Defining Services Dependencies Automatically (Autowiring) ¶
Autowiring allows you to manage services in the container with minimal configuration. It reads the type-hints on your constructor (or other methods) and automatically passes the correct services to each method. Symfony's autowiring is designed to be predictable: if it is not absolutely clear which dependency should be passed, you'll see an actionable exception.
Tip
Thanks to Symfony's compiled container, there is no runtime overhead for using autowiring.
An Autowiring Example ¶
Imagine you're building an API to publish statuses on a Twitter feed, obfuscated with ROT13, a fun encoder that shifts all characters 13 letters forward in the alphabet.
Start by creating a ROT13 transformer class:
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// src/Util/Rot13Transformer.php
namespace App\Util;
class Rot13Transformer
{
public function transform(string $value): string
{
return str_rot13($value);
}
}
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And now a Twitter client using this transformer:
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// src/Service/TwitterClient.php
namespace App\Service;
use App\Util\Rot13Transformer;
// ...
class TwitterClient
{
private $transformer;
public function __construct(Rot13Transformer $transformer)
{
$this->transformer = $transformer;
}
public function tweet(User $user, string $key, string $status): void
{
$transformedStatus = $this->transformer->transform($status);
// ... connect to Twitter and send the encoded status
}
}
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If you're using the default services.yaml configuration, both classes are automatically registered as services and configured to be autowired. This means you can use them immediately without any configuration.
However, to understand autowiring better, the following examples explicitly configure both services:
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# config/services.yaml
services:
_defaults:
autowire: true
autoconfigure: true
# ...
App\Service\TwitterClient:
# redundant thanks to _defaults, but value is overridable on each service
autowire: true
App\Util\Rot13Transformer:
autowire: true
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Now, you can use the TwitterClient service immediately in a controller:
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// src/Controller/DefaultController.php
namespace App\Controller;
use App\Service\TwitterClient;
use Symfony\Bundle\FrameworkBundle\Controller\AbstractController;
use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpFoundation\Response;
use Symfony\Component\Routing\Annotation\Route;
class DefaultController extends AbstractController
{
#[Route('/tweet')]
public function tweet(TwitterClient $twitterClient, Request $request): Response
{
// fetch $user, $key, $status from the POST'ed data
$twitterClient->tweet($user, $key, $status);
// ...
}
}
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This works automatically! The container knows to pass the Rot13Transformer service
as the first argument when creating the TwitterClient service.
Autowiring Logic Explained ¶
Autowiring works by reading the Rot13Transformer type-hint in TwitterClient:
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// src/Service/TwitterClient.php
namespace App\Service;
// ...
use App\Util\Rot13Transformer;
class TwitterClient
{
// ...
public function __construct(Rot13Transformer $transformer)
{
$this->transformer = $transformer;
}
}
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The autowiring system looks for a service whose id exactly matches the type-hint:
so App\Util\Rot13Transformer. In this case, that exists! When you configured
the Rot13Transformer service, you used its fully-qualified class name as its
id. Autowiring isn't magic: it looks for a service whose id matches the type-hint.
If you load services automatically,
each service's id is its class name.
If there is not a service whose id exactly matches the type, a clear exception will be thrown.
Autowiring is a great way to automate configuration, and Symfony tries to be as predictable and as clear as possible.
Using Aliases to Enable Autowiring ¶
The main way to configure autowiring is to create a service whose id exactly matches
its class. In the previous example, the service's id is App\Util\Rot13Transformer,
which allows us to autowire this type automatically.
This can also be accomplished using an alias. Suppose that
for some reason, the id of the service was instead app.rot13.transformer. In
this case, any arguments type-hinted with the class name (App\Util\Rot13Transformer)
can no longer be autowired.
No problem! To fix this, you can create a service whose id matches the class by adding a service alias:
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# config/services.yaml
services:
# ...
# the id is not a class, so it won't be used for autowiring
app.rot13.transformer:
class: App\Util\Rot13Transformer
# ...
# but this fixes it!
# the ``app.rot13.transformer`` service will be injected when
# an ``App\Util\Rot13Transformer`` type-hint is detected
App\Util\Rot13Transformer: '@app.rot13.transformer'
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This creates a service "alias", whose id is App\Util\Rot13Transformer.
Thanks to this, autowiring sees this and uses it whenever the Rot13Transformer
class is type-hinted.
Tip
Aliases are used by the core bundles to allow services to be autowired. For
example, MonologBundle creates a service whose id is logger. But it also
adds an alias: Psr\Log\LoggerInterface that points to the logger service.
This is why arguments type-hinted with Psr\Log\LoggerInterface can be autowired.
Working with Interfaces ¶
You might also find yourself type-hinting abstractions (e.g. interfaces) instead of concrete classes as it replaces your dependencies with other objects.
To follow this best practice, suppose you decide to create a TransformerInterface:
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// src/Util/TransformerInterface.php
namespace App\Util;
interface TransformerInterface
{
public function transform(string $value): string;
}
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Then, you update Rot13Transformer to implement it:
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// ...
class Rot13Transformer implements TransformerInterface
{
// ...
}
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Now that you have an interface, you should use this as your type-hint:
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class TwitterClient
{
public function __construct(TransformerInterface $transformer)
{
// ...
}
// ...
}
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But now, the type-hint (App\Util\TransformerInterface) no longer matches
the id of the service (App\Util\Rot13Transformer). This means that the
argument can no longer be autowired.
To fix that, add an alias:
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# config/services.yaml
services:
# ...
App\Util\Rot13Transformer: ~
# the ``App\Util\Rot13Transformer`` service will be injected when
# an ``App\Util\TransformerInterface`` type-hint is detected
App\Util\TransformerInterface: '@App\Util\Rot13Transformer'
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Thanks to the App\Util\TransformerInterface alias, the autowiring subsystem
knows that the App\Util\Rot13Transformer service should be injected when
dealing with the TransformerInterface.
Tip
When using a service definition prototype, if only one service is discovered that implements an interface, configuring the alias is not mandatory and Symfony will automatically create one.
Dealing with Multiple Implementations of the Same Type ¶
Suppose you create a second class - UppercaseTransformer that implements
TransformerInterface:
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// src/Util/UppercaseTransformer.php
namespace App\Util;
class UppercaseTransformer implements TransformerInterface
{
public function transform(string $value): string
{
return strtoupper($value);
}
}
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If you register this as a service, you now have two services that implement the
App\Util\TransformerInterface type. Autowiring subsystem can not decide
which one to use. Remember, autowiring isn't magic; it looks for a service
whose id matches the type-hint. So you need to choose one by creating an alias
from the type to the correct service id (see Defining Services Dependencies Automatically (Autowiring)).
Additionally, you can define several named autowiring aliases if you want to use
one implementation in some cases, and another implementation in some
other cases.
For instance, you may want to use the Rot13Transformer
implementation by default when the TransformerInterface interface is
type hinted, but use the UppercaseTransformer implementation in some
specific cases. To do so, you can create a normal alias from the
TransformerInterface interface to Rot13Transformer, and then
create a named autowiring alias from a special string containing the
interface followed by a variable name matching the one you use when doing
the injection:
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// src/Service/MastodonClient.php
namespace App\Service;
use App\Util\TransformerInterface;
class MastodonClient
{
private $transformer;
public function __construct(TransformerInterface $shoutyTransformer)
{
$this->transformer = $shoutyTransformer;
}
public function toot(User $user, string $key, string $status): void
{
$transformedStatus = $this->transformer->transform($status);
// ... connect to Mastodon and send the transformed status
}
}
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# config/services.yaml
services:
# ...
App\Util\Rot13Transformer: ~
App\Util\UppercaseTransformer: ~
# the ``App\Util\UppercaseTransformer`` service will be
# injected when an ``App\Util\TransformerInterface``
# type-hint for a ``$shoutyTransformer`` argument is detected.
App\Util\TransformerInterface $shoutyTransformer: '@App\Util\UppercaseTransformer'
# If the argument used for injection does not match, but the
# type-hint still matches, the ``App\Util\Rot13Transformer``
# service will be injected.
App\Util\TransformerInterface: '@App\Util\Rot13Transformer'
App\Service\TwitterClient:
# the Rot13Transformer will be passed as the $transformer argument
autowire: true
# If you wanted to choose the non-default service and do not
# want to use a named autowiring alias, wire it manually:
# arguments:
# $transformer: '@App\Util\UppercaseTransformer'
# ...
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Thanks to the App\Util\TransformerInterface alias, any argument type-hinted
with this interface will be passed the App\Util\Rot13Transformer service.
If the argument is named $shoutyTransformer,
App\Util\UppercaseTransformer will be used instead.
But, you can also manually wire any other service by specifying the argument
under the arguments key.
Fixing Non-Autowireable Arguments ¶
Autowiring only works when your argument is an object. But if you have a scalar argument (e.g. a string), this cannot be autowired: Symfony will throw a clear exception.
To fix this, you can manually wire the problematic argument in the service configuration. You wire up only the difficult arguments, Symfony takes care of the rest.
You can also use the #[Autowire] parameter attribute to instruct the autowiring
logic about those arguments:
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// src/Service/MessageGenerator.php
namespace App\Service;
use Psr\Log\LoggerInterface;
use Symfony\Component\DependencyInjection\Attribute\Autowire;
class MessageGenerator
{
public function __construct(
#[Autowire(service: 'monolog.logger.request')] LoggerInterface $logger
) {
// ...
}
}
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6.1
The #[Autowire] attribute was introduced in Symfony 6.1.
The #[Autowire] attribute can also be used for parameters
and even complex expressions:
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// src/Service/MessageGenerator.php
namespace App\Service;
use Psr\Log\LoggerInterface;
use Symfony\Component\DependencyInjection\Attribute\Autowire;
class MessageGenerator
{
public function __construct(
// use the %...% syntax for parameters
#[Autowire('%kernel.project_dir%/data')]
string $dataDir,
#[Autowire('%kernel.debug%')]
bool $debugMode,
// and expressions
#[Autowire(expression: 'service("App\\Mail\\MailerConfiguration").getMailerMethod()')]
string $mailerMethod
) {
}
// ...
}
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Autowiring other Methods (e.g. Setters and Public Typed Properties) ¶
When autowiring is enabled for a service, you can also configure the container
to call methods on your class when it's instantiated. For example, suppose you want
to inject the logger service, and decide to use setter-injection:
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Attributes
属性
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// src/Util/Rot13Transformer.php
namespace App\Util;
use Symfony\Contracts\Service\Attribute\Required;
class Rot13Transformer
{
private $logger;
#[Required]
public function setLogger(LoggerInterface $logger): void
{
$this->logger = $logger;
}
public function transform($value): string
{
$this->logger->info('Transforming '.$value);
// ...
}
}
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Autowiring will automatically call any method with the #[Required] attribute
above it, autowiring each argument. If you need to manually wire some of the arguments
to a method, you can always explicitly configure the method call.
If your PHP version doesn't support attributes (they were introduced in PHP 8),
you can use the @required annotation instead.
Despite property injection having some drawbacks,
autowiring with #[Required] or @required can also be applied to public
typed properties:
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属性
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namespace App\Util;
use Symfony\Contracts\Service\Attribute\Required;
class Rot13Transformer
{
#[Required]
public LoggerInterface $logger;
public function transform($value)
{
$this->logger->info('Transforming '.$value);
// ...
}
}
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Autowiring Controller Action Methods ¶
If you're using the Symfony Framework, you can also autowire arguments to your controller action methods. This is a special case for autowiring, which exists for convenience. See Controller for more details.
Performance Consequences ¶
Thanks to Symfony's compiled container, there is no performance penalty for using
autowiring. However, there is a small performance penalty in the dev environment,
as the container may be rebuilt more often as you modify classes. If rebuilding
your container is slow (possible on very large projects), you may not be able to
use autowiring.
Public and Reusable Bundles ¶
Public bundles should explicitly configure their services and not rely on autowiring. Autowiring depends on the services that are available in the container and bundles have no control over the service container of applications they are included in. You can use autowiring when building reusable bundles within your company, as you have full control over all code.