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Building your own Framework with the MicroKernelTrait

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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 so flexible that let you control your application structure and features quite easily.

A Single-File Symfony Application

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

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$ composer require symfony/config symfony/http-kernel \
  symfony/http-foundation symfony/routing \
  symfony/dependency-injection symfony/framework-bundle

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

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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 as BaseKernel;
use Symfony\Component\Routing\RouteCollectionBuilder;

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

class Kernel extends BaseKernel
{
    use MicroKernelTrait;

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

    protected function configureContainer(ContainerBuilder $c, LoaderInterface $loader)
    {
        // PHP equivalent of config/packages/framework.yaml
        $c->loadFromExtension('framework', array(
            '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:randomNumber');
    }

    public function randomNumber($limit)
    {
        return new JsonResponse(array(
            'number' => rand(0, $limit)
        ));
    }
}

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

That's it! To test it, you can start the built-in web server:

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$ php -S localhost:8000

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/packages/* 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).

Advanced Example: Twig, Annotations and the Web Debug Toolbar

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": {
            "App\\": "src/"
        }
    }
}

Now, suppose you want to 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 Symfony\Bundle\FrameworkBundle\Kernel\MicroKernelTrait;
use Symfony\Component\Config\Loader\LoaderInterface;
use Symfony\Component\DependencyInjection\ContainerBuilder;
use Symfony\Component\HttpKernel\Kernel as BaseKernel;
use Symfony\Component\Routing\RouteCollectionBuilder;
use Doctrine\Common\Annotations\AnnotationRegistry;

$loader = require __DIR__.'/../vendor/autoload.php';
// auto-load annotations
AnnotationRegistry::registerLoader(array($loader, 'loadClass'));

class Kernel extends BaseKernel
{
    use MicroKernelTrait;

    public function registerBundles()
    {
        $bundles = array(
            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/framework.yaml');

        // configure WebProfilerBundle only if the bundle is enabled
        if (isset($this->bundles['WebProfilerBundle'])) {
            $c->loadFromExtension('web_profiler', array(
                '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/log';
    }
}

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

  • YAML
  • XML
  • PHP 
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# config/framework.yaml
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 randomNumber($limit)
    {
        $number = rand(0, $limit);

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

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

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<!-- src/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 public/index.php:

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// public/index.php

use Symfony\Component\HttpFoundation\Request;

require __DIR__.'/../src/Kernel.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/
|  ├─ Kernel.php
|  ├─ Controller
|  |  └─ MicroController.php
│  └─ Resources
|     └─ views
|        └─ micro
|           └─ random.html.twig
├─ var/
|  ├─ cache/
│  └─ log/
├─ vendor/
│  └─ ...
├─ composer.json
└─ composer.lock

As before you can use PHP built-in server:

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cd public/
$ php -S localhost:8000

Then see webpage in browser:

http://localhost:8000/random/10

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