PageCacheAcceptHeaderController.php

<?php

namespace Drupal\system_test\Controller;

use Drupal\Core\Cache\CacheableJsonResponse;
use Drupal\Core\Cache\CacheableResponse;
use Symfony\Component\HttpFoundation\Request;

/**
 * Defines a controller to respond the page cache accept header test.
 */
class PageCacheAcceptHeaderController {
    
    /**
     * Processes a request that will vary with Accept header.
     *
     * @param \Symfony\Component\HttpFoundation\Request $request
     *   The current request object.
     *
     * @return mixed
     */
    public function content(Request $request) {
        if ($request->getRequestFormat() === 'json') {
            return new CacheableJsonResponse([
                'content' => 'oh hai this is json',
            ]);
        }
        else {
            return new CacheableResponse("<p>oh hai this is html.</p>");
        }
    }

}