Built-in Symfony Service Tags

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Built-in Symfony Service Tags

Service tags are the mechanism used by the DependencyInjection component to flag services that require special processing, like console commands or Twig extensions.

This article shows the most common tags provided by Symfony components, but in your application there could be more tags available provided by third-party bundles.

Run this command to display tagged services in your application:

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$ php bin/console debug:container --tags

To search for a specific tag, re-run this command with a search term:

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$ php bin/console debug:container --tag=form.type

assets.package

Purpose: Add an asset package to the application

This is an alternative way to declare an asset package. The name of the package is set in this order:

  • first, the package attribute of the tag;
  • then, the value returned by the static method getDefaultPackageName() if defined;
  • finally, the service name.
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services:
    App\Assets\AvatarPackage:
        tags:
            - { name: assets.package, package: avatars }

Now you can use the avatars package in your templates:

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<img src="{{ asset('...', 'avatars') }}">

auto_alias

Purpose: Define aliases based on the value of container parameters

Consider the following configuration that defines three different but related services:

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services:
    app.mysql_lock:
        class: App\Lock\MysqlLock
    app.postgresql_lock:
        class: App\Lock\PostgresqlLock
    app.sqlite_lock:
        class: App\Lock\SqliteLock

Instead of dealing with these three services, your application needs a generic app.lock service that will be an alias to one of these services, depending on some configuration. Thanks to the auto_alias option, you can automatically create that alias based on the value of a configuration parameter.

Considering that a configuration parameter called database_type exists. Then, the generic app.lock service can be defined as follows:

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services:
    app.mysql_lock:
        # ...
    app.postgresql_lock:
        # ...
    app.sqlite_lock:
        # ...
    app.lock:
        tags:
            - { name: auto_alias, format: "app.%database_type%_lock" }

The format option defines the expression used to construct the name of the service to alias. This expression can use any container parameter (as usual, wrapping their names with % characters).

Note

When using the auto_alias tag, it's not mandatory to define the aliased services as private. However, doing that (like in the above example) makes sense most of the times to prevent accessing those services directly instead of using the generic service alias.

console.command

Purpose: Add a command to the application

For details on registering your own commands in the service container, read How to Define Commands as Services.

container.hot_path

Purpose: Add to list of always needed services

This tag identifies the services that are always needed. It is only applied to a very short list of bootstrapping services (like router, event_dispatcher, http_kernel, request_stack, etc.). Then, it is propagated to all dependencies of these services, with a special case for event listeners, where only listed events are propagated to their related listeners.

It will replace, in cache for generated service factories, the PHP autoload by plain inlined include_once. The benefit is a complete bypass of the autoloader for services and their class hierarchy. The result is a significant performance improvement.

Use this tag with great caution, you have to be sure that the tagged service is always used.

container.no_preload

Purpose: Remove a class from the list of classes preloaded by PHP

Add this tag to a service and its class won't be preloaded when using PHP class preloading:

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services:
    App\SomeNamespace\SomeService:
        tags: ['container.no_preload']

If you add some service tagged with container.no_preload as an argument of another service, the container.no_preload tag is applied automatically to that service too.

container.preload

Purpose: Add some class to the list of classes preloaded by PHP

When using PHP class preloading, this tag allows you to define which PHP classes should be preloaded. This can improve performance by making some of the classes used by your service always available for all requests (until the server is restarted):

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services:
    App\SomeNamespace\SomeService:
        tags:
            - { name: 'container.preload', class: 'App\SomeClass' }
            - { name: 'container.preload', class: 'App\Some\OtherClass' }
            # ...

controller.argument_value_resolver

Purpose: Register a value resolver for controller arguments such as Request

Value resolvers implement the ValueResolverInterface and are used to resolve argument values for controllers as described here: Built-in Symfony Service Tags.

6.2

The ValueResolverInterface was introduced in Symfony 6.2. Prior to 6.2, you had to use the ArgumentValueResolverInterface, which defines different methods.

data_collector

Purpose: Create a class that collects custom data for the profiler

For details on creating your own custom data collection, read the Profiler article.

doctrine.event_listener

Purpose: Add a Doctrine event listener

For details on creating Doctrine event listeners, read the Doctrine events article.

doctrine.event_subscriber

Purpose: Add a Doctrine event subscriber

For details on creating Doctrine event subscribers, read the Doctrine events article.

form.type

Purpose: Create a custom form field type

For details on creating your own custom form type, read the How to Create a Custom Form Field Type article.

form.type_extension

Purpose: Create a custom "form extension"

For details on creating Form type extensions, read the How to Create a Form Type Extension article.

form.type_guesser

Purpose: Add your own logic for "form type guessing"

This tag allows you to add your own logic to the form guessing process. By default, form guessing is done by "guessers" based on the validation metadata and Doctrine metadata (if you're using Doctrine) or Propel metadata (if you're using Propel).

See also

For information on how to create your own type guesser, see Creating a custom Type Guesser.

kernel.cache_clearer

Purpose: Register your service to be called during the cache clearing process

Cache clearing occurs whenever you call cache:clear command. If your bundle caches files, you should add a custom cache clearer for clearing those files during the cache clearing process.

In order to register your custom cache clearer, first you must create a service class:

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// src/Cache/MyClearer.php
namespace App\Cache;

use Symfony\Component\HttpKernel\CacheClearer\CacheClearerInterface;

class MyClearer implements CacheClearerInterface
{
    public function clear(string $cacheDirectory): void
    {
        // clear your cache
    }
}

If you're using the default services.yaml configuration, your service will be automatically tagged with kernel.cache_clearer. But, you can also register it manually:

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services:
    App\Cache\MyClearer:
        tags: [kernel.cache_clearer]

kernel.cache_warmer

Purpose: Register your service to be called during the cache warming process

Cache warming occurs whenever you run the cache:warmup or cache:clear command (unless you pass --no-warmup to cache:clear). It is also run when handling the request, if it wasn't done by one of the commands yet.

The purpose is to initialize any cache that will be needed by the application and prevent the first user from any significant "cache hit" where the cache is generated dynamically.

To register your own cache warmer, first create a service that implements the CacheWarmerInterface interface:

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// src/Cache/MyCustomWarmer.php
namespace App\Cache;

use App\Foo\Bar;
use Symfony\Component\HttpKernel\CacheWarmer\CacheWarmerInterface;

class MyCustomWarmer implements CacheWarmerInterface
{
    public function warmUp($cacheDirectory): array
    {
        // ... do some sort of operations to "warm" your cache

        $filesAndClassesToPreload = [];
        $filesAndClassesToPreload[] = Bar::class;

        foreach (scandir($someCacheDir) as $file) {
            if (!is_dir($file = $someCacheDir.'/'.$file)) {
                $filesAndClassesToPreload[] = $file;
            }
        }

        return $filesAndClassesToPreload;
    }

    public function isOptional(): bool
    {
        return true;
    }
}

The warmUp() method must return an array with the files and classes to preload. Files must be absolute paths and classes must be fully-qualified class names. The only restriction is that files must be stored in the cache directory. If you don't need to preload anything, return an empty array.

The isOptional() method should return true if it's possible to use the application without calling this cache warmer. In Symfony, optional warmers are always executed by default (you can change this by using the --no-optional-warmers option when executing the command).

If you're using the default services.yaml configuration, your service will be automatically tagged with kernel.cache_warmer. But, you can also register it manually:

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services:
    App\Cache\MyCustomWarmer:
        tags:
            - { name: kernel.cache_warmer, priority: 0 }

Note

The priority is optional and its value is a positive or negative integer that defaults to 0. The higher the number, the earlier that warmers are executed.

Caution

If your cache warmer fails its execution because of any exception, Symfony won't try to execute it again for the next requests. Therefore, your application and/or bundles should be prepared for when the contents generated by the cache warmer are not available.

In addition to your own cache warmers, Symfony components and third-party bundles define cache warmers too for their own purposes. You can list them all with the following command:

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$ php bin/console debug:container --tag=kernel.cache_warmer

kernel.event_listener

Purpose: To listen to different events/hooks in Symfony

During the execution of a Symfony application, different events are triggered and you can also dispatch custom events. This tag allows you to hook your own classes into any of those events.

For a full example of this listener, read the Events and Event Listeners article.

Core Event Listener Reference

For the reference of Event Listeners associated with each kernel event, see the Symfony Events Reference.

kernel.event_subscriber

Purpose: To subscribe to a set of different events/hooks in Symfony

This is an alternative way to create an event listener, and is the recommended way (instead of using kernel.event_listener). See Events and Event Listeners.

kernel.fragment_renderer

Purpose: Add a new HTTP content rendering strategy

To add a new rendering strategy - in addition to the core strategies like EsiFragmentRenderer - create a class that implements FragmentRendererInterface, register it as a service, then tag it with kernel.fragment_renderer.

kernel.locale_aware

Purpose: To access and use the current locale

Setting and retrieving the locale can be done via configuration or using container parameters, listeners, route parameters or the current request.

Thanks to the Translation contract, the locale can be set via services.

To register your own locale aware service, first create a service that implements the LocaleAwareInterface interface:

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// src/Locale/MyCustomLocaleHandler.php
namespace App\Locale;

use Symfony\Contracts\Translation\LocaleAwareInterface;

class MyCustomLocaleHandler implements LocaleAwareInterface
{
    public function setLocale(string $locale): void
    {
        $this->locale = $locale;
    }

    public function getLocale(): string
    {
        return $this->locale;
    }
}

If you're using the default services.yaml configuration, your service will be automatically tagged with kernel.locale_aware. But, you can also register it manually:

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services:
    App\Locale\MyCustomLocaleHandler:
        tags: [kernel.locale_aware]

kernel.reset

Purpose: Clean up services between requests

During the kernel.terminate event, Symfony looks for any service tagged with the kernel.reset tag to reinitialize their state. This is done by calling to the method whose name is configured in the method argument of the tag.

This is mostly useful when running your projects in application servers that reuse the Symfony application between requests to improve performance. This tag is applied for example to the built-in data collectors of the profiler to delete all their information.

mime.mime_type_guesser

Purpose: Add your own logic for guessing MIME types

This tag is used to register your own MIME type guessers in case the guessers provided by the Mime component don't fit your needs.

monolog.logger

Purpose: To use a custom logging channel with Monolog

Monolog allows you to share its handlers between several logging channels. The logger service uses the channel app but you can change the channel when injecting the logger in a service.

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services:
    App\Log\CustomLogger:
        arguments: ['@logger']
        tags:
            - { name: monolog.logger, channel: app }

Tip

You can create custom channels and even autowire logging channels.

monolog.processor

Purpose: Add a custom processor for logging

Monolog allows you to add processors in the logger or in the handlers to add extra data in the records. A processor receives the record as an argument and must return it after adding some extra data in the extra attribute of the record.

The built-in IntrospectionProcessor can be used to add the file, the line, the class and the method where the logger was triggered.

You can add a processor globally:

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services:
    Monolog\Processor\IntrospectionProcessor:
        tags: [monolog.processor]

Tip

If your service is not a callable (using __invoke()) you can add the method attribute in the tag to use a specific method.

You can add also a processor for a specific handler by using the handler attribute:

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services:
    Monolog\Processor\IntrospectionProcessor:
        tags:
            - { name: monolog.processor, handler: firephp }

You can also add a processor for a specific logging channel by using the channel attribute. This will register the processor only for the security logging channel used in the Security component:

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services:
    Monolog\Processor\IntrospectionProcessor:
        tags:
            - { name: monolog.processor, channel: security }

Note

You cannot use both the handler and channel attributes for the same tag as handlers are shared between all channels.

routing.loader

Purpose: Register a custom service that loads routes

To enable a custom routing loader, add it as a regular service in one of your configuration and tag it with routing.loader:

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services:
    App\Routing\CustomLoader:
        tags: [routing.loader]

For more information, see How to Create a custom Route Loader.

routing.expression_language_provider

Purpose: Register a provider for expression language functions in routing

This tag is used to automatically register expression function providers for the routing expression component. Using these providers, you can add custom functions to the routing expression language.

security.expression_language_provider

Purpose: Register a provider for expression language functions in security

This tag is used to automatically register expression function providers for the security expression component. Using these providers, you can add custom functions to the security expression language.

security.voter

Purpose: To add a custom voter to Symfony's authorization logic

When you call isGranted() on Symfony's authorization checker, a system of "voters" is used behind the scenes to determine if the user should have access. The security.voter tag allows you to add your own custom voter to that system.

For more information, read the How to Use Voters to Check User Permissions article.

serializer.encoder

Purpose: Register a new encoder in the serializer service

The class that's tagged should implement the EncoderInterface and DecoderInterface.

For more details, see How to Use the Serializer.

serializer.normalizer

Purpose: Register a new normalizer in the Serializer service

The class that's tagged should implement the NormalizerInterface and DenormalizerInterface.

For more details, see How to Use the Serializer.

Run the following command to check the priorities of the default normalizers:

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$ php bin/console debug:container --tag serializer.normalizer

translation.loader

Purpose: To register a custom service that loads translations

By default, translations are loaded from the filesystem in a variety of different formats (YAML, XLIFF, PHP, etc).

Now, register your loader as a service and tag it with translation.loader:

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services:
    App\Translation\MyCustomLoader:
        tags:
            - { name: translation.loader, alias: bin }

The alias option is required and very important: it defines the file "suffix" that will be used for the resource files that use this loader. For example, suppose you have some custom bin format that you need to load. If you have a bin file that contains French translations for the messages domain, then you might have a file translations/messages.fr.bin.

When Symfony tries to load the bin file, it passes the path to your custom loader as the $resource argument. You can then perform any logic you need on that file in order to load your translations.

If you're loading translations from a database, you'll still need a resource file, but it might either be blank or contain a little bit of information about loading those resources from the database. The file is key to trigger the load() method on your custom loader.

translation.extractor

Purpose: To register a custom service that extracts messages from a file

When executing the translation:extract command, it uses extractors to extract translation messages from a file. By default, the Symfony Framework has a TwigExtractor and a PHP extractor to find and extract translation keys from Twig templates and PHP files.

Symfony includes two PHP extractors: PhpExtractor and PhpAstExtractor. The first one is simple but doesn't require to install any packages; the second one is much more advanced, but requires to install this dependency in your project:

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$ composer require nikic/php-parser

6.2

The PhpExtractor class is deprecated since Symfony 6.2. The PhpAstExtractor class will be the only PHP extractor available starting from Symfony 7.0.

You can create your own extractor by creating a class that implements ExtractorInterface and tagging the service with translation.extractor. The tag has one required option: alias, which defines the name of the extractor:

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// src/Acme/DemoBundle/Translation/FooExtractor.php
namespace Acme\DemoBundle\Translation;

use Symfony\Component\Translation\Extractor\ExtractorInterface;
use Symfony\Component\Translation\MessageCatalogue;

class FooExtractor implements ExtractorInterface
{
    protected string $prefix;

    /**
     * Extracts translation messages from a template directory to the catalog.
     */
    public function extract(string $directory, MessageCatalogue $catalog): void
    {
        // ...
    }

    /**
     * Sets the prefix that should be used for new found messages.
     */
    public function setPrefix(string $prefix): void
    {
        $this->prefix = $prefix;
    }
}
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services:
    App\Translation\CustomExtractor:
        tags:
            - { name: translation.extractor, alias: foo }

translation.dumper

Purpose: To register a custom service that dumps messages to a file

After a translation extractor has extracted all messages from the templates, the dumpers are executed to dump the messages to a translation file in a specific format.

Symfony already comes with many dumpers:

You can create your own dumper by extending FileDumper or implementing DumperInterface and tagging the service with translation.dumper. The tag has one option: alias This is the name that's used to determine which dumper should be used.

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services:
    App\Translation\JsonFileDumper:
        tags:
            - { name: translation.dumper, alias: json }

twig.extension

Purpose: To register a custom Twig Extension

To enable a Twig extension, add it as a regular service in one of your configuration and tag it with twig.extension. If you're using the default services.yaml configuration, the service is auto-registered and auto-tagged. But, you can also register it manually:

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services:
    App\Twig\AppExtension:
        tags: [twig.extension]

    # optionally you can define the priority of the extension (default = 0).
    # Extensions with higher priorities are registered earlier. This is mostly
    # useful to register late extensions that override other extensions.
    App\Twig\AnotherExtension:
        tags: [{ name: twig.extension, priority: -100 }]

For information on how to create the actual Twig Extension class, see Twig's documentation on the topic or read the Creating and Using Templates article.

twig.loader

Purpose: Register a custom service that loads Twig templates

By default, Symfony uses only one Twig Loader - FilesystemLoader. If you need to load Twig templates from another resource, you can create a service for the new loader and tag it with twig.loader.

If you use the default services.yaml configuration, the service will be automatically tagged thanks to autoconfiguration. But, you can also register it manually:

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services:
    App\Twig\CustomLoader:
        tags:
            - { name: twig.loader, priority: 0 }

Note

The priority is optional and its value is a positive or negative integer that defaults to 0. Loaders with higher numbers are tried first.

twig.runtime

Purpose: To register a custom Lazy-Loaded Twig Extension

Lazy-Loaded Twig Extensions are defined as regular services but they need to be tagged with twig.runtime. If you're using the default services.yaml configuration, the service is auto-registered and auto-tagged. But, you can also register it manually:

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services:
    App\Twig\AppExtension:
        tags: [twig.runtime]

validator.constraint_validator

Purpose: Create your own custom validation constraint

This tag allows you to create and register your own custom validation constraint. For more information, read the How to Create a Custom Validation Constraint article.

validator.initializer

Purpose: Register a service that initializes objects before validation

This tag provides a very uncommon piece of functionality that allows you to perform some sort of action on an object right before it's validated. For example, it's used by Doctrine to query for all of the lazily-loaded data on an object before it's validated. Without this, some data on a Doctrine entity would appear to be "missing" when validated, even though this is not really the case.

If you do need to use this tag, just make a new class that implements the ObjectInitializerInterface interface. Then, tag it with the validator.initializer tag (it has no options).

For an example, see the DoctrineInitializer class inside the Doctrine Bridge.

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