Contributed by
Ryan Weaver
in #22234.

In Symfony 3.3, configuring services is much simpler thanks to these new configuration options:

  • _defaults: defines the default value for the public, tags and autowire options of the services defined in a given file;
  • _instanceof: defines the default configuration of services depending on their classes (e.g. add the twig.extension tag to any service that implements Twig_ExtensionInterface).

The evolution of this simplification is the new autoconfigure option, which is like an automated version of _instanceof. If enabled, this option adds some default configuration depending on the class implemented by the service.

Let's suppose that you want to add tags automatically to your security voters:

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services:
    _defaults:
        autowire: true

    _instanceof:
        Symfony\Component\Security\Core\Authorization\Voter\VoterInterface:
            tags: [security.voter]

    AppBundle\Security\PostVoter: ~

Now ask yourself: if you are registering a service with a class that implements VoterInterface, when would you ever not want that to be tagged with security.voter? In other words, a service implementing VoterInterface can only be a security voter, unless you are doing some seriously weird things.

The same example using autoconfigure looks like this:

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services:
    _defaults:
        autowire: true
        autoconfigure: true

    AppBundle\Security\PostVoter: ~

This works because each enabled bundle has the opportunity to add one or more automated _instanceof definitions. Of course we've already enabled this for all the common Symfony services: commands, form types, event subscribers, etc.

It's not magic

Whenever we introduce a new feature to automatize the configuration of services, some developers quickly discredit it for being "magic". For us, "magic" means that something happened without you explicitly asking for it. Magic is bad because it leads to WTF moments and hard-to-debug issues.

However, this feature won't work unless you explicitly include the autoconfigure option in your configuration file. Besides, it only applies to the services defined in the same file where you include autoconfigure, so there will be no side-effects. In short, this is not magic, just automation.