22.1.1 Using the Built-In Constraints to Make a New Constraint

Bean Validation includes several built-in constraints that can be combined to create new, reusable constraints. This can simplify constraint definition by allowing developers to define a custom constraint made up of several built-in constraints that may then be applied to component attributes with a single annotation.

@Pattern.List({
  @Pattern(regexp = "[a-z0-9!#$%&'*+/=?^_`{|}~-]+(?:\\."
    +"[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*"
    +"@(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?\\.)+[a-z0-9](?:[a-z0-9-]*[a-z0-9])?")
})
@Constraint(validatedBy = {})
@Documented
@Target({ElementType.METHOD,
    ElementType.FIELD,
    ElementType.ANNOTATION_TYPE,
    ElementType.CONSTRUCTOR,
    ElementType.PARAMETER})
@Retention(RetentionPolicy.RUNTIME)
public @interface Email {

    String message() default "{invalid.email}";

    Class<?>[] groups() default {};

    Class<? extends Payload>[] payload() default {};

    @Target({ElementType.METHOD,
        ElementType.FIELD,
        ElementType.ANNOTATION_TYPE,
        ElementType.CONSTRUCTOR,
        ElementType.PARAMETER})
    @Retention(RetentionPolicy.RUNTIME)
    @Documented
    @interface List {
        Email[] value();
    }
}

This custom constraint can then be applied to an attribute.

...
@Email
protected String email;
...

22.1.2 Removing Ambiguity in Constraint Targets

Custom constraints that can be applied to both return values and method parameters require a validationAppliesTo element to identify the target of the constraint.

@Constraint(validatedBy=MyConstraintValidator.class)
@Target({ METHOD, FIELD, TYPE, ANNOTATION_TYPE, CONSTRUCTOR, PARAMETER })
@Retention(RUNTIME)
public @interface MyConstraint {
  String message() default "{com.example.constraint.MyConstraint.message}";
  Class<?>[] groups() default {};
  ConstraintTarget validationAppliesTo() default ConstraintTarget.PARAMETERS;
...
}

This constraint sets the validationAppliesTo target by default to the method parameters.

@MyConstraint(validationAppliesTo=ConstraintTarget.RETURN_TYPE)
public String doSomething(String param1, String param2) { ... }

In the preceding example, the target is set to the return value of the method.