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:
1
$ composer require symfony/framework-bundle symfony/runtime
Next, create an index.php
file that defines the kernel class and runs it:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34
// 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:
1
$ 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:
1 2 3 4 5 6 7 8 9 10 11 12
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 theFrameworkBundle
. If you need to register more bundles, override this method:1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
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 normalconfig/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:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
// ...
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',
];
}
}
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 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:
1 2 3 4 5 6 7 8 9 10
{
"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:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77
// 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:
1
$ 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:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
// 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:
1 2 3 4
# config/framework.yaml
framework:
secret: S0ME_SECRET
profiler: { only_exceptions: false }
This also loads attribute routes from an src/Controller/
directory, which
has one file in it:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
// 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
:
1 2 3 4 5 6 7 8 9 10
<!-- 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
:
1 2 3 4 5 6 7 8 9 10 11
// 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:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
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:
1
$ symfony server:start
Then visit the page in your browser: http://localhost:8000/random/10