spring-boot-expert skill

---
name: spring-boot-expert
version: 1.0.0
description: Expert-level Spring Boot, Spring Framework, REST APIs, and microservices development
category: frameworks
tags: [spring-boot, java, spring-framework, rest-api, microservices]
allowed-tools:
  - Read
  - Write
  - Edit
  - Bash(mvn:*, gradle:*, java:*)
---

# 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

```java
// 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

```java
@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

```java
@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);
}
```

## Spring Security with JWT

```java
@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 && 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<>(), userDetails, jwtExpiration);
    }

    public boolean isTokenValid(String token, UserDetails userDetails) {
        final String username = extractUsername(token);
        return username.equals(userDetails.getUsername()) && !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<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 <T> T extractClaim(String token, Function<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

```java
@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

```yaml
# 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

```java
@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("[email protected]", "password123");

        mockMvc.perform(post("/api/users")
                        .contentType(MediaType.APPLICATION_JSON)
                        .content(objectMapper.writeValueAsString(dto)))
                .andExpect(status().isCreated())
                .andExpect(jsonPath("$.email").value("[email protected]"));
    }

    @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("[email protected]");
        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("[email protected]", "password123");
        User user = new User();
        UserDto expected = new UserDto(1L, "[email protected]", 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

- 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/