عرض عملي: كتابة قيد مخصص @ValidSku

خطوة بخطوة

1. Define the SKU format (for example three letters, a dash, four digits: ABC-1234).
2. Write the @ValidSku annotation and SkuValidator.
3. Put @ValidSku on CreateProductRequest.sku.
4. Planned mistake: forget @Valid on the controller parameter; a bad SKU gets through. Fix: add @Valid.
5. Post a bad SKU and see a 400; post a good one and see success.