Agent Skills: Spring Boot REST API Standards

Spring Boot REST API Standards

Overview

REST API design standards for Spring Boot covering URL design, HTTP methods, status codes, DTOs, validation, error handling, pagination, and security headers.

When to Use

• Creating REST endpoints and API routes
• Designing DTOs and API contracts
• Implementing error handling and validation
• Setting up pagination and filtering
• Configuring security headers and CORS
• Reviewing REST API architecture

Instructions

To Build RESTful API Endpoints

Follow these steps to create well-designed REST API endpoints:

1. Design Resource-Based URLs

   • Use plural nouns for resource names
   • Follow REST conventions: GET /users, POST /users, PUT /users/{id}
   • Avoid action-based URLs like /getUserList

2. Implement Proper HTTP Methods

   • GET: Retrieve resources (safe, idempotent)
   • POST: Create resources (not idempotent)
   • PUT: Replace entire resources (idempotent)
   • PATCH: Partial updates (not idempotent)
   • DELETE: Remove resources (idempotent)

3. Use Appropriate Status Codes

   • 200 OK: Successful GET/PUT/PATCH
   • 201 Created: Successful POST with Location header
   • 204 No Content: Successful DELETE
   • 400 Bad Request: Invalid request data
   • 404 Not Found: Resource doesn't exist
   • 409 Conflict: Duplicate resource
   • 500 Internal Server Error: Unexpected errors

4. Create Request/Response DTOs

   • Separate API contracts from domain entities
   • Use Java records or Lombok @Data/@Value
   • Apply Jakarta validation annotations
   • Keep DTOs immutable when possible

5. Implement Validation

   • Use @Valid annotation on @RequestBody parameters
   • Apply validation constraints (@NotBlank, @Email, @Size, etc.)
   • Handle validation errors with MethodArgumentNotValidException

6. Set Up Error Handling

   • Use @RestControllerAdvice for global exception handling
   • Return standardized error responses with status, error, message, and timestamp
   • Use ResponseStatusException for specific HTTP status codes

7. Configure Pagination

   • Use Pageable for large datasets
   • Include page, size, sort parameters
   • Return metadata with total elements, totalPages, etc.

8. Add Security Headers

   • Configure CORS policies
   • Set content security policy
   • Include X-Frame-Options, X-Content-Type-Options

Validation checkpoints:

• After step 1-2: Verify URL structure follows REST conventions (/users not /getUsers)
• After step 3: Test each endpoint returns correct status codes
• After step 4-5: Validate DTOs with curl or HTTPie before proceeding
• After step 6: Confirm error responses match standardized format

Examples

Basic CRUD Controller

@RestController
@RequestMapping("/v1/users")
@RequiredArgsConstructor
@Slf4j
public class UserController {
    private final UserService userService;

    @GetMapping
    public ResponseEntity<Page<UserResponse>> getAllUsers(
            @RequestParam(defaultValue = "0") int page,
            @RequestParam(defaultValue = "10") int pageSize) {
        log.debug("Fetching users page {} size {}", page, pageSize);
        Page<UserResponse> users = userService.getAll(page, pageSize);
        return ResponseEntity.ok(users);
    }

    @GetMapping("/{id}")
    public ResponseEntity<UserResponse> getUserById(@PathVariable Long id) {
        return ResponseEntity.ok(userService.getById(id));
    }

    @PostMapping
    public ResponseEntity<UserResponse> createUser(@Valid @RequestBody CreateUserRequest request) {
        UserResponse created = userService.create(request);
        return ResponseEntity.status(HttpStatus.CREATED).body(created);
    }

    @PutMapping("/{id}")
    public ResponseEntity<UserResponse> updateUser(
            @PathVariable Long id,
            @Valid @RequestBody UpdateUserRequest request) {
        return ResponseEntity.ok(userService.update(id, request));
    }

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

Request/Response DTOs

// Request DTO
@Data
@NoArgsConstructor
@AllArgsConstructor
public class CreateUserRequest {
    @NotBlank(message = "User name cannot be blank")
    private String name;

    @Email(message = "Valid email required")
    private String email;
}

// Response DTO
@Data
@NoArgsConstructor
@AllArgsConstructor
public class UserResponse {
    private Long id;
    private String name;
    private String email;
    private LocalDateTime createdAt;
}

Global Exception Handler

@RestControllerAdvice
@Slf4j
public class GlobalExceptionHandler {

    @ExceptionHandler(MethodArgumentNotValidException.class)
    public ResponseEntity<ErrorResponse> handleValidationException(
            MethodArgumentNotValidException ex, WebRequest request) {
        String errors = ex.getBindingResult().getFieldErrors().stream()
                .map(f -> f.getField() + ": " + f.getDefaultMessage())
                .collect(Collectors.joining(", "));

        ErrorResponse errorResponse = new ErrorResponse(
                HttpStatus.BAD_REQUEST.value(),
                "Validation Error",
                "Validation failed: " + errors,
                request.getDescription(false).replaceFirst("uri=", "")
        );
        return new ResponseEntity<>(errorResponse, HttpStatus.BAD_REQUEST);
    }

    @ExceptionHandler(ResponseStatusException.class)
    public ResponseEntity<ErrorResponse> handleResponseStatusException(
            ResponseStatusException ex, WebRequest request) {
        ErrorResponse error = new ErrorResponse(
            ex.getStatusCode().value(),
            ex.getStatusCode().toString(),
            ex.getReason(),
            request.getDescription(false).replaceFirst("uri=", "")
        );
        return new ResponseEntity<>(error, ex.getStatusCode());
    }
}

Best Practices

1. Use Constructor Injection

@Service
@RequiredArgsConstructor
public class UserService {
    private final UserRepository userRepository;
}

2. Prefer Immutable DTOs (Java Records or @Value)

public record UserResponse(Long id, String name, String email) {}

3. Implement Proper Transaction Management

@Service
@Transactional
public class UserService {
    @Transactional(readOnly = true)
    public Optional<User> findById(Long id) { return userRepository.findById(id); }

    @Transactional
    public User create(User user) { return userRepository.save(user); }
}

Constraints and Warnings

1. Never expose entities directly - Use DTOs to separate API contracts from domain models
2. Follow REST conventions - Use nouns for resources (/users), correct HTTP methods, plural names, proper status codes
3. Handle all exceptions globally - Use @RestControllerAdvice, never let raw exceptions bubble up
4. Always paginate large result sets - Prevent performance issues and DDoS vulnerabilities
5. Validate all input data - Use Jakarta validation annotations on request DTOs
6. Never expose sensitive data - Don't log or expose passwords, tokens, PII

References

• See references/ directory for comprehensive reference material including HTTP status codes, Spring annotations, and detailed examples
• Refer to the developer-kit-java:spring-boot-code-review-expert agent for code review guidelines
• Review spring-boot-dependency-injection/SKILL.md for dependency injection patterns
• Check ../spring-boot-test-patterns/SKILL.md for testing REST APIs