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 andautowire 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 implementsVoterInterface, when would you ever not want that to be tagged withsecurity.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:trueautoconfigure:trueAppBundle\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.