--- /dev/null
+<?php
+
+namespace Drupal\Tests\Core\EventSubscriber;
+
+use Drupal\Core\EventSubscriber\ExceptionJsonSubscriber;
+use Drupal\Tests\UnitTestCase;
+use Symfony\Component\HttpFoundation\JsonResponse;
+use Symfony\Component\HttpFoundation\Request;
+use Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent;
+use Symfony\Component\HttpKernel\Exception\MethodNotAllowedHttpException;
+use Symfony\Component\HttpKernel\HttpKernelInterface;
+
+/**
+ * @coversDefaultClass \Drupal\Core\EventSubscriber\ExceptionJsonSubscriber
+ * @group EventSubscriber
+ */
+class ExceptionJsonSubscriberTest extends UnitTestCase {
+
+ /**
+ * @covers ::on4xx
+ */
+ public function testOn4xx() {
+ $kernel = $this->prophesize(HttpKernelInterface::class);
+ $request = Request::create('/test');
+ $e = new MethodNotAllowedHttpException(['POST', 'PUT'], 'test message');
+ $event = new GetResponseForExceptionEvent($kernel->reveal(), $request, 'GET', $e);
+ $subscriber = new ExceptionJsonSubscriber();
+ $subscriber->on4xx($event);
+ $response = $event->getResponse();
+
+ $this->assertInstanceOf(JsonResponse::class, $response);
+ $this->assertEquals('{"message":"test message"}', $response->getContent());
+ $this->assertEquals(405, $response->getStatusCode());
+ $this->assertEquals('POST, PUT', $response->headers->get('Allow'));
+ $this->assertEquals('application/json', $response->headers->get('Content-Type'));
+ }
+
+}