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Testing Services

When to Use

Verifying service logic, dependency injection, container integration.

Decision

Service uses... Test type Pattern
No Drupal dependencies Unit Mock all dependencies
Database, config, entities Kernel Use real container
HTTP context, current route Browser Full Drupal install

Pattern

Unit test with mocked dependencies: 

namespace Drupal\Tests\my_module\Unit;

use Drupal\my_module\Service\DataProcessor;
use Drupal\Tests\UnitTestCase;

class DataProcessorTest extends UnitTestCase {

  public function testProcessData(): void {
    $mock_logger = $this->createMock(LoggerInterface::class);
    $mock_logger->expects($this->once())->method('info');

    $service = new DataProcessor($mock_logger);
    $result = $service->processData(['key' => 'value']);

    $this->assertEquals('processed', $result['status']);
  }
}

Kernel test with real container: 

namespace Drupal\Tests\my_module\Kernel;

use Drupal\KernelTests\KernelTestBase;

class MyServiceTest extends KernelTestBase {

  protected static $modules = ['system', 'my_module'];

  public function testServiceIntegration(): void {
    $service = $this->container->get('my_module.my_service');
    $result = $service->doSomething();
    $this->assertNotEmpty($result);
  }
}

Replacing service with mock in Kernel test: 

use Drupal\Core\DependencyInjection\ServiceModifierInterface;

class MyServiceTest extends KernelTestBase implements ServiceModifierInterface {

  public function alter(ContainerBuilder $container): void {
    $container->getDefinition('my_module.external_api')
      ->setClass(MockExternalApi::class);
  }
}

Reference: /core/modules/system/tests/src/Unit/Routing/AdminRouteSubscriberTest.php

Common Mistakes

  • Using \Drupal::service() in service class -- impossible to unit test (inject dependencies via constructor)
  • Not mocking external API calls -- tests fail when API down, slow tests
  • Testing service in isolation when integration matters -- use Kernel test to verify container wiring
  • Over-mocking makes test meaningless -- if mocking everything, test verifies nothing
  • Forgetting to call expects() on mock -- mock created but never verified

See Also