Example of how to handle entity deserialization and validation without a form type class in Symfony

Symfony request handler.php

<?php
/** DefaultController.php **/

namespace AppBundle\Controller;

use AppBundle\Entity\Person;
use AppBundle\Service\RequestHandler;
use Sensio\Bundle\FrameworkExtraBundle\Configuration\Route;
use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpFoundation\Response;
use Twig\Environment;

class DefaultController
{
    /** @var  Environment */
    private $twig;
    /** @var  RequestHandler */
    private $handler;

    /**
     * DefaultController constructor.
     * @param Environment $twig
     * @param RequestHandler $handler
     */
    public function __construct(Environment $twig, RequestHandler $handler)
    {
        $this->twig = $twig;
        $this->handler = $handler;
    }

    /**
     * @Route("/", name="homepage")
     * @param Request $request
     * @return Response
     */
    public function indexAction(Request $request)
    {
        $person = $this->handler->handle($request, Person::class);
        // replace this example code with whatever you need
        return new Response($this->twig->render('default/index.html.twig', ['person' => $person]));
    }
}

/** RequestHandler.php **/

namespace AppBundle\Service;

use Symfony\Component\HttpFoundation\HeaderBag;
use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpKernel\Exception\BadRequestHttpException;
use Symfony\Component\Serializer\SerializerInterface;
use Symfony\Component\Validator\Validator\ValidatorInterface;

class RequestHandler
{
    /** @var  ValidatorInterface */
    private $validator;
    /** @var  SerializerInterface */
    private $serializer;

    /**
     * RequestHandler constructor.
     * @param ValidatorInterface $validator
     * @param SerializerInterface $serializer
     */
    public function __construct(ValidatorInterface $validator, SerializerInterface $serializer)
    {
        $this->validator = $validator;
        $this->serializer = $serializer;
    }

    public function handle(Request $request, string $class)
    {
        $format = $this->getFormat($request->headers);
        $entity = $this->serializer->deserialize($request->getContent(),$class,$format);
        $errors = $this->validator->validate($entity);
        if ($errors->count() > 0){
            throw new BadRequestHttpException('Validation failed!');
        }
        return $entity;
    }

    private function getFormat(HeaderBag $headers)
    {
        $type = $headers->get('content-type');
        switch ($type){
            case 'application/json':
            default:
                return 'json';
        }
    }
}