Why Lazy Services?

In some cases, you may want to inject a service that is a bit heavy to instantiate, but is not always used inside your object. For example, imagine you have a NewsletterManager and you inject a mailer service into it. Only a few methods on your NewsletterManager actually use the mailer, but even when you don't need it, a mailer service is always instantiated in order to construct your NewsletterManager.

Configuring lazy services is one answer to this. With a lazy service, a "proxy" of the mailer service is actually injected. It looks and acts like the mailer, except that the mailer isn't actually instantiated until you interact with the proxy in some way.

Configuration

You can mark the service as lazy by manipulating its definition:

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# config/services.yaml
services:
    App\Twig\AppExtension:
        lazy: true

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<!-- config/services.xml -->
<?xml version="1.0" encoding="UTF-8" ?>
<container xmlns="http://symfony.com/schema/dic/services"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://symfony.com/schema/dic/services
        https://symfony.com/schema/dic/services/services-1.0.xsd">

    <services>
        <service id="App\Twig\AppExtension" lazy="true"/>
    </services>
</container>

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// config/services.php
namespace Symfony\Component\DependencyInjection\Loader\Configurator;

use App\Twig\AppExtension;

return function(ContainerConfigurator $container): void {
    $services = $container->services();

    $services->set(AppExtension::class)->lazy();
};

Once you inject the service into another service, a lazy ghost object with the same signature of the class representing the service should be injected. A lazy ghost object is an object that is created empty and that is able to initialize itself when being accessed for the first time). The same happens when calling Container::get() directly.

To check if your lazy service works you can check the interface of the received object:

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dump(class_implements($service));
// the output should include "Symfony\Component\VarExporter\LazyObjectInterface"

You can also configure your service's laziness thanks to the Autoconfigure attribute. For example, to define your service as lazy use the following:

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namespace App\Twig;

use Symfony\Component\DependencyInjection\Attribute\Autoconfigure;
use Twig\Extension\ExtensionInterface;

#[Autoconfigure(lazy: true)]
class AppExtension implements ExtensionInterface
{
    // ...
}

Interface Proxifying

Under the hood, proxies generated to lazily load services inherit from the class used by the service. However, sometimes this is not possible at all (e.g. because the class is final and can not be extended) or not convenient.

To workaround this limitation, you can configure a proxy to only implement specific interfaces.

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# config/services.yaml
services:
    App\Twig\AppExtension:
        lazy: 'Twig\Extension\ExtensionInterface'
        # or a complete definition:
        lazy: true
        tags:
            - { name: 'proxy', interface: 'Twig\Extension\ExtensionInterface' }

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<!-- config/services.xml -->
<?xml version="1.0" encoding="UTF-8" ?>
<container xmlns="http://symfony.com/schema/dic/services"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://symfony.com/schema/dic/services
        https://symfony.com/schema/dic/services/services-1.0.xsd">

    <services>
        <service id="App\Twig\AppExtension" lazy="Twig\Extension\ExtensionInterface"/>
        <!-- or a complete definition: -->
        <service id="App\Twig\AppExtension" lazy="true">
            <tag name="proxy" interface="Twig\Extension\ExtensionInterface"/>
        </service>
    </services>
</container>

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// config/services.php
namespace Symfony\Component\DependencyInjection\Loader\Configurator;

use App\Twig\AppExtension;
use Twig\Extension\ExtensionInterface;

return function(ContainerConfigurator $container): void {
    $services = $container->services();

    $services->set(AppExtension::class)
        ->lazy()
        ->tag('proxy', ['interface' => ExtensionInterface::class])
    ;
};

Just like in the Configuration section, you can use the Autoconfigure attribute to configure the interface to proxify by passing its FQCN as the lazy parameter value:

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namespace App\Twig;

use Symfony\Component\DependencyInjection\Attribute\Autoconfigure;
use Twig\Extension\ExtensionInterface;

#[Autoconfigure(lazy: ExtensionInterface::class)]
class AppExtension implements ExtensionInterface
{
    // ...
}

The virtual proxy injected into other services will only implement the specified interfaces and will not extend the original service class, allowing to lazy load services using final classes. You can configure the proxy to implement multiple interfaces by adding new "proxy" tags.