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Auto-Adding Services

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At this point... we have a directory with a PHP class inside. And, honestly, we could just move this into its own repository, put it on Packagist and be done! But in that case, it wouldn't be a bundle, it would simply be a library, which is more or less defined as: a directory full of PHP classes.

So what is the difference between a library and a bundle? What does a bundle give is that a library does not? The "mostly-accurate" answer is simple: services. If we only created a library, people could use our classes, but it would be up to them to add configuration to register them as services in Symfony's container. But if we make a bundle, we can automatically add services to the container as soon as our bundle is installed. Sure, bundles can also do a few other things - like provide translations and other config - but providing services is their main super power.

So, we're going to create a bundle. Actually, the perfect solution would be to create a library with only the KnpUIpsum class, and then also a bundle that requires that library and adds the Symfony service configuration. A good example of this is KnpMenu and KnpMenuBundle.

Creating the Bundle Class

To make this a bundle, create a new class called KnpULoremIpsumBundle. This could be called anything... but usually it's the vendor namespace plus the directory name.

Make this extend Bundle and... that's it! You almost never need to have any logic in here.

// ... lines 1 - 2
namespace KnpU\LoremIpsumBundle;
// ... lines 4 - 6
class KnpULoremIpsumBundle extends Bundle
// ... lines 8 - 11

To enable this in our app, open bundles.php and configure it for all environments. I'll remove the use statement for consistency. Normally, this happens automatically when we install a bundle... but since we just added the bundle manually, we gotta do it by hand.

17 lines | config/bundles.php
// ... lines 1 - 2
return [
// ... lines 4 - 14
KnpU\LoremIpsumBundle\KnpULoremIpsumBundle::class => ['all' => true],

And, congratulations! We now have a bundle!

Creating the Extension Class

So.... what the heck does that give us? Remember: the super-power of a bundle is that it can automatically add services to the container, without the user needing to configure anything. How does that work? Let me show you.

Next to the bundle class, create a new directory called DependencyInjection. Then, add a new class inside with the same name of the bundle, except ending in Extension. So, KnpULoremIpsumExtension. Make this extend Extension from HttpKernel. This forces us to implement one method. I'll go to the Code -> Generate menu, or Cmd+N on a Mac, choose "Implement Methods" and select the one we need. Inside, just var_dump that we're alive and... die!

// ... lines 1 - 2
namespace KnpU\LoremIpsumBundle\DependencyInjection;
// ... lines 4 - 7
class KnpULoremIpsumExtension extends Extension
public function load(array $configs, ContainerBuilder $container)
var_dump('We\'re alive!');die;

Now move over and refresh. Yes! It hits our new code!

This is really important. Whenever Symfony builds the container, it loops over all the bundles and, inside of each, looks for a DependencyInjection directory and then inside of that, a class with the same name of the bundle, but ending in Extension. Woh. If that class exists, it instantiates it and calls load(). This is our big chance to add any services we want! We can go crazy!

See this $container variable? It's not really a container, it's a container builder: something we can add services to.

Adding services.xml

Right now, our service is defined in the config/services.yaml file of the application. Delete that! We're going to put a service configuration file inside the bundle instead. Create a Resources/ directory and another config/ directory inside: this is the best-practice location for service config. Then, add services.xml. Yep, I said XML. Wait, don't run away!

You can use YAML to configure your services, but XML is the best-practice for re-usable bundles... though it doesn't matter much. Using XML does have one tiny advantage: it doesn't require the symfony/yaml component, which, at least in theory, makes your bundle feel a bit lighter.

To fill this in... um, I cheat. Google for "Symfony Services", open the documentation, search for XML, and stop when you find a code block that defines a service. Click the XML tab and steal this! Paste it into our code. The only thing we need to do is configure a single service whose id is the class of the service. So, use KnpU\LoremIpsumBundle\KnpUIpsum. We're not passing any arguments, so we can use the short XML syntax for now.

<?xml version="1.0" encoding="UTF-8" ?>
<container xmlns=""
<service id="KnpU\LoremIpsumBundle\KnpUIpsum" />

But this file isn't processed automatically. Go to the extension class and remove the var_dump(). The code to load the config file looks a little funny: $loader = new XmlFileLoader() from the DependencyInjection component. Pass this a new FileLocator - the one from the Config component - with the path to that directory: ../Resources/config. Below that, add $loader->load('services.xml').

// ... lines 1 - 9
class KnpULoremIpsumExtension extends Extension
public function load(array $configs, ContainerBuilder $container)
$loader = new XmlFileLoader($container, new FileLocator(__DIR__.'/../Resources/config'));

Voilà! Refresh the page. It works! When the container builds, the load() method is called and our bundle adds its service.

Next, let's talk about service id best-practices, how to support autowiring and public versus private services.