How do you handle exceptions globally with @ControllerAdvice?

Question

IT Interview Practice · Accepted Answer

Instead of wrapping every controller method in try/catch, Spring lets you centralize error handling in one class annotated `@ControllerAdvice` (or `@RestControllerAdvice`). Methods marked `@ExceptionHandler` catch exceptions thrown anywhere in your controllers and turn them into consistent HTTP responses.

## A global handler

```java
@RestControllerAdvice // = @ControllerAdvice + @ResponseBody
public class GlobalExceptionHandler {

// Catches this exception from ANY controller and returns 404.
    @ExceptionHandler(EntityNotFoundException.class)
    @ResponseStatus(HttpStatus.NOT_FOUND)
    public ApiError notFound(EntityNotFoundException ex) {
        return new ApiError("NOT_FOUND", ex.getMessage());
    }

// Validation failures (@Valid) throw this — turn field errors into 400.
    @ExceptionHandler(MethodArgumentNotValidException.class)
    @ResponseStatus(HttpStatus.BAD_REQUEST)
    public ApiError invalid(MethodArgumentNotValidException ex) {
        String msg = ex.getBindingResult().getFieldErrors().stream()
            .map(e -> e.getField() + " " + e.getDefaultMessage())
            .collect(Collectors.joining(", "));
        return new ApiError("VALIDATION", msg);
    }
}
```

Any controller can now just `throw new EntityNotFoundException(...)` and trust the advice to shape the response.

## The modern option: ProblemDetail

Spring 6 / Boot 3 ship RFC 9457 `ProblemDetail`, a standardized error body:

```java
@ExceptionHandler(PaymentDeclinedException.class)
public ProblemDetail declined(PaymentDeclinedException ex) {
    var pd = ProblemDetail.forStatusAndDetail(HttpStatus.PAYMENT_REQUIRED, ex.getMessage());
    pd.setTitle("Payment declined");
    return pd; // serialized as application/problem+json
}
```

## Why it matters

Consistent, machine-readable error responses are what separate a professional API from one that leaks stack traces. Interviewers want to see you centralize handling (DRY), map exceptions to the *right* status codes, and never expose internal details to clients. Mentioning `ProblemDetail` shows you are current with Boot 3.

A global handler

The modern option: ProblemDetail

Why it matters

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