Building your own Framework with the MicroKernelTrait

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Read the updated version of this page for Symfony 7.2 (the current stable version).

A traditional Symfony app contains a sensible directory structure, various configuration files and an AppKernel with several bundles already-registered. This is a fully-featured app that's ready to go.

But did you know, you can create a fully-functional Symfony application in as little as one file? This is possible thanks to the new MicroKernelTrait. This allows you to start with a tiny application, and then add features and structure as you need to.

A Single-File Symfony Application

Start with a completely empty directory. Get symfony/symfony as a dependency via Composer:

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        $ composer require symfony/symfony
    

Next, create an index.php file that creates a kernel class and executes it:

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        // index.php
use Symfony\Bundle\FrameworkBundle\Kernel\MicroKernelTrait;
use Symfony\Component\Config\Loader\LoaderInterface;
use Symfony\Component\DependencyInjection\ContainerBuilder;
use Symfony\Component\HttpFoundation\JsonResponse;
use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpKernel\Kernel;
use Symfony\Component\Routing\RouteCollectionBuilder;

// require Composer's autoloader
require __DIR__.'/vendor/autoload.php';

class AppKernel extends Kernel
{
    use MicroKernelTrait;

    public function registerBundles()
    {
        return [
            new Symfony\Bundle\FrameworkBundle\FrameworkBundle()
        ];
    }

    protected function configureContainer(ContainerBuilder $c, LoaderInterface $loader)
    {
        // PHP equivalent of config.yml
        $c->loadFromExtension('framework', [
            'secret' => 'S0ME_SECRET'
        ]);
    }

    protected function configureRoutes(RouteCollectionBuilder $routes)
    {
        // kernel is a service that points to this class
        // optional 3rd argument is the route name
        $routes->add('/random/{limit}', 'kernel:randomAction');
    }

    public function randomAction($limit)
    {
        return new JsonResponse([
            'number' => rand(0, $limit)
        ]);
    }
}

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

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

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.
configureContainer(ContainerBuilder $c, LoaderInterface $loader): This method builds and configures the container. In practice, you will use loadFromExtension to configure different bundles (this is the equivalent of what you see in a normal config.yml file). You can also register services directly in PHP or load external configuration files (shown below).
configureRoutes(RouteCollectionBuilder $routes): Your job in this method is to add routes to the application. The RouteCollectionBuilder has methods that make adding routes in PHP more fun. You can also load external routing files (shown below).

The purpose of the MicroKernelTrait is not to have a single-file application. Instead, its goal 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": {
            "": "src/"
        }
    }
}
    

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

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

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        // app/AppKernel.php
use Doctrine\Common\Annotations\AnnotationRegistry;
use Symfony\Bundle\FrameworkBundle\Kernel\MicroKernelTrait;
use Symfony\Component\Config\Loader\LoaderInterface;
use Symfony\Component\DependencyInjection\ContainerBuilder;
use Symfony\Component\HttpKernel\Kernel;
use Symfony\Component\Routing\RouteCollectionBuilder;

// require Composer's autoloader
$loader = require __DIR__.'/../vendor/autoload.php';
// auto-load annotations
AnnotationRegistry::registerLoader([$loader, 'loadClass']);

class AppKernel extends Kernel
{
    use MicroKernelTrait;

    public function registerBundles()
    {
        $bundles = [
            new Symfony\Bundle\FrameworkBundle\FrameworkBundle(),
            new Symfony\Bundle\TwigBundle\TwigBundle(),
        ];

        if ($this->getEnvironment() == 'dev') {
            $bundles[] = new Symfony\Bundle\WebProfilerBundle\WebProfilerBundle();
        }

        return $bundles;
    }

    protected function configureContainer(ContainerBuilder $c, LoaderInterface $loader)
    {
        $loader->load(__DIR__.'/config/config.yml');

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

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

        // load the annotation routes
        $routes->import(__DIR__.'/../src/Controller/', '/', 'annotation');
    }

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

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

3.4

Support for annotation routing without an external bundle was introduced in Symfony 3.4. Prior to version 3.4, you needed to install the SensioFrameworkExtraBundle.

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

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        # app/config/config.yml
framework:
    secret: S0ME_SECRET
    templating:
        engines: ['twig']
    profiler: { only_exceptions: false }
    

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        <!-- app/config/config.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:templating>
            <framework:engine>twig</framework:engine>
        </framework:templating>
        <framework:profiler only-exceptions="false"/>
    </framework:config>
</container>
    

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        // app/config/config.php
$container->loadFromExtension('framework', [
    'secret' => 'S0ME_SECRET',
    'templating' => [
        'engines' => ['twig'],
    ],
    'profiler' => [
        'only_exceptions' => false,
    ],
]);
    

This also loads annotation 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\Controller;
use Symfony\Component\Routing\Annotation\Route;

class MicroController extends Controller
{
    /**
     * @Route("/random/{limit}")
     */
    public function randomAction($limit)
    {
        $number = rand(0, $limit);

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

Template files should live in the Resources/views directory of whatever directory your kernel lives in. Since AppKernel lives in app/, this template lives at app/Resources/views/micro/random.html.twig:

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        <!-- app/Resources/views/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 web/index.php:

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        // web/index.php
use Symfony\Component\HttpFoundation\Request;

require __DIR__.'/../app/AppKernel.php';

$kernel = new AppKernel('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/
├─ app/
|  ├─ AppKernel.php
│  ├─ config/
│  └─ Resources
|     └─ views
|        └─ micro
|           └─ random.html.twig
├─ src/
│  └─ App
|     └─ Controller
|        └─ MicroController.php
├─ var/
|  ├─ cache/
│  └─ logs/
├─ vendor/
│  └─ ...
├─ web/
|  └─ index.php
├─ composer.json
└─ composer.lock
    

As before you can use the Symfony Local Web Server:

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        cd web/
$ symfony server:start
    

Then visit the page in your browser:

http://localhost:8000/random/10

Hey, that looks a lot like a traditional Symfony application! You're right: the MicroKernelTrait is still Symfony: but you can control your structure and features with less boilerplate configuration and code.

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

Advanced Example: Twig, Annotations and the Web Debug Toolbar