> ## Documentation Index
> Fetch the complete documentation index at: https://docs.activeviam.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Add custom UI exceptions

> How to display custom error messages in the Atoti Limits UI by returning ProblemDetail responses from RestControllerAdvice exception handlers, with an example based on the built-in RestExceptionHandlerControllerAdvice

To do this, return a response type in the structure of a [ProblemDetail](https://datatracker.ietf.org/doc/html/rfc7807) object.

## Steps

1. Add a method annotated with `@ExceptionHandler` in a class annotated with `@RestControllerAdvice` or `@ControllerAdvice`. The `@ExceptionHandler` annotation should contain the classes of exceptions you’d like to handle.
2. Make that method return a `ProblemDetail` object
3. Throw your desired exception from within the application. Note that most common exceptions are already handled in `RestExceptionHandlerControllerAdvice` and  `LimitsProcessExceptionHandler`.

## How it works

For an example of how we do this on the server side in Atoti Limits see `RestExceptionHandlerControllerAdvice`, which looks as follows:

```java theme={"languages":{"custom":["/engine/python-sdk/0.9/languages/pycon.tmLanguage.json"]}}
@RestControllerAdvice
public class RestExceptionHandlerControllerAdvice {

   @ExceptionHandler({ LimitsServiceException.class })
   ProblemDetail handleRestServiceException(LimitsServiceException ex) {
      final ProblemDetail problemDetail = ProblemDetail.forStatusAndDetail(ex.getHttpStatus(), ex.getMessage());
      problemDetail.setProperty("action", ex.getAction());
      problemDetail.setProperty("stacktrace", ExceptionUtils.getStackTrace(ex));
      return problemDetail;
   }
}
```

This simple class is annotated with `@RestControllerAdvice`, meaning it will catch exceptions thrown from REST methods. In reality, it inherits from the global
exception handler `@ControllerAdvice`, which handles exceptions thrown from any method.
The single method in `RestExceptionHandlerControllerAdvice` catches the generic service exception `LimitsServiceException` using the `@ExceptionHandler` annotation and returns a `ProblemDetail` object, which our UI will parse appropriately.

The `ProblemDetail` object accepts a map of variables that may hold custom fields of interest. For example, we supply the following fields:

* `action`: a string you can use to suggest a course of action to the user, displayed by the UI.
* `stacktrace`: a string that contains the stack trace of the exception, not displayed by the UI but may be useful for debugging.

<Note>
  The `@ExceptionHandler` annotation accepts multiple classes of exception for a single method.
</Note>

For more server-side information, see the [Spring Framework Documentation](https://docs.spring.io/spring-framework/reference/):

* [Error responses](https://docs.spring.io/spring-framework/reference/web/webmvc/mvc-ann-rest-exceptions.)
* [ProblemDetail](https://docs.spring.io/spring-framework/docs/current/javadoc-api/org/springframework/http/ProblemDetail.)
* [ControllerAdvice](https://spring.io/blog/2013/11/01/exception-handling-in-spring-mvc#global-exception-handling)