Custom Entity REST Resource in Drupal 9 with controller

// 1. Create a custom module (e.g., custom_rest_module).

// 2. Define the module in custom_rest_module.info.yml.
name: 'Custom Rest Module'
type: module
description: 'Provides a custom REST resource for managing entities.'
core_version_requirement: ^8 || ^9

// 3. Create a file custom_rest_module.routing.yml for defining routes.
custom_rest_module.entity:
  path: '/custom-rest-endpoint/{entity_id}'
  defaults:
    _controller: '\Drupal\custom_rest_module\Controller\CustomRestController::handle'
  methods:
    - 'GET'
    - 'POST'
    - 'DELETE'
    - 'PATCH'
  requirements:
    _access: 'TRUE'

// 4. Create a controller CustomRestController.php.
namespace Drupal\custom_rest_module\Controller;

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

/**
 * CustomRestController controller.
 */
class CustomRestController extends ControllerBase {

  /**
   * Handle requests for the custom REST resource.
   */
  public function handle(Request $request, $entity_id) {
    switch ($request->getMethod()) {
      case 'GET':
        // Implement GET method logic.
        $response = $this->getEntity($entity_id);
        break;

      case 'POST':
        // Implement POST method logic.
        $response = $this->createEntity($request->getContent());
        break;

      case 'DELETE':
        // Implement DELETE method logic.
        $response = $this->deleteEntity($entity_id);
        break;

      case 'PATCH':
        // Implement PATCH (update) method logic.
        $response = $this->updateEntity($entity_id, $request->getContent());
        break;

      default:
        $response = new JsonResponse(['error' => 'Invalid request method.'], 400);
    }

    return $response;
  }

  // Implement CRUD methods (getEntity, createEntity, deleteEntity, updateEntity).

  // Example:
  private function getEntity($entity_id) {
    // Implement logic to fetch entity by ID.
    // ...

    return new JsonResponse(['data' => 'Entity data for ID ' . $entity_id]);
  }

  private function createEntity($data) {
    // Implement logic to create a new entity.
    // ...

    return new JsonResponse(['message' => 'Entity created successfully.']);
  }

  private function deleteEntity($entity_id) {
    // Implement logic to delete entity by ID.
    // ...

    return new JsonResponse(['message' => 'Entity deleted successfully.']);
  }

  private function updateEntity($entity_id, $data) {
    // Implement logic to update entity by ID.
    // ...

    return new JsonResponse(['message' => 'Entity updated successfully.']);
  }

}

Remember to replace custom_rest_module with your actual module name and modify the code according to your entity type and business logic. This example provides a basic structure; you may need to enhance it based on your specific requirements.