Service Container

LaravelDoc 2021-04

Introduction

The Laravel service container is a powerful tool for managing class dependencies and performing dependency injection. Dependency injection is a fancy phrase that essentially means this: class dependencies are "injected" into the class via the constructor or, in some cases, "setter" methods.

Let's look at a simple example:

<?php
namespace App\Http\Controllers;
use App\Http\Controllers\Controller;
use App\Repositories\UserRepository;
use App\Models\User;
class UserController extends Controller
{
    /**
     * The user repository implementation.
     * 
     * @var UserRepository
     */
    protected $users;
    /**
     * Create a new controller instance.
     * 
     * @param UserRepository $users
     * @return void
     */
    public function __construct(UserRepository $users)
    {
        $this->users = $users;
    }
    /**
     * show the profile for the given user.
     * 
     * @param int $id
     * @return Response
     */
    public function show($id)
    {
        $user = $this->users->find($id);
        return view('user.profile', ['user' => $user]);
    }
}

In this example, the UserController needs to retrieve users from a data source. So, we will inject a service that is able to retrieve users. In this context, our UserRepository most likely uses Eloquent to retrieve user information from the database. However, since the repository is injected, we are able to easily swap it out with another implementation. We are also able to easily "mock", or create a dummy implementation of the UserRepository when testing our application.

A deep understanding of the Laravel service container is essential to building a powerful, large application, as well as for contributing to the Laravel core itself.

Zero Configuration Resolution

if a class has no dependencies or only depends on other concrete classes(not interfaces), the container does not need to be instructed on how to resolve that class. For example, you may place the following code in your routes/web.php file:

<?php
class Service
{
    //
}
Route::get('/', function (Service $service) {
    die(get_class($service));
});

In this example, hitting your application's / route will automatically resolve the Service class and inject it into your route's handler. This is game changing. It means you can develop your application and take advantage of dependency injection without worrying about bloated configuration files.

Thankfully, many of the classes you will be writing when building a Laravel application automatically receive their dependencies via the container, including controllers, event listeners, middleware, and more. Additionally, you may type-hint dependencies in the handle method of queued jobs. Once you taste the power of automatic and zero configuration dependency injection it feels impossible to develop whthout it.

When To Use The Container

Thanks to zero configuration resolution, you will often type-hint dependecies on routes, controllers, event listeners, and elsewhere without ever manually interactiong with the container. For example, you might type-hint the Illuminate\Http\Request object on your route definition so that you can easily access the current request. Even though we never have to interact with the container to write thsi code, it is managing the injection of these dependencies behind the scenes:

use Illuminate\Http\Request;
Route::get('/', function (Request $request) {
    // ...
});

In many cases, thanks to automatic dependency injection and facades, you can build Laravel applications without ever manually binding or resolving anything from the container. So, when would you ever manually interact with the container? Let's examine two situations.

First, if you write a class that implements an interface and you wish to type-hint that interface on a route or class constructor, you must tell the container how to resolve that interface. Secondly, if you are writing a Laravel Package that you plan to share with other Laravel developers, you may need to bind your package's services into the container.

Binding

Binding Basics

Simple Bindings

Almost all of your service container bindings will be registered within service Providers, so most of these examples will demonstrate using the container in that context.

Within a service provider, you always have access to the container via the $this->app property. We can register a binding using the bind method, passing the class or interface name that we wish to register along with a closure that returns an instance of the class:

use App\Services\Transistor;
use App\Services\PodcastParser;
$this->app->bind(Transistor::class, function($app) {
    return new Transistor($app->make(PodcastParser::class));
});

Note that we receive the container itself as an grgument to the resolver. We can then use the container to resolve sub-dependencies of the object we are building.

As mentioned, you will typically be interacting with the container within service providers; however, if you would like to interact with the container outside of a service provider, you may do so via the App facade:

use App\Services\Transistor;
use Illuminate\Support\Facades\App;
App::bind(Transistor::class, function ($app) {
    // ...
});

There is no need to bind classes into the container if they do not depend on any interfaces. The container does not need to be instructed on how to build these objects, since it can automatically resolve these objects using reflection.

Binding A Singleton

The singleton method binds a class or interface into the container that should only be resolved one time. Once a sigleton binding is resolved, the same object instance will be returned on subsequent calls into the container:

use App\Services\Transistor;
use App\Services\PodcastParser;
$this->app->singleton(Transistor::class, function($app){
    return new Transistor($app->make(PodcastParser::class));
});

Binding Instances

You may also bind an existing object instance into the container using the instance method. The given instance will always be returned on subsequent calls into the container:

use App\Services\Transistor;
use App\Services\PodcastParser;
$service = new Transistor(new PodcastParser);
$this->app->instance(Transistor::class, $service);

Binding Interfaces To Implementations

A very powerful feature of the service container is its ability to bind an interface to a given implementation. For example, Let's assume we have an EventPusher interface and a RedisEventPusher implementation. Once we have coded our RedisEventPusher implementation of this interface, we can register it with the service container like so:

use App\contracts\EventPusher;
use App\Services\RedisEventPusher;
$this->app->bind(EventPusher::class, RedisEventPusher::class);

This statement tells the container that it should inject the RedisEventPusher when a class needs an implementation of EventPusher. Now we can type-hint the EventPusher interface in the constructor of a class that is resolved by the container. Remember, controllers, event listeners, middleware, and various other types of classes within Laravel applications are always resolved using the container:

use App\Contracts\EventPusher;
/**
 * Create a new class instance.
 * 
 * @param \App\Contracts\EventPusher $pusher
 * @return void
 */
 public function __construct(EventPusher $pusher)
 {
    $this->pusher = $pusher;
 }

Contextual Binding

Sometimes you may have two classes that utilize the same interface, but you wish to inject defferent implementations into each class. For example, two controllers may depend on different implementations of the Illuminate\Contracts\Filesystem\Filesystem contract. Laravel provides a simple, fluent interface for defining this behavior:

use App\Http\Controllers\PhotoController;
use App\Http\Controllers\UploadController;
use App\Http\Controllers\VideoController;
use Illuminate\Contracts\Filesystem\Filesystem;
use Illuminate\Support\Facades\Storage;
$this->app->when(PhotoController::class)
    ->needs(Filesystem::class)
    ->give(function () {
        return Storage::disk('local');
    });
$this->app->when([VideoController::class, UploadController::class])
    ->needs(Filesystem::class)
    ->give(function () {
        return Storage::disk('s3');
    });

Binding Primitives

Sometimes you may have a class that receives some injected classes, but also needs an injected primitive value such as an integer. You may easily use contextual binding to inject any value class may need:

$this->app->when('App\Http\Controllers\UserController')
    ->needs('$variableName')
    ->give($value);

Sometimes a class may depend on an array of tagged instances. Using the giveTagged method, you may easily inject all of the container bindings with that tag:

$this->app->when(ReportAggregator::class)
    ->needs('$reports')
    ->giveTagged('reprots');

if you need to inject a value from one of your application's configuration files, you may use the giveConfig method:

$this->app->when(ReportAggregator::class)
    ->needs('$timezone')
    ->giveConfig('app.timezone');

Binding Typed Variadics

Occasionally, you may have a class that receives an array of typed objects using a variadic constructor argument:

use App\Models\Filter;
use App\Services\Logger;
class Firewall
{
    /**
     * The logger instance.
     * 
     * @var \App\Services\Logger
     */
     protected $logger;
     /**
      * The filter instances.
      * 
      * @var array
      */
      protected $filters;
      /**
       * Create a new class instance.
       * 
       * @param \App\Services\Logger $logger
       * @param array $filters
       * @return void
       */
       public function __construct(Logger $logger, Filter ...$filters)
       {
            $this->logger = $logger;
            $this->filters = $filters;
       }
}

Using contextual binding, you may resolve this dependency by providing the give method with a closure that returns an array of resolved Filter instances:

$this->app->when(Firewall::class)
    ->needs(Filter::class)
    ->give(function($app){
        return [
            $app->make(NullFilter::class),
            $app->make(ProfanityFilter::class),
            $app->make(TooLongFilter::class),
        ];
    })

For convenience, you may also just provide an array of class names to be resolved by the container whenever Firewall needs Filter instances:

$this->app->when(Firewall::class)
    ->needs(Filter::class)
    ->give([
        NullFilter::class,
        ProfanityFilter::class,
        TooLongFilter::class,
    ]);

Variadic Tag Dependencies

Sometimes a class may have a variadic dependency that is type-hinted as a given class(Report ...$reports). Using the needs and giveTagged methods, you may easily inject all of the container bindings with taht tag for the given dependency:

$this->app->when(ReportAggregator::class)
    ->needs(Reprot::class)
    ->giveTagged('reprots);

Tagging

Occasionally, you may need to resolve all of a certain "category" of binding. For example, perhaps you are building a report analyzer that receives an array of many different Report interface implementations. After registering the Report implementations, you can assign them a tag using the tag method:

$this->app->bind(CpuReport::class, function () {
    //
});
$this->app->bind(MemoryReport::class, function () {
    //
});
$this->app->tag([CpuReport::class, MemoryReport::class], 'reports');

Once the services have been tagged, you may easily resolve them all via the container's tagged method:

$this->app->bind(ReportAnalyzer::class, function ($app) {
    return new ReportAnalyzer($app->tagged('reports));
});

Extending Bindings

The extend method allows the modification of resolved services. For example, when a service is resolved, you may run additional code to decorate or configure the service. The extend method accepts a closure, which should return the modified service, as its only argument. The closure receives the service being resolved and the container instance:

$this->app->extend(Service::class, function ($service, $app) {
    return new DecoratedService($service);
});

Resolving

The make Method

You may use the make method to resolve a class instance from the container. The make method accepts the name of the class or interface you wish to resolve:

use App\Services\Transistor;
$transistor = $this->app->make(Transistor::class);

if some of your class' dependencies are not resolvable via the container, you may inject them by passing them as an associative array into the makeWith method. For example, we may manually pass the $id constructor argument required by the Transistor service:

use App\Services\Transistor;
$transistor = $this->app->makeWith(Transistor::class, ['id' => 1]);

If you are outside of a service provider in a location of your code that does not have access to the $app variable, you may use the App facade to resolve a class instance from the container:

use App\Services\Transistor;
use Illuminate\Support\Facades\App;
$transistor = App::make(Transisitor::class);

if you would like to have the Laravel container instance itself injected into a class that is being resolved by the container, you may type-hint the Illuminat\Container\Container class on your class' constructor:

use Illuminate\Container\Container;
/**
 * Create a new class instance.
 * 
 * @param \Illuminate\Container\Container $container
 * @return void
 */
 public function __construct(Container $container)
 {
    $this->container = $container;
 }

Automatic Injection

Alternatively, and importantly, you may type-hint the dependency in the constructor of a class that is resolved by the container, including controllers, event listeners, middleware, and more. Additionally, you may type-hint dependencies in the handle method of queued jobs. In practice, this is how most of your objects should be resolved by the container.

For example, you may type-hint a repository defined by your application in a controller's constructor. The repository will automatically be resolved and injected into the class:

<?php
namespace App\Http\Controllers;
use App\Repositories\UserRepository;
class UserController extends Controller
{
    /**
     * The user repository instance.
     * 
     * @var \App\Repositories\UserRepository
     */
     pritected $users;
     /**
      * Create a new controller instance.
      * 
      * @param \App\Repositories\UserRepository $users
      * @return void
      */
      public function __construct(UserRepository $users)
      {
            $this->users = $users;
      }
      public function show($id)
      {
        //
      }
}

Container Events

The service container fires an event each time it resolves an object. You may listen to this event using the resolving method:

use App\Services\Transistor;
$this->app->resolving(Transistor::class, function ($transistor, $app) {
    // called when container resolves objects of type "Transistor"...
});
$this->app->resolving(function ($object, $app) {
    // Called when container resolves object of any type...
});

As you can see, the object being resolved will be passed to the callback, allowing you to set any additional properties on the object before it is given to its consumer.

PSR-11

Laravel's service container implements the PSR-11 interface. Therefore, you may type-hint the PSR-11 container interface to obtain an instance of the Laravel container:

use App\Services\Transistor;
use Psr\Container\ContainerInterface;
Route::get('/', function (ContainerInterface $container) {
    $service = $container->get(Transistor::class);
});

An exception is thrown if the given identifier can't be resolved. The exception will be an instance pof Psr\Container\NotFoundExceptionInterface if the identifier was never bound. if the identifier was bound but was unable to be resolved, an instance of Psr\Container\ContainerExceptionInterface will be thrown.

Service Container