Defining Services Dependencies Automatically (Autowiring)

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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);
    }
}

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
{
    public function __construct(
        private Rot13Transformer $transformer,
    ) {
    }

    public function tweet(User $user, string $key, string $status): void
    {
        $transformedStatus = $this->transformer->transform($status);

        // ... connect to Twitter and send the encoded status
    }
}

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

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\Attribute\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);

        // ...
    }
}

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(
        private Rot13Transformer $transformer,
    ) {
    }
}

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'

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;
}

Then, you update Rot13Transformer to implement it:

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// ...
class Rot13Transformer implements TransformerInterface
{
    // ...
}

Now that you have an interface, you should use this as your type-hint:

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class TwitterClient
{
    public function __construct(
        private TransformerInterface $transformer,
    ) {
        // ...
    }

    // ...
}

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'

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.

Tip

Autowiring is powerful enough to guess which service to inject even when using union and intersection types. This means you're able to type-hint argument with complex types like this:

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use Symfony\Component\Serializer\Normalizer\DenormalizerInterface;
use Symfony\Component\Serializer\Normalizer\NormalizerInterface;
use Symfony\Component\Serializer\SerializerInterface;

class DataFormatter
{
    public function __construct(
        private (NormalizerInterface&DenormalizerInterface)|SerializerInterface $transformer,
    ) {
        // ...
    }

    // ...
}

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);
    }
}

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 an argument 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
{
    public function __construct(
        private TransformerInterface $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'
        # ...

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.

Another option is to use the #[Target] attribute. By adding this attribute to the argument you want to autowire, you can specify which service to inject by passing the name of the argument used in the named alias. This way, you can have multiple services implementing the same interface and keep the argument name separate from any implementation name (like shown in the example above). In addition, you'll get an exception in case you make any typo in the target name.

Warning

The #[Target] attribute only accepts the name of the argument used in the named alias; it does not accept service ids or service aliases.

You can get a list of named autowiring aliases by running the debug:autowiring command:

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$ php bin/console debug:autowiring LoggerInterface

Autowirable Types
=================

 The following classes & interfaces can be used as type-hints when autowiring:
 (only showing classes/interfaces matching LoggerInterface)

 Describes a logger instance.
 Psr\Log\LoggerInterface - alias:monolog.logger
 Psr\Log\LoggerInterface $assetMapperLogger - target:asset_mapperLogger - alias:monolog.logger.asset_mapper
 Psr\Log\LoggerInterface $cacheLogger - alias:monolog.logger.cache
 Psr\Log\LoggerInterface $httpClientLogger - target:http_clientLogger - alias:monolog.logger.http_client
 Psr\Log\LoggerInterface $mailerLogger - alias:monolog.logger.mailer

 [...]

Suppose you want to inject the App\Util\UppercaseTransformer service. You would use the #[Target] attribute by passing the name of the $shoutyTransformer argument:

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// src/Service/MastodonClient.php
namespace App\Service;

use App\Util\TransformerInterface;
use Symfony\Component\DependencyInjection\Attribute\Target;

class MastodonClient
{
    public function __construct(
        #[Target('shoutyTransformer')]
        private TransformerInterface $transformer,
    ) {
    }
}

Tip

Since the #[Target] attribute normalizes the string passed to it to its camelCased form, name variations (e.g. shouty.transformer) also work.

Note

Some IDEs will show an error when using #[Target] as in the previous example: "Attribute cannot be applied to a property because it does not contain the 'Attribute::TARGET_PROPERTY' flag". The reason is that thanks to PHP constructor promotion this constructor argument is both a parameter and a class property. You can safely ignore this error message.

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')]
        private LoggerInterface $logger,
    ) {
        // ...
    }
}

The #[Autowire] attribute can also be used for parameters, complex expressions and even environment variables , including env variable processors:

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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,

        // or use argument "param"
        #[Autowire(param: 'kernel.debug')]
        bool $debugMode,

        // expressions
        #[Autowire(expression: 'service("App\\\Mail\\\MailerConfiguration").getMailerMethod()')]
        string $mailerMethod,

        // environment variables
        #[Autowire(env: 'SOME_ENV_VAR')]
        string $senderName,

        // environment variables with processors
        #[Autowire(env: 'bool:SOME_BOOL_ENV_VAR')]
        bool $allowAttachments,
    ) {
    }
    // ...
}

Generate Closures With Autowiring

A service closure is an anonymous function that returns a service. This type of instantiation is handy when you are dealing with lazy-loading. It is also useful for non-shared service dependencies.

Automatically creating a closure encapsulating the service instantiation can be done with the AutowireServiceClosure attribute:

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// src/Service/Remote/MessageFormatter.php
namespace App\Service\Remote;

use Symfony\Component\DependencyInjection\Attribute\AsAlias;

#[AsAlias('third_party.remote_message_formatter')]
class MessageFormatter
{
    public function __construct()
    {
        // ...
    }

    public function format(string $message): string
    {
        // ...
    }
}

// src/Service/MessageGenerator.php
namespace App\Service;

use App\Service\Remote\MessageFormatter;
use Symfony\Component\DependencyInjection\Attribute\AutowireServiceClosure;

class MessageGenerator
{
    public function __construct(
        #[AutowireServiceClosure('third_party.remote_message_formatter')]
        private \Closure $messageFormatterResolver,
    ) {
    }

    public function generate(string $message): void
    {
        $formattedMessage = ($this->messageFormatterResolver)()->format($message);

        // ...
    }
}

It is common that a service accepts a closure with a specific signature. In this case, you can use the AutowireCallable attribute to generate a closure with the same signature as a specific method of a service. When this closure is called, it will pass all its arguments to the underlying service function. If the closure needs to be called more than once, the service instance is reused for repeated calls. Unlike a service closure, this will not create extra instances of a non-shared service:

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// src/Service/MessageGenerator.php
namespace App\Service;

use Symfony\Component\DependencyInjection\Attribute\AutowireCallable;

class MessageGenerator
{
    public function __construct(
        #[AutowireCallable(service: 'third_party.remote_message_formatter', method: 'format')]
        private \Closure $formatCallable,
    ) {
    }

    public function generate(string $message): void
    {
        $formattedMessage = ($this->formatCallable)($message);

        // ...
    }
}

Finally, you can pass the lazy: true option to the AutowireCallable attribute. By doing so, the callable will automatically be lazy, which means that the encapsulated service will be instantiated only at the closure's first call.

The AutowireMethodOf attribute provides a simpler way of specifying the name of the service method by using the property name as method name:

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// src/Service/MessageGenerator.php
namespace App\Service;

use Symfony\Component\DependencyInjection\Attribute\AutowireMethodOf;

class MessageGenerator
{
    public function __construct(
        #[AutowireMethodOf('third_party.remote_message_formatter')]
        private \Closure $format,
    ) {
    }

    public function generate(string $message): void
    {
        $formattedMessage = ($this->format)($message);

        // ...
    }
}

7.1

The AutowireMethodOf attribute was introduced in Symfony 7.1.

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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// src/Util/Rot13Transformer.php
namespace App\Util;

use Symfony\Contracts\Service\Attribute\Required;

class Rot13Transformer
{
    private LoggerInterface $logger;

    #[Required]
    public function setLogger(LoggerInterface $logger): void
    {
        $this->logger = $logger;
    }

    public function transform($value): string
    {
        $this->logger->info('Transforming '.$value);
        // ...
    }
}

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.

Despite property injection having some drawbacks, autowiring with #[Required] can also be applied to public typed properties:

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namespace App\Util;

use Symfony\Contracts\Service\Attribute\Required;

class Rot13Transformer
{
    #[Required]
    public LoggerInterface $logger;

    public function transform($value): void
    {
        $this->logger->info('Transforming '.$value);
        // ...
    }
}

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.

This work, including the code samples, is licensed under a Creative Commons BY-SA 3.0 license.
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