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I would like to handle my exceptions correctly in Spring, so I have a question about exceptionHandler syntax : Is it ok to throw specific exceptions in the controller, if they are caught by the exception handler ?

More specifically :

Here is the Exception :

public class UnknownUserException extends Exception {

    private static final long serialVersionUID = 1L;

    public UnknownUserException(String message) {
        super(message);
    }
}

Here is the ExceptionHandler with the specific method for UnknownUserException :

@ControllerAdvice
@ResponseBody
public class ControllerExceptionHandler {

    @ExceptionHandler(UnknownUserException.class)
    @ResponseStatus(value = HttpStatus.NOT_FOUND)
    public ErrorMessage unknownUserExceptionMessage(UnknownUserException ex, WebRequest request) {
       ErrorMessage message = new ErrorMessage("The user doesn't exist: " +ex.getLocalizedMessage(), ex);
       return message;
      }
}

Here is one example of a mapping that may raise that exception :

@GetMapping({"/user/{id}"})
    public ResponseEntity<UserProfileDto> getById(@PathVariable Long id) throws UnknownUserException {
            UserProfileDto user = userService.findById(id);
            return ResponseEntity.ok(user);
    }

The userService.findById(id) may throw the UnknownUserException.

From what I understood, the controllerAdvice "overrides" the controller in case of that specific exception thrown by a service, but then, what should I do with my controller ? should I throw the exception again (like above) or catch the specific exception and return a ResponseEntity(HttpStatus.NOT_FOUND) ?

FloranePlo
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1 Answers1

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In an ideal scenario exception should be thrown immediately when it is known in your case as you mentioned service method will throw UnknownUserException that the right thing to do. Your Controller Advice should be able to handle that exception. ContollerAdvice will handle any matching exception that is thrown during execution of the request irrespective of the origin of exception.

Refer to this link for other options on handling exception

Ravik
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  • Thank you for the confirmation, I'm still having a bit of trouble as to when to throw/ catch exceptions. – FloranePlo Oct 14 '21 at 17:06
  • You should catch exception when you have some alternative to serve. In other words catch those exception which will not have any impact in serving the request. E.g A request to get available books you may get the list of books from database or cache. In case database is down you can serve (catch db exception) the request from cache. But make sure you add information in the response that it is not a complete/upto date response. – Ravik Oct 15 '21 at 01:51
  • I see, thank you, it's very helpful. I have not built a cache for the application yet, but that brings it to my attention, so I'll look it up now, as it's an interesting and useful thing to implement. And at least, I see I was using the catch in the right way because for now, I only catched Exception in my storage service to return a default image in case of error in fetching my files (broken src in the front are pretty ugly ^^). – FloranePlo Oct 16 '21 at 03:40