Marks an annotation as being a Jakarta Validation constraint.

A given constraint annotation must be annotated by a @Constraint annotation which refers to its list of constraint validation implementations.

Each constraint annotation must host the following attributes:

• String message() default [...]; which should default to an error message key made of the fully-qualified class name of the constraint followed by .message. For example "{com.acme.constraints.NotSafe.message}"
• Class<?>[] groups() default {}; for user to customize the targeted groups
• Class<? extends Payload>[] payload() default {}; for extensibility purposes

When building a constraint that is both generic and cross-parameter, the constraint annotation must host the validationAppliesTo() property. A constraint is generic if it targets the annotated element and is cross-parameter if it targets the array of parameters of a method or constructor.

    ConstraintTarget validationAppliesTo() default ConstraintTarget.IMPLICIT;

This property allows the constraint user to choose whether the constraint targets the return type of the executable or its array of parameters. A constraint is both generic and cross-parameter if

• two kinds of ConstraintValidators are attached to the constraint, one targeting ValidationTarget#ANNOTATED_ELEMENT and one targeting ValidationTarget#PARAMETERS,
• or if a ConstraintValidator targets both ANNOTATED_ELEMENT and PARAMETERS.

Such dual constraints are rare. See SupportedValidationTarget for more info.

Here is an example of constraint definition:

@Documented
@Constraint(validatedBy = OrderNumberValidator.class)
@Target({ METHOD, FIELD, ANNOTATION_TYPE, CONSTRUCTOR, PARAMETER, TYPE_USE })
@Retention(RUNTIME)
public @interface OrderNumber {
    String message() default "{com.acme.constraint.OrderNumber.message}";
    Class<?>[] groups() default {};
    Class<? extends Payload>[] payload() default {};
}