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Validation Layer
The Validation Layer enforces business rules and data constraints. Validation occurs at multiple levels: DTO fields, service business rules, and database constraints.
Custom Validators
Domain-specific validation logic:
@Component
public class SupplierValidator {
private final SupplierRepository repository;
public void validateUniquenessOnCreate(String name) {
if (repository.existsByNameIgnoreCase(name)) {
throw new IllegalStateException(
"Supplier with name '" + name + "' already exists"
);
}
}
public void validateRequiredFields(CreateSupplierDTO dto) {
if (dto.getName() == null || dto.getName().isBlank()) {
throw new IllegalArgumentException("Supplier name is required");
}
if (dto.getContactName() == null || dto.getContactName().isBlank()) {
throw new IllegalArgumentException("Contact name is required");
}
}
}@Component
public class InventoryItemValidator {
private final InventoryItemRepository itemRepository;
private final SupplierRepository supplierRepository;
public void validateBeforeCreate(CreateInventoryItemDTO dto) {
if (!supplierRepository.existsById(dto.getSupplierId())) {
throw new IllegalArgumentException(
"Supplier with ID '" + dto.getSupplierId() + "' not found"
);
}
if (itemRepository.existsByNameIgnoreCase(dto.getName())) {
throw new IllegalStateException(
"Item with name '" + dto.getName() + "' already exists"
);
}
if (dto.getInitialQuantity() < 0) {
throw new IllegalArgumentException(
"Initial quantity cannot be negative"
);
}
}
}Validation Flow
HTTP Request
↓
Controller with @Valid
↓
DTO Field Validation (@NotNull, @NotBlank, etc.)
↓ Valid?
├─ No → Validation Error Response (400)
└─ Yes → Service Layer
↓
Custom Validator Checks
↓ Business Rules Met?
├─ No → Exception → Error Response
└─ Yes → Execute Operation
↓
Database Constraints
↓ Valid?
├─ No → Constraint Violation
└─ Yes → Success