abstract class HttpExceptionSubscriberBase in Drupal 8

Utility base class for exception subscribers.

A subscriber may extend this class and implement getHandledFormats() to indicate which request formats it will respond to. Then implement an on*() method for any error code (HTTP response code) that should be handled. For example, to handle a specific error code like 404 Not Found messages add the method:

public function on404(GetResponseForExceptionEvent $event) {}

To implement a fallback for the entire 4xx class of codes, implement the method:

public function on4xx(GetResponseForExceptionEvent $event) {}

That method should then call $event->setResponse() to set the response object for the exception. Alternatively, it may opt not to do so and then other listeners will have the opportunity to handle the exception.

Note: Core provides several important exception listeners by default. In most cases, setting the priority of a contrib listener to the default of 0 will do what you expect and handle the exceptions you'd expect it to handle. If a custom priority is set, be aware of the following core-registered listeners.

• Fast404ExceptionHtmlSubscriber: 200. This subscriber will return a minimalist, high-performance 404 page for HTML requests. It is not recommended to have a priority higher than this one as it will only slow down that use case.
• ExceptionLoggingSubscriber: 50. This subscriber logs all exceptions but does not handle them. Do not register a listener with a higher priority unless you want exceptions to not get logged, which makes debugging more difficult.
• DefaultExceptionSubscriber: -256. The subscriber of last resort, this will provide generic handling for any exception. A listener with a lower priority will never get called.

All other core-provided exception handlers have negative priorities so most module-provided listeners will naturally take precedence over them.

Hierarchy

• class \Drupal\Core\EventSubscriber\HttpExceptionSubscriberBase implements \Symfony\Component\EventDispatcher\EventSubscriberInterface

1 file declares its use of HttpExceptionSubscriberBase

DefaultExceptionSubscriber.php in core/modules/serialization/src/EventSubscriber/DefaultExceptionSubscriber.php

File

core/lib/Drupal/Core/EventSubscriber/HttpExceptionSubscriberBase.php, line 55

Namespace

Drupal\Core\EventSubscriber

abstract class HttpExceptionSubscriberBase implements EventSubscriberInterface {

  /**
   * Specifies the request formats this subscriber will respond to.
   *
   * @return array
   *   An indexed array of the format machine names that this subscriber will
   *   attempt to process, such as "html" or "json". Returning an empty array
   *   will apply to all formats.
   *
   * @see \Symfony\Component\HttpFoundation\Request
   */
  protected abstract function getHandledFormats();

  /**
   * Specifies the priority of all listeners in this class.
   *
   * The default priority is 1, which is very low. To have listeners that have
   * a "first attempt" at handling exceptions return a higher priority.
   *
   * @return int
   *   The event priority of this subscriber.
   */
  protected static function getPriority() {
    return 0;
  }

  /**
   * Handles errors for this subscriber.
   *
   * @param \Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent $event
   *   The event to process.
   */
  public function onException(GetResponseForExceptionEvent $event) {
    $exception = $event
      ->getException();

    // Make the exception available for example when rendering a block.
    $request = $event
      ->getRequest();
    $request->attributes
      ->set('exception', $exception);
    $handled_formats = $this
      ->getHandledFormats();
    $format = $request->query
      ->get(MainContentViewSubscriber::WRAPPER_FORMAT, $request
      ->getRequestFormat());
    if ($exception instanceof HttpExceptionInterface && (empty($handled_formats) || in_array($format, $handled_formats))) {
      $method = 'on' . $exception
        ->getStatusCode();

      // Keep just the leading number of the status code to produce either a
      // on400 or a 500 method callback.
      $method_fallback = 'on' . substr($exception
        ->getStatusCode(), 0, 1) . 'xx';

      // We want to allow the method to be called and still not set a response
      // if it has additional filtering logic to determine when it will apply.
      // It is therefore the method's responsibility to set the response on the
      // event if appropriate.
      if (method_exists($this, $method)) {
        $this
          ->{$method}($event);
      }
      elseif (method_exists($this, $method_fallback)) {
        $this
          ->{$method_fallback}($event);
      }
    }
  }

  /**
   * Registers the methods in this class that should be listeners.
   *
   * @return array
   *   An array of event listener definitions.
   */
  public static function getSubscribedEvents() {
    $events[KernelEvents::EXCEPTION][] = [
      'onException',
      static::getPriority(),
    ];
    return $events;
  }

}

Members

Name	Modifiers	Type	Description	Overrides
HttpExceptionSubscriberBase::getHandledFormats	abstract protected	function	Specifies the request formats this subscriber will respond to.	4
HttpExceptionSubscriberBase::getPriority	protected static	function	Specifies the priority of all listeners in this class.	4
HttpExceptionSubscriberBase::getSubscribedEvents	public static	function	Registers the methods in this class that should be listeners.
HttpExceptionSubscriberBase::onException	public	function	Handles errors for this subscriber.