Building your own Framework with the MicroKernelTrait

The default Kernel class included in Symfony applications uses a MicroKernelTrait to configure the bundles, the routes and the service container in the same class.

This micro-kernel approach is flexible, allowing you to control your application structure and features.

A Single-File Symfony Application

Start with a completely empty directory and install these Symfony components via Composer:

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        $ composer symfony/framework-bundle symfony/runtime
    

Next, create an index.php file that defines the kernel class and runs it:

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        // index.php
use Symfony\Bundle\FrameworkBundle\Kernel\MicroKernelTrait;
use Symfony\Component\DependencyInjection\Loader\Configurator\ContainerConfigurator;
use Symfony\Component\HttpFoundation\JsonResponse;
use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpKernel\Kernel as BaseKernel;
use Symfony\Component\Routing\Attribute\Route;

require_once dirname(__DIR__).'/vendor/autoload_runtime.php';

class Kernel extends BaseKernel
{
    use MicroKernelTrait;

    protected function configureContainer(ContainerConfigurator $container): void
    {
        // PHP equivalent of config/packages/framework.yaml
        $container->extension('framework', [
            'secret' => 'S0ME_SECRET'
        ]);
    }

    #[Route('/random/{limit}', name: 'random_number')]
    public function randomNumber(int $limit): JsonResponse
    {
        return new JsonResponse([
            'number' => random_int(0, $limit),
        ]);
    }
}

return static function (array $context) {
    return new Kernel($context['APP_ENV'], (bool) $context['APP_DEBUG']);
};
    

That's it! To test it, start the Symfony Local Web Server:

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        $ symfony server:start
    

Then see the JSON response in your browser: http://localhost:8000/random/10

Tip

If your kernel only defines a single controller, you can use an invokable method:

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        class Kernel extends BaseKernel
{
    use MicroKernelTrait;

    // ...

    #[Route('/random/{limit}', name: 'random_number')]
    public function __invoke(int $limit): JsonResponse
    {
        // ...
    }
}
    

The Methods of a "Micro" Kernel

When you use the MicroKernelTrait, your kernel needs to have exactly three methods that define your bundles, your services and your routes:

registerBundles()

This is the same registerBundles() that you see in a normal kernel. By default, the micro kernel only registers the FrameworkBundle. If you need to register more bundles, override this method:

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        use Symfony\Bundle\FrameworkBundle\FrameworkBundle;
use Symfony\Bundle\TwigBundle\TwigBundle;
// ...

class Kernel extends BaseKernel
{
    use MicroKernelTrait;

    // ...

    public function registerBundles(): array
    {
        yield new FrameworkBundle();
        yield new TwigBundle();
    }
}
    

configureContainer(ContainerConfigurator $container)

This method builds and configures the container. In practice, you will use extension() to configure different bundles (this is the equivalent of what you see in a normal config/packages/* file). You can also register services directly in PHP or load external configuration files (shown below).

configureRoutes(RoutingConfigurator $routes)

In this method, you can use the RoutingConfigurator object to define routes in your application and associate them to the controllers defined in this very same file.

However, it's more convenient to define the controller routes using PHP attributes, as shown above. That's why this method is commonly used only to load external routing files (e.g. from bundles) as shown below.

Adding Interfaces to "Micro" Kernel

When using the MicroKernelTrait, you can also implement the CompilerPassInterface to automatically register the kernel itself as a compiler pass as explained in the dedicated compiler pass section. If the ExtensionInterface is implemented when using the MicroKernelTrait, then the kernel will be automatically registered as an extension. You can learn more about it in the dedicated section about managing configuration with extensions.

It is also possible to implement the EventSubscriberInterface to handle events directly from the kernel, again it will be registered automatically:

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        // ...
use App\Exception\Danger;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Symfony\Component\HttpKernel\Event\ExceptionEvent;
use Symfony\Component\HttpKernel\KernelEvents;

class Kernel extends BaseKernel implements EventSubscriberInterface
{
    use MicroKernelTrait;

    // ...

    public function onKernelException(ExceptionEvent $event): void
    {
        if ($event->getThrowable() instanceof Danger) {
            $event->setResponse(new Response('It\'s dangerous to go alone. Take this ⚔'));
        }
    }

    public static function getSubscribedEvents(): array
    {
        return [
            KernelEvents::EXCEPTION => 'onKernelException',
        ];
    }
}
    

The purpose of the MicroKernelTrait is not to have a single-file application. Instead, its goal is to give you the power to choose your bundles and structure.

First, you'll probably want to put your PHP classes in an src/ directory. Configure your composer.json file to load from there:

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        {
    "require": {
        "...": "..."
    },
    "autoload": {
        "psr-4": {
            "App\\": "src/"
        }
    }
}
    

Then, run composer dump-autoload to dump your new autoload config.

Now, suppose you want to define a custom configuration for your app, use Twig and load routes via annotations. Instead of putting everything in index.php, create a new src/Kernel.php to hold the kernel. Now it looks like this:

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        // src/Kernel.php
namespace App;

use App\DependencyInjection\AppExtension;
use Symfony\Bundle\FrameworkBundle\FrameworkBundle;
use Symfony\Bundle\FrameworkBundle\Kernel\MicroKernelTrait;
use Symfony\Bundle\TwigBundle\TwigBundle;
use Symfony\Bundle\WebProfilerBundle\WebProfilerBundle;
use Symfony\Component\DependencyInjection\ContainerBuilder;
use Symfony\Component\DependencyInjection\Loader\Configurator\ContainerConfigurator;
use Symfony\Component\HttpKernel\Kernel as BaseKernel;
use Symfony\Component\Routing\Loader\Configurator\RoutingConfigurator;

class Kernel extends BaseKernel
{
    use MicroKernelTrait;

    public function registerBundles(): iterable
    {
        yield FrameworkBundle();
        yield TwigBundle();

        if ('dev' === $this->getEnvironment()) {
            yield WebProfilerBundle();
        }
    }

    protected function build(ContainerBuilder $containerBuilder): void
    {
        $containerBuilder->registerExtension(new AppExtension());
    }

    protected function configureContainer(ContainerConfigurator $container): void
    {
        $container->import(__DIR__.'/../config/framework.yaml');

        // register all classes in /src/ as service
        $container->services()
            ->load('App\\', __DIR__.'/*')
            ->autowire()
            ->autoconfigure()
        ;

        // configure WebProfilerBundle only if the bundle is enabled
        if (isset($this->bundles['WebProfilerBundle'])) {
            $container->extension('web_profiler', [
                'toolbar' => true,
                'intercept_redirects' => false,
            ]);
        }
    }

    protected function configureRoutes(RoutingConfigurator $routes): void
    {
        // import the WebProfilerRoutes, only if the bundle is enabled
        if (isset($this->bundles['WebProfilerBundle'])) {
            $routes->import('@WebProfilerBundle/Resources/config/routing/wdt.xml')->prefix('/_wdt');
            $routes->import('@WebProfilerBundle/Resources/config/routing/profiler.xml')->prefix('/_profiler');
        }

        // load the routes defined as PHP attributes
        // (use 'annotation' as the second argument if you define routes as annotations)
        $routes->import(__DIR__.'/Controller/', 'attribute');
    }

    // optional, to use the standard Symfony cache directory
    public function getCacheDir(): string
    {
        return __DIR__.'/../var/cache/'.$this->getEnvironment();
    }

    // optional, to use the standard Symfony logs directory
    public function getLogDir(): string
    {
        return __DIR__.'/../var/log';
    }
}
    

Before continuing, run this command to add support for the new dependencies:

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        $ composer require symfony/yaml symfony/twig-bundle symfony/web-profiler-bundle
    

Next, create a new extension class that defines your app configuration and add a service conditionally based on the foo value:

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        // src/DependencyInjection/AppExtension.php
namespace App\DependencyInjection;

use Symfony\Component\Config\Definition\Configurator\DefinitionConfigurator;
use Symfony\Component\DependencyInjection\ContainerBuilder;
use Symfony\Component\DependencyInjection\Extension\AbstractExtension;
use Symfony\Component\DependencyInjection\Loader\Configurator\ContainerConfigurator;

class AppExtension extends AbstractExtension
{
    public function configure(DefinitionConfigurator $definition): void
    {
        $definition->rootNode()
            ->children()
                ->booleanNode('foo')->defaultTrue()->end()
            ->end();
    }

    public function loadExtension(array $config, ContainerConfigurator $containerConfigurator, ContainerBuilder $containerBuilder): void
    {
        if ($config['foo']) {
            $containerBuilder->register('foo_service', \stdClass::class);
        }
    }
}
    

Unlike the previous kernel, this loads an external config/framework.yaml file, because the configuration started to get bigger:

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        # config/framework.yaml
framework:
    secret: S0ME_SECRET
    profiler: { only_exceptions: false }
    

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        <!-- config/framework.xml -->
<?xml version="1.0" encoding="UTF-8" ?>
<container xmlns="http://symfony.com/schema/dic/services"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:framework="http://symfony.com/schema/dic/symfony"
    xsi:schemaLocation="http://symfony.com/schema/dic/services https://symfony.com/schema/dic/services/services-1.0.xsd
        http://symfony.com/schema/dic/symfony https://symfony.com/schema/dic/symfony/symfony-1.0.xsd">

    <framework:config secret="S0ME_SECRET">
        <framework:profiler only-exceptions="false"/>
    </framework:config>
</container>
    

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        // config/framework.php
use Symfony\Config\FrameworkConfig;

return static function (FrameworkConfig $framework): void {
    $framework
        ->secret('SOME_SECRET')
        ->profiler()
            ->onlyExceptions(false)
    ;
};
    

This also loads attribute routes from an src/Controller/ directory, which has one file in it:

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        // src/Controller/MicroController.php
namespace App\Controller;

use Symfony\Bundle\FrameworkBundle\Controller\AbstractController;
use Symfony\Component\HttpFoundation\Response;
use Symfony\Component\Routing\Attribute\Route;

class MicroController extends AbstractController
{
    #[Route('/random/{limit}')]
    public function randomNumber(int $limit): Response
    {
        $number = random_int(0, $limit);

        return $this->render('micro/random.html.twig', [
            'number' => $number,
        ]);
    }
}
    

Template files should live in the templates/ directory at the root of your project. This template lives at templates/micro/random.html.twig:

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        <!-- templates/micro/random.html.twig -->
<!DOCTYPE html>
<html>
    <head>
        <title>Random action</title>
    </head>
    <body>
        <p>{{ number }}</p>
    </body>
</html>
    

Finally, you need a front controller to boot and run the application. Create a public/index.php:

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        // public/index.php
use App\Kernel;
use Symfony\Component\HttpFoundation\Request;

require __DIR__.'/../vendor/autoload.php';

$kernel = new Kernel('dev', true);
$request = Request::createFromGlobals();
$response = $kernel->handle($request);
$response->send();
$kernel->terminate($request, $response);
    

That's it! This /random/10 URL will work, Twig will render, and you'll even get the web debug toolbar to show up at the bottom. The final structure looks like this:

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        your-project/
├─ config/
│  └─ framework.yaml
├─ public/
|  └─ index.php
├─ src/
|  ├─ Controller
|  |  └─ MicroController.php
|  └─ Kernel.php
├─ templates/
|  └─ micro/
|     └─ random.html.twig
├─ var/
|  ├─ cache/
│  └─ log/
├─ vendor/
│  └─ ...
├─ composer.json
└─ composer.lock
    

As before you can use the Symfony Local Web Server:

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        $ symfony server:start
    

Then visit the page in your browser: http://localhost:8000/random/10

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

TOC

Version

Building your own Framework with the MicroKernelTrait

A Single-File Symfony Application

The Methods of a "Micro" Kernel

Adding Interfaces to "Micro" Kernel

Advanced Example: Twig, Annotations and the Web Debug Toolbar