依赖注入不需要的服务

Question

Imagine we are using some Dependency injection and we use it in some controllers.

For example Laravel or any other framework with DI (doesn't matter the language in this case PHP).

Let's say we have this controller:

class UserController extends Controller
{
    public function __construct(UserRepository $users, Mailer $mailer, Logger $logger)
    {
        $this->users = $users;
        $this->mailer = $mailer;
        $this->logger = $logger;       
    }
    public function method1(){...}
    public function method2(){...}
    public function index(UserRepository $users){...}
}

Everybody say this is good design, but my question is this:

If the controller calls method1 and it uses only mailer for example, whats the point to construct the object with all the 3 dependencies. Isn't this not optimal even it doesn't take a lot of memory or performance?

Somebody can think that this controller needs all of them but in reality if I call only method1 it will need only mailer .

If I try to inject the dependencies on the methods like "index" method, how can I do it because If i extend the same method and need different dependencies PHP will blow up because the method signatures are not the same.

What is the best way to handle this.

If I extract each method in separate controllers I'll have to do a lot of repetitions.

Thanks

Answer 1

There are a few things to consider:

• First of all, when injection constructors are simple , it shouldn't matter that not all dependencies are used all the time. This shouldn't cause any measurable performance penalty, because the performance bottleneck will typically be caused by I/O.
• If, however, your class contains many dependencies, and there are a lot of dependencies that are only used in some methods, it might be an indication that the methods on the class are not cohesive. That again could be an indication of the class violating the Single Responsibility Principle. In other words, the class might be too big, has multiple responsibilities, and if that's the case, it should be split into multiple smaller classes. Grouping class functionality around the concept of an entity (such as UserController does) is often a cause by SRP violations. Instead, a better solution typically is to group classes around functionality; eg DeleteUserController , PromoteUserController , BlockUserController , etc.