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. You can read about how to create them in the components section ":ref:`components-di-separate-compiler-passes`".

Compiler passes are registered 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());
    }
}

One of the most common use-cases of compiler passes is to work with :doc:`tagged services </service_container/tags>`. In those cases, instead of creating a compiler pass, you can make the kernel implement :class:`Symfony\\Component\\DependencyInjection\\Compiler\\CompilerPassInterface` and process the services 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) {
            // ...
        }
    }
}

Working with Compiler Passes in Bundles

:doc:`Bundles </bundles>` can define compiler passes in the build() method of the main bundle class (this is not needed when implementing the process() method in the extension):

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

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

class MyBundle extends Bundle
{
    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 then by convention, tag names consist of the name of the bundle (lowercase, underscores as separators), followed by a dot, and finally the "real" name. For example, if you want to introduce some sort of "transport" tag in your AcmeMailerBundle, you should call it acme_mailer.transport.