package com.springmvc.validator;
...

@Target({METHOD, FIELD, ANNOTATION_TYPE})
@Retention(RUNTIME)
@Constraint(validatedBy=BookIdValidator.class)
public @interface BookId {
    String message() default "{BookId.NewBook.bookId}"; ➊
    Class<?>[] groups() default {};
    public abstract Class<? extends Payload>[] payload() default {};
}