diff --git a/core/core.services.yml b/core/core.services.yml
index 183f7e2..30093f3 100644
--- a/core/core.services.yml
+++ b/core/core.services.yml
@@ -714,16 +714,44 @@ services:
     tags:
       - { name: event_subscriber }
     arguments: ['@config.manager', '@config.storage', '@config.storage.snapshot']
-  exception_controller:
-    class: Drupal\Core\Controller\ExceptionController
-    arguments: ['@content_negotiation', '@title_resolver', '@html_page_renderer', '@html_fragment_renderer', '@string_translation', '@url_generator', '@logger.factory']
-    calls:
-      - [setContainer, ['@service_container']]
-  exception_listener:
-    class: Drupal\Core\EventSubscriber\ExceptionListener
+  exception.default_json:
+    class: Drupal\Core\EventSubscriber\ExceptionJsonSubscriber
+    tags:
+      - { name: event_subscriber }
+  exception.default_html:
+    class: Drupal\Core\EventSubscriber\DefaultExceptionHtmlSubscriber
+    tags:
+      - { name: event_subscriber }
+    arguments: ['@html_fragment_renderer', '@html_page_renderer', '@config.factory']
+  exception.default:
+    class: Drupal\Core\EventSubscriber\DefaultExceptionSubscriber
+    tags:
+      - { name: event_subscriber }
+    arguments: ['@html_fragment_renderer', '@html_page_renderer', '@config.factory']
+  exception.logger:
+    class: Drupal\Core\EventSubscriber\ExceptionLoggingSubscriber
+    tags:
+      - { name: event_subscriber }
+    arguments: ['@logger.factory']
+  exception.custom_page_html:
+    class: Drupal\Core\EventSubscriber\ExceptionJsonSubscriber
+    tags:
+      - { name: event_subscriber }
+    arguments: ['@config.factory', '@path.alias_manager', '@http_kernel']
+  exception.custom_page_html:
+    class: Drupal\Core\EventSubscriber\CustomPageExceptionHtmlSubscriber
+    tags:
+      - { name: event_subscriber }
+    arguments: ['@config.factory', '@path.alias_manager', '@http_kernel']
+  exception.fast_404_html:
+    class: Drupal\Core\EventSubscriber\Fast404ExceptionHtmlSubscriber
+    tags:
+      - { name: event_subscriber }
+    arguments: ['@config.factory', '@http_kernel']
+  exception.test_site:
+    class: Drupal\Core\EventSubscriber\ExceptionTestSiteSubscriber
     tags:
       - { name: event_subscriber }
-    arguments: [['@exception_controller', execute]]
   route_processor_manager:
     class: Drupal\Core\RouteProcessor\RouteProcessorManager
     tags:
diff --git a/core/lib/Drupal/Core/Controller/ExceptionController.php b/core/lib/Drupal/Core/Controller/ExceptionController.php
deleted file mode 100644
index 69a477b..0000000
--- a/core/lib/Drupal/Core/Controller/ExceptionController.php
+++ /dev/null
@@ -1,452 +0,0 @@
-<?php
-
-/**
- * @file
- * Contains \Drupal\Core\Controller\ExceptionController.
- */
-
-namespace Drupal\Core\Controller;
-
-use Drupal\Core\Logger\LoggerChannelFactoryInterface;
-use Drupal\Core\Page\DefaultHtmlPageRenderer;
-use Drupal\Core\Page\HtmlFragmentRendererInterface;
-use Drupal\Core\Page\HtmlPageRendererInterface;
-use Drupal\Core\Routing\UrlGeneratorInterface;
-use Symfony\Component\DependencyInjection\ContainerAwareInterface;
-use Symfony\Component\DependencyInjection\ContainerInterface;
-use Symfony\Component\HttpFoundation\Request;
-use Symfony\Component\HttpFoundation\Response;
-use Symfony\Component\HttpFoundation\JsonResponse;
-use Symfony\Component\HttpKernel\HttpKernelInterface;
-use Drupal\Component\Utility\SafeMarkup;
-use Drupal\Component\Utility\String;
-use Symfony\Component\Debug\Exception\FlattenException;
-use Drupal\Core\ContentNegotiation;
-use Drupal\Core\Utility\Error;
-use Drupal\Core\StringTranslation\StringTranslationTrait;
-use Drupal\Core\StringTranslation\TranslationInterface;
-
-/**
- * This controller handles HTTP errors generated by the routing system.
- */
-class ExceptionController extends HtmlControllerBase implements ContainerAwareInterface {
-  use StringTranslationTrait;
-
-  /**
-   * The content negotiation library.
-   *
-   * @var \Drupal\Core\ContentNegotiation
-   */
-  protected $negotiation;
-
-  /**
-   * The service container.
-   *
-   * @var \Symfony\Component\DependencyInjection\ContainerInterface
-   */
-  protected $container;
-
-  /**
-   * The page rendering service.
-   *
-   * @var \Drupal\Core\Page\HtmlPageRendererInterface
-   */
-  protected $htmlPageRenderer;
-
-  /**
-   * The fragment rendering service.
-   *
-   * @var \Drupal\Core\Page\HtmlFragmentRendererInterface
-   */
-  protected $fragmentRenderer;
-
-  /**
-   * The logger factory service.
-   *
-   * @var \Drupal\Core\Logger\LoggerChannelFactoryInterface
-   */
-  protected $loggerFactory;
-
-  /**
-   * Constructor.
-   *
-   * @param \Drupal\Core\ContentNegotiation $negotiation
-   *   The content negotiation library to use to determine the correct response
-   *   format.
-   * @param \Drupal\Core\Controller\TitleResolverInterface $title_resolver
-   *   The title resolver.
-   * @param \Drupal\Core\Page\HtmlPageRendererInterface $renderer
-   *   The page renderer.
-   * @param \Drupal\Core\Page\HtmlFragmentRendererInterface $fragment_renderer
-   *   The fragment rendering service.
-   * @param \Drupal\Core\StringTranslation\TranslationInterface $string_translation
-   *   The url generator.
-   * @param \Drupal\Core\Routing\UrlGeneratorInterface $url_generator
-   *   The URL generator.
-   * @param \Drupal\Core\Logger\LoggerChannelFactoryInterface $logger_factory
-   *   The logger factory.
-   */
-  public function __construct(ContentNegotiation $negotiation, TitleResolverInterface $title_resolver, HtmlPageRendererInterface $renderer, HtmlFragmentRendererInterface $fragment_renderer, TranslationInterface $string_translation, UrlGeneratorInterface $url_generator, LoggerChannelFactoryInterface $logger_factory) {
-    parent::__construct($title_resolver, $url_generator);
-    $this->negotiation = $negotiation;
-    $this->htmlPageRenderer = $renderer;
-    $this->fragmentRenderer = $fragment_renderer;
-    $this->stringTranslation = $string_translation;
-    $this->loggerFactory = $logger_factory;
-  }
-
-  /**
-   * Sets the Container associated with this Controller.
-   *
-   * @param \Symfony\Component\DependencyInjection\ContainerInterface $container
-   *   A ContainerInterface instance.
-   *
-   * @api
-   */
-  public function setContainer(ContainerInterface $container = NULL) {
-    $this->container = $container;
-  }
-
-  /**
-   * Handles an exception on a request.
-   *
-   * @param \Symfony\Component\Debug\Exception\FlattenException $exception
-   *   The flattened exception.
-   * @param \Symfony\Component\HttpFoundation\Request $request
-   *   The request that generated the exception.
-   *
-   * @return \Symfony\Component\HttpFoundation\Response
-   *   A response object.
-   */
-  public function execute(FlattenException $exception, Request $request) {
-    $method = 'on' . $exception->getStatusCode() . $this->negotiation->getContentType($request);
-
-    if (method_exists($this, $method)) {
-      return $this->$method($exception, $request);
-    }
-
-    return new Response('A fatal error occurred: ' . $exception->getMessage(), $exception->getStatusCode(), $exception->getHeaders());
-  }
-
-  /**
-   * Processes a MethodNotAllowed exception into an HTTP 405 response.
-   *
-   * @param \Symfony\Component\Debug\Exception\FlattenException $exception
-   *   The flattened exception.
-   * @param \Symfony\Component\HttpFoundation\Request $request
-   *   The request object that triggered this exception.
-   *
-   * @return \Symfony\Component\HttpFoundation\Response
-   *   A response object.
-   */
-  public function on405Html(FlattenException $exception, Request $request) {
-    return new Response('Method Not Allowed', 405);
-  }
-
-  /**
-   * Processes an AccessDenied exception into an HTTP 403 response.
-   *
-   * @param \Symfony\Component\Debug\Exception\FlattenException $exception
-   *   The flattened exception.
-   * @param \Symfony\Component\HttpFoundation\Request $request
-   *   The request object that triggered this exception.
-   *
-   * @return \Symfony\Component\HttpFoundation\Response
-   *   A response object.
-   */
-  public function on403Html(FlattenException $exception, Request $request) {
-    // @todo Remove dependency on the internal _system_path attribute:
-    //   https://www.drupal.org/node/2293523.
-    $system_path = $request->attributes->get('_system_path');
-    $this->loggerFactory->get('access denied')->warning($system_path);
-
-    $system_config = $this->container->get('config.factory')->get('system.site');
-    $path = $this->container->get('path.alias_manager')->getPathByAlias($system_config->get('page.403'));
-    if ($path && $path != $system_path) {
-      if ($request->getMethod() === 'POST') {
-        $subrequest = Request::create($request->getBaseUrl() . '/' . $path, 'POST', array('destination' => $system_path, '_exception_statuscode' => 403) + $request->request->all(), $request->cookies->all(), array(), $request->server->all());
-      }
-      else {
-        $subrequest = Request::create($request->getBaseUrl() . '/' . $path, 'GET', array('destination' => $system_path, '_exception_statuscode' => 403), $request->cookies->all(), array(), $request->server->all());
-      }
-
-      $response = $this->container->get('http_kernel')->handle($subrequest, HttpKernelInterface::SUB_REQUEST);
-      $response->setStatusCode(403, 'Access denied');
-    }
-    else {
-      $page_content = array(
-        '#markup' => $this->t('You are not authorized to access this page.'),
-        '#title' => $this->t('Access denied'),
-      );
-
-      $fragment = $this->createHtmlFragment($page_content, $request);
-      $page = $this->fragmentRenderer->render($fragment, 403);
-      $response = new Response($this->htmlPageRenderer->render($page), $page->getStatusCode());
-      return $response;
-    }
-
-    return $response;
-  }
-
-  /**
-   * Processes a NotFound exception into an HTTP 404 response.
-   *
-   * @param \Symfony\Component\Debug\Exception\FlattenException $exception
-   *   The flattened exception.
-   * @param \Symfony\Component\HttpFoundation\Request $request
-   *   The request object that triggered this exception.
-   *
-   * @return \Symfony\Component\HttpFoundation\Response
-   *   A response object.
-   */
-  public function on404Html(FlattenException $exception, Request $request) {
-    $this->loggerFactory->get('page not found')->warning(String::checkPlain($request->attributes->get('_system_path')));
-
-    // Check for and return a fast 404 page if configured.
-    $config = \Drupal::config('system.performance');
-
-    $exclude_paths = $config->get('fast_404.exclude_paths');
-    if ($config->get('fast_404.enabled') && $exclude_paths && !preg_match($exclude_paths, $request->getPathInfo())) {
-      $fast_paths = $config->get('fast_404.paths');
-      if ($fast_paths && preg_match($fast_paths, $request->getPathInfo())) {
-        $fast_404_html = $config->get('fast_404.html');
-        $fast_404_html = strtr($fast_404_html, array('@path' => String::checkPlain($request->getUri())));
-        return new Response($fast_404_html, 404);
-      }
-    }
-
-    // @todo Remove dependency on the internal _system_path attribute:
-    //   https://www.drupal.org/node/2293523.
-    $system_path = $request->attributes->get('_system_path');
-
-    $path = $this->container->get('path.alias_manager')->getPathByAlias(\Drupal::config('system.site')->get('page.404'));
-    if ($path && $path != $system_path) {
-      // @todo Um, how do I specify an override URL again? Totally not clear. Do
-      //   that and sub-call the kernel rather than using meah().
-      // @todo The create() method expects a slash-prefixed path, but we store a
-      //   normal system path in the site_404 variable.
-      if ($request->getMethod() === 'POST') {
-        $subrequest = Request::create($request->getBaseUrl() . '/' . $path, 'POST', array('destination' => $system_path, '_exception_statuscode' => 404) + $request->request->all(), $request->cookies->all(), array(), $request->server->all());
-      }
-      else {
-        $subrequest = Request::create($request->getBaseUrl() . '/' . $path, 'GET', array('destination' => $system_path, '_exception_statuscode' => 404), $request->cookies->all(), array(), $request->server->all());
-      }
-
-      $response = $this->container->get('http_kernel')->handle($subrequest, HttpKernelInterface::SUB_REQUEST);
-      $response->setStatusCode(404, 'Not Found');
-    }
-    else {
-      $page_content = array(
-        '#markup' => $this->t('The requested page "@path" could not be found.', array('@path' => $request->getPathInfo())),
-        '#title' => $this->t('Page not found'),
-      );
-
-      $fragment = $this->createHtmlFragment($page_content, $request);
-      $page = $this->fragmentRenderer->render($fragment, 404);
-      $response = new Response($this->htmlPageRenderer->render($page), $page->getStatusCode());
-      return $response;
-    }
-
-    return $response;
-  }
-
-  /**
-   * Processes a generic exception into an HTTP 500 response.
-   *
-   * @param \Symfony\Component\Debug\Exception\FlattenException $exception
-   *   Metadata about the exception that was thrown.
-   * @param \Symfony\Component\HttpFoundation\Request $request
-   *   The request object that triggered this exception.
-   *
-   * @return \Symfony\Component\HttpFoundation\Response
-   *   A response object.
-   */
-  public function on500Html(FlattenException $exception, Request $request) {
-    $error = $this->decodeException($exception);
-
-    // Because the kernel doesn't run until full bootstrap, we know that
-    // most subsystems are already initialized.
-
-    $headers = array();
-
-    // When running inside the testing framework, we relay the errors
-    // to the tested site by the way of HTTP headers.
-    if (DRUPAL_TEST_IN_CHILD_SITE && !headers_sent() && (!defined('SIMPLETEST_COLLECT_ERRORS') || SIMPLETEST_COLLECT_ERRORS)) {
-      // $number does not use drupal_static as it should not be reset
-      // as it uniquely identifies each PHP error.
-      static $number = 0;
-      $assertion = array(
-        $error['!message'],
-        $error['%type'],
-        array(
-          'function' => $error['%function'],
-          'file' => $error['%file'],
-          'line' => $error['%line'],
-        ),
-      );
-      $headers['X-Drupal-Assertion-' . $number] = rawurlencode(serialize($assertion));
-      $number++;
-    }
-
-    $this->loggerFactory->get('php')->log($error['severity_level'], '%type: !message in %function (line %line of %file).', $error);
-
-    // Display the message if the current error reporting level allows this type
-    // of message to be displayed, and unconditionnaly in update.php.
-    if (error_displayable($error)) {
-      $class = 'error';
-
-      // If error type is 'User notice' then treat it as debug information
-      // instead of an error message.
-      // @see debug()
-      if ($error['%type'] == 'User notice') {
-        $error['%type'] = 'Debug';
-        $class = 'status';
-      }
-
-      // Attempt to reduce verbosity by removing DRUPAL_ROOT from the file path
-      // in the message. This does not happen for (false) security.
-      $root_length = strlen(DRUPAL_ROOT);
-      if (substr($error['%file'], 0, $root_length) == DRUPAL_ROOT) {
-        $error['%file'] = substr($error['%file'], $root_length + 1);
-      }
-      // Should not translate the string to avoid errors producing more errors.
-      $message = String::format('%type: !message in %function (line %line of %file).', $error);
-
-      // Check if verbose error reporting is on.
-      $error_level = $this->container->get('config.factory')->get('system.logging')->get('error_level');
-
-      if ($error_level == ERROR_REPORTING_DISPLAY_VERBOSE) {
-        $backtrace_exception = $exception;
-        while ($backtrace_exception->getPrevious()) {
-          $backtrace_exception = $backtrace_exception->getPrevious();
-        }
-        $backtrace = $backtrace_exception->getTrace();
-        // First trace is the error itself, already contained in the message.
-        // While the second trace is the error source and also contained in the
-        // message, the message doesn't contain argument values, so we output it
-        // once more in the backtrace.
-        array_shift($backtrace);
-        // Generate a backtrace containing only scalar argument values.
-        $message .= '<pre class="backtrace">' . Error::formatFlattenedBacktrace($backtrace) . '</pre>';
-      }
-      drupal_set_message(SafeMarkup::set($message), $class, TRUE);
-    }
-
-    $content = $this->t('The website has encountered an error. Please try again later.');
-    $output = DefaultHtmlPageRenderer::renderPage($content, $this->t('Error'));
-    $response = new Response($output);
-    $response->setStatusCode(500, '500 Service unavailable (with message)');
-
-    return $response;
-  }
-
-  /**
-   * Processes an AccessDenied exception that occurred on a JSON request.
-   *
-   * @param \Symfony\Component\Debug\Exception\FlattenException $exception
-   *   The flattened exception.
-   * @param \Symfony\Component\HttpFoundation\Request $request
-   *   The request object that triggered this exception.
-   *
-   * @return \Symfony\Component\HttpFoundation\JsonResponse
-   *   A JSON response object.
-   */
-  public function on403Json(FlattenException $exception, Request $request) {
-    $response = new JsonResponse();
-    $response->setStatusCode(403, 'Access Denied');
-    return $response;
-  }
-
-  /**
-   * Processes a NotFound exception that occurred on a JSON request.
-   *
-   * @param \Symfony\Component\Debug\Exception\FlattenException $exception
-   *   The flattened exception.
-   * @param \Symfony\Component\HttpFoundation\Request $request
-   *   The request object that triggered this exception.
-   *
-   * @return \Symfony\Component\HttpFoundation\JsonResponse
-   *   A JSON response object.
-   */
-  public function on404Json(FlattenException $exception, Request $request) {
-    $response = new JsonResponse();
-    $response->setStatusCode(404, 'Not Found');
-    return $response;
-  }
-
-  /**
-   * Processes a MethodNotAllowed exception that occurred on a JSON request.
-   *
-   * @param \Symfony\Component\Debug\Exception\FlattenException $exception
-   *   The flattened exception.
-   * @param \Symfony\Component\HttpFoundation\Request $request
-   *   The request object that triggered this exception.
-   *
-   * @return \Symfony\Component\HttpFoundation\JsonResponse
-   *   A JSON response object.
-   */
-  public function on405Json(FlattenException $exception, Request $request) {
-    $response = new JsonResponse();
-    $response->setStatusCode(405, 'Method Not Allowed');
-    return $response;
-  }
-
-
-  /**
-   * This method is a temporary port of _drupal_decode_exception().
-   *
-   * @todo This should get refactored. FlattenException could use some
-   *   improvement as well.
-   *
-   * @param \Symfony\Component\Debug\Exception\FlattenException $exception
-   *  The flattened exception.
-   *
-   * @return array
-   *   An array of string-substitution tokens for formatting a message about the
-   *   exception.
-   */
-  protected function decodeException(FlattenException $exception) {
-    $message = $exception->getMessage();
-
-    $backtrace = $exception->getTrace();
-
-    // This value is missing from the stack for some reason in the
-    // FlattenException version of the backtrace.
-    $backtrace[0]['line'] = $exception->getLine();
-
-    // For database errors, we try to return the initial caller,
-    // skipping internal functions of the database layer.
-    if (strpos($exception->getClass(), 'DatabaseExceptionWrapper') !== FALSE) {
-      // A DatabaseExceptionWrapper exception is actually just a courier for
-      // the original PDOException.  It's the stack trace from that exception
-      // that we care about.
-      $backtrace = $exception->getPrevious()->getTrace();
-      $backtrace[0]['line'] = $exception->getLine();
-
-      // The first element in the stack is the call, the second element gives us the caller.
-      // We skip calls that occurred in one of the classes of the database layer
-      // or in one of its global functions.
-      $db_functions = array('db_query',  'db_query_range');
-      while (!empty($backtrace[1]) && ($caller = $backtrace[1]) &&
-          ((strpos($caller['namespace'], 'Drupal\Core\Database') !== FALSE || strpos($caller['class'], 'PDO') !== FALSE)) ||
-          in_array($caller['function'], $db_functions)) {
-        // We remove that call.
-        array_shift($backtrace);
-      }
-    }
-
-    $caller = Error::getLastCaller($backtrace);
-
-    return array(
-      '%type' => $exception->getClass(),
-      // The standard PHP exception handler considers that the exception message
-      // is plain-text. We mimick this behavior here.
-      '!message' => String::checkPlain($message),
-      '%function' => $caller['function'],
-      '%file' => $caller['file'],
-      '%line' => $caller['line'],
-      'severity_level' => WATCHDOG_ERROR,
-    );
-  }
-
-}
diff --git a/core/lib/Drupal/Core/EventSubscriber/CustomPageExceptionHtmlSubscriber.php b/core/lib/Drupal/Core/EventSubscriber/CustomPageExceptionHtmlSubscriber.php
new file mode 100644
index 0000000..fa20119
--- /dev/null
+++ b/core/lib/Drupal/Core/EventSubscriber/CustomPageExceptionHtmlSubscriber.php
@@ -0,0 +1,126 @@
+<?php
+/**
+ * @file
+ * Contains Drupal\Core\EventSubscriber\DefaultExceptionHtmlSubscriber
+ */
+
+namespace Drupal\Core\EventSubscriber;
+
+use Drupal\Core\Config\ConfigFactoryInterface;
+use Drupal\Core\Path\AliasManagerInterface;
+use Symfony\Component\HttpFoundation\Response;
+use Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent;
+use Symfony\Component\HttpKernel\HttpKernelInterface;
+
+/**
+ * Exception subscriber for handling core custom error pages.
+ */
+class CustomPageExceptionHtmlSubscriber extends HttpExceptionSubscriberBase {
+
+  /**
+   * The site settings configuration.
+   *
+   * @var \Drupal\Core\Config\Config
+   */
+  protected $systemSiteSettings;
+
+  /**
+   * @var \Drupal\Core\Path\AliasManagerInterface
+   */
+  protected $aliasManager;
+
+  /**
+   * @var \Symfony\Component\HttpKernel\HttpKernelInterface
+   */
+  protected $httpKernel;
+
+  /**
+   * Constructs a new CustomPageExceptionHtmlSubscriber.
+   *
+   * @param ConfigFactoryInterface $config_factory
+   *   The configuration factory.
+   * @param AliasManagerInterface $alias_manager
+   *   The alias manager service.
+   * @param HttpKernelInterface $http_kernel
+   *   The HTTP Kernel service.
+   */
+  public function __construct(ConfigFactoryInterface $config_factory, AliasManagerInterface $alias_manager, HttpKernelInterface $http_kernel) {
+    $this->systemSiteSettings = $config_factory->get('system.site');
+    $this->aliasManager = $alias_manager;
+    $this->httpKernel = $http_kernel;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static function getPriority() {
+    return -50;
+  }
+
+  /**
+   * {@inheritDoc}
+   */
+  protected function getHandledFormats() {
+    return ['html'];
+  }
+
+  /**
+   * Handles a 403 error for HTML.
+   *
+   * @param \Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent $event
+   *   The event to process.
+   */
+  public function on403(GetResponseForExceptionEvent $event) {
+    $request = $event->getRequest();
+
+    // @todo Remove dependency on the internal _system_path attribute:
+    //   https://www.drupal.org/node/2293523.
+    $system_path = $request->attributes->get('_system_path');
+
+    $path = $this->aliasManager->getPathByAlias($this->systemSiteSettings->get('page.403'));
+    if ($path && $path != $system_path) {
+      if ($request->getMethod() === 'POST') {
+        $subrequest = Request::create($request->getBaseUrl() . '/' . $path, 'POST', array('destination' => $system_path, '_exception_statuscode' => Response::HTTP_FORBIDDEN) + $request->request->all(), $request->cookies->all(), array(), $request->server->all());
+      }
+      else {
+        $subrequest = Request::create($request->getBaseUrl() . '/' . $path, 'GET', array('destination' => $system_path, '_exception_statuscode' => Response::HTTP_FORBIDDEN), $request->cookies->all(), [], $request->server->all());
+      }
+
+      $response = $this->httpKernel->handle($subrequest, HttpKernelInterface::SUB_REQUEST);
+      $response->setStatusCode(Response::HTTP_FORBIDDEN, 'Access denied');
+      $event->setResponse($response);
+    }
+  }
+
+  /**
+   * Handles a 404 error for HTML.
+   *
+   * @param \Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent $event
+   *   The event to process.
+   */
+  public function on404(GetResponseForExceptionEvent $event) {
+    $request = $event->getRequest();
+
+    // @todo Remove dependency on the internal _system_path attribute:
+    //   https://www.drupal.org/node/2293523.
+    $system_path = $request->attributes->get('_system_path');
+
+    $path = $this->aliasManager->getPathByAlias($this->systemSiteSettings->get('page.404'));
+    if ($path && $path != $system_path) {
+      // @todo Um, how do I specify an override URL again? Totally not clear. Do
+      //   that and sub-call the kernel rather than using meah().
+      // @todo The create() method expects a slash-prefixed path, but we store a
+      //   normal system path in the site_404 variable.
+      if ($request->getMethod() === 'POST') {
+        $subrequest = Request::create($request->getBaseUrl() . '/' . $path, 'POST', array('destination' => $system_path, '_exception_statuscode' => 404) + $request->request->all(), $request->cookies->all(), array(), $request->server->all());
+      }
+      else {
+        $subrequest = Request::create($request->getBaseUrl() . '/' . $path, 'GET', array('destination' => $system_path, '_exception_statuscode' => 404), $request->cookies->all(), array(), $request->server->all());
+      }
+
+      $response = $this->httpKernel->handle($subrequest, HttpKernelInterface::SUB_REQUEST);
+      $response->setStatusCode(Response::HTTP_NOT_FOUND, 'Not Found');
+      $event->setResponse($response);
+    }
+  }
+}
diff --git a/core/lib/Drupal/Core/EventSubscriber/DefaultExceptionHtmlSubscriber.php b/core/lib/Drupal/Core/EventSubscriber/DefaultExceptionHtmlSubscriber.php
new file mode 100644
index 0000000..8f020f8
--- /dev/null
+++ b/core/lib/Drupal/Core/EventSubscriber/DefaultExceptionHtmlSubscriber.php
@@ -0,0 +1,123 @@
+<?php
+/**
+ * @file
+ * Contains Drupal\Core\EventSubscriber\DefaultExceptionHtmlSubscriber
+ */
+
+namespace Drupal\Core\EventSubscriber;
+
+use Drupal\Core\Config\ConfigFactoryInterface;
+use Drupal\Core\Page\HtmlFragment;
+use Drupal\Core\Page\HtmlFragmentRendererInterface;
+use Drupal\Core\Page\HtmlPageRendererInterface;
+use Drupal\Core\StringTranslation\StringTranslationTrait;
+use Symfony\Component\HttpFoundation\Response;
+use Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent;
+
+/**
+ * Handle most HTTP errors for HTML.
+ */
+class DefaultExceptionHtmlSubscriber extends HttpExceptionSubscriberBase {
+  use StringTranslationTrait;
+
+  /**
+   * @var \Drupal\Core\Page\HtmlFragmentRendererInterface
+   */
+  protected $fragmentRenderer;
+
+  /**
+   * @var \Drupal\Core\Page\HtmlPageRendererInterface
+   */
+  protected $htmlPageRenderer;
+
+  /**
+   * @var string
+   *
+   * One of the error level constants defined in bootstrap.inc.
+   */
+  protected $errorLevel;
+
+  /**
+   * Constructs a new DefaultExceptionHtmlSubscriber.
+   *
+   * @param HtmlFragmentRendererInterface $fragment_renderer
+   *   The fragment renderer.
+   * @param HtmlPageRendererInterface $page_renderer
+   *   The page renderer.
+   * @param ConfigFactoryInterface $config_factory
+   *   The configuration factory.
+   */
+  public function __construct(HtmlFragmentRendererInterface $fragment_renderer, HtmlPageRendererInterface $page_renderer, ConfigFactoryInterface $config_factory) {
+    $this->fragmentRenderer = $fragment_renderer;
+    $this->htmlPageRenderer = $page_renderer;
+    $this->errorLevel = $config_factory->get('system.logging')->get('error_level');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static function getPriority() {
+    // A very low priority so that custom handlers are almost certain to fire
+    // before it, even if someone forgets to set a priority.
+    return -128;
+  }
+
+  /**
+   * {@inheritDoc}
+   */
+  protected function getHandledFormats() {
+    return ['html'];
+  }
+
+  /**
+   * Handles a 403 error for HTML.
+   *
+   * @param \Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent $event
+   *   The event to process.
+   */
+  public function on403(GetResponseForExceptionEvent $event) {
+    $response = $this->createResponse($this->t('Access denied'), $this->t('You are not authorized to access this page.'), Response::HTTP_FORBIDDEN);
+    $event->setResponse($response);
+  }
+
+  /**
+   * Handles a 404 error for HTML.
+   *
+   * @param \Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent $event
+   *   The event to process.
+   */
+  public function on404(GetResponseForExceptionEvent $event) {
+    $path = $event->getRequest()->getPathInfo();
+    $response = $this->createResponse($this->t('Page not found'), $this->t('The requested page "@path" could not be found.', ['@path' => $path]), Response::HTTP_NOT_FOUND);
+    $event->setResponse($response);
+  }
+
+  /**
+   * Handles a 405 error for HTML.
+   *
+   * @param \Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent $event
+   *   The event to process.
+   */
+  public function on405(GetResponseForExceptionEvent $event) {
+    $response = new Response('Method Not Allowed', Response::HTTP_METHOD_NOT_ALLOWED);
+    $event->setResponse($response);
+  }
+
+  /**
+   * @param $title
+   *   The page title of the response.
+   * @param $body
+   *   The body of the error page.
+   * @param $response_code
+   *   The HTTP response code of the response.
+   * @return Response
+   *   An error Response object ready to return to the browser.
+   */
+  protected function createResponse($title, $body, $response_code) {
+    $fragment = new HtmlFragment($body);
+    $fragment->setTitle($title);
+
+    $page = $this->fragmentRenderer->render($fragment, $response_code);
+    return new Response($this->htmlPageRenderer->render($page), $page->getStatusCode());
+  }
+}
\ No newline at end of file
diff --git a/core/lib/Drupal/Core/EventSubscriber/DefaultExceptionSubscriber.php b/core/lib/Drupal/Core/EventSubscriber/DefaultExceptionSubscriber.php
new file mode 100644
index 0000000..2b8425a
--- /dev/null
+++ b/core/lib/Drupal/Core/EventSubscriber/DefaultExceptionSubscriber.php
@@ -0,0 +1,197 @@
+<?php
+/**
+ * @file
+ * Contains Drupal\Core\EventSubscriber\DefaultExceptionSubscriber
+ */
+
+namespace Drupal\Core\EventSubscriber;
+
+use Drupal\Component\Utility\SafeMarkup;
+use Drupal\Component\Utility\String;
+use Drupal\Core\Config\ConfigFactoryInterface;
+use Drupal\Core\Page\DefaultHtmlPageRenderer;
+use Drupal\Core\Page\HtmlFragment;
+use Drupal\Core\Page\HtmlFragmentRendererInterface;
+use Drupal\Core\Page\HtmlPageRendererInterface;
+use Drupal\Core\StringTranslation\StringTranslationTrait;
+use Drupal\Core\Utility\Error;
+use Symfony\Component\EventDispatcher\EventSubscriberInterface;
+use Symfony\Component\HttpFoundation\JsonResponse;
+use Symfony\Component\HttpFoundation\Response;
+use Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent;
+use Symfony\Component\HttpKernel\KernelEvents;
+
+/**
+ * Last-chance handler for exceptions.
+ *
+ * This handler will catch any exceptions not caught elsewhere and report
+ * them as an error page.
+ */
+class DefaultExceptionSubscriber implements EventSubscriberInterface {
+  use StringTranslationTrait;
+
+  /**
+   * @var \Drupal\Core\Page\HtmlFragmentRendererInterface
+   */
+  protected $fragmentRenderer;
+
+  /**
+   * @var \Drupal\Core\Page\HtmlPageRendererInterface
+   */
+  protected $htmlPageRenderer;
+
+  /**
+   * @var string
+   *
+   * One of the error level constants defined in bootstrap.inc.
+   */
+  protected $errorLevel;
+
+  /**
+   * Constructs a new DefaultExceptionHtmlSubscriber.
+   *
+   * @param \Drupal\Core\Page\HtmlFragmentRendererInterface $fragment_renderer
+   *   The fragment renderer.
+   * @param \Drupal\Core\Page\HtmlPageRendererInterface $page_renderer
+   *   The page renderer.
+   * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
+   *   The configuration factory.
+   */
+  public function __construct(HtmlFragmentRendererInterface $fragment_renderer, HtmlPageRendererInterface $page_renderer, ConfigFactoryInterface $config_factory) {
+    $this->fragmentRenderer = $fragment_renderer;
+    $this->htmlPageRenderer = $page_renderer;
+    $this->errorLevel = $config_factory->get('system.logging')->get('error_level');
+  }
+
+  /**
+   * Handles any exception as a generic error page for HTML.
+   *
+   * @param \Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent $event
+   *   The event to process.
+   */
+  protected function onHtml(GetResponseForExceptionEvent $event) {
+    $exception = $event->getException();
+    $error = Error::decodeException($exception);
+
+    // Display the message if the current error reporting level allows this type
+    // of message to be displayed, and unconditionnaly in update.php.
+    if (error_displayable($error)) {
+      $class = 'error';
+
+      // If error type is 'User notice' then treat it as debug information
+      // instead of an error message.
+      // @see debug()
+      if ($error['%type'] == 'User notice') {
+        $error['%type'] = 'Debug';
+        $class = 'status';
+      }
+
+      // Attempt to reduce verbosity by removing DRUPAL_ROOT from the file path
+      // in the message. This does not happen for (false) security.
+      $root_length = strlen(DRUPAL_ROOT);
+      if (substr($error['%file'], 0, $root_length) == DRUPAL_ROOT) {
+        $error['%file'] = substr($error['%file'], $root_length + 1);
+      }
+      // Do not translate the string to avoid errors producing more errors.
+      $message = String::format('%type: !message in %function (line %line of %file).', $error);
+
+      // Check if verbose error reporting is on.
+      if ($this->errorLevel == ERROR_REPORTING_DISPLAY_VERBOSE) {
+        $backtrace_exception = $exception;
+        while ($backtrace_exception->getPrevious()) {
+          $backtrace_exception = $backtrace_exception->getPrevious();
+        }
+        $backtrace = $backtrace_exception->getTrace();
+        // First trace is the error itself, already contained in the message.
+        // While the second trace is the error source and also contained in the
+        // message, the message doesn't contain argument values, so we output it
+        // once more in the backtrace.
+        array_shift($backtrace);
+        // Generate a backtrace containing only scalar argument values.
+        $message .= '<pre class="backtrace">' . Error::formatFlattenedBacktrace($backtrace) . '</pre>';
+      }
+      drupal_set_message(SafeMarkup::set($message), $class, TRUE);
+    }
+
+    $content = $this->t('The website has encountered an error. Please try again later.');
+    $output = DefaultHtmlPageRenderer::renderPage($content, $this->t('Error'));
+    $response = new Response($output);
+    $response->setStatusCode(Response::HTTP_INTERNAL_SERVER_ERROR, '500 Service unavailable (with message)');
+
+    $event->setResponse($response);
+  }
+
+  /**
+   * Handles any exception as a generic error page for JSON.
+   *
+   * @todo This should probably check the error reporting leve.
+   *
+   * @param \Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent $event
+   *   The event to process.
+   */
+  protected function onJson(GetResponseForExceptionEvent $event) {
+    $exception = $event->getException();
+    $error = Error::decodeException($exception);
+
+    // Display the message if the current error reporting level allows this type
+    // of message to be displayed,
+    $message = error_displayable($error) ? $exception->getMessage() : '';
+
+    $response = new JsonResponse([$message], Response::HTTP_INTERNAL_SERVER_ERROR);
+    $event->setResponse($response);
+  }
+
+  /**
+   * Creates an Html response for the provided criteria.
+   *
+   * @param $title
+   *   The page title of the response.
+   * @param $body
+   *   The body of the error page.
+   * @param $response_code
+   *   The HTTP response code of the response.
+   * @return \Symfony\Component\HttpFoundation\Response
+   *   An error Response object ready to return to the browser.
+   */
+  protected function createHtmlResponse($title, $body, $response_code) {
+    $fragment = new HtmlFragment($body);
+    $fragment->setTitle($title);
+
+    $page = $this->fragmentRenderer->render($fragment, $response_code);
+    return new Response($this->htmlPageRenderer->render($page), $page->getStatusCode());
+  }
+
+  /**
+   * Handles errors for this subscriber.
+   *
+   * @param \Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent $event
+   *   The event to process.
+   */
+  public function onException(GetResponseForExceptionEvent $event) {
+    $format = $event->getRequest()->getRequestFormat();
+
+    // These are all JSON errors for our purposes. Any special handling for
+    // them can/should happen in earlier listeners if desired.
+    if (in_array($format, ['drupal_modal', 'drupal_dialog', 'drupal_ajax'])) {
+      $format = 'json';
+    }
+
+    // If it's an unrecognized format, assume HTML.
+    $method = 'on' . $format;
+    if (!method_exists($this, $method)) {
+      $method = 'onHtml';
+    }
+    $this->$method($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', -256];
+    return $events;
+  }
+}
diff --git a/core/lib/Drupal/Core/EventSubscriber/ExceptionJsonSubscriber.php b/core/lib/Drupal/Core/EventSubscriber/ExceptionJsonSubscriber.php
new file mode 100644
index 0000000..18af507
--- /dev/null
+++ b/core/lib/Drupal/Core/EventSubscriber/ExceptionJsonSubscriber.php
@@ -0,0 +1,67 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Core\EventSubscriber\ExceptionJsonSubscriber.
+ */
+
+namespace Drupal\Core\EventSubscriber;
+
+use Symfony\Component\HttpFoundation\JsonResponse;
+use Symfony\Component\HttpFoundation\Response;
+use Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent;
+
+/**
+ * Default handling for JSON errors.
+ */
+class ExceptionJsonSubscriber extends HttpExceptionSubscriberBase {
+
+  /**
+   * {@inheritDoc}
+   */
+  protected function getHandledFormats() {
+    return ['json'];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static function getPriority() {
+    // This will fire after the most common HTML handler, since HTML requests
+    // are still more common than JSON requests.
+    return -75;
+  }
+
+  /**
+   * Handles a 403 error for JSON.
+   *
+   * @param \Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent $event
+   *   The event to process.
+   */
+  public function on403(GetResponseForExceptionEvent $event) {
+    $response = new JsonResponse(NULL, Response::HTTP_FORBIDDEN);
+    $event->setResponse($response);
+  }
+
+  /**
+   * Handles a 404 error for JSON.
+   *
+   * @param \Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent $event
+   *   The event to process.
+   */
+  public function on404(GetResponseForExceptionEvent $event) {
+    $response = new JsonResponse(NULL, Response::HTTP_NOT_FOUND);
+    $event->setResponse($response);
+  }
+
+  /**
+   * Handles a 405 error for JSON.
+   *
+   * @param \Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent $event
+   *   The event to process.
+   */
+  public function on405(GetResponseForExceptionEvent $event) {
+    $response = new JsonResponse(NULL, Response::HTTP_METHOD_NOT_ALLOWED);
+    $event->setResponse($response);
+  }
+}
diff --git a/core/lib/Drupal/Core/EventSubscriber/ExceptionListener.php b/core/lib/Drupal/Core/EventSubscriber/ExceptionListener.php
deleted file mode 100644
index 0944703..0000000
--- a/core/lib/Drupal/Core/EventSubscriber/ExceptionListener.php
+++ /dev/null
@@ -1,36 +0,0 @@
-<?php
-
-/**
- * @file
- * Contains \Drupal\Core\EventSubscriber\ExceptionListener.
- */
-
-namespace Drupal\Core\EventSubscriber;
-
-use Symfony\Component\Debug\Exception\FlattenException;
-use Symfony\Component\HttpFoundation\Request;
-use Symfony\Component\HttpKernel\EventListener\ExceptionListener as ExceptionListenerBase;
-use Symfony\Component\HttpKernel\Log\DebugLoggerInterface;
-
-/**
- * Extends the symfony exception listener to support POST subrequests.
- */
-class ExceptionListener extends ExceptionListenerBase {
-
-  /**
-   * {@inheritdoc}
-   *
-   * In contrast to the symfony base class, do not override POST requests to GET
-   * requests.
-   */
-  protected function duplicateRequest(\Exception $exception, Request $request) {
-    $attributes = array(
-      '_controller' => $this->controller,
-      'exception' => FlattenException::create($exception),
-      'logger' => $this->logger instanceof DebugLoggerInterface ? $this->logger : NULL,
-      'format' => $request->getRequestFormat(),
-    );
-    return $request->duplicate(NULL, NULL, $attributes);
-  }
-
-}
diff --git a/core/lib/Drupal/Core/EventSubscriber/ExceptionLoggingSubscriber.php b/core/lib/Drupal/Core/EventSubscriber/ExceptionLoggingSubscriber.php
new file mode 100644
index 0000000..22ce88f
--- /dev/null
+++ b/core/lib/Drupal/Core/EventSubscriber/ExceptionLoggingSubscriber.php
@@ -0,0 +1,104 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Core\EventSubscriber\ExceptionLoggingSubscriber.
+ */
+
+namespace Drupal\Core\EventSubscriber;
+
+use Drupal\Component\Utility\String;
+use Drupal\Core\Logger\LoggerChannelFactoryInterface;
+use Drupal\Core\Utility\Error;
+use Symfony\Component\EventDispatcher\EventSubscriberInterface;
+use Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent;
+use Symfony\Component\HttpKernel\Exception\HttpException;
+use Symfony\Component\HttpKernel\KernelEvents;
+
+/**
+ * Log exceptions without further handling.
+ */
+class ExceptionLoggingSubscriber implements EventSubscriberInterface {
+
+  /**
+   * @var \Drupal\Core\Logger\LoggerChannelFactoryInterface
+   */
+  protected $logger;
+
+  /**
+   * Constructs a new ExceptionLoggingSubscriber.
+   *
+   * @param \Drupal\Core\Logger\LoggerChannelFactoryInterface $logger
+   */
+  public function __construct(LoggerChannelFactoryInterface $logger) {
+    $this->logger = $logger;
+  }
+
+  /**
+   * Log 403 errors.
+   *
+   * @param \Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent $event
+   *   The event to process.
+   */
+  public function on403(GetResponseForExceptionEvent $event) {
+    $request = $event->getRequest();
+    // @todo Remove dependency on the internal _system_path attribute:
+    //   https://www.drupal.org/node/2293523.
+    $this->logger->get('access denied')->warning(String::checkPlain($request->attributes->get('_system_path')));
+  }
+
+  /**
+   * Log 404 errors.
+   *
+   * @param \Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent $event
+   *   The event to process.
+   */
+  public function on404(GetResponseForExceptionEvent $event) {
+    $request = $event->getRequest();
+    // @todo Remove dependency on the internal _system_path attribute:
+    //   https://www.drupal.org/node/2293523.
+    $this->logger->get('page not found')->warning(String::checkPlain($request->attributes->get('_system_path')));
+  }
+
+  /**
+   * Log not-otherwise-specified errors, including HTTP 500.
+   *
+   * @param \Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent $event
+   *   The event to process.
+   */
+  public function onError(GetResponseForExceptionEvent $event) {
+    $exception = $event->getException();
+    $error = Error::decodeException($exception);
+    $this->logger->get('php')->log($error['severity_level'], '%type: !message in %function (line %line of %file).', $error);
+  }
+
+  /**
+   * Log all exceptions.
+   *
+   * @param \Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent $event
+   *   The event to process.
+   */
+  public function onException(GetResponseForExceptionEvent $event) {
+    $exception = $event->getException();
+
+    $method = 'onError';
+
+    // Treat any non-HTTP exception as if it were one, so we log it the same.
+    if ($exception instanceof HttpException) {
+      $possible_method = 'on500';
+      if (method_exists($this, $possible_method)) {
+        $method = $possible_method;
+      }
+    }
+
+    $this->$method($event);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function getSubscribedEvents() {
+    $events[KernelEvents::EXCEPTION][] = ['onException', 50];
+    return $events;
+  }
+}
diff --git a/core/lib/Drupal/Core/EventSubscriber/ExceptionTestSiteSubscriber.php b/core/lib/Drupal/Core/EventSubscriber/ExceptionTestSiteSubscriber.php
new file mode 100644
index 0000000..96a2e4a
--- /dev/null
+++ b/core/lib/Drupal/Core/EventSubscriber/ExceptionTestSiteSubscriber.php
@@ -0,0 +1,65 @@
+<?php
+/**
+ * @file
+ * Contains Drupal\Core\EventSubscriber\ExceptionTestSiteSubscriber
+ */
+
+namespace Drupal\Core\EventSubscriber;
+
+use Drupal\Core\Utility\Error;
+use Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent;
+
+/**
+ * Custom handling of errors when in a system-under-test.
+ */
+class ExceptionTestSiteSubscriber extends HttpExceptionSubscriberBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static function getPriority() {
+    return 3;
+  }
+
+  /**
+   * {@inheritDoc}
+   */
+  protected function getHandledFormats() {
+    return ['html'];
+  }
+
+  /**
+   * Checks for special handling of errors inside Simpletest.
+   *
+   * @todo The $headers array appears to not actually get used at all in the
+   * original code. It's quite possible that this entire method is now
+   * vestigial and can be removed.
+   *
+   * @param \Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent $event
+   */
+  public function on500(GetResponseForExceptionEvent $event) {
+    $exception = $event->getException();
+    $error = Error::decodeException($exception);
+
+    $headers = array();
+
+    // When running inside the testing framework, we relay the errors
+    // to the tested site by the way of HTTP headers.
+    if (DRUPAL_TEST_IN_CHILD_SITE && !headers_sent() && (!defined('SIMPLETEST_COLLECT_ERRORS') || SIMPLETEST_COLLECT_ERRORS)) {
+      // $number does not use drupal_static as it should not be reset
+      // as it uniquely identifies each PHP error.
+      static $number = 0;
+      $assertion = array(
+        $error['!message'],
+        $error['%type'],
+        array(
+          'function' => $error['%function'],
+          'file' => $error['%file'],
+          'line' => $error['%line'],
+        ),
+      );
+      $headers['X-Drupal-Assertion-' . $number] = rawurlencode(serialize($assertion));
+      $number++;
+    }
+  }
+}
diff --git a/core/lib/Drupal/Core/EventSubscriber/Fast404ExceptionHtmlSubscriber.php b/core/lib/Drupal/Core/EventSubscriber/Fast404ExceptionHtmlSubscriber.php
new file mode 100644
index 0000000..d00023c
--- /dev/null
+++ b/core/lib/Drupal/Core/EventSubscriber/Fast404ExceptionHtmlSubscriber.php
@@ -0,0 +1,84 @@
+<?php
+/**
+ * @file
+ * Contains Drupal\Core\EventSubscriber\Fast404ExceptionHtmlSubscriber
+ */
+
+namespace Drupal\Core\EventSubscriber;
+
+use Drupal\Core\Config\ConfigFactoryInterface;
+use Drupal\Core\StringTranslation\StringTranslationTrait;
+use Drupal\Component\Utility\String;
+use Symfony\Component\HttpFoundation\Response;
+use Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent;
+use Symfony\Component\HttpKernel\HttpKernelInterface;
+
+/**
+ * High-performance 404 exception subscriber.
+ *
+ * This subscriber will return a minimalist 404 response for HTML requests
+ * without running a full page theming operation.
+ */
+class Fast404ExceptionHtmlSubscriber extends HttpExceptionSubscriberBase {
+
+  /**
+   * The site performance configuration.
+   *
+   * @var \Drupal\Core\Config\Config
+   */
+  protected $systemPerformance;
+
+  /**
+   * @var \Symfony\Component\HttpKernel\HttpKernelInterface
+   */
+  protected $httpKernel;
+
+  /**
+   * Constructs a new CustomPageExceptionHtmlSubscriber.
+   *
+   * @param \Drupal\Core\Config\ConfigFactoryInterfac $config_factory
+   *   The configuration factory.
+   * @param \Symfony\Component\HttpKernel\HttpKernelInterface $http_kernel
+   *   The HTTP Kernel service.
+   */
+  public function __construct(ConfigFactoryInterface $config_factory, HttpKernelInterface $http_kernel) {
+    $this->systemPerformance = $config_factory->get('system.performance');
+    $this->httpKernel = $http_kernel;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static function getPriority() {
+    // A very high priority so that it can take precedent over anything else,
+    // and thus be fast.
+    return 100;
+  }
+
+  /**
+   * {@inheritDoc}
+   */
+  protected function getHandledFormats() {
+    return ['html'];
+  }
+
+  /**
+   * Handles a 404 error for HTML.
+   *
+   * @param \Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent $event
+   *   The event to process.
+   */
+  public function on404(GetResponseForExceptionEvent $event) {
+    $request = $event->getRequest();
+
+    $exclude_paths = $this->systemPerformance->get('fast_404.exclude_paths');
+    if ($this->systemPerformance->get('fast_404.enabled') && $exclude_paths && !preg_match($exclude_paths, $request->getPathInfo())) {
+      $fast_paths = $this->systemPerformance->get('fast_404.paths');
+      if ($fast_paths && preg_match($fast_paths, $request->getPathInfo())) {
+        $fast_404_html = strtr($this->systemPerformance->get('fast_404.html'), ['@path' => String::checkPlain($request->getUri())]);
+        $response = new Response($fast_404_html, Response::HTTP_NOT_FOUND);
+        $event->setResponse($response);
+      }
+    }
+  }
+}
diff --git a/core/lib/Drupal/Core/EventSubscriber/HttpExceptionSubscriberBase.php b/core/lib/Drupal/Core/EventSubscriber/HttpExceptionSubscriberBase.php
new file mode 100644
index 0000000..b550399
--- /dev/null
+++ b/core/lib/Drupal/Core/EventSubscriber/HttpExceptionSubscriberBase.php
@@ -0,0 +1,89 @@
+<?php
+/**
+ * @file
+ * Contains \Drupal\Core\EventSubscriber\HttpExceptionSubscriberBase.
+ */
+
+namespace Drupal\Core\EventSubscriber;
+
+use Symfony\Component\EventDispatcher\EventSubscriberInterface;
+use Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent;
+use Symfony\Component\HttpKernel\Exception\HttpException;
+use Symfony\Component\HttpKernel\KernelEvents;
+
+/**
+ * Utilty 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 404 Not Found messages add a method:
+ *
+ * @code
+ * public function on404(GetResponseForExceptionEvent $event) {}
+ * @endcode
+ *
+ * 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.
+ */
+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 ot process,such as "html" or "json". Returning an empty array
+   *   will apply to all formats.
+   *
+   * @see \Symfony\Component\HttpFoundation\Request
+   */
+  abstract protected 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 1;
+  }
+
+  /**
+   * Handles errors for this subscriber.
+   *
+   * @param \Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent $event
+   *   The event to process.
+   */
+  public function onException(GetResponseForExceptionEvent $event) {
+    $exception = $event->getException();
+    $handled_formats = $this->getHandledFormats();
+    if ($exception instanceof HttpException && (empty($handled_formats) || in_array($event->getRequest()->getRequestFormat(), $handled_formats))) {
+      $method = 'on' . $exception->getStatusCode();
+      // We want to allow the method to be called and still not set a response
+      // if it has additional filtering logic to determine when it will apply.
+      // It is therefore the method's responsibility to set the response on the
+      // event if appropriate.
+      if (method_exists($this, $method)) {
+        $this->$method($event);
+      }
+    }
+  }
+
+  /**
+   * 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;
+  }
+
+}
diff --git a/core/lib/Drupal/Core/Routing/Enhancer/ParamConversionEnhancer.php b/core/lib/Drupal/Core/Routing/Enhancer/ParamConversionEnhancer.php
index 614ac96..e94a249 100644
--- a/core/lib/Drupal/Core/Routing/Enhancer/ParamConversionEnhancer.php
+++ b/core/lib/Drupal/Core/Routing/Enhancer/ParamConversionEnhancer.php
@@ -86,7 +86,7 @@ public function onException(GetResponseForExceptionEvent $event) {
    * {@inheritdoc}
    */
   public static function getSubscribedEvents() {
-    $events[KernelEvents::EXCEPTION][] = array('onException', 0);
+    $events[KernelEvents::EXCEPTION][] = array('onException', 75);
     return $events;
   }
 
diff --git a/core/tests/Drupal/Tests/Core/Controller/ExceptionControllerTest.php b/core/tests/Drupal/Tests/Core/Controller/ExceptionControllerTest.php
deleted file mode 100644
index 3a796ec..0000000
--- a/core/tests/Drupal/Tests/Core/Controller/ExceptionControllerTest.php
+++ /dev/null
@@ -1,58 +0,0 @@
-<?php
-
-/**
- * @file
- * Contains \Drupal\Tests\Core\Controller\ExceptionControllerTest
- */
-
-namespace Drupal\Tests\Core\Controller {
-
-use Drupal\Core\Controller\ExceptionController;
-use Drupal\Tests\UnitTestCase;
-use Symfony\Component\HttpFoundation\Request;
-use Symfony\Component\Debug\Exception\FlattenException;
-
-/**
- * @coversDefaultClass \Drupal\Core\Controller\ExceptionController
- * @group Controller
- */
-class ExceptionControllerTest extends UnitTestCase {
-
-  /**
-   * Ensure the execute() method returns a valid response on 405 exceptions.
-   */
-  public function test405HTML() {
-    $exception = new \Exception('Test exception');
-    $flat_exception = FlattenException::create($exception, 405);
-    $html_page_renderer = $this->getMock('Drupal\Core\Page\HtmlPageRendererInterface');
-    $html_fragment_renderer = $this->getMock('Drupal\Core\Page\HtmlFragmentRendererInterface');
-    $title_resolver = $this->getMock('Drupal\Core\Controller\TitleResolverInterface');
-    $translation = $this->getMock('Drupal\Core\StringTranslation\TranslationInterface');
-    $url_generator = $this->getMock('Drupal\Core\Routing\UrlGeneratorInterface');
-    $logger_factory = $this->getMock('Drupal\Core\Logger\LoggerChannelFactoryInterface');
-
-    $content_negotiation = $this->getMock('Drupal\Core\ContentNegotiation');
-    $content_negotiation->expects($this->any())
-      ->method('getContentType')
-      ->will($this->returnValue('html'));
-
-    $exception_controller = new ExceptionController($content_negotiation, $title_resolver, $html_page_renderer, $html_fragment_renderer, $translation, $url_generator, $logger_factory);
-    $response = $exception_controller->execute($flat_exception, new Request());
-    $this->assertEquals($response->getStatusCode(), 405, 'HTTP status of response is correct.');
-    $this->assertEquals($response->getContent(), 'Method Not Allowed', 'HTTP response body is correct.');
-  }
-
-}
-
-}
-
-namespace {
-  use Drupal\Core\Language\Language;
-
-  if (!function_exists('language_default')) {
-    function language_default() {
-      $language = new Language(array('langcode' => 'en'));
-      return $language;
-    }
-  }
-}
diff --git a/core/tests/Drupal/Tests/Core/EventSubscriber/ExceptionListenerTest.php b/core/tests/Drupal/Tests/Core/EventSubscriber/ExceptionListenerTest.php
deleted file mode 100644
index 5815ee1..0000000
--- a/core/tests/Drupal/Tests/Core/EventSubscriber/ExceptionListenerTest.php
+++ /dev/null
@@ -1,98 +0,0 @@
-<?php
-
-/**
- * @file
- * Contains \Drupal\Tests\Core\EventSubscriber\ExceptionListenerTest.
- */
-
-namespace Drupal\Tests\Core\EventSubscriber;
-
-use Drupal\Component\Utility\UrlHelper;
-use Drupal\Core\EventSubscriber\ExceptionListener;
-use Drupal\Tests\UnitTestCase;
-use Symfony\Component\HttpFoundation\Request;
-use Symfony\Component\HttpFoundation\Response;
-use Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent;
-
-/**
- * @coversDefaultClass \Drupal\Core\EventSubscriber\ExceptionListener
- * @group EventSubscriber
- */
-class ExceptionListenerTest extends UnitTestCase {
-
-  /**
-   * The tested exception listener.
-   *
-   * @var \Drupal\Core\EventSubscriber\ExceptionListener
-   */
-  protected $exceptionListener;
-
-  /**
-   * The mocked HTTP kernel.
-   *
-   * @var \Symfony\Component\HttpKernel\HttpKernelInterface|\PHPUnit_Framework_MockObject_MockObject
-   */
-  protected $kernel;
-
-  /**
-   * The PHP error log settings before the test.
-   *
-   * @var string
-   */
-  protected $errorLog;
-
-  /**
-   * {@inheritdoc}
-   */
-  protected function setUp() {
-    $this->exceptionListener = new ExceptionListener('example');
-    $this->kernel = $this->getMock('Symfony\Component\HttpKernel\HttpKernelInterface');
-
-    // You can't create an exception in PHP without throwing it. Store the
-    // current error_log, and disable it temporarily.
-    $this->errorLog = ini_set('error_log', file_exists('/dev/null') ? '/dev/null' : 'nul');
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  protected function tearDown() {
-    ini_set('error_log', $this->errorLog);
-  }
-
-  /**
-   * Tests onHandleException with a POST request.
-   */
-  public function testHandleWithPostRequest() {
-    $request = Request::create('/test', 'POST', array('name' => 'druplicon', 'pass' => '12345'));
-
-    $this->kernel->expects($this->once())->method('handle')->will($this->returnCallback(function (Request $request) {
-      return new Response($request->getMethod());
-    }));
-
-    $event = new GetResponseForExceptionEvent($this->kernel, $request, 'foo', new \Exception('foo'));
-
-    $this->exceptionListener->onKernelException($event);
-
-    $response = $event->getResponse();
-    $this->assertEquals('POST name=druplicon&pass=12345', $response->getContent() . " " . UrlHelper::buildQuery($request->request->all()));
-  }
-
-  /**
-   * Tests onHandleException with a GET request.
-   */
-  public function testHandleWithGetRequest() {
-    $request = Request::create('/test', 'GET', array('name' => 'druplicon', 'pass' => '12345'));
-
-    $this->kernel->expects($this->once())->method('handle')->will($this->returnCallback(function (Request $request) {
-      return new Response($request->getMethod() . ' ' . UrlHelper::buildQuery($request->query->all()));
-    }));
-
-    $event = new GetResponseForExceptionEvent($this->kernel, $request, 'foo', new \Exception('foo'));
-    $this->exceptionListener->onKernelException($event);
-
-    $response = $event->getResponse();
-    $this->assertEquals('GET name=druplicon&pass=12345 ', $response->getContent() . " " . UrlHelper::buildQuery($request->request->all()));
-  }
-
-}