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Integration with Other Layers

The Integration responsibility describes how the Controller Layer connects to downstream layers in the application.

Layered Architecture Integration

Controller Layer (You are here)
       ↓
Service Layer (Business Logic)
       ↓
Repository Layer (Data Access)
       ↓
Database

Interaction Pattern

1. Controller Receives Request

@PostMapping
public ResponseEntity<SupplierDTO> create(@Valid @RequestBody CreateSupplierDTO dto) {
    // Request received, DTO validated

2. Controller Delegates to Service

The controller calls service methods but never implements business logic:

    // NO business logic here
    SupplierDTO created = supplierService.create(dto);  // Delegate

3. Service Performs Validation

The service layer validates business rules:

// In SupplierService
@Transactional
public SupplierDTO create(CreateSupplierDTO dto) {
    // Business validation
    supplierValidator.validateRequiredFields(dto);
    supplierValidator.validateUniquenessOnCreate(dto.getName());
    
    // Convert DTO → Entity
    Supplier entity = mapper.toEntity(dto);

4. Service Calls Repository

The service persists data via repository methods:

    // Persist to database
    Supplier saved = repository.save(entity);
    
    // Return DTO to controller
    return mapper.toDTO(saved);
}

5. Controller Builds HTTP Response

The controller receives the result and wraps it in a response:

    // Build HTTP response
    URI location = ServletUriComponentsBuilder.fromCurrentRequest()
        .path("/{id}")
        .buildAndExpand(created.getId())
        .toUri();
    
    return ResponseEntity.created(location).body(created);  // 201 CREATED
}

Data Flow Diagram

Client (JSON)
    ↓
Controller
    ├─ Receives HTTP request
    ├─ Validates with @Valid
    ├─ Checks authorization with @PreAuthorize
    └─ Delegates to service
    ↓
Service Layer
    ├─ Validates business rules
    ├─ Transforms DTO → Entity
    ├─ Calls repository
    └─ Transforms Entity → DTO
    ↓
Repository Layer
    ├─ Executes SQL/JPQL queries
    └─ Returns entities
    ↓
Database
    └─ Persists/retrieves data
    ↑
Entity is returned through layers
    ↓
Controller (DTO)
    └─ Returns HTTP response (JSON)

Responsibility Boundaries

Controller DOES: - ✅ Handle HTTP concerns (routing, status codes) - ✅ Validate input with @Valid - ✅ Check authorization with @PreAuthorize - ✅ Convert DTO ↔︎ JSON - ✅ Call service methods

Controller DOES NOT: - ❌ Implement business logic - ❌ Access repositories directly - ❌ Connect to databases - ❌ Validate business rules - ❌ Manage transactions

Service DOES: - ✅ Implement business logic - ✅ Validate business rules - ✅ Manage transactions - ✅ Call repositories - ✅ Handle exceptions

Repository DOES: - ✅ Execute database queries - ✅ Manage entities - ✅ Handle pagination/sorting - ✅ Translate to domain exceptions

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