ClassResolver.php

<?php

namespace Drupal\Core\DependencyInjection;

use Symfony\Component\DependencyInjection\ContainerAwareTrait;
use Symfony\Component\DependencyInjection\ContainerAwareInterface;

/**
 * Implements the class resolver interface supporting class names and services.
 */
class ClassResolver implements ClassResolverInterface, ContainerAwareInterface {
    use DependencySerializationTrait;
    use ContainerAwareTrait;
    
    /**
     * {@inheritdoc}
     */
    public function getInstanceFromDefinition($definition) {
        if ($this->container
            ->has($definition)) {
            $instance = $this->container
                ->get($definition);
        }
        else {
            if (!class_exists($definition)) {
                throw new \InvalidArgumentException(sprintf('Class "%s" does not exist.', $definition));
            }
            if (is_subclass_of($definition, 'Drupal\\Core\\DependencyInjection\\ContainerInjectionInterface')) {
                $instance = $definition::create($this->container);
            }
            else {
                $instance = new $definition();
            }
        }
        if ($instance instanceof ContainerAwareInterface) {
            $instance->setContainer($this->container);
        }
        return $instance;
    }

}