services.html.twig

    <p>A Service Container (or dependency injection container) is simply a PHP object that manages the instantiation of services (i.e. objects).</p>

{% raw %}<pre><code># src/Acme/HelloBundle/Resources/config/services.yml
parameters:
    newsletter_manager.class: Acme\HelloBundle\Newsletter\NewsletterManager
    my_mailer.transport: sendmail
    my_mailer.gateways:
        - mail1
        - mail2
        - mail3

services:
    my_mailer:
        class: "%my_mailer.class%"
        arguments: [%my_mailer.transport%]
    newsletter_manager:
        class: "%newsletter_manager.class%"
        arguments: [@my_mailer] //required contructor args. Use @ to refer another service.
        calls:
        - [ setMailer, [ @my_mailer ] ] //Optional dependencies.
        tags:
        - { name: twig.extension } //Twig finds all services tagged with twig.extension and automatically registers them as extensions.
</code></pre>{% endraw %}

    <p>Now we can set our class to be a real service:</p>

{% raw %}<pre><code>namespace Acme\HelloBundle\Newsletter;

use Symfony\Component\Templating\EngineInterface;

class NewsletterManager
{
    protected $mailer;
    protected $templating;

    public function __construct(\Swift_Mailer $mailer, EngineInterface $templating)
    {
        $this-&gt;mailer = $mailer;
        $this-&gt;templating = $templating;
    }

    // ...
}
</code></pre>{% endraw %}

    <p>And for this particual service the corresponding services.yml would be:</p>

{% raw %}<pre><code> services:
    newsletter_manager:
        class: "%newsletter_manager.class%"
        arguments: [@mailer, @templating]
</code></pre>{% endraw %}

    <p>In YAML, the special @my_mailer syntax tells the container to look for a service named my_mailer and to pass that object into the constructor of NewsletterManager. In this case, however, the specified service my_mailer must exist. If it does not, an exception will be thrown. You can mark your dependencies as optional - this will be discussed in the next section</p>

    <p><strong>Making References Optional</strong></p>

{% raw %}<pre><code>services:
    newsletter_manager:
        class: "%newsletter_manager.class%"
        arguments: [@?my_mailer]
</code></pre>{% endraw %}

    <h3>Debugging services</h3>

    <p>You can find out what services are registered with the container using the console. To show all services and the class for each service, run:</p>

{% raw %}<pre><code>$ php app/console container:debug
$ php app/console container:debug --show-private
$ php app/console container:debug my_mailer
</code></pre>{% endraw %}

    <p>See also:</p>

    <ul>
        <li>
            <a href="http://symfony.com/doc/current/components/dependency_injection/parentservices.html">Managing Common Dependencies with Parent Services</a>
        </li>
    </ul>