# 3. Validation errors that say something

[← 2. Choosing a mechanism](02-choosing-a-mechanism.md) · [Index](../README.md) · Next: [4. Types and message codes →](04-i18n-and-types.md)

Two exceptions, depending on where the constraint is:

| Constraint on | Exception | Stock `detail` |
|---|---|---|
| a `@Valid @RequestBody` object | `MethodArgumentNotValidException` | `Invalid request content.` |
| a parameter directly (`@RequestParam @Max(100) int limit`) | `HandlerMethodValidationException` | `Validation failure` |

The second is Spring Framework 6.1's built-in method validation: a constraint annotation on a
controller parameter is enough, no `@Validated` on the class.

## The stock body tells the client nothing

From [`matrix-boot-flag.txt`](output/matrix-boot-flag.txt) - three violations in the request, none in
the response:

```
HTTP 400  Content-Type: application/problem+json
{"detail":"Invalid request content.","instance":"/orders","status":400,"title":"Bad Request"}
```

## Adding the violations

Override the protected hook, keep the body the base class built, add an extension member, and hand
it back to `handleExceptionInternal` so headers and status stay right
([`GlobalExceptionHandler`](../src/main/java/com/ankurm/problems/advice/GlobalExceptionHandler.java)):

```java
@Override
protected ResponseEntity<Object> handleMethodArgumentNotValid(MethodArgumentNotValidException ex,
        HttpHeaders headers, HttpStatusCode status, WebRequest request) {
    ProblemDetail pd = ex.getBody();
    pd.setDetail("The request body has " + ex.getErrorCount() + " invalid field(s)");
    pd.setProperty("errors", ex.getFieldErrors().stream()
            .map(e -> Map.of("pointer", "#/" + e.getField().replace('.', '/'),
                             "detail", String.valueOf(e.getDefaultMessage())))
            .toList());
    return handleExceptionInternal(ex, pd, headers, status, request);
}
```

The `{"pointer","detail"}` shape is not invented: it is the example RFC 9457 itself uses in
section 3. From [`matrix-advice.txt`](output/matrix-advice.txt):

```
{"detail":"The request body has 3 invalid field(s)","instance":"/orders","status":400,"title":"Bad Request","errors":[{"pointer":"#/quantity","detail":"must be greater than or equal to 1"},{"pointer":"#/customerEmail","detail":"must be a well-formed email address"},{"pointer":"#/sku","detail":"must not be blank"}]}
```

## Do not assert on the order

The order in that line is not declaration order, and it differed between runs of the same
request. The contract
test asserts membership, not position.

## Nested fields

`FieldError.getField()` uses dots for nesting (`address.city`) and brackets for indexes
(`lines[0].sku`). The `replace('.', '/')` here handles the first; if your API has lists of objects,
convert `[n]` to `/n` as well so the pointer is a valid JSON Pointer.