Lazy Services

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See also

Other ways to inject services lazily are via a service closure or service subscriber.

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.

Caution

Lazy services do not support final or readonly classes, but you can use Interface Proxifying to work around this limitation.

In PHP versions prior to 8.0 lazy services do not support parameters with default values for built-in PHP classes (e.g. PDO).

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

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
{
    // ...
}

You can also configure laziness when your service is injected with the Autowire attribute:

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namespace App\Service;

use App\Twig\AppExtension;
use Symfony\Component\DependencyInjection\Attribute\Autowire;

class MessageGenerator
{
    public function __construct(
        #[Autowire(service: 'app.twig.app_extension', lazy: true)] ExtensionInterface $extension
    ) {
        // ...
    }
}

This attribute also allows you to define the interfaces to proxy when using laziness, and supports lazy-autowiring of union types:

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public function __construct(
    #[Autowire(service: 'foo', lazy: FooInterface::class)]
    FooInterface|BarInterface $foo,
) {
}

Another possibility is to use the Lazy attribute:

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namespace App\Twig;

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

#[Lazy]
class AppExtension implements ExtensionInterface
{
    // ...
}

This attribute can be applied to both class and parameters that should be lazy-loaded. It defines an optional parameter used to define interfaces for proxy and intersection types:

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public function __construct(
    #[Lazy(FooInterface::class)]
    FooInterface|BarInterface $foo,
) {
}

7.1

The #[Lazy] attribute was introduced in Symfony 7.1.

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' }

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.

Tip

This feature can also act as a safe guard: given that the proxy does not extend the original class, only the methods defined by the interface can be called, preventing to call implementation specific methods. It also prevents injecting the dependency at all if you type-hinted a concrete implementation instead of the interface.

This work, including the code samples, is licensed under a Creative Commons BY-SA 3.0 license.
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