Cannot properly test ErrorController Spring Boot - spring

Question

due to this tutorial - https://www.baeldung.com/spring-boot-custom-error-page I wanted to customize my error page ie. when someone go to www.myweb.com/blablablalb3 I want to return page with text "wrong url request".
All works fine:
#Controller
public class ApiServerErrorController implements ErrorController {
#Override
public String getErrorPath() {
return "error";
}
#RequestMapping("/error")
public String handleError() {
return "forward:/error-page.html";
}
}
But I dont know how to test it:
#Test
public void makeRandomRequest__shouldReturnErrorPage() throws Exception {
this.mockMvc.perform(get(RANDOM_URL))
.andDo(print());
}
print() returns:
MockHttpServletResponse:
Status = 404
Error message = null
Headers = {X-Application-Context=[application:integration:-1]}
Content type = null
Body =
Forwarded URL = null
Redirected URL = null
Cookies = []
So I cant created something like this:
.andExpect(forwardedUrl("error-page"));
because it fails, but on manual tests error-page is returned.

Answer 1

Testing of a custom ErrorController with MockMvc is unfortunately not supported.
For a detailed explanation, see the official recommendation from the Spring Boot team (source).
To be sure that any error handling is working fully, it's necessary to
involve the servlet container in that testing as it's responsible for
error page registration etc. Even if MockMvc itself or a Boot
enhancement to MockMvc allowed forwarding to an error page, you'd be
testing the testing infrastructure not the real-world scenario that
you're actually interested in.
Our recommendation for tests that want to be sure that error handling
is working correctly, is to use an embedded container and test with
WebTestClient, RestAssured, or TestRestTemplate.

Answer 2

My suggestion is to use #ControllerAdvice
In this way you can work around the problem and you can continue to use MockMvc with the big advantage that you are not required to have a running server.
Of course to test explicitly the error page management you need a running server. My suggestion is mainly for those who implemented ErrorController but still want to use MockMvc for unit testing.
#ControllerAdvice
public class MyControllerAdvice {
#ExceptionHandler(FileSizeLimitExceededException.class)
public ResponseEntity<Throwable> handleFileException(HttpServletRequest request, FileSizeLimitExceededException ex) {
return new ResponseEntity<>(ex, HttpStatus.PAYLOAD_TOO_LARGE);
}
#ExceptionHandler(Throwable.class)
public ResponseEntity<Throwable> handleUnexpected(HttpServletRequest request, Throwable throwable) {
return new ResponseEntity<>(throwable, HttpStatus.INTERNAL_SERVER_ERROR);
}
}