spring-boot-rest-api-standards

What this skill does

# 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

```java
@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

```java
// 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

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

### 2. Prefer Immutable DTOs (Java Records or `@Value`)
```java
public record UserResponse(Long id, String name, String email) {}
```

### 3. Implement Proper Transaction Management
```java
@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