Add custom header to REST PUT request - spring-boot

Question

I have an application that uses Spring Boot 2.1.8 with an Angular front end. I have defined my repositories using #RepositoryRestResource. The Spring Boot app basically provides a REST API for the Angular piece. The application defines several business entities and the REST respo provides CRUD functionality for each entity. Here is a typical repo interface for one of those entities:
// REST resouce /api/privgroups
#RepositoryRestResource(collectionResourceRel = "privgroups", path = "privgroups")
public interface PrivGroupRepository extends CrudRepository<PrivGroup, Long>
{
List<PrivGroup> findAll();
}
GETing and single entity, POSTing (creating an entity) and PUTing (updating an entity) are all working fine, but I would like to return a custom header when updating (HTTP PUT) an entity. The header would be consumed by the Angular side to display a custom toastr message specific to that entity. Since the repositories also implement the REST interface I am unsure how to add a specific header that would change based on the target entity.
I have developed applications that include a REST controller that calls against a service which in turn calls against a repository. In this case, I have more control and can easily return custom headers like so:
#PutMapping("/{id}")
public ResponseEntity<MyEntity> updateMyEntity(#PathVariable("id") Long id, #RequestBody MyEntity myEntity)
{
MyEntity updatedEntity = this.MyEntityService.updateMyEntity(MyEntity);
return ResponseEntity.ok()
.header("X-Save", "MyEntity")
.body(updatedEntity);
}
Is there a newer "built-in" technique I can use to achieve this? I know I can add headers using a filter and I've read a couple posts on the subject. I think it would be difficult to identify the entity being updated and I'm unsure this is the best approach.
NOTE that this post:
Rest API - how add custom headers?
is really old.
The spring data rest docs https://docs.spring.io/spring-data/rest/docs/current/reference/html/
don't have anything specific on the subject.

Answer 1

I have used Spring Data Rest recently and I didn't find any "built-in" technique for that. However you can achieve that by implementing the ResponseBodyAdvice interface in a class annotated with #ControllerAdvice. This is how I got it:
#ControllerAdvice
public class PutMyEntityBodyAdvice implements ResponseBodyAdvice<Object> {
#Override
public boolean supports(MethodParameter returnType, Class<? extends HttpMessageConverter<?>> converterType) {
// Write the condition to check if beforyBodyWrite should be called
return returnType.getParameterType().equals(ResponseEntity.class);
}
#Override
public Object beforeBodyWrite(Object object, MethodParameter returnType, MediaType selectedContentType,
Class<? extends HttpMessageConverter<?>> selectedConverterType,
ServerHttpRequest request, ServerHttpResponse response) {
// Check if it is a PUT request for MyEntity object
if (HttpMethod.PUT.equals(request.getMethod()) && object instanceof PersistentEntityResource) {
PersistentEntityResource persistentEntityResource = (PersistentEntityResource) object;
if (persistentEntityResource.getContent().getClass().equals(MyEntity.class)) {
// Add custom header or manipulate response object
response.getHeaders().add("X-Save", "MyEntity");
}
}
return object;
}
}

Answer 2

The answer by #pepevalbe looks promising.
An alternative might be - as you suggested - using a standard Servlet Filter or Spring HandlerInterceptor.
To address the issue of getting a reference to the modified entity in that case you could register a Spring Data Rest event listener that would simply store a reference to the modified entity in ThreadLocal storage from where it could be retrieved in the Filter or HandlerInterceptor.
https://www.baeldung.com/java-threadlocal
https://docs.spring.io/spring-data/rest/docs/current/reference/html/#events.application-listener