Validation: fail fast at the edge

Garbage in, structured rejection out

A controller is a boundary. Inputs should be validated before they reach service code — that way the service has already been handed clean domain values and can focus on business rules. Spring's Bean Validation integration (@Valid + JSR-380 annotations) makes this almost free.

record CreateInvoiceRequest(
    @NotBlank String customerName,
    @Email    String email,
    @NotNull @Positive Long amountCents,
    @NotBlank String currency
) {}

@PostMapping
ResponseEntity<InvoiceDto> create(@Valid @RequestBody CreateInvoiceRequest req) { /* ... */ }

Translate MethodArgumentNotValidException once

Write a single @ControllerAdvice that converts validation failures into your standard error response shape. Clients get consistent, actionable messages with field-level detail; your controllers remain a handful of lines each.