Compiling the Container

Managing Configuration with Extensions

As well as loading configuration directly into the container as shown in The DependencyInjection Component, you can manage it by registering extensions with the container. The first step in the compilation process is to load configuration from any extension classes registered with the container. Unlike the configuration loaded directly, they are only processed when the container is compiled. If your application is modular then extensions allow each module to register and manage their own service configuration.

The extensions must implement ExtensionInterface and can be registered with the container with:

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$container->registerExtension($extension);

The main work of the extension is done in the load() method. In the load() method you can load configuration from one or more configuration files as well as manipulate the container definitions using the methods shown in How to work with Service Definition Objects.

The load() method is passed a fresh container to set up, which is then merged afterwards into the container it is registered with. This allows you to have several extensions managing container definitions independently. The extensions do not add to the containers configuration when they are added but are processed when the container's compile() method is called.

A very simple extension may just load configuration files into the container:

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use Symfony\Component\Config\FileLocator;
use Symfony\Component\DependencyInjection\ContainerBuilder;
use Symfony\Component\DependencyInjection\Extension\ExtensionInterface;
use Symfony\Component\DependencyInjection\Loader\XmlFileLoader;

class AcmeDemoExtension implements ExtensionInterface
{
    public function load(array $configs, ContainerBuilder $container)
    {
        $loader = new XmlFileLoader(
            $container,
            new FileLocator(__DIR__.'/../Resources/config')
        );
        $loader->load('services.xml');
    }

    // ...
}

This does not gain very much compared to loading the file directly into the overall container being built. It just allows the files to be split up among the modules/bundles. Being able to affect the configuration of a module from configuration files outside of the module/bundle is needed to make a complex application configurable. This can be done by specifying sections of config files loaded directly into the container as being for a particular extension. These sections on the config will not be processed directly by the container but by the relevant Extension.

The Extension must specify a getAlias() method to implement the interface:

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// ...

class AcmeDemoExtension implements ExtensionInterface
{
    // ...

    public function getAlias()
    {
        return 'acme_demo';
    }
}

For YAML configuration files specifying the alias for the extension as a key will mean that those values are passed to the Extension's load() method:

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# ...
acme_demo:
    foo: fooValue
    bar: barValue

If this file is loaded into the configuration then the values in it are only processed when the container is compiled at which point the Extensions are loaded:

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use Symfony\Component\Config\FileLocator;
use Symfony\Component\DependencyInjection\ContainerBuilder;
use Symfony\Component\DependencyInjection\Loader\YamlFileLoader;

$container = new ContainerBuilder();
$container->registerExtension(new AcmeDemoExtension);

$loader = new YamlFileLoader($container, new FileLocator(__DIR__));
$loader->load('config.yaml');

// ...
$container->compile();

The values from those sections of the config files are passed into the first argument of the load() method of the extension:

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public function load(array $configs, ContainerBuilder $container)
{
    $foo = $configs[0]['foo']; //fooValue
    $bar = $configs[0]['bar']; //barValue
}

The $configs argument is an array containing each different config file that was loaded into the container. You are only loading a single config file in the above example but it will still be within an array. The array will look like this:

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[
    [
        'foo' => 'fooValue',
        'bar' => 'barValue',
    ],
]

While you can manually manage merging the different files, it is much better to use the Config component to merge and validate the config values. Using the configuration processing you could access the config value this way:

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use Symfony\Component\Config\Definition\Processor;
// ...

public function load(array $configs, ContainerBuilder $container)
{
    $configuration = new Configuration();
    $processor = new Processor();
    $config = $processor->processConfiguration($configuration, $configs);

    $foo = $config['foo']; //fooValue
    $bar = $config['bar']; //barValue

    // ...
}

There are a further two methods you must implement. One to return the XML namespace so that the relevant parts of an XML config file are passed to the extension. The other to specify the base path to XSD files to validate the XML configuration:

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public function getXsdValidationBasePath()
{
    return __DIR__.'/../Resources/config/';
}

public function getNamespace()
{
    return 'http://www.example.com/symfony/schema/';
}

The XML version of the config would then look like this:

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<?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:acme-demo="http://www.example.com/schema/dic/acme_demo"
    xsi:schemaLocation="http://symfony.com/schema/dic/services
        https://symfony.com/schema/dic/services/services-1.0.xsd
        http://www.example.com/schema/dic/acme_demo
        https://www.example.com/schema/dic/acme_demo/acme_demo-1.0.xsd"
>
    <acme-demo:config>
        <acme_demo:foo>fooValue</acme_demo:foo>
        <acme_demo:bar>barValue</acme_demo:bar>
    </acme-demo:config>
</container>

The processed config value can now be added as container parameters as if it were listed in a parameters section of the config file but with the additional benefit of merging multiple files and validation of the configuration:

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public function load(array $configs, ContainerBuilder $container)
{
    $configuration = new Configuration();
    $processor = new Processor();
    $config = $processor->processConfiguration($configuration, $configs);

    $container->setParameter('acme_demo.FOO', $config['foo']);

    // ...
}

More complex configuration requirements can be catered for in the Extension classes. For example, you may choose to load a main service configuration file but also load a secondary one only if a certain parameter is set:

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public function load(array $configs, ContainerBuilder $container)
{
    $configuration = new Configuration();
    $processor = new Processor();
    $config = $processor->processConfiguration($configuration, $configs);

    $loader = new XmlFileLoader(
        $container,
        new FileLocator(__DIR__.'/../Resources/config')
    );
    $loader->load('services.xml');

    if ($config['advanced']) {
        $loader->load('advanced.xml');
    }
}

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use Symfony\Component\DependencyInjection\ContainerBuilder;

$container = new ContainerBuilder();
$extension = new AcmeDemoExtension();
$container->registerExtension($extension);
$container->loadFromExtension($extension->getAlias());
$container->compile();

Prepending Configuration Passed to the Extension

An Extension can prepend the configuration of any Bundle before the load() method is called by implementing PrependExtensionInterface:

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use Symfony\Component\DependencyInjection\Extension\PrependExtensionInterface;
// ...

class AcmeDemoExtension implements ExtensionInterface, PrependExtensionInterface
{
    // ...

    public function prepend(ContainerBuilder $container)
    {
        // ...

        $container->prependExtensionConfig($name, $config);

        // ...
    }
}

For more details, see How to Simplify Configuration of Multiple Bundles, which is specific to the Symfony Framework, but contains more details about this feature.

Execute Code During Compilation

You can also execute custom code during compilation by writing your own compiler pass. By implementing CompilerPassInterface in your extension, the added process() method will be called during compilation:

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// ...
use Symfony\Component\DependencyInjection\Compiler\CompilerPassInterface;

class AcmeDemoExtension implements ExtensionInterface, CompilerPassInterface
{
    public function process(ContainerBuilder $container)
    {
        // ... do something during the compilation
    }

    // ...
}

As process() is called after all extensions are loaded, it allows you to edit service definitions of other extensions as well as retrieving information about service definitions.

The container's parameters and definitions can be manipulated using the methods described in How to work with Service Definition Objects.

A common use-case of compiler passes is to search for all service definitions that have a certain tag, in order to dynamically plug each one into other services. See the section on service tags for an example.

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use Symfony\Component\DependencyInjection\Compiler\CompilerPassInterface;
use Symfony\Component\DependencyInjection\ContainerBuilder;

class CustomPass implements CompilerPassInterface
{
    public function process(ContainerBuilder $container)
    {
        // ... do something during the compilation
    }
}

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use Symfony\Component\DependencyInjection\ContainerBuilder;

$container = new ContainerBuilder();
$container->addCompilerPass(new CustomPass());

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// ...
$container->addCompilerPass(
    new CustomPass(),
    PassConfig::TYPE_AFTER_REMOVING
);

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// ...
// FirstPass is executed after SecondPass because its priority is lower
$container->addCompilerPass(
    new FirstPass(), PassConfig::TYPE_AFTER_REMOVING, 10
);
$container->addCompilerPass(
    new SecondPass(), PassConfig::TYPE_AFTER_REMOVING, 30
);

Dumping the Configuration for Performance

Using configuration files to manage the service container can be much easier to understand than using PHP once there are a lot of services. This ease comes at a price though when it comes to performance as the config files need to be parsed and the PHP configuration built from them. The compilation process makes the container more efficient but it takes time to run. You can have the best of both worlds though by using configuration files and then dumping and caching the resulting configuration. The PhpDumper serves at dumping the compiled container:

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use Symfony\Component\DependencyInjection\ContainerBuilder;
use Symfony\Component\DependencyInjection\Dumper\PhpDumper;

$file = __DIR__ .'/cache/container.php';

if (file_exists($file)) {
    require_once $file;
    $container = new ProjectServiceContainer();
} else {
    $container = new ContainerBuilder();
    // ...
    $container->compile();

    $dumper = new PhpDumper($container);
    file_put_contents($file, $dumper->dump());
}

ProjectServiceContainer is the default name given to the dumped container class. However, you can change this with the class option when you dump it:

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// ...
$file = __DIR__ .'/cache/container.php';

if (file_exists($file)) {
    require_once $file;
    $container = new MyCachedContainer();
} else {
    $container = new ContainerBuilder();
    // ...
    $container->compile();

    $dumper = new PhpDumper($container);
    file_put_contents(
        $file,
        $dumper->dump(['class' => 'MyCachedContainer'])
    );
}

You will now get the speed of the PHP configured container with the ease of using configuration files. Additionally dumping the container in this way further optimizes how the services are created by the container.

In the above example you will need to delete the cached container file whenever you make any changes. Adding a check for a variable that determines if you are in debug mode allows you to keep the speed of the cached container in production but getting an up to date configuration whilst developing your application:

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// ...

// based on something in your project
$isDebug = ...;

$file = __DIR__ .'/cache/container.php';

if (!$isDebug && file_exists($file)) {
    require_once $file;
    $container = new MyCachedContainer();
} else {
    $container = new ContainerBuilder();
    // ...
    $container->compile();

    if (!$isDebug) {
        $dumper = new PhpDumper($container);
        file_put_contents(
            $file,
            $dumper->dump(['class' => 'MyCachedContainer'])
        );
    }
}

This could be further improved by only recompiling the container in debug mode when changes have been made to its configuration rather than on every request. This can be done by caching the resource files used to configure the container in the way described in "Caching based on Resources" in the config component documentation.

You do not need to work out which files to cache as the container builder keeps track of all the resources used to configure it, not just the configuration files but the extension classes and compiler passes as well. This means that any changes to any of these files will invalidate the cache and trigger the container being rebuilt. You need to ask the container for these resources and use them as metadata for the cache:

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// ...

// based on something in your project
$isDebug = ...;

$file = __DIR__ .'/cache/container.php';
$containerConfigCache = new ConfigCache($file, $isDebug);

if (!$containerConfigCache->isFresh()) {
    $container = new ContainerBuilder();
    // ...
    $container->compile();

    $dumper = new PhpDumper($container);
    $containerConfigCache->write(
        $dumper->dump(['class' => 'MyCachedContainer']),
        $container->getResources()
    );
}

require_once $file;
$container = new MyCachedContainer();

Now the cached dumped container is used regardless of whether debug mode is on or not. The difference is that the ConfigCache is set to debug mode with its second constructor argument. When the cache is not in debug mode the cached container will always be used if it exists. In debug mode, an additional metadata file is written with all the involved resource files. These are then checked to see if their timestamps have changed, if they have the cache will be considered stale.