rest-error-handler

REST Error Handler — Scaffold

One advice class so every endpoint shares one error shape. Clients always get an RFC 7807 ProblemDetail (application/problem+json), and the server never leaks a stack trace or internal message on an unexpected failure.

Step 1 — Detect conventions (do not assume)

Read before generating:

• Existing advice: search for @RestControllerAdvice / @ControllerAdvice or a class extending ResponseEntityExceptionHandler. If one exists, ADD the missing handlers to it (show a diff) rather than creating a competing advice — two global advices fighting over the same exception is non-deterministic.
• Base package: mirror where existing *Controller.java live; place the advice in the …​.web package alongside them.
• Existing not-found exception: if the project already has a NotFoundException / ResourceNotFoundException, reuse it; don't generate a duplicate.
• Error type base URI: pick a stable base for the type field (e.g. https://<app>/problems); fill {{PROBLEM_BASE}}.

Step 2 — Render templates

From assets/, substitute {{PACKAGE}} and {{PROBLEM_BASE}}:

• GlobalExceptionHandler.java.tmpl → <pkg>/web/GlobalExceptionHandler.java
• NotFoundException.java.tmpl → <pkg>/NotFoundException.java (skip if one exists)

The advice maps three cases:

1. MethodArgumentNotValidException → 400, body carries an errors map of field → message built from getBindingResult().getFieldErrors().
2. NotFoundException → 404 with the domain message.
3. Exception (fallback) → 500, logged server-side, body is an opaque "Unexpected error" — no ex.getMessage(), no stack trace.

Step 3 — Wire it up

• Throw NotFoundException from services (e.g. on findById(...).orElseThrow(...)), never return null to the controller.
• Spring 6 serializes ProblemDetail as application/problem+json automatically; no extra config needed. Confirm the app is Spring Boot 3.x (Spring 6) — ProblemDetail does not exist in Spring 5 / Boot 2.x.