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Code Partition for Beau Simensen

Symfony Kernel Application From Scratch: Overriding the Root Directory

The Kernel is the heart of the Symfony system. It manages an environment made of bundles.

— Symfony's Kernel Docblock

One of the best things that happened to Sculpin was Rob Loach suggesting it support Symfony's Bundle interface for plugins. What started out as an interesting experiment in trying to bolt Bundles into a completely custom application resulted in rebuilding Sculpin from scratch with Symfony's Kernel at its core.

Since then I've become a huge fan of Symfony's Kernel. I believe that it can make a great base for applications of any type. I'd like to share some of the things that I've learned in the hopes that it will help to inspire the community to build more applications around Symfony's Kernel.

Your Root Directory Is Where Now?

One of the design goals for Sculpin was that I wanted to have configuration (Kernel or otherwise) completely optional. Someone should be able to download a phar and run it without having to explicitly configure anything.

With respect to Kernel, this meant that I wanted the Kernel configuration (app/config/kernel.yml) and the Kernel class definition (app/AppKernel.php) to be completely optional. Anyone familiar with Kernel will know that this was going to be a problem.

The base Kernel implementation uses reflection magic to determine the kernel.root based on the file that defines the Kernel instance. This works when you know your Kernel is always going to live in app/AppKernel.php. This does not work so well when your Kernel class actually lives deep under vendor/sculpin/sculpin/src/....

This is one of the rare cases that I have run into that Symfony Kernel is actually tied to how Symfony Standard Edition is setup. Or maybe Symfony Standard Edition is setup based on this restrction? Either way, there is no way to configure the root directory; It is always going to be setup by Magic.

Decorating the Kernel instance was not going to be an option due to when and how the Kernel handles setting the root directory and due to the fact that as a protected property other people (and indeed Kernel itself) access $this->rootDir directly.

This meant that overriding the Kernel root directory has to be handled by the constructor. I had hoped that the ability to specify a custom root directory could be added to Symfony core. Alas, it was not meant to be.

All is not lost. It is still possible even if it is not pretty.

class CustomKernel extends Kernel
    public function __construct($environment, $debug, $rootDir = null)
        if (null !== $rootDir) {
            $this->rootDir = $rootDir;

        parent::__construct($environment, $debug);

While not the prettiest it does get the job done. This custom Kernel can now exist anywhere, whether deep inside vendor/... or embedded in a phar.

We now have a base for building our application around our custom Kernel with no restrictions on where the Kernel lives in relation to our application directory.

Tags: symfony, sculpin

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