Consistent form validation across a Spring Boot application is best achieved by centralizing rules in shared model classes and reusable validators rather than duplicating checks in each controller.
Key Points: • Declare standard constraints like @NotBlank and @Size directly on shared DTO or model fields so every endpoint using that type inherits the same rules. • Build a custom ConstraintValidator for cross-field or domain-specific logic that annotations alone can't express. • Apply @Valid consistently on controller parameters, and use @Validated at the service layer for method-level validation. • Centralize error handling with a @ControllerAdvice that converts validation failures into a uniform error response format. • Use validation groups when the same DTO needs different rules for create versus update flows.
Example: Instead of writing separate null and length checks in each of five different form controllers, the same AddressDto class carrying @NotBlank and @Pattern annotations is reused everywhere addresses are submitted, guaranteeing identical rules.
Interview Tip: A concise interview answer is:
"I keep validation consistent by putting standard constraint annotations on shared model classes instead of duplicating checks per controller, writing a custom ConstraintValidator for any cross-field logic, and centralizing error formatting in a @ControllerAdvice so every form gets the same rules and the same error response shape."