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<User> findRecentUsers(@Param("date") LocalDateTime date);
}

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

Exception Handling

@RestControllerAdvice
public class GlobalExceptionHandler {

    @ExceptionHandler(ResourceNotFoundException.class)
    public ResponseEntity<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<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<ValidationErrorResponse> handleValidation(
            MethodArgumentNotValidException ex
    ) {
        Map<String, String> errors = new HashMap<>();
        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<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

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

Reference Documentation

Detailed material lives alongside this skill and is read on demand:

• Spring Security with JWT
• Testing

Resources

• Spring Boot Documentation: https://spring.io/projects/spring-boot
• Spring Framework: https://spring.io/projects/spring-framework
• Spring Data JPA: https://spring.io/projects/spring-data-jpa
• Spring Security: https://spring.io/projects/spring-security
• Baeldung: https://www.baeldung.com/