Dependency injection in controllers in Laravel - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when injecting a service in a Laravel controller?

Consider a Laravel controller that injects a service class via its constructor. What will be the output when calling the controller method that uses this service to return a greeting message?

Laravel

<?php
namespace App\Http\Controllers;
use App\Services\GreetingService;

class WelcomeController extends Controller
{
    protected $greetingService;

    public function __construct(GreetingService $greetingService)
    {
        $this->greetingService = $greetingService;
    }

    public function greet()
    {
        return $this->greetingService->sayHello();
    }
}

// GreetingService.php
namespace App\Services;

class GreetingService
{
    public function sayHello()
    {
        return 'Hello, Laravel!';
    }
}

A "Hello, Laravel!"

B An error: Class 'GreetingService' not found

C "Hello from controller!"

D Null or empty response

💡 Hint

Think about how Laravel automatically resolves classes injected in the constructor.

Laravel's service container automatically injects the GreetingService instance into the controller constructor. Calling sayHello() returns the string 'Hello, Laravel!'.

❓ state_output

intermediate

What is the value of the injected repository property after controller instantiation?

Given a Laravel controller that injects a UserRepository interface, what will be the value of the repository property after the controller is created?

Laravel

<?php
namespace App\Http\Controllers;
use App\Repositories\UserRepositoryInterface;

class UserController extends Controller
{
    private UserRepositoryInterface $repository;

    public function __construct(UserRepositoryInterface $repository)
    {
        $this->repository = $repository;
    }

    public function index()
    {
        return $this->repository->allUsers();
    }
}

A An empty array

B An instance of a class implementing UserRepositoryInterface

C An error: Cannot instantiate interface UserRepositoryInterface

D Null because interfaces cannot be instantiated

💡 Hint

Laravel binds interfaces to concrete classes in the service container.

Laravel's service container resolves the interface to a concrete class bound in a service provider, so the repository property holds an instance of that class.

📝 Syntax

advanced

Which option correctly injects a service into a Laravel controller method?

You want to inject a Logger service directly into a controller method instead of the constructor. Which code snippet correctly achieves this?

Laravel

<?php
namespace App\Http\Controllers;
use App\Services\Logger;

class LogController extends Controller
{
    public function logAction( /* what goes here? */ )
    {
        // Use $logger to log a message
        $logger->info('Action logged');
        return 'Logged';
    }
}

A Logger $logger

B public Logger $logger

C Logger $logger = new Logger()

D Logger $logger = null

💡 Hint

Method injection requires type hinting the parameter without visibility or default values.

Laravel supports method injection by type hinting the parameter. Visibility keywords or default values are not allowed in method parameters.

🔧 Debug

advanced

Why does this Laravel controller injection cause an error?

Examine the following controller code. Why does Laravel throw a binding resolution exception when trying to instantiate this controller?

Laravel

<?php
namespace App\Http\Controllers;
use App\Services\PaymentGateway;

class PaymentController extends Controller
{
    public function __construct(PaymentGateway $gateway)
    {
        $this->gateway = $gateway;
    }
}

A The constructor is missing a return type

B The property $gateway is not declared

C PaymentGateway is not bound in the service container

D The namespace declaration is incorrect

💡 Hint

Check if the service container knows how to create PaymentGateway.

If PaymentGateway is a class that requires parameters or is not bound in the container, Laravel cannot resolve it automatically, causing the error.

🧠 Conceptual

expert

What is the main benefit of using dependency injection in Laravel controllers?

Choose the best explanation for why dependency injection is used in Laravel controllers.

A It prevents the use of service providers

B It forces all dependencies to be global singletons

C It makes controllers run faster by caching dependencies

D It allows automatic resolution and easier testing by decoupling dependencies

💡 Hint

Think about how dependency injection affects code flexibility and testing.

Dependency injection decouples the controller from concrete implementations, enabling easier swapping of dependencies and unit testing.