AppFactory, PSR-17 Auto-Discovery & Kernel Bootstrap Internals

Slim 4 intentionally decouples its core routing and middleware kernel from any concrete PSR-7 HTTP message library. Instead of hardcoding new Slim\Psr7\Response(), Slim provides a zero-configuration bootstrap layer in Slim\Factory\AppFactory (Slim/Factory/AppFactory.php), Slim\Factory\ServerRequestCreatorFactory (Slim/Factory/ServerRequestCreatorFactory.php), and Slim\Factory\Psr17\* (10 classes) that probes installed Composer packages at startup, optionally wraps them in slim/http convenience decorators, and wires the default Slim\App object graph.

app_factory_internals_1
AppFactory::determineResponseFactory() & Psr17FactoryProvider Ordered Auto-Discovery and Slim-Http Decoration Flow
Source Code 
from drawlib.canvas import save, setup
from drawlib.icons import phosphor
from drawlib.smartarts import CardList, Panel
from drawlib.styles import Colors, Styles

setup(width=196, height=92, dpi=150)

# Left Panel: Psr17FactoryProvider Ordered Probe Queue
p_left = Panel(
    "Psr17FactoryProvider::$factories Probe Order",
    "",
    style=Styles.Neutral.patch(shape_r=2.0),
    header_style=Styles.PrimaryNeutral.patch(shape_line_width=1.2),
    text_style=Styles.DarkBold.patch(text_size=10.8),
    icon=phosphor.magnifying_glass,
    icon_style=Styles.PrimaryBold,
    header_variant="filled",
    header_height=8.5,
)
p_left.draw((4.0, 4.0), width=102.0, height=84.0)
lx, lw, lh = p_left.get_content_box((4.0, 4.0), width=102.0, height=84.0, padding=(3.5, 2.8))

providers = CardList(
    style=Styles.White,
    text_style=Styles.DarkBold.patch(text_size=10.2),
    description_style=Styles.Dark.patch(text_size=9.8),
    badge_shape="rounded_rect",
    badge_style=Styles.PrimaryNeutral,
    badge_text_style=Styles.PrimaryBold.patch(text_size=9.8),
    icon_style=Styles.PrimaryBold,
    direction="vertical",
    spacing=2.4,
    show_arrows=True,
    arrow_style=Styles.DarkBold.patch(line_arrow_head_scale=10, line_width=1.3),
)
providers.add(
    "1. SlimPsr17Factory (slim/psr7)",
    "Slim\\Psr7\\Factory\\{Response,Stream,ServerRequest}",
    badge_text="#1",
    icon=phosphor.star,
    style=Styles.PrimaryFlat,
    text_style=Styles.WhiteBold.patch(text_size=10.2),
    description_style=Styles.White.patch(text_size=9.8),
    badge_style=Styles.White,
    badge_text_style=Styles.PrimaryBold.patch(text_size=9.8),
    icon_style=Styles.WhiteBold,
)
providers.add(
    "2. HttpSoftPsr17Factory (httpsoft)",
    "HttpSoft\\Message\\{Response,Stream,ServerRequest}",
    badge_text="#2",
    icon=phosphor.package,
    style=Styles.PrimaryNeutral,
)
providers.add(
    "3. NyholmPsr17Factory (nyholm/psr7)",
    "Unified Nyholm\\Psr7\\Factory\\Psr17Factory (4x)",
    badge_text="#3",
    icon=phosphor.lightning,
    style=Styles.SecondaryNeutral,
    badge_style=Styles.White.patch(shape_line_color=Colors.Gray4),
    badge_text_style=Styles.DarkBold.patch(text_size=9.8),
)
providers.add(
    "4. LaminasDiactorosPsr17Factory",
    "Laminas\\Diactoros\\{Response,Stream,ServerRequest}",
    badge_text="#4",
    icon=phosphor.cube,
)
providers.add(
    "5. GuzzlePsr17Factory (guzzlehttp/psr7)",
    "GuzzleHttp\\Psr7\\HttpFactory & fromGlobals()",
    badge_text="#5",
    icon=phosphor.globe,
)
providers.draw(xy=lx, width=lw, height=lh)

# Right Panel: Decoration & Closure::fromCallable Bridge
p_right = Panel(
    "Slim-Http Decoration & Global Creator",
    "",
    style=Styles.Neutral.patch(shape_r=2.0),
    header_style=Styles.SecondaryNeutral.patch(shape_line_width=1.2),
    text_style=Styles.DarkBold.patch(text_size=10.8),
    icon=phosphor.magic_wand,
    icon_style=Styles.SecondaryBold,
    header_variant="filled",
    header_height=8.5,
)
p_right.draw((110.0, 4.0), width=82.0, height=84.0)
rx, rw, rh = p_right.get_content_box((110.0, 4.0), width=82.0, height=84.0, padding=(3.5, 3.0))

dec_list = CardList(
    style=Styles.White,
    text_style=Styles.DarkBold.patch(text_size=10.4),
    description_style=Styles.Dark.patch(text_size=9.8),
    badge_shape="rounded_rect",
    badge_style=Styles.SecondaryNeutral,
    badge_text_style=Styles.DarkBold.patch(text_size=9.8),
    icon_style=Styles.SecondaryBold,
    direction="vertical",
    spacing=3.0,
)
dec_list.add(
    "SlimHttpPsr17Factory",
    "Wraps into DecoratedResponseFactory",
    badge_text="D1",
    icon=phosphor.paint_brush,
    style=Styles.PrimaryNeutral,
)
dec_list.add(
    "SlimHttpServerRequestCreator",
    "Wraps into Slim\\Http\\ServerRequest",
    badge_text="D2",
    icon=phosphor.arrows_out_simple,
)
dec_list.add(
    "Closure::fromCallable() Adapter",
    "Unifies static & instance fromGlobals()",
    badge_text="D3",
    icon=phosphor.brackets_curly,
    style=Styles.AccentFlat,
    text_style=Styles.WhiteBold.patch(text_size=10.4),
    description_style=Styles.White.patch(text_size=9.8),
    badge_style=Styles.White,
    badge_text_style=Styles.DarkBold.patch(text_size=9.8),
    icon_style=Styles.WhiteBold,
)
dec_list.add(
    "createFromContainer(ContainerInterface)",
    "Resolves 5 Slim core interfaces from PSR-11 container before fallback",
    badge_text="DI",
    icon=phosphor.database,
)
dec_list.draw(xy=rx, width=rw, height=lh)

save()

1. AppFactory::create() vs AppFactory::createFromContainer()

Slim\Factory\AppFactory (Slim/Factory/AppFactory.php:28-217) holds nine protected static properties ($psr17FactoryProvider, $responseFactory, $streamFactory, $container, $callableResolver, $routeCollector, $routeResolver, $middlewareDispatcher, and $slimHttpDecoratorsAutomaticDetectionEnabled = true) so applications can either pass collaborators directly to AppFactory::create(...) or pre-configure static defaults before calling AppFactory::create().

Direct Static Bootstrap (Slim/Factory/AppFactory.php:53-70)

public static function create(
    ?ResponseFactoryInterface $responseFactory = null,
    ?ContainerInterface $container = null,
    ?CallableResolverInterface $callableResolver = null,
    ?RouteCollectorInterface $routeCollector = null,
    ?RouteResolverInterface $routeResolver = null,
    ?MiddlewareDispatcherInterface $middlewareDispatcher = null
): App {
    static::$responseFactory = $responseFactory ?? static::$responseFactory;
    return new App(
        self::determineResponseFactory(),
        $container ?? static::$container,
        $callableResolver ?? static::$callableResolver,
        $routeCollector ?? static::$routeCollector,
        $routeResolver ?? static::$routeResolver,
        $middlewareDispatcher ?? static::$middlewareDispatcher
    );
}

Container-First Bootstrap (Slim/Factory/AppFactory.php:77-122)

When bootstrapping from a PSR-11 container via AppFactory::createFromContainer(ContainerInterface $container), AppFactory probes the container using $container->has(...) and instanceof checks for five contract interfaces in strict order:

  1. ResponseFactoryInterface::class (lines 79–84): falls back to self::determineResponseFactory() if absent from the container.
  2. CallableResolverInterface::class (lines 86–91): falls back to null (which triggers new CallableResolver($container) inside App::__construct()).
  3. RouteCollectorInterface::class (lines 93–98): falls back to null.
  4. RouteResolverInterface::class (lines 100–105): falls back to null.
  5. MiddlewareDispatcherInterface::class (lines 107–112): falls back to null.

2. PSR-17 Candidate Discovery & Slim\Http Decoration

Psr17FactoryProvider::$factories Lookup Table (Slim/Factory/Psr17/Psr17FactoryProvider.php:22-28)

When AppFactory::determineResponseFactory() (Slim/Factory/AppFactory.php:127-157) is called without an explicit $responseFactory, it instantiates Psr17FactoryProvider and loops over Psr17FactoryProvider::getFactories(). Notice the exact ordering in Psr17FactoryProvider::$factories (and how Psr17FactoryProvider::addFactory() uses array_unshift() at line 51 so custom user factories take precedence at index 0):

Priority Adapter Class (Slim\Factory\Psr17\*) $responseFactoryClass $streamFactoryClass $serverRequestCreatorClass & $serverRequestCreatorMethod
#1 SlimPsr17Factory Slim\Psr7\Factory\ResponseFactory Slim\Psr7\Factory\StreamFactory Slim\Psr7\Factory\ServerRequestFactory :: createFromGlobals (static)
#2 HttpSoftPsr17Factory HttpSoft\Message\ResponseFactory HttpSoft\Message\StreamFactory HttpSoft\ServerRequest\ServerRequestCreator :: createFromGlobals (static)
#3 NyholmPsr17Factory Nyholm\Psr7\Factory\Psr17Factory Nyholm\Psr7\Factory\Psr17Factory Nyholm\Psr7Server\ServerRequestCreator -> fromGlobals (instance)
#4 LaminasDiactorosPsr17Factory Laminas\Diactoros\ResponseFactory Laminas\Diactoros\StreamFactory Laminas\Diactoros\ServerRequestFactory :: fromGlobals (static)
#5 GuzzlePsr17Factory GuzzleHttp\Psr7\HttpFactory GuzzleHttp\Psr7\HttpFactory GuzzleHttp\Psr7\ServerRequest :: fromGlobals (static)

Why NyholmPsr17Factory Overrides getServerRequestCreator() (Slim/Factory/Psr17/NyholmPsr17Factory.php:19-46)

Four of the five PSR-17 providers expose a static factory method (createFromGlobals() or fromGlobals()) that requires zero constructor arguments. Nyholm\Psr7Server\ServerRequestCreator, however, requires four PSR-17 factory interfaces (ServerRequestFactoryInterface, UriFactoryInterface, UploadedFileFactoryInterface, StreamFactoryInterface) injected into its constructor. Because Nyholm\Psr7\Factory\Psr17Factory implements all four PSR-17 interfaces in a single unified class, NyholmPsr17Factory::getServerRequestCreator() instantiates one $psr17Factory and passes it four times:

// Slim/Factory/Psr17/NyholmPsr17Factory.php:19-35
public static function getServerRequestCreator(): ServerRequestCreatorInterface
{
    $psr17Factory = new static::$responseFactoryClass();

    $serverRequestCreator = new static::$serverRequestCreatorClass(
        $psr17Factory,
        $psr17Factory,
        $psr17Factory,
        $psr17Factory
    );

    return new ServerRequestCreator($serverRequestCreator, static::$serverRequestCreatorMethod);
}

And inside Slim\Factory\Psr17\ServerRequestCreator::createServerRequestFromGlobals() (Slim/Factory/Psr17/ServerRequestCreator.php:38-45), Slim unifies both class-name strings (['Slim\Psr7\Factory\ServerRequestFactory', 'createFromGlobals']) and object instances ([$nyholmCreatorInstance, 'fromGlobals']) using Closure::fromCallable():

public function createServerRequestFromGlobals(): ServerRequestInterface
{
    /** @var callable $callable */
    $callable = [$this->serverRequestCreator, $this->serverRequestCreatorMethod];

    /** @var ServerRequestInterface */
    return (Closure::fromCallable($callable))();
}

Automatic slim/http Decoration (SlimHttpPsr17Factory & SlimHttpServerRequestCreator)

If static::$slimHttpDecoratorsAutomaticDetectionEnabled is true (the default), Slim checks whether the optional slim/http package is installed:

  • Response Decoration (Slim/Factory/AppFactory.php:159-171 & Slim/Factory/Psr17/SlimHttpPsr17Factory.php:17-39): When both a ResponseFactoryInterface and a StreamFactoryInterface are available and class_exists('Slim\Http\Factory\DecoratedResponseFactory') returns true, SlimHttpPsr17Factory::createDecoratedResponseFactory($responseFactory, $streamFactory) wraps them so route handlers receive Slim\Http\Response (adding helpers like withJson(), withRedirect(), and write()).
  • Request Decoration (Slim/Factory/ServerRequestCreatorFactory.php:61-72 & Slim/Factory/Psr17/SlimHttpServerRequestCreator.php:19-56): When class_exists('Slim\Http\ServerRequest') returns true, attemptServerRequestCreatorDecoration() wraps the underlying creator in new SlimHttpServerRequestCreator($serverRequestCreator), decorating the resulting PSR-7 request in new Slim\Http\ServerRequest($request) (line 43).

3. Default Component Graph Assembly in App::__construct()

Once AppFactory resolves $responseFactory, Slim\App::__construct() (Slim/App.php:55-80) and its parent constructor Slim\Routing\RouteCollectorProxy::__construct() (Slim/Routing/RouteCollectorProxy.php:41-53) wire the seven core framework objects into a ready-to-run PSR-15 pipeline:

app_factory_internals_2
Slim\App::__construct() Default Object Graph Assembly & MiddlewareDispatcher Kernel Seeding
Source Code 
from drawlib.canvas import save, setup
from drawlib.icons import phosphor
from drawlib.smartarts import CardList, Panel
from drawlib.styles import Colors, Styles

setup(width=196, height=90, dpi=150)

# Left Panel: 1. RouteCollectorProxy::__construct() (Parent Constructor)
p_left = Panel(
    "1. RouteCollectorProxy::__construct()",
    "Slim/Routing/RouteCollectorProxy.php:41-53",
    style=Styles.Neutral.patch(shape_r=2.0),
    header_style=Styles.PrimaryNeutral.patch(shape_line_width=1.2),
    text_style=Styles.DarkBold.patch(text_size=10.8),
    description_style=Styles.Dark.patch(text_size=9.8),
    icon=phosphor.plugs_connected,
    icon_style=Styles.PrimaryBold,
    header_variant="filled",
    header_height=8.5,
)
p_left.draw((4.0, 4.0), width=92.0, height=82.0)
lx, lw, lh = p_left.get_content_box((4.0, 4.0), width=92.0, height=82.0, padding=(3.5, 3.0))

proxy_list = CardList(
    style=Styles.White,
    text_style=Styles.DarkBold.patch(text_size=10.2),
    description_style=Styles.Dark.patch(text_size=9.8),
    badge_shape="rounded_rect",
    badge_style=Styles.PrimaryNeutral,
    badge_text_style=Styles.PrimaryBold.patch(text_size=9.8),
    icon_style=Styles.PrimaryBold,
    direction="vertical",
    spacing=2.6,
    show_arrows=True,
    arrow_style=Styles.DarkBold.patch(line_arrow_head_scale=10, line_width=1.3),
)
proxy_list.add(
    "ResponseFactoryInterface & ContainerInterface",
    "Stores responseFactory & optional PSR-11 container [L49-50]",
    badge_text="P1",
    icon=phosphor.database,
)
proxy_list.add(
    "CallableResolver(container) [App.php:65]",
    "Instantiates default CallableResolver if none was injected",
    badge_text="P2",
    icon=phosphor.funnel,
    style=Styles.PrimaryNeutral,
)
proxy_list.add(
    "RouteCollector(responseFactory, resolver, container)",
    "Creates central route registry on RouteCollectorProxy [L51]",
    badge_text="P3",
    icon=phosphor.git_branch,
    style=Styles.PrimaryFlat,
    text_style=Styles.WhiteBold.patch(text_size=10.2),
    description_style=Styles.White.patch(text_size=9.8),
    badge_style=Styles.White,
    badge_text_style=Styles.PrimaryBold.patch(text_size=9.8),
    icon_style=Styles.WhiteBold,
)
proxy_list.add(
    "RequestResponse & RouteParser [L99-100]",
    "RouteCollector initializes default Strategy & RouteParser(this)",
    badge_text="P4",
    icon=phosphor.compass,
)
proxy_list.draw(xy=lx, width=lw, height=lh)

# Right Panel: 2. App::__construct() Kernel Wiring (Slim/App.php:70-79)
p_right = Panel(
    "2. App::__construct() Kernel Wiring",
    "Slim/App.php:70-79",
    style=Styles.Neutral.patch(shape_r=2.0),
    header_style=Styles.SecondaryNeutral.patch(shape_line_width=1.2),
    text_style=Styles.DarkBold.patch(text_size=10.8),
    description_style=Styles.Dark.patch(text_size=9.8),
    icon=phosphor.rocket_launch,
    icon_style=Styles.SecondaryBold,
    header_variant="filled",
    header_height=8.5,
)
p_right.draw((100.0, 4.0), width=92.0, height=82.0)
rx, rw, rh = p_right.get_content_box((100.0, 4.0), width=92.0, height=82.0, padding=(3.5, 3.0))

app_list = CardList(
    style=Styles.White,
    text_style=Styles.DarkBold.patch(text_size=10.2),
    description_style=Styles.Dark.patch(text_size=9.8),
    badge_shape="rounded_rect",
    badge_style=Styles.SecondaryNeutral,
    badge_text_style=Styles.DarkBold.patch(text_size=9.8),
    icon_style=Styles.SecondaryBold,
    direction="vertical",
    spacing=2.6,
    show_arrows=True,
    arrow_style=Styles.DarkBold.patch(line_arrow_head_scale=10, line_width=1.3),
)
app_list.add(
    "RouteResolver(routeCollector) [L70]",
    "Creates RouteResolver wrapping Slim\\Routing\\Dispatcher",
    badge_text="K1",
    icon=phosphor.magnifying_glass,
)
app_list.add(
    "RouteRunner(routeResolver, routeParser, this) [L71]",
    "Passes App (this) as 3rd arg so RouteRunner can read basePath",
    badge_text="K2",
    icon=phosphor.play_circle,
    style=Styles.SecondaryNeutral,
    badge_style=Styles.White.patch(shape_line_color=Colors.Gray4),
    badge_text_style=Styles.DarkBold.patch(text_size=9.8),
)
app_list.add(
    "MiddlewareDispatcher(routeRunner, ...) [L74]",
    "Creates default dispatcher or calls seedMiddlewareStack(routeRunner)",
    badge_text="K3",
    icon=phosphor.stack,
    style=Styles.AccentFlat,
    text_style=Styles.WhiteBold.patch(text_size=10.2),
    description_style=Styles.White.patch(text_size=9.8),
    badge_style=Styles.White,
    badge_text_style=Styles.DarkBold.patch(text_size=9.8),
    icon_style=Styles.WhiteBold,
)
app_list.add(
    "Innermost Kernel: this->tip = routeRunner",
    "Ready for LIFO app->add() wrapping & app->run() execution",
    badge_text="K4",
    icon=phosphor.check_circle,
    style=Styles.PrimaryNeutral,
)
app_list.draw(xy=rx, width=rw, height=lh)

save()

Step-by-Step Walkthrough of Slim/App.php:55-80

// Slim/App.php:55-80
public function __construct(
    ResponseFactoryInterface $responseFactory,
    ?ContainerInterface $container = null,
    ?CallableResolverInterface $callableResolver = null,
    ?RouteCollectorInterface $routeCollector = null,
    ?RouteResolverInterface $routeResolver = null,
    ?MiddlewareDispatcherInterface $middlewareDispatcher = null
) {
    parent::__construct(
        $responseFactory,
        $callableResolver ?? new CallableResolver($container),
        $container,
        $routeCollector
    );

    $this->routeResolver = $routeResolver ?? new RouteResolver($this->routeCollector);
    $routeRunner = new RouteRunner($this->routeResolver, $this->routeCollector->getRouteParser(), $this);

    if (!$middlewareDispatcher) {
        $middlewareDispatcher = new MiddlewareDispatcher($routeRunner, $this->callableResolver, $container);
    } else {
        $middlewareDispatcher->seedMiddlewareStack($routeRunner);
    }

    $this->middlewareDispatcher = $middlewareDispatcher;
}
  1. Line 63–68 (parent::__construct(...)): Delegates to RouteCollectorProxy::__construct() (Slim/Routing/RouteCollectorProxy.php:41-53). If $callableResolver is null, it instantiates new CallableResolver($container). Inside RouteCollectorProxy::__construct() (line 51), if $routeCollector is null, it instantiates new RouteCollector($responseFactory, $callableResolver, $container), which in turn defaults $this->defaultInvocationStrategy = new RequestResponse() and $this->routeParser = new RouteParser($this) (Slim/Routing/RouteCollector.php:99-100).
  2. Line 70 ($this->routeResolver): Defaults to new RouteResolver($this->routeCollector), which lazily wraps new Dispatcher($this->routeCollector) (Slim/Routing/RouteResolver.php:34).
  3. Line 71 ($routeRunner): Instantiates new RouteRunner($this->routeResolver, $this->routeCollector->getRouteParser(), $this). Passing $this (Slim\App as RouteCollectorProxyInterface) enables RouteRunner::handle() (Slim/Routing/RouteRunner.php:65-70) to read $this->routeCollectorProxy->getBasePath() at dispatch time and inject RouteContext::BASE_PATH (__basePath__) into the request!
  4. Line 73–79 ($this->middlewareDispatcher): Either constructs new MiddlewareDispatcher($routeRunner, $this->callableResolver, $container) or—when a custom $middlewareDispatcher was injected via AppFactory::createFromContainer()—calls $middlewareDispatcher->seedMiddlewareStack($routeRunner) so $routeRunner becomes the innermost kernel ($this->tip) of the middleware stack.

Continue to Chapter 3: MiddlewareDispatcher & CallableResolver Internals to examine how MiddlewareDispatcher wraps $this->tip in LIFO closures and how CallableResolver resolves container services and class:method strings.