How to pass information from ConstraintValidator to Controller?

Question

I have a custom ConstraintValidator.

public class UrlValidator implements ConstraintValidator<ValidUrl, RemoteFile> {

  @Override
  public boolean isValid(RemoteFile value, ConstraintValidatorContext context) {
    return true;
  }
}

The RemoteFile is a very simple DTO.

public record RemoteFile(String url) {}

I'm using UrlValidator with my custom annotation.

@Documented
@Target(ElementType.PARAMETER)
@Retention(RetentionPolicy.RUNTIME)
@Constraint(validatedBy = UrlValidator.class)
public @interface ValidUrl {
  String message() default "invalid file";

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

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

Finally I'm using the annotation inside my controller.

  @PostMapping("/hello")
  public String remote(@ValidUrl @RequestBody RemoteFile body) {
    // do stuff inside the controller
  }

I'd like to pass some information from my validator to my controller. I found a way to do this but I was wondering why the other way does not work.

Here is what works.

  @Override
  public boolean isValid(RemoteFile value, ConstraintValidatorContext context) {
    RequestContextHolder.getRequestAttributes()
        .setAttribute("key", "value", RequestAttributes.SCOPE_REQUEST);

    return true;
  }

I can afterwards access the information in my controller.

  @PostMapping("/hello")
  public String remote(@ValidUrl @RequestBody RemoteFile body) {

    var value =
        (String)
            RequestContextHolder.getRequestAttributes()
                .getAttribute("key", RequestAttributes.SCOPE_REQUEST);

    // do stuff inside the controller
  }

Then I tried something different because I thought there is a simpler way. However this DOES not work and I do not understand why. Use the HttpServletRequest and set an attribute.

  @Autowired HttpServletRequest request;


  @Override
  public boolean isValid(RemoteFile value, ConstraintValidatorContext context) {

    request.setAttribute("key", "value");

    return true;
  }

Use the attribute directly in the controller method.

  @PostMapping("/remote")
  public String remote(
    @ValidUrl @RequestBody RemoteFile body,
    @RequestAttribute String key
  ) {
    // do stuff in the controller
  }

However I'm always getting the error message

Resolved [org.springframework.web.bind.ServletRequestBindingException: Missing request attribute 'key' of type String]

I do not understand the error message. The attribute should be there because the validator should have set it. Any ideas? I'm lost!