
            .setIssuedAt(new Date(System.currentTimeMillis()))
            .setExpiration(new Date(System.currentTimeMillis() + expiration))
            .signWith(getSignInKey(), SignatureAlgorithm.HS256)
            .compact();
}

private &#x3C;T> T extractClaim(String token, Function&#x3C;Claims, T> claimsResolver) {
    final Claims claims = extractAllClaims(token);
    return claimsResolver.apply(claims);
}

private Claims extractAllClaims(String token) {
    return Jwts
            .parserBuilder()
            .setSigningKey(getSignInKey())
            .build()
            .parseClaimsJws(token)
            .getBody();
}

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

}

Exception Handling

@RestControllerAdvice public class GlobalExceptionHandler {

@ExceptionHandler(ResourceNotFoundException.class)
public ResponseEntity&#x3C;ErrorResponse> handleNotFound(ResourceNotFoundException ex) {
    ErrorResponse error = new ErrorResponse(
            "NOT_FOUND",
            ex.getMessage(),
            LocalDateTime.now()
    );
    return ResponseEntity.status(HttpStatus.NOT_FOUND).body(error);
}

@ExceptionHandler(DuplicateEmailException.class)
public ResponseEntity&#x3C;ErrorResponse> handleDuplicateEmail(DuplicateEmailException ex) {
    ErrorResponse error = new ErrorResponse(
            "DUPLICATE_EMAIL",
            ex.getMessage(),
            LocalDateTime.now()
    );
    return ResponseEntity.status(HttpStatus.CONFLICT).body(error);
}

@ExceptionHandler(MethodArgumentNotValidException.class)
public ResponseEntity&#x3C;ValidationErrorResponse> handleValidation(
        MethodArgumentNotValidException ex
) {
    Map&#x3C;String, String> errors = new HashMap&#x3C;>();
    ex.getBindingResult().getFieldErrors().forEach(error ->
            errors.put(error.getField(), error.getDefaultMessage())
    );

    ValidationErrorResponse response = new ValidationErrorResponse(
            "VALIDATION_ERROR",
            "Request validation failed",
            errors,
            LocalDateTime.now()
    );

    return ResponseEntity.status(HttpStatus.BAD_REQUEST).body(response);
}

@ExceptionHandler(Exception.class)
public ResponseEntity&#x3C;ErrorResponse> handleGeneral(Exception ex) {
    ErrorResponse error = new ErrorResponse(
            "INTERNAL_ERROR",
            "An unexpected error occurred",
            LocalDateTime.now()
    );
    return ResponseEntity.status(HttpStatus.INTERNAL_SERVER_ERROR).body(error);
}

}

public record ErrorResponse( String code, String message, LocalDateTime timestamp ) {}

public record ValidationErrorResponse( String code, String message, Map<String, String> errors, LocalDateTime timestamp ) {}

Configuration

application.yml

spring: application: name: user-service

datasource: url: jdbc:postgresql://localhost:5432/mydb username: ${DB_USERNAME:postgres} password: ${DB_PASSWORD:postgres} driver-class-name: org.postgresql.Driver

jpa: hibernate: ddl-auto: validate show-sql: false properties: hibernate: format_sql: true dialect: org.hibernate.dialect.PostgreSQLDialect

flyway: enabled: true baseline-on-migrate: true

server: port: 8080 error: include-message: always include-binding-errors: always

jwt: secret: ${JWT_SECRET:your-secret-key-here} expiration: 3600000 # 1 hour

logging: level: root: INFO com.example: DEBUG

Testing

@SpringBootTest @AutoConfigureMockMvc @TestPropertySource(locations = "classpath:application-test.properties") class UserControllerTest {

@Autowired
private MockMvc mockMvc;

@Autowired
private ObjectMapper objectMapper;

@Autowired
private UserRepository userRepository;

@BeforeEach
void setUp() {
    userRepository.deleteAll();
}

@Test
void shouldCreateUser() throws Exception {
    UserCreateDto dto = new UserCreateDto("test@example.com", "password123");

    mockMvc.perform(post("/api/users")
                    .contentType(MediaType.APPLICATION_JSON)
                    .content(objectMapper.writeValueAsString(dto)))
            .andExpect(status().isCreated())
            .andExpect(jsonPath("$.email").value("test@example.com"));
}

@Test
void shouldGetUser() throws Exception {
    User user = createTestUser();

    mockMvc.perform(get("/api/users/{id}", user.getId()))
            .andExpect(status().isOk())
            .andExpect(jsonPath("$.email").value(user.getEmail()));
}

@Test
void shouldReturnNotFoundForInvalidId() throws Exception {
    mockMvc.perform(get("/api/users/999"))
            .andExpect(status().isNotFound());
}

private User createTestUser() {
    User user = new User();
    user.setEmail("test@example.com");
    user.setPassword("hashed-password");
    return userRepository.save(user);
}

}

// Service unit test @ExtendWith(MockitoExtension.class) class UserServiceTest {

@Mock
private UserRepository userRepository;

@Mock
private PasswordEncoder passwordEncoder;

@Mock
private UserMapper userMapper;

@InjectMocks
private UserService userService;

@Test
void shouldCreateUser() {
    UserCreateDto dto = new UserCreateDto("test@example.com", "password123");
    User user = new User();
    UserDto expected = new UserDto(1L, "test@example.com", LocalDateTime.now());

    when(userRepository.existsByEmail(dto.email())).thenReturn(false);
    when(passwordEncoder.encode(dto.password())).thenReturn("hashed");
    when(userRepository.save(any(User.class))).thenReturn(user);
    when(userMapper.toDto(user)).thenReturn(expected);

    UserDto result = userService.create(dto);

    assertNotNull(result);
    assertEquals(expected.email(), result.email());
    verify(userRepository).save(any(User.class));
}

}

Best Practices

  • Use constructor injection

  • Separate concerns (Controller/Service/Repository)

  • Implement proper exception handling

  • Use DTOs for API layer

  • Write comprehensive tests

  • Use database migrations (Flyway/Liquibase)

  • Implement security properly

  • Use profiles for different environments

  • Enable Spring Boot Actuator for monitoring

  • Use connection pooling

  • Implement caching where appropriate

  • Follow RESTful conventions

Anti-Patterns

❌ Field injection ❌ Business logic in controllers ❌ No exception handling ❌ Exposing entities directly ❌ Hardcoded configuration ❌ No transaction management ❌ Missing validation

Resources

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