Laravel DI值为空

namespace App\Http\Controllers;

use App\Services\ExampleService;

class ExampleController extends Controller
{
    protected $exampleService;

    public function __construct(ExampleService $exampleService)
    {
        $this->exampleService = $exampleService;
    }

    public function index()
    {
        return $this->exampleService->doSomething();
    }
}

// 在 AppServiceProvider 的 register 方法中注册服务
public function register()
{
    $this->app->singleton(ExampleService::class, function ($app) {
        return new ExampleService();
    });
}

'providers' => [
    // ...
    Illuminate\Contracts\Container\BindingResolutionException::class,
],

php artisan config:clear
php artisan route:clear

namespace App\Services;

class ExampleService
{
    public function doSomething()
    {
        return 'Hello, World!';
    }
}

namespace App\Providers;

use Illuminate\Support\ServiceProvider;
use App\Services\ExampleService;

class AppServiceProvider extends ServiceProvider
{
    public function register()
    {
        $this->app->singleton(ExampleService::class, function ($app) {
            return new ExampleService();
        });
    }

    public function boot()
    {
        //
    }
}

namespace App\Http\Controllers;

use App\Services\ExampleService;

class ExampleController extends Controller
{
    protected $exampleService;

    public function __construct(ExampleService $exampleService)
    {
        $this->exampleService = $exampleService;
    }

    public function index()
    {
        return $this->exampleService->doSomething();
    }
}