Laravel create a route method GET with param in url

I created a route & contronller:

Route::group(['prefix' => 'test'], function () {
        Route::get('product/{id}', ['uses' => 'ProductController@getProduct']);
});

ProductController:

class ProductController extends MyController {

public $_params = null;
public function __construct(Request $request) {
    $this->request = $request;
    $this->_params = $request->all();
    $options = array();
    parent::__construct($options);
}

public function getProduct() {
    dd($this->_params);
}
}

I requested: http://localhost/test/product/123

But the id = 123 not exist in $this->_params