FooBarController.php

<?php

namespace Drupal\mymodule\Controller;

use Drupal\Core\Controller\ControllerBase;
use Symfony\Component\HttpFoundation\JsonResponse;
use Symfony\Component\HttpFoundation\Request;

/**
 * Class FooBarController.
 *
 * @package Drupal\mymodule\Controller
 */
class FooBarController extends ControllerBase {
  /**
   * Provides content for the /mymodule/foobar path.
   *
   * @param \Symfony\Component\HttpFoundation\Request $request
   *   The current request
   */
  public function foobar(Request $request) {
    $content = $request->getContent();
    $data = array();
    $data['a'] = $request->get('foo');
    $data['b'] = $request->request->get('foo');
    $data['c'] = \Drupal::requestStack()->getCurrentRequest()->request->get('foo');
    $data['d'] = $request->request->all();
    $data['e'] = $content;

    $request = \Drupal::requestStack()->getCurrentRequest();
    $content = $request->getContent();
    $data['f'] = $request->get('foo');
    $data['g'] = $request->request->get('foo');
    $data['h'] = $request->request->all();
    $data['i'] = $content;
    return new JsonResponse((object) $data);
  }
// Return value:
// {
//   "a": null,
//   "b": null,
//   "c": null,
//   "d": [],
//   "e": "",
//   "f": null,
//  "g": null,
//   "h": [],
//   "i": ""
// }
}