# API Error Handling Guidelines

Every REST error this server returns is an RFC-7807 `application/problem+json` body produced by a `@RestControllerAdvice`. A controller throws a meaningful exception. An advice class translates it into HTTP semantics and decides exactly what detail leaves the server. No controller formats an error itself, and no stack trace reaches a client.

## How `WorkspaceControllerAdvice` works

1. `@RestControllerAdvice(basePackages = "de.tum.cit.aet.hephaestus.workspace")` scopes the advice to workspace endpoints only, but covers the entire workspace package (controllers for repositories, labels, lifecycle, etc.). One mapper handles every route in that package, which guarantees consistency.
2. Each `@ExceptionHandler` method matches a domain exception (for example `EntityNotFoundException`) and returns a configured `ProblemDetail` object. Spring automatically serializes it to JSON with the proper status code.
3. The private helpers sanitize exception messages through `LoggingUtils` before they return them. This prevents leaks of stack traces or SQL fragments. The server logs any unexpected `IllegalStateException` as a server error. Domain-specific violations, such as slug conflicts and lifecycle issues, have their own exception classes. Thus, the advice can return the correct HTTP semantics.
4. Controllers simply throw exceptions (or let services throw) and never build ad-hoc `Map.of("error", ...)` responses again.

## Rolling the pattern out elsewhere

When you touch another REST controller:

1. **Identify the domain-specific exceptions** the controller or underlying services already throw.

2. **Create a dedicated advice class** in the same package. If multiple controllers share the same error model, use a shared module instead. Annotate it with `@RestControllerAdvice(basePackages = "your.package")`. Thus, every controller in that bounded context reuses the mapper.

3. **Map exceptions to HTTP semantics**:

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   * Validation or invariant violations → `400 Bad Request`
   * Missing resources → `404 Not Found`
   * Conflicts (duplicates, state transitions) → `409 Conflict`
   * Authorization issues should still bubble up to Spring Security so we get `401/403` uniformly.

4. **Return `ProblemDetail` everywhere**. Include a user-actionable `title` plus a `detail` sourced from the exception where it is safe to expose.

5. **Document the mapping** (update this file or the feature README) so clients know what to expect.

### Shared advice for multiple controllers

If controllers reuse the same exception hierarchy, point `@RestControllerAdvice` at the shared base package. Git provider controllers are an example. Every endpoint in that bounded context then inherits the mapper. Keep advice classes focused. Too many unrelated handlers in one class make maintenance difficult.

## Validation errors deserve structure, too

Spring surfaces method-argument validation failures as `MethodArgumentNotValidException` (body binding) or `ConstraintViolationException` (query/path parameters). Both become structured JSON, so a client can highlight the field that failed rather than print a sentence. See [RFC 9457](https://www.rfc-editor.org/rfc/rfc9457) for the media type. Our convention:

* Use the same advice class to `@ExceptionHandler` both exception types.
* Return `ProblemDetail` with `title = "Validation failed"`, `status = 400`, and a short descriptive `detail`.
* Attach a machine-friendly `errors` map through `problem.setProperty("errors", …)`. Each key is the field or parameter that failed. We remove method prefixes so only the field name remains. Each value is a list of human-readable messages.

### Path-variable validation

Slug parameters across `WorkspaceController` use the shared `@WorkspaceSlug` constraint, which `WorkspaceSlugValidator` implements. The controller carries `@Validated`. Invalid slugs raise a `ConstraintViolationException`. The advice handles this exception and returns the same `errors` map shape as body validation failures.

## Testing expectations

* Add WebTestClient or MockMvc tests that trigger the exception path and assert the `ProblemDetail` body.
* Cover the advice indirectly through integration tests so we catch serialization regressions when upgrading Spring Boot.