Dependency injection in controllers in Laravel - Step-by-Step Execution

Concept Flow - Dependency injection in controllers

Request comes in

↓

Laravel Router matches route

↓

Controller instantiated

↓

Dependencies injected via constructor or method

↓

Controller method executes with dependencies

↓

Response returned

Laravel receives a request, matches it to a route, creates the controller, injects needed dependencies automatically, then runs the controller method.

Execution Sample

Laravel

class UserController {
  protected $repo;

  public function __construct(UserRepository $repo) {
    $this->repo = $repo;
  }
  public function index() {
    return $this->repo->all();
  }
}

This controller receives a UserRepository automatically injected, then uses it to get all users.

Execution Table

Step	Action	Dependency	Controller State	Output
1	Request received for /users	None yet	No controller instance	None
2	Route matched to UserController@index	None yet	No controller instance	None
3	Instantiate UserController	UserRepository created and injected	repo = UserRepository instance	None
4	Call index() method	repo available	repo = UserRepository instance	Calls repo->all()
5	UserRepository->all() returns user list	repo used	repo unchanged	User list returned
6	Response sent to client	repo used	repo unchanged	User list JSON

💡 Controller method finishes and response is sent back to client

Variable Tracker

Variable	Start	After Step 3	After Step 4	Final
repo	null	UserRepository instance	UserRepository instance	UserRepository instance

Key Moments - 3 Insights

How does Laravel know what to inject into the controller constructor?

Laravel uses type hints in the constructor to automatically create and inject the correct dependency, as shown in step 3 of the execution_table.

What if the dependency requires its own dependencies?

Laravel resolves nested dependencies automatically by checking the constructor of each dependency, so UserRepository's dependencies are also injected before UserRepository is passed to the controller.

Can dependencies be injected into controller methods instead of constructor?

Yes, Laravel supports method injection where dependencies are passed as parameters to controller methods, resolved automatically before method execution (similar to constructor injection shown in step 4).

Visual Quiz - 3 Questions

Look at the execution_table, what is the state of the controller after step 3?

A repo is null

B Controller is not instantiated

C repo is a UserRepository instance

D repo is a string

💡 Hint

Check the 'Controller State' column at step 3 in the execution_table

At which step does Laravel inject the UserRepository dependency?

A Step 1

B Step 3

C Step 2

D Step 5

💡 Hint

Look for when the controller is instantiated and dependencies are created in the execution_table

If the UserRepository had another dependency, how would the execution_table change?

A Step 3 would include creating that dependency first

B Step 4 would inject the new dependency

C Step 1 would create all dependencies

D No change needed

💡 Hint

Recall Laravel resolves nested dependencies before injecting the main dependency at controller instantiation

Concept Snapshot

Dependency injection in Laravel controllers:
- Laravel auto-injects dependencies by type hinting in constructors or methods.
- Controller is instantiated with dependencies resolved first.
- Dependencies can have their own dependencies, resolved recursively.
- This keeps controllers clean and testable.
- Injection happens before controller method runs.

Full Transcript

When Laravel receives a request, it matches the route to a controller method. Before calling the method, Laravel creates the controller instance. It looks at the constructor's type hints and automatically creates and injects those dependencies. For example, if the controller needs a UserRepository, Laravel creates it and passes it in. Then the controller method runs using that dependency. This process makes code cleaner and easier to test because dependencies are managed automatically. If dependencies have their own dependencies, Laravel resolves those too before injection. This can also happen in controller methods via method injection.