Custom JSON Controller

AccountController.php

<?php

namespace Drupal\jsonapi_custom\Controller;

use Drupal\Core\Controller\ControllerBase;
use Psr\Http\Message\ServerRequestInterface;
use Symfony\Component\DependencyInjection\ContainerInterface;
use Symfony\Component\HttpFoundation\JsonResponse;
use Drupal\jsonapi_extras\EntityToJsonApi;

class AccountController extends ControllerBase {

  /**
   * @var \Drupal\jsonapi_extras\EntityToJsonApi
   */
  private $entityToJsonApi;

  /**
   * CustomController constructor.
   *
   * @param \Drupal\jsonapi_extras\EntityToJsonApi $entity_to_json_api
   */
  public function __construct(EntityToJsonApi $entity_to_json_api) {
    $this->entityToJsonApi = $entity_to_json_api;
  }

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container) {
    return new static(
      $container->get('jsonapi_extras.entity.to_jsonapi')
    );
  }

  /**
   * Processes a GET request.
   */
  public function me(ServerRequestInterface $request) {
    $user = $this->currentUser();
    return new JsonResponse(
      json_decode($this->entityToJsonApi->serialize($user->getAccount()))
    );
  }

}

jsonapi_custom.routing.yml

oauth2_token.current_user_info:
  path: '/oauth/me'
  defaults:
    _controller: 'Drupal\jsonapi_custom\Controller\AccountController::me'
  methods: [GET]
  requirements:
    _access: 'TRUE'
    _format: 'json'
  options:
    _auth: ['oauth2']
    no_cache: TRUE