Autowiring works automatically for MOST cases – this chapter covers the cases where manual configuration becomes necessary, and the central services.yaml file behind it.

Understanding config/services.yaml

services:
    _defaults:
        autowire: true
        autoconfigure: true

    App\:
        resource: '../src/'
        exclude:
            - '../src/DependencyInjection/'
            - '../src/Entity/'
            - '../src/Kernel.php'

This default configuration (already set up by Symfony Flex at install time) AUTOMATICALLY registers EVERY class under src/ as a service – EXACTLY why ProjectStatsService from chapter 33 worked WITHOUT manual registration. exclude excludes directories that contain NO services (entities are pure data objects, not kernel bootstrap code).

autowire and autoconfigure in detail

When autowiring ISN'T enough: multiple implementations

If SEVERAL classes implement the SAME interface, Symfony CAN'T automatically decide which one is meant:

interface NotificationChannelInterface
{
    public function send(string $message): void;
}

class EmailChannel implements NotificationChannelInterface { /* ... */ }
class SlackChannel implements NotificationChannelInterface { /* ... */ }

If a service constructor requests NotificationChannelInterface $channel, Symfony throws an error: "ambiguous service". Solution: EXPLICIT binding in services.yaml, via a named argument:

services:
    App\Service\AlertService:
        arguments:
            $channel: '@App\Service\Notification\EmailChannel'

$channel MUST exactly match the constructor's parameter name, @ references a service by its ID (usually the full class name).

Injecting scalar values via bind

For configuration values (not objects), like a maximum number of tasks per project:

services:
    _defaults:
        autowire: true
        autoconfigure: true
        bind:
            $maxTasksPerProject: '%env(int:MAX_TASKS_PER_PROJECT)%'

class ProjectLimitService
{
    public function __construct(
        private readonly int $maxTasksPerProject,
    ) {
    }
}

%env(int:...)% reads the environment variable (EXACTLY like DATABASE_URL in chapter 4), int: automatically converts the string value to int. bind in _defaults applies to ALL services whose constructor has a parameter with EXACTLY this name – handy for widely used configuration values.

Tags: Symfony's plugin mechanism

Tags mark services as an "extension of a specific system" – with autoconfigure: true, this MOSTLY happens automatically via interface detection (event subscribers in chapter 37, voters from chapter 29 e.g. get automatically recognized via the security.voter tag), manual tagging is only needed for your own, more advanced extension points.