```
@Documented
@Target(ANNOTATION_TYPE)
@Retention(RUNTIME)
public @interface Constraint
```
Marks an annotation as being a Bean 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 })
 @Retention(RUNTIME)
 public @interface OrderNumber {
     String message() default "{com.acme.constraint.OrderNumber.message}";
     Class[] groups() default {};
     Class[] payload() default {};
 }
 
```

Required Element Summary

Required Elements
Modifier and Type	Required Element	Description
`java.lang.Class<? extends ConstraintValidator<?,?>>[]`	`validatedBy`	`ConstraintValidator` classes must reference distinct target types for a given `ValidationTarget` If two `ConstraintValidator`s refer to the same type, an exception will occur.

- Element Detail
  - validatedBy
```
java.lang.Class<? extends ConstraintValidator<?,?>>[] validatedBy
```
    ConstraintValidator classes must reference distinct target types for a given ValidationTarget If two ConstraintValidators refer to the same type, an exception will occur.
    At most one ConstraintValidator targeting the array of parameters of methods or constructors (aka cross-parameter) is accepted. If two or more are present, an exception will occur.
    
    Returns:
    
    array of (@code ConstraintValidator} classes implementing the constraint

Annotation Type Constraint

Required Element Summary

Element Detail

validatedBy