The HttpKernel Component ¶
The HttpKernel component provides a structured process for converting a
Requestinto aResponseby making use of the EventDispatcher component. It's flexible enough to create a full-stack framework (Symfony), a micro-framework (Silex) or an advanced CMS (Drupal).HttpKernel コンポーネントは、EventDispatcher コンポーネントを利用して、リクエストをレスポンスに変換するための構造化されたプロセスを提供します。フルスタック フレームワーク (Symfony)、マイクロ フレームワーク (Silex)、または高度な CMS (Drupal) を作成するのに十分な柔軟性があります。
Installation ¶
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$ composer require symfony/http-kernel
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Note
If you install this component outside of a Symfony application, you must
require the vendor/autoload.php file in your code to enable the class
autoloading mechanism provided by Composer. Read
this article for more details.
The Workflow of a Request ¶
See also
This article explains how to use the HttpKernel features as an independent component in any PHP application. In Symfony applications everything is already configured and ready to use. Read the Controller and Events and Event Listeners articles to learn about how to use it to create controllers and define events in Symfony applications.
Every HTTP web interaction begins with a request and ends with a response. Your job as a developer is to create PHP code that reads the request information (e.g. the URL) and creates and returns a response (e.g. an HTML page or JSON string). This is a simplified overview of the request workflow in Symfony applications:
- The user asks for a resource in a browser;ユーザーはブラウザでリソースを要求します。
- The browser sends a request to the server;ブラウザはサーバーにリクエストを送信します。
- Symfony gives the application a Request object;symfony はアプリケーションに Request オブジェクトを提供します。
- The application generates a Response object using the data of the Request object;アプリケーションは、Request オブジェクトのデータを使用して Response オブジェクトを生成します。
- The server sends back the response to the browser;サーバーはブラウザに応答を返します。
- The browser displays the resource to the user.ブラウザはリソースをユーザーに表示します。
Typically, some sort of framework or system is built to handle all the repetitive tasks (e.g. routing, security, etc) so that a developer can build each page of the application. Exactly how these systems are built varies greatly. The HttpKernel component provides an interface that formalizes the process of starting with a request and creating the appropriate response. The component is meant to be the heart of any application or framework, no matter how varied the architecture of that system:
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namespace Symfony\Component\HttpKernel;
use Symfony\Component\HttpFoundation\Request;
interface HttpKernelInterface
{
// ...
/**
* @return Response A Response instance
*/
public function handle(
Request $request,
int $type = self::MAIN_REQUEST,
bool $catch = true
): Response;
}
|
Internally, HttpKernel::handle() - the concrete implementation of HttpKernelInterface::handle() - defines a workflow that starts with a Request and ends with a Response.
The exact details of this workflow are the key to understanding how the kernel (and the Symfony Framework or any other library that uses the kernel) works.
HttpKernel: Driven by Events ¶
The HttpKernel::handle() method works internally by dispatching events.
This makes the method both flexible, but also a bit abstract, since all the
"work" of a framework/application built with HttpKernel is actually done
in event listeners.
To help explain this process, this document looks at each step of the process and talks about how one specific implementation of the HttpKernel - the Symfony Framework - works.
Initially, using the HttpKernel does not take many steps. You create an event dispatcher and a controller and argument resolver (explained below). To complete your working kernel, you'll add more event listeners to the events discussed below:
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use Symfony\Component\EventDispatcher\EventDispatcher;
use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpFoundation\RequestStack;
use Symfony\Component\HttpKernel\Controller\ArgumentResolver;
use Symfony\Component\HttpKernel\Controller\ControllerResolver;
use Symfony\Component\HttpKernel\HttpKernel;
// create the Request object
$request = Request::createFromGlobals();
$dispatcher = new EventDispatcher();
// ... add some event listeners
// create your controller and argument resolvers
$controllerResolver = new ControllerResolver();
$argumentResolver = new ArgumentResolver();
// instantiate the kernel
$kernel = new HttpKernel($dispatcher, $controllerResolver, new RequestStack(), $argumentResolver);
// actually execute the kernel, which turns the request into a response
// by dispatching events, calling a controller, and returning the response
$response = $kernel->handle($request);
// send the headers and echo the content
$response->send();
// trigger the kernel.terminate event
$kernel->terminate($request, $response);
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See "The HttpKernel Component" for a more concrete implementation.
For general information on adding listeners to the events below, see The HttpKernel Component.
Caution
As of 3.1 the HttpKernel accepts a fourth argument, which must be an instance of ArgumentResolverInterface. In 4.0 this argument will become mandatory.
See also
There is a wonderful tutorial series on using the HttpKernel component and other Symfony components to create your own framework. See Introduction.
1) The kernel.request Event
¶
Typical Purposes: To add more information to the Request, initialize
parts of the system, or return a Response if possible (e.g. a security
layer that denies access).
Kernel Events Information Table
The first event that is dispatched inside HttpKernel::handle
is kernel.request, which may have a variety of different listeners.
Listeners of this event can be quite varied. Some listeners - such as a security
listener - might have enough information to create a Response object immediately.
For example, if a security listener determined that a user doesn't have access,
that listener may return a RedirectResponse
to the login page or a 403 Access Denied response.
If a Response is returned at this stage, the process skips directly to
the kernel.response event.
Other listeners initialize things or add more information to the request.
For example, a listener might determine and set the locale on the Request
object.
Another common listener is routing. A router listener may process the Request
and determine the controller that should be rendered (see the next section).
In fact, the Request object has an "attributes"
bag which is a perfect spot to store this extra, application-specific data
about the request. This means that if your router listener somehow determines
the controller, it can store it on the Request attributes (which can be used
by your controller resolver).
Overall, the purpose of the kernel.request event is either to create and
return a Response directly, or to add information to the Request
(e.g. setting the locale or setting some other information on the Request
attributes).
Note
When setting a response for the kernel.request event, the propagation
is stopped. This means listeners with lower priority won't be executed.
2) Resolve the Controller ¶
Assuming that no kernel.request listener was able to create a Response,
the next step in HttpKernel is to determine and prepare (i.e. resolve) the
controller. The controller is the part of the end-application's code that
is responsible for creating and returning the Response for a specific page.
The only requirement is that it is a PHP callable - i.e. a function, method
on an object or a Closure.
But how you determine the exact controller for a request is entirely up
to your application. This is the job of the "controller resolver" - a class
that implements ControllerResolverInterface
and is one of the constructor arguments to HttpKernel.
Your job is to create a class that implements the interface and fill in its
method: getController(). In fact, one default implementation already
exists, which you can use directly or learn from:
ControllerResolver.
This implementation is explained more in the sidebar below:
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namespace Symfony\Component\HttpKernel\Controller;
use Symfony\Component\HttpFoundation\Request;
interface ControllerResolverInterface
{
public function getController(Request $request);
}
|
Internally, the HttpKernel::handle() method first calls
getController()
on the controller resolver. This method is passed the Request and is responsible
for somehow determining and returning a PHP callable (the controller) based
on the request's information.
3) The kernel.controller Event
¶
Typical Purposes: Initialize things or change the controller just before the controller is executed.
Kernel Events Information Table
After the controller callable has been determined, HttpKernel::handle()
dispatches the kernel.controller event. Listeners to this event might initialize
some part of the system that needs to be initialized after certain things
have been determined (e.g. the controller, routing information) but before
the controller is executed.
Another typical use-case for this event is to retrieve the attributes from the controller using the getAttributes() method. See the Symfony section below for some examples.
6.2
The ControllerEvent::getAttributes() method was introduced in Symfony 6.2.
Listeners to this event can also change the controller callable completely by calling ControllerEvent::setController on the event object that's passed to listeners on this event.
4) Getting the Controller Arguments ¶
Next, HttpKernel::handle() calls
ArgumentResolverInterface::getArguments().
Remember that the controller returned in getController() is a callable.
The purpose of getArguments() is to return the array of arguments that
should be passed to that controller. Exactly how this is done is completely
up to your design, though the built-in ArgumentResolver
is a good example.
At this point the kernel has a PHP callable (the controller) and an array of arguments that should be passed when executing that callable.
5) Calling the Controller ¶
The next step of HttpKernel::handle() is executing the controller.
The job of the controller is to build the response for the given resource. This could be an HTML page, a JSON string or anything else. Unlike every other part of the process so far, this step is implemented by the "end-developer", for each page that is built.
Usually, the controller will return a Response object. If this is true,
then the work of the kernel is just about done! In this case, the next step
is the kernel.response event.
But if the controller returns anything besides a Response, then the kernel
has a little bit more work to do - kernel.view
(since the end goal is always to generate a Response object).
Note
A controller must return something. If a controller returns null,
an exception will be thrown immediately.
6) The kernel.view Event
¶
Typical Purposes: Transform a non-Response return value from a controller
into a Response
Kernel Events Information Table
If the controller doesn't return a Response object, then the kernel dispatches
another event - kernel.view. The job of a listener to this event is to
use the return value of the controller (e.g. an array of data or an object)
to create a Response.
This can be useful if you want to use a "view" layer: instead of returning
a Response from the controller, you return data that represents the page.
A listener to this event could then use this data to create a Response that
is in the correct format (e.g HTML, JSON, etc).
At this stage, if no listener sets a response on the event, then an exception
is thrown: either the controller or one of the view listeners must always
return a Response.
Note
When setting a response for the kernel.view event, the propagation
is stopped. This means listeners with lower priority won't be executed.
7) The kernel.response Event
¶
Typical Purposes: Modify the Response object just before it is sent
Kernel Events Information Table
The end goal of the kernel is to transform a Request into a Response. The
Response might be created during the kernel.request
event, returned from the controller,
or returned by one of the listeners to the kernel.view
event.
Regardless of who creates the Response, another event - kernel.response
is dispatched directly afterwards. A typical listener to this event will modify
the Response object in some way, such as modifying headers, adding cookies,
or even changing the content of the Response itself (e.g. injecting some
JavaScript before the end </body> tag of an HTML response).
After this event is dispatched, the final Response object is returned
from handle(). In the
most typical use-case, you can then call the send()
method, which sends the headers and prints the Response content.
8) The kernel.terminate Event
¶
Typical Purposes: To perform some "heavy" action after the response has been streamed to the user
Kernel Events Information Table
The final event of the HttpKernel process is kernel.terminate and is unique
because it occurs after the HttpKernel::handle() method, and after the
response is sent to the user. Recall from above, then the code that uses
the kernel, ends like this:
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// sends the headers and echoes the content
$response->send();
// triggers the kernel.terminate event
$kernel->terminate($request, $response);
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As you can see, by calling $kernel->terminate after sending the response,
you will trigger the kernel.terminate event where you can perform certain
actions that you may have delayed in order to return the response as quickly
as possible to the client (e.g. sending emails).
Caution
Internally, the HttpKernel makes use of the fastcgi_finish_request
PHP function. This means that at the moment, only the PHP FPM server
API is able to send a response to the client while the server's PHP process
still performs some tasks. With all other server APIs, listeners to kernel.terminate
are still executed, but the response is not sent to the client until they
are all completed.
Note
Using the kernel.terminate event is optional, and should only be
called if your kernel implements TerminableInterface.
Handling Exceptions: the kernel.exception Event
¶
Typical Purposes: Handle some type of exception and create an appropriate
Response to return for the exception
Kernel Events Information Table
If an exception is thrown at any point inside HttpKernel::handle(), another
event - kernel.exception is dispatched. Internally, the body of the handle()
method is wrapped in a try-catch block. When any exception is thrown, the
kernel.exception event is dispatched so that your system can somehow respond
to the exception.
Each listener to this event is passed a ExceptionEvent
object, which you can use to access the original exception via the
getThrowable()
method. A typical listener on this event will check for a certain type of
exception and create an appropriate error Response.
For example, to generate a 404 page, you might throw a special type of exception
and then add a listener on this event that looks for this exception and
creates and returns a 404 Response. In fact, the HttpKernel component
comes with an ErrorListener,
which if you choose to use, will do this and more by default (see the sidebar
below for more details).
Note
When setting a response for the kernel.exception event, the propagation
is stopped. This means listeners with lower priority won't be executed.
Creating an Event Listener ¶
As you've seen, you can create and attach event listeners to any of the events
dispatched during the HttpKernel::handle() cycle. Typically a listener is a PHP
class with a method that's executed, but it can be anything. For more information
on creating and attaching event listeners, see The EventDispatcher Component.
The name of each of the "kernel" events is defined as a constant on the KernelEvents class. Additionally, each event listener is passed a single argument, which is some subclass of KernelEvent. This object contains information about the current state of the system and each event has their own event object:
| Name | KernelEvents Constant |
Argument passed to the listener |
|---|---|---|
| kernel.request | KernelEvents::REQUEST |
RequestEvent |
| kernel.controller | KernelEvents::CONTROLLER |
ControllerEvent |
| kernel.controller_arguments | KernelEvents::CONTROLLER_ARGUMENTS |
ControllerArgumentsEvent |
| kernel.view | KernelEvents::VIEW |
ViewEvent |
| kernel.response | KernelEvents::RESPONSE |
ResponseEvent |
| kernel.finish_request | KernelEvents::FINISH_REQUEST |
FinishRequestEvent |
| kernel.terminate | KernelEvents::TERMINATE |
TerminateEvent |
| kernel.exception | KernelEvents::EXCEPTION |
ExceptionEvent |
A full Working Example ¶
When using the HttpKernel component, you're free to attach any listeners to the core events, use any controller resolver that implements the ControllerResolverInterface and use any argument resolver that implements the ArgumentResolverInterface. However, the HttpKernel component comes with some built-in listeners and everything else that can be used to create a working example:
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use Symfony\Component\EventDispatcher\EventDispatcher;
use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpFoundation\RequestStack;
use Symfony\Component\HttpFoundation\Response;
use Symfony\Component\HttpKernel\Controller\ArgumentResolver;
use Symfony\Component\HttpKernel\Controller\ControllerResolver;
use Symfony\Component\HttpKernel\EventListener\RouterListener;
use Symfony\Component\HttpKernel\HttpKernel;
use Symfony\Component\Routing\Matcher\UrlMatcher;
use Symfony\Component\Routing\RequestContext;
use Symfony\Component\Routing\Route;
use Symfony\Component\Routing\RouteCollection;
$routes = new RouteCollection();
$routes->add('hello', new Route('/hello/{name}', [
'_controller' => function (Request $request) {
return new Response(
sprintf("Hello %s", $request->get('name'))
);
}]
));
$request = Request::createFromGlobals();
$matcher = new UrlMatcher($routes, new RequestContext());
$dispatcher = new EventDispatcher();
$dispatcher->addSubscriber(new RouterListener($matcher, new RequestStack()));
$controllerResolver = new ControllerResolver();
$argumentResolver = new ArgumentResolver();
$kernel = new HttpKernel($dispatcher, $controllerResolver, new RequestStack(), $argumentResolver);
$response = $kernel->handle($request);
$response->send();
$kernel->terminate($request, $response);
|
Sub Requests ¶
In addition to the "main" request that's sent into HttpKernel::handle(),
you can also send a so-called "sub request". A sub request looks and acts like
any other request, but typically serves to render just one small portion of
a page instead of a full page. You'll most commonly make sub-requests from
your controller (or perhaps from inside a template, that's being rendered by
your controller).
To execute a sub request, use HttpKernel::handle(), but change the second
argument as follows:
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use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpKernel\HttpKernelInterface;
// ...
// create some other request manually as needed
$request = new Request();
// for example, possibly set its _controller manually
$request->attributes->set('_controller', '...');
$response = $kernel->handle($request, HttpKernelInterface::SUB_REQUEST);
// do something with this response
|
This creates another full request-response cycle where this new Request is
transformed into a Response. The only difference internally is that some
listeners (e.g. security) may only act upon the main request. Each listener
is passed some subclass of KernelEvent,
whose isMainRequest()
method can be used to check if the current request is a "main" or "sub" request.
For example, a listener that only needs to act on the main request may look like this:
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use Symfony\Component\HttpKernel\Event\RequestEvent;
// ...
public function onKernelRequest(RequestEvent $event)
{
if (!$event->isMainRequest()) {
return;
}
// ...
}
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Locating Resources ¶
The HttpKernel component is responsible of the bundle mechanism used in Symfony applications. The key feature of the bundles is that they allow to override any resource used by the application (config files, templates, controllers, translation files, etc.)
This overriding mechanism works because resources are referenced not by their
physical path but by their logical path. For example, the services.xml file
stored in the Resources/config/ directory of a bundle called FooBundle is
referenced as @FooBundle/Resources/config/services.xml. This logical path
will work when the application overrides that file and even if you change the
directory of FooBundle.
The HttpKernel component provides a method called locateResource() which can be used to transform logical paths into physical paths:
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$path = $kernel->locateResource('@FooBundle/Resources/config/services.xml');
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Learn more ¶
- Built-in Symfony Events組み込みの symfony イベント