dependency_injection.rst

.. index::
    single: Dependency Injection

The Dependency Injection Component

The Dependency Injection component allows you to standardize and centralize the way objects are constructed in your application.

For an introduction to Dependency Injection and service containers see :doc:`/book/service_container`

Installation

You can install the component in many different ways:

• Use the official Git repository (https://github.com/symfony/DependencyInjection);
• Install it via PEAR ( pear.symfony.com/DependencyInjection);
• Install it via Composer (symfony/dependency-injection on Packagist).

Basic Usage

You might have a simple class like the following Mailer that you want to make available as a service:

class Mailer
{
    private $transport;

    public function __construct()
    {
        $this->transport = 'sendmail';
    }

    // ...
}

You can register this in the container as a service:

use Symfony\Component\DependencyInjection\ContainerBuilder;

$sc = new ContainerBuilder();
$sc->register('mailer', 'Mailer');

An improvement to the class to make it more flexible would be to allow the container to set the transport used. If you change the class so this is passed into the constructor:

class Mailer
{
    private $transport;

    public function __construct($transport)
    {
        $this->transport = $transport;
    }

    // ...
}

Then you can set the choice of transport in the container:

use Symfony\Component\DependencyInjection\ContainerBuilder;

$sc = new ContainerBuilder();
$sc->register('mailer', 'Mailer')
    ->addArgument('sendmail'));

This class is now much more flexible as we have separated the choice of transport out of the implementation and into the container.

Which mail transport you have chosen may be something other services need to know about. You can avoid having to change it in multiple places by making it a parameter in the container and then referring to this parameter for the Mailer service's constructor argument:

use Symfony\Component\DependencyInjection\ContainerBuilder;

$sc = new ContainerBuilder();
$sc->setParameter('mailer.transport', 'sendmail');
$sc->register('mailer', 'Mailer')
    ->addArgument('%mailer.transport%'));

Now that the mailer service is in the container you can inject it as a dependency of other classes. If you have a NewsletterManager class like this:

use Mailer;

class NewsletterManager
{
    private $mailer;

    public function __construct(Mailer $mailer)
    {
        $this->mailer = $mailer;
    }

    // ...
}

Then you can register this as a service as well and pass the mailer service into it:

use Symfony\Component\DependencyInjection\ContainerBuilder;
use Symfony\Component\DependencyInjection\Reference;

$sc = new ContainerBuilder();

$sc->setParameter('mailer.transport', 'sendmail');
$sc->register('mailer', 'Mailer')
    ->addArgument('%mailer.transport%'));

$sc->register('newsletter_manager', 'NewsletterManager')
    ->addArgument(new Reference('mailer'));

If the NewsletterManager did not require the Mailer and injecting it was only optional then you could use setter injection instead:

use Mailer;

class NewsletterManager
{
    private $mailer;

    public function setMailer(Mailer $mailer)
    {
        $this->mailer = $mailer;
    }

    // ...
}

You can now choose not to inject a Mailer into the NewsletterManager. If you do want to though then the container can call the setter method:

use Symfony\Component\DependencyInjection\ContainerBuilder;
use Symfony\Component\DependencyInjection\Reference;

$sc = new ContainerBuilder();

$sc->setParameter('mailer.transport', 'sendmail');
$sc->register('mailer', 'Mailer')
    ->addArgument('%mailer.transport%'));

$sc->register('newsletter_manager', 'NewsletterManager')
    ->addMethodCall('setMailer', new Reference('mailer'));

You could then get your newsletter_manager service from the container like this:

use Symfony\Component\DependencyInjection\ContainerBuilder;
use Symfony\Component\DependencyInjection\Reference;

$sc = new ContainerBuilder();

//--

$newsletterManager = $sc->get('newsletter_manager');

Avoiding Your Code Becoming Dependent on the Container

Whilst you can retrieve services from the container directly it is best to minimize this. For example, in the NewsletterManager we injected the mailer service in rather than asking for it from the container. We could have injected the container in and retrieved the mailer service from it but it would then be tied to this particular container making it difficult to reuse the class elsewhere.

You will need to get a service from the container at some point but this should be as few times as possible at the entry point to your application.

Setting Up the Container with Configuration Files

As well as setting up the services using PHP as above you can also use configuration files. To do this you also need to install the Config component:

• Use the official Git repository (https://github.com/symfony/Config);
• Install it via PEAR ( pear.symfony.com/Config);
• Install it via Composer (symfony/config on Packagist).

Loading an xml config file:

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

$sc = new ContainerBuilder();
$loader = new XmlFileLoader($container, new FileLocator(__DIR__));
$loader->load('services.xml');

Loading a yaml config file:

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

$sc = new ContainerBuilder();
$loader = new YamlFileLoader($container, new FileLocator(__DIR__));
$loader->load('services.yml');

The newsletter_manager and `` mailer`` services can be set up using config files:

.. configuration-block::

    .. code-block:: yaml

        # src/Acme/HelloBundle/Resources/config/services.yml
        parameters:
            # ...
            mailer.transport: sendmail

        services:
            my_mailer:
                class:     Mailer
                arguments: [@mailer]
            newsletter_manager:
                class:     NewsletterManager
                calls:
                    - [ setMailer, [ @mailer ] ]

    .. code-block:: xml

        <!-- src/Acme/HelloBundle/Resources/config/services.xml -->
        <parameters>
            <!-- ... -->
            <parameter key="mailer.transport">sendmail</parameter>
        </parameters>

        <services>
            <service id="mailer" class="Mailer">
                <argument>%mailer.transport%</argument>
            </service>

            <service id="newsletter_manager" class="NewsletterManager">
                <call method="setMailer">
                     <argument type="service" id="mailer" />
                </call>
            </service>
        </services>

    .. code-block:: php

        use Symfony\Component\DependencyInjection\Reference;

        // ...
        $sc->setParameter('mailer.transport', 'sendmail');
        $sc->register('mailer', 'Mailer')
           ->addArgument('%mailer.transport%'));

        $sc->register('newsletter_manager', 'NewsletterManager')
           ->addMethodCall('setMailer', new Reference('mailer'));

Learn more from the Cookbook

• :doc:`/cookbook/service_container/factories`
• :doc:`/cookbook/service_container/parentservices`