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\Jobs;

use App\User;
use Illuminate\Contracts\Mail\Mailer;
use Illuminate\Contracts\Bus\SelfHandling;

class PurchasePodcast implements SelfHandling
{
    /**
     * The mailer implementation.
     */
    protected $mailer;

    /**
     * Create a new instance.
     *
     * @param  Mailer  $mailer
     * @return void
     */
    public function __construct(Mailer $mailer)
    {
        $this->mailer = $mailer;
    }

    /**
     * Purchase a podcast.
     *
     * @return void
     */
    public function handle()
    {
        //
    }
}

In this example, the PurchasePodcast job needs to send e-mails when a podcast is purchased. So, we will inject a service that is able to send e-mails. Since the service 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 mailer 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.

Binding

Almost all of your service container bindings will be registered within service providers, so all of these examples will demonstrate using the container in that context. However, 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 such "concrete" objects using PHP's reflection services.

Within a service provider, you always have access to the container via the $this->app instance variable. 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:

$this->app->bind('HelpSpot\API', function ($app) {
    return new HelpSpot\API($app['HttpClient']);
});

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

Binding A Singleton

The singleton method binds a class or interface into the container that should only be resolved one time, and then that same instance will be returned on subsequent calls into the container:

$this->app->singleton('FooBar', function ($app) {
    return new FooBar($app['SomethingElse']);
});

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:

$fooBar = new FooBar(new SomethingElse);

$this->app->instance('FooBar', $fooBar);

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:

$this->app->bind('App\Contracts\EventPusher', 'App\Services\RedisEventPusher');

This 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 a constructor, or any other location where dependencies are injected by the service container:

use App\Contracts\EventPusher;

/**
 * Create a new class instance.
 *
 * @param  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 different implementations into each class. For example, when our system receives a new Order, we may want to send an event via PubNub rather than Pusher. Laravel provides a simple, fluent interface for defining this behavior:

$this->app->when('App\Handlers\Commands\CreateOrderHandler')
          ->needs('App\Contracts\EventPusher')
          ->give('App\Services\PubNubEventPusher');

You may even pass a Closure to the give method:

$this->app->when('App\Handlers\Commands\CreateOrderHandler')
          ->needs('App\Contracts\EventPusher')
          ->give(function () {
                  // Resolve dependency...
              });

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 your class may need:

$this->app->when('App\Handlers\Commands\CreateOrderHandler')
          ->needs('$maxOrderCount')
          ->give(10);

Tagging

Occasionally, you may need to resolve all of a certain "category" of binding. For example, perhaps you are building a report aggregator 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('SpeedReport', function () {
    //
});

$this->app->bind('MemoryReport', function () {
    //
});

$this->app->tag(['SpeedReport', 'MemoryReport'], 'reports');

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

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

Resolving

There are several ways to resolve something out of the container. First, you may use the make method, which accepts the name of the class or interface you wish to resolve:

$fooBar = $this->app->make('FooBar');

Secondly, you may access the container like an array, since it implements PHP's ArrayAccess interface:

$fooBar = $this->app['FooBar'];

Lastly, but most importantly, you may simply "type-hint" the dependency in the constructor of a class that is resolved by the container, including controllers, event listeners, queue jobs, middleware, and more. In practice, this is how most of your objects are resolved by the container.

The container will automatically inject dependencies for the classes it resolves. 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\Users\Repository as UserRepository;

class UserController extends Controller
{
    /**
     * The user repository instance.
     */
    protected $users;

    /**
     * Create a new controller instance.
     *
     * @param  UserRepository  $users
     * @return void
     */
    public function __construct(UserRepository $users)
    {
        $this->users = $users;
    }

    /**
     * Show the user with the given ID.
     *
     * @param  int  $id
     * @return Response
     */
    public function show($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:

$this->app->resolving(function ($object, $app) {
    // Called when container resolves object of any type...
});

$this->app->resolving(FooBar::class, function (FooBar $fooBar, $app) {
    // Called when container resolves objects of type "FooBar"...
});

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.