How can I decorate the REST response in Spring (Boot)?

Question

I've got a Spring Boot application that returns various objects that get encoded as JSON responses and I'd like to post-process them and add information to certain super classes.

Is there a way to filter, intercept, etc. the object responses from my REST endpoints before they get encoded to JSON with Jackson.

A filter won't work since it operates at the HttpServlet{Request,Response} level.

Answer 1

I guess ResponseBodyAdvice is your friend. Basically it:

Allows customizing the response after the execution of an @ResponseBody or a ResponseEntity controller method but before the body is written with an HttpMessageConverter. Implementations may be may be registered directly with RequestMappingHandlerAdapter and ExceptionHandlerExceptionResolver or more likely annotated with @ControllerAdvice in which case they will be auto-detected by both.

Here i'm intercepting all the returned Strings and make them uppercase:

@ControllerAdvice
public class MyResponseBodyAdvisor implements ResponseBodyAdvice<String> {
    @Override
    public boolean supports(MethodParameter returnType, Class<? extends HttpMessageConverter<?>> converterType) {
        return returnType.getParameterType().equals(String.class);
    }

    @Override
    public String beforeBodyWrite(String body, MethodParameter returnType, MediaType selectedContentType, Class<? extends HttpMessageConverter<?>> selectedConverterType, ServerHttpRequest request, ServerHttpResponse response) {
        return body.toUpperCase();
    }
}