How to Work with Compiler Passes

Compiler passes give you an opportunity to manipulate other :doc:`service definitions </service_container/definitions>` that have been registered with the service container.

If your compiler pass is relatively small, you can define it inside the application's Kernel class instead of creating a :ref:`separate compiler pass class <components-di-separate-compiler-passes>`.

To do so, make your kernel implement :class:`Symfony\\Component\\DependencyInjection\\Compiler\\CompilerPassInterface` and add the compiler pass code inside the process() method:

// src/Kernel.php
namespace App;

use Symfony\Bundle\FrameworkBundle\Kernel\MicroKernelTrait;
use Symfony\Component\DependencyInjection\Compiler\CompilerPassInterface;
use Symfony\Component\DependencyInjection\ContainerBuilder;
use Symfony\Component\HttpKernel\Kernel as BaseKernel;

class Kernel extends BaseKernel implements CompilerPassInterface
{
    use MicroKernelTrait;

    // ...

    public function process(ContainerBuilder $container): void
    {
        // in this method you can manipulate the service container:
        // for example, changing some container service:
        $container->getDefinition('app.some_private_service')->setPublic(true);

        // or processing tagged services:
        foreach ($container->findTaggedServiceIds('some_tag') as $id => $tags) {
            // ...
        }
    }
}

If you create separate compiler pass classes, enable them in the build() method of the application kernel:

// src/Kernel.php
namespace App;

use App\DependencyInjection\Compiler\CustomPass;
use Symfony\Bundle\FrameworkBundle\Kernel\MicroKernelTrait;
use Symfony\Component\DependencyInjection\ContainerBuilder;
use Symfony\Component\HttpKernel\Kernel as BaseKernel;

class Kernel extends BaseKernel
{
    use MicroKernelTrait;

    // ...

    protected function build(ContainerBuilder $container): void
    {
        $container->addCompilerPass(new CustomPass());
    }
}

Working with Compiler Passes in Bundles

If your compiler pass is relatively small, you can add it directly in the main bundle class. To do so, make your bundle implement the :class:`Symfony\\Component\\DependencyInjection\\Compiler\\CompilerPassInterface` and place the compiler pass code inside the process() method of the main bundle class:

// src/MyBundle/MyBundle.php
namespace App\MyBundle;

use App\DependencyInjection\Compiler\CustomPass;
use Symfony\Component\DependencyInjection\Compiler\CompilerPassInterface;
use Symfony\Component\DependencyInjection\ContainerBuilder;
use Symfony\Component\HttpKernel\Bundle\AbstractBundle;

class MyBundle extends AbstractBundle implements CompilerPassInterface
{
    public function process(ContainerBuilder $container): void
    {
        // in this method you can manipulate the service container:
        // for example, changing some container service:
        $container->getDefinition('app.some_private_service')->setPublic(true);

        // or processing tagged services:
        foreach ($container->findTaggedServiceIds('some_tag') as $id => $tags) {
            // ...
        }
    }
}

Alternatively, when using :ref:`separate compiler pass classes <components-di-separate-compiler-passes>`, bundles can enable them in the build() method of their main bundle class:

// src/MyBundle/MyBundle.php
namespace App\MyBundle;

use App\DependencyInjection\Compiler\CustomPass;
use Symfony\Component\DependencyInjection\ContainerBuilder;
use Symfony\Component\HttpKernel\Bundle\AbstractBundle;

class MyBundle extends AbstractBundle
{
    public function build(ContainerBuilder $container): void
    {
        parent::build($container);

        $container->addCompilerPass(new CustomPass());
    }
}

If you are using custom :doc:`service tags </service_container/tags>` in a bundle, the convention is to format tag names by starting with the bundle's name in lowercase (using underscores as separators), followed by a dot, and finally the specific tag name. For example, to introduce a "transport" tag in your AcmeMailerBundle, you would name it acme_mailer.transport.