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 as you can access it via:

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

However, there are use-cases when you don't want a service to be public. This is common when a service is only defined because it could be used as an argument for another service.

Simply said: A service will be private when you do not want to access it directly from your code.

Here is an example:

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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);

Now that the service is private, you cannot call:

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$container->get('foo');

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.