drupal-service-di

Drupal Service & Dependency Injection

Always use constructor injection — never use \Drupal:: static calls in classes. Always check the structural index before creating new services to avoid duplicates. Prefer interface type-hints over concrete classes.

Service Definition

# my_module.services.yml
services:
  my_module.content_manager:
    class: Drupal\my_module\ContentManager
    arguments:
      - '@entity_type.manager'
      - '@current_user'
      - '@logger.channel.my_module'

  my_module.event_subscriber:
    class: Drupal\my_module\EventSubscriber\MyEventSubscriber
    arguments:
      - '@my_module.content_manager'
    tags:
      - { name: event_subscriber }

Constructor Property Promotion

declare(strict_types=1);

namespace Drupal\my_module;

use Drupal\Core\Entity\EntityTypeManagerInterface;
use Drupal\Core\Session\AccountProxyInterface;
use Psr\Log\LoggerInterface;

final class ContentManager implements ContentManagerInterface {

  public function __construct(
    private readonly EntityTypeManagerInterface $entityTypeManager,
    private readonly AccountProxyInterface $currentUser,
    private readonly LoggerInterface $logger,
  ) {}

}

Interface Design

declare(strict_types=1);

namespace Drupal\my_module;

interface ContentManagerInterface {

  /**
   * Loads content items for the current user.
   *
   * @return \Drupal\node\NodeInterface[]
   *   Array of node entities.
   */
  public function loadUserContent(): array;

}

Plugin Dependency Injection

declare(strict_types=1);

namespace Drupal\my_module\Plugin\Block;

use Drupal\Core\Block\BlockBase;
use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
use Drupal\my_module\ContentManagerInterface;
use Symfony\Component\DependencyInjection\ContainerInterface;

/**
 * Provides a content list block.
 *
 * @Block(
 *   id = "my_module_content_list",
 *   admin_label = @Translation("Content List"),
 * )
 */
final class ContentListBlock extends BlockBase implements ContainerFactoryPluginInterface {

  public function __construct(
    array $configuration,
    $plugin_id,
    $plugin_definition,
    private readonly ContentManagerInterface $contentManager,
  ) {
    parent::__construct($configuration, $plugin_id, $plugin_definition);
  }

  public static function create(
    ContainerInterface $container,
    array $configuration,
    $plugin_id,
    $plugin_definition,
  ): static {
    return new static(
      $configuration,
      $plugin_id,
      $plugin_definition,
      $container->get('my_module.content_manager'),
    );
  }

  public function build(): array {
    return $this->contentManager->loadUserContent();
  }

}

Anti-Patterns

// Bad: static service call.
$node = \Drupal::entityTypeManager()->getStorage('node')->load($nid);

// Good: injected dependency.
$node = $this->entityTypeManager->getStorage('node')->load($nid);

// Bad: no interface type-hint.
class MyService {
  public function __construct(private readonly ContentManager $manager) {}
}

// Good: interface type-hint.
class MyService {
  public function __construct(private readonly ContentManagerInterface $manager) {}
}

Service Name Discovery

Service names follow no universal convention. Don't guess -- verify.

# Quick check: does a service exist?
ddev drush eval 'print json_encode(["exists" => Drupal::hasService("module_name.service_name")]);'

When a service name fails: Read the module's *.services.yml directly rather than guessing variations. The module prefix may be singular (group_permission.checker) when you expect plural (group_permissions.checker).

Finding the Loaded File (Patched Modules)

When a module exists in both vendor/drupal/ and web/modules/contrib/, only one is actually loaded by PHP.

// Find which file is running at runtime
$ref = new \ReflectionMethod($service, 'methodName');
echo $ref->getFileName();
// Edit the file that ReflectionMethod reports, not the one you assume.

Common scenario: A composer-patched module. The original sits in vendor/, the patched version in web/modules/contrib/. Editing the vendor copy has no effect because the autoloader loads from contrib.

Common Drupal Services

Service ID	Interface	Purpose
entity_type.manager	EntityTypeManagerInterface	Entity CRUD
current_user	AccountProxyInterface	Current user
config.factory	ConfigFactoryInterface	Configuration
logger.factory	LoggerChannelFactoryInterface	Logging
messenger	MessengerInterface	User messages
module_handler	ModuleHandlerInterface	Module operations
state	StateInterface	Key-value state
cache.default	CacheBackendInterface	Default cache