Bug: /api/cards/compare returns 400 due to routing conflict with /{id} path variable

[yortch]

Problem

Requests to GET /api/cards/compare are incorrectly matched by the @GetMapping("/{id}") endpoint in CreditCardController, causing a MethodArgumentTypeMismatchException when Spring attempts to convert the string "compare" to java.lang.Long. This results in HTTP 400 Bad Request errors and breaks the card comparison feature.

Evidence

Production logs (2026-04-25 ~00:01 UTC) — 3 occurrences within 10 seconds:

WARN DefaultHandlerExceptionResolver: Resolved [MethodArgumentTypeMismatchException:
Failed to convert value of type 'java.lang.String' to required type 'java.lang.Long';
For input string: "compare"]

Detected by automated health check on ca-banking-demo-backend (revision ca-banking-demo-backend--azd-1774558182).

Root Cause

CreditCardController maps @GetMapping("/{id}") which acts as a catch-all for any path under /api/cards/. When /api/cards/compare is requested, Spring matches it to /{id} and fails the type conversion from String to Long.

Affected file: backend/src/main/java/com/threeriversbank/controller/CreditCardController.java (line 42)

Fix (Ready on branch fix/cards-compare-routing-bug)

1. New /compare endpoint (CreditCardController.java)

Add @GetMapping("/compare") mapped before /{id} to prevent route ambiguity. Accepts optional ids query parameter for targeted comparison:

@GetMapping("/compare")
@Operation(summary = "Compare credit cards")
public ResponseEntity<List<CreditCardDto>> compareCards(
        @RequestParam(required = false) List<Long> ids) {
    List<CreditCardDto> cards;
    if (ids != null && !ids.isEmpty()) {
        cards = creditCardService.getCardsByIds(ids);
    } else {
        cards = creditCardService.getAllCreditCards();
    }
    return ResponseEntity.ok(cards);
}

2. New service method (CreditCardService.java)

@Transactional(readOnly = true)
public List<CreditCardDto> getCardsByIds(List<Long> ids) {
    return creditCardRepository.findAllById(ids).stream()
            .map(this::convertToDtoWithDetails)
            .collect(Collectors.toList());
}

3. Global Exception Handler (GlobalExceptionHandler.java)

New @RestControllerAdvice class that handles MethodArgumentTypeMismatchException and RuntimeException with structured JSON error responses.

4. Unit tests (CreditCardControllerTest.java)

• compareCards_WithIds_ShouldReturnSelectedCards
• compareCards_WithoutIds_ShouldReturnAllCards

Impact

• Severity: Medium — card comparison feature is broken for users navigating to /api/cards/compare
• Scope: Backend only; frontend comparison page (/cards) works via GET /api/cards but direct API access fails

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This issue was created by sre-sre-three-rivers-bswqe--b2b14894
Tracked by the SRE agent here