# Errors Handling

API Platform comes with a powerful error system. It handles expected (such as faulty JSON documents sent by the
client or validation errors) as well as unexpected errors (PHP exceptions and errors).
API Platform automatically sends the appropriate HTTP status code to the client: `400` for expected errors, `500` for
unexpected ones. It also provides a description of the error in [the Hydra error format](https://www.hydra-cg.com/spec/latest/core/#description-of-http-status-codes-and-errors)
or in the format described in the [RFC 7807](https://tools.ietf.org/html/rfc7807), depending of the format selected during the [content negotiation](content-negotiation.md).

## Converting PHP Exceptions to HTTP Errors

The framework also allows you configure the HTTP status code sent to the clients when custom exceptions are thrown.

In the following example, we throw a domain exception from the business layer of the application and
configure API Platform to convert it to a `404 Not Found` error:

```php
<?php
// api/src/Exception/ProductNotFoundException.php

namespace App\Exception;

final class ProductNotFoundException extends \Exception
{
}
```

```php
<?php
// api/src/EventSubscriber/ProductManager.php

namespace App\EventSubscriber;

use ApiPlatform\Core\EventListener\EventPriorities;
use App\Entity\Product;
use App\Exception\ProductNotFoundException;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpKernel\Event\ViewEvent;
use Symfony\Component\HttpKernel\KernelEvents;

final class ProductManager implements EventSubscriberInterface
{
    public static function getSubscribedEvents(): array
    {
        return [
            KernelEvents::VIEW => ['checkProductAvailability', EventPriorities::PRE_VALIDATE],
        ];
    }

    public function checkProductAvailability(ViewEvent $event): void
    {
        $product = $event->getControllerResult();
        if (!$product instanceof Product || !$event->getRequest()->isMethodSafe(false)) {
            return;
        }

        if (!$product->isPubliclyAvailable()) {
            // Using internal codes for a better understanding of what's going on
            throw new ProductNotFoundException(sprintf('The product "%s" does not exist.', $product->getId()));
        }
    }
}
```

If you use the standard distribution of API Platform, this event listener will be automatically registered. If you use a
custom installation, [learn how to register listeners](events.md#custom-event-listeners).

Then, configure the framework to catch `App\Exception\ProductNotFoundException` exceptions and convert them into `404`
errors:

```yaml
# config/packages/api_platform.yaml
api_platform:
    # ...
    exception_to_status:
        # The 4 following handlers are registered by default, keep those lines to prevent unexpected side effects
        Symfony\Component\Serializer\Exception\ExceptionInterface: 400 # Use a raw status code (recommended)
        ApiPlatform\Core\Exception\InvalidArgumentException: !php/const Symfony\Component\HttpFoundation\Response::HTTP_BAD_REQUEST
        ApiPlatform\Core\Exception\FilterValidationException: 400
        Doctrine\ORM\OptimisticLockException: 409

        # Custom mapping
        App\Exception\ProductNotFoundException: 404 # Here is the handler for our custom exception
```

Any type of `Exception` can be thrown, API Platform will convert it to a Symfony's `HttpException`. The framework also takes
care of serializing the error description according to the request format. For instance, if the API should respond in JSON-LD,
the error will be returned in this format as well:

`GET /products/1234`

```json
{
  "@context": "/contexts/Error",
  "@type": "Error",
  "hydra:title": "An error occurred",
  "hydra:description": "The product \"1234\" does not exist."
}
```