Constraints as Annotations

Jakarta Bean Validation lets a constraint live on the field it applies to, right next to the type — no separate validation function to write and keep in sync with the class.

java

record RideRequest(
    @NotBlank(message = "Pickup location is required") String pickup,
    @Min(value = 1, message = "Passengers must be at least 1") int passengers
) {}

Common constraint annotations
Annotation	Requires
`@NotNull`	Not `null` — says nothing about emptiness
`@NotBlank`	Not `null`, not empty, not only whitespace — for `String`
`@NotEmpty`	Not `null` and not empty — for a `String` or a collection
`@Min` / `@Max`	A numeric value within a bound
`@Size`	A `String`, collection or array within a length range
`@Pattern`	A `String` matching a regular expression

Every constraint here is just metadata on its own — nothing reads it or acts on it until the next card's annotation is added to the controller method itself.

Spring Boot
Input Validation