Marking Services as public / private

When defining services, you'll usually want to be able to access these definitions within your application code. These services are called public. For example, the doctrine service registered with the container when using the DoctrineBundle is a public service. This means that you can fetch it from the container using the get() method:

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$doctrine = $container->get('doctrine');

In some cases, a service only exists to be injected into another service and is not intended to be fetched directly from the container as shown above.

In these cases, to get a minor performance boost, you can set the service to be not public (i.e. private):

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services:
   foo:
     class: Example\Foo
     public: false

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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"
    xsi:schemaLocation="http://symfony.com/schema/dic/services http://symfony.com/schema/dic/services/services-1.0.xsd">

    <services>
        <service id="foo" class="Example\Foo" public="false" />
    </services>
</container>

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use Symfony\Component\DependencyInjection\Definition;

$definition = new Definition('Example\Foo');
$definition->setPublic(false);
$container->setDefinition('foo', $definition);

What makes private services special is that, if they are only injected once, they are converted from services to inlined instantiations (e.g. new PrivateThing()). This increases the container's performance.

Now that the service is private, you should not fetch the service directly from the container:

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$container->get('foo');

This may or may not work, depending on if the service could be inlined. Simply said: A service can be marked as private if you do not want to access it directly from your code.

However, if a service has been marked as private, you can still alias it (see below) to access this service (via the alias).

Synthetic Services

Synthetic services are services that are injected into the container instead of being created by the container.

For example, if you're using the HttpKernel component with the DependencyInjection component, then the request service is injected in the ContainerAwareHttpKernel::handle() method when entering the request scope. The class does not exist when there is no request, so it can't be included in the container configuration. Also, the service should be different for every subrequest in the application.

To create a synthetic service, set synthetic to true:

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services:
    request:
        synthetic: true

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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"
    xsi:schemaLocation="http://symfony.com/schema/dic/services http://symfony.com/schema/dic/services/services-1.0.xsd">

    <services>
        <service id="request" synthetic="true" />
    </services>
</container>

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use Symfony\Component\DependencyInjection\Definition;

$container
    ->setDefinition('request', new Definition())
    ->setSynthetic(true);

As you see, only the synthetic option is set. All other options are only used to configure how a service is created by the container. As the service isn't created by the container, these options are omitted.

Now, you can inject the class by using Container::set:

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// ...
$container->set('request', new MyRequest(...));

Aliasing

You may sometimes want to use shortcuts to access some services. You can do so by aliasing them and, furthermore, you can even alias non-public services.

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services:
   foo:
     class: Example\Foo
   bar:
     alias: foo

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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"
    xsi:schemaLocation="http://symfony.com/schema/dic/services http://symfony.com/schema/dic/services/services-1.0.xsd">

    <services>
        <service id="foo" class="Example\Foo" />

        <service id="bar" alias="foo" />
    </services>
</container>

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use Symfony\Component\DependencyInjection\Definition;

$container->setDefinition('foo', new Definition('Example\Foo'));

$containerBuilder->setAlias('bar', 'foo');

This means that when using the container directly, you can access the foo service by asking for the bar service like this:

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$container->get('bar'); // Would return the foo service

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services:
   foo:
     class: Example\Foo
   bar: "@foo"

Requiring Files

There might be use cases when you need to include another file just before the service itself gets loaded. To do so, you can use the file directive.

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services:
   foo:
     class: Example\Foo\Bar
     file: "%kernel.root_dir%/src/path/to/file/foo.php"

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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"
    xsi:schemaLocation="http://symfony.com/schema/dic/services http://symfony.com/schema/dic/services/services-1.0.xsd">

    <services>
        <service id="foo" class="Example\Foo\Bar">
            <file>%kernel.root_dir%/src/path/to/file/foo.php</file>
        </service>
    </services>
</container>

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use Symfony\Component\DependencyInjection\Definition;

$definition = new Definition('Example\Foo\Bar');
$definition->setFile('%kernel.root_dir%/src/path/to/file/foo.php');
$container->setDefinition('foo', $definition);

Notice that Symfony will internally call the PHP statement require_once, which means that your file will be included only once per request.

Decorating Services

When overriding an existing definition, the old service is lost:

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$container->register('foo', 'FooService');

// this is going to replace the old definition with the new one
// old definition is lost
$container->register('foo', 'CustomFooService');

Most of the time, that's exactly what you want to do. But sometimes, you might want to decorate the old one instead. In this case, the old service should be kept around to be able to reference it in the new one. This configuration replaces foo with a new one, but keeps a reference of the old one as bar.inner:

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bar:
  public: false
  class: stdClass
  decorates: foo
  arguments: ["@bar.inner"]

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<service id="bar" class="stdClass" decorates="foo" public="false">
    <argument type="service" id="bar.inner" />
</service>

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use Symfony\Component\DependencyInjection\Reference;

$container->register('bar', 'stdClass')
    ->addArgument(new Reference('bar.inner'))
    ->setPublic(false)
    ->setDecoratedService('foo');

Here is what's going on here: the setDecoratedService() method tells the container that the bar service should replace the foo service, renaming foo to bar.inner. By convention, the old foo service is going to be renamed bar.inner, so you can inject it into your new service.

You can change the inner service name if you want to:

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bar:
  class: stdClass
  public: false
  decorates: foo
  decoration_inner_name: bar.wooz
  arguments: ["@bar.wooz"]

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<service id="bar" class="stdClass" decorates="foo" decoration-inner-name="bar.wooz" public="false">
    <argument type="service" id="bar.wooz" />
</service>

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use Symfony\Component\DependencyInjection\Reference;

$container->register('bar', 'stdClass')
    ->addArgument(new Reference('bar.wooz'))
    ->setPublic(false)
    ->setDecoratedService('foo', 'bar.wooz');