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The Critical kernel.exception Event Listeners

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Back at the browser, refresh to get the normal not found page, click to open the profiler... and go into Events. Because this was a 404 page, the kernel.exception event was dispatched. The most important listener - and the one that eventually will render this page - is ErrorListener.

Let's see how it works! Hit Shift + Shift and open ErrorListener.php: get the one from http-kernel/, not console/. Look down here for the getSubscribedEvents() method. Interesting: it listens to KernelEvents::CONTROLLER_ARGUMENTS and it listens to KernelEvents::EXCEPTION twice. We won't look at the CONTROLLER_ARGUMENTS listener method - but if you want to look back at it after finishing the entire tutorial, it should make sense. What it does is minor, but interesting.

When the kernel.exception event is dispatched, logKernelException() will be called first and then, later, onKernelException(), because it has a -128 priority.

How Exceptions are Logged

Find logKernelException() up on top. Its job is simple: log that an exception was thrown. If you follow the logException() logic, you'll see that it logs at a different level based on the status code. We're going to talk more soon about how different exceptions get different status codes. But the important piece here is that all 500 status code exceptions log at the critical() level, and 400 status code exceptions log at error(). If you're like us, you've probably used this fact before in your Monolog config to send 500 error logs to somewhere where you can be notified, like a Slack channel.

The Error Controller

The other listener method is onKernelException(). This is what's responsible for rendering the error page: both the nice development error page and the boring, production error page. It has a priority of -128 because it will eventually set the Response on the event, which will stop event propagation. The low priority makes it easy to register other listeners before this happens. Heck, you could easily create a listener that replaces this one, by setting the Response itself... though, there are better ways to customize the error process.

Go find this method. Hmm. The first thing it does is reference some $this->controller property. Let's find out what that is. dd($this->controller), then spin over to your browser, make sure you're on a 404 page and refresh.

// ... lines 1 - 29
class ErrorListener implements EventSubscriberInterface
// ... lines 32 - 49
public function onKernelException(ExceptionEvent $event, string $eventName = null, EventDispatcherInterface $eventDispatcher = null)
// ... lines 52 - 55
// ... lines 57 - 89
// ... lines 91 - 149

Interesting: it's a string: error_controller. Find your terminal and run:

php bin/console debug:container error_controller

Surprise! error_controller is the id of a service! And its job apparently is to:

Render error or exception pages from a given FlattenException

Ok, we don't know what a FlattenException is yet, but apparently this is a controller that's good at rendering error pages. Let's see what it looks like!

Hit Shift + Shift to open ErrorController.php. Ooooo. It has an __invoke() method! This is an invokable controller! We talked about those earlier when we were inside the controller resolver. Usually a controller will have the format ClassName::methodName. Well, we learned that this is really ServiceId::methodName.

Anyways, for an invokable controller - a controller class that has an __invoke() method - the syntax is simpler: just, ServiceId: no :: stuff. That is what's happening here.

How the ErrorController is Called

Ok cool, so Symfony is going to execute this error_controller as a controller... and it will render the page. But... how? You can't normally just call a controller directly... or at least, you shouldn't do this.

Back in ErrorListener, take out the dd(). The logic here is fascinating. It says $request = $this->duplicateRequest() and passes the $exception and $request objects. Let's jump down to that method. Apparently, the Request class has a duplicate() method on it, which does exactly what you think - it effectively clones the object.

But, it passes this $attributes value to the third argument. This says:

Please create an exact copy of this Request. When you do that, keep the same query parameters as the original, the same POST parameters as the original, but replace the original request attributes with this new array.

So... it's a clone, but with different request attributes. Most importantly, the new attributes have an _controller key set to that error_controller string.

Move back up to the onKernelException() method. We have a Request object that has an _controller request attribute. Here's the magic: $response = $event->getKernel()->handle($request).

Yea! It's calling the HttpKernel::handle() method! The same one that we use in index.php and the same one we've been studying. Inside of handling the original request, it's handling a second request and getting back the response. And notice that it mentions something called a "sub request". We'll talk more about that soon.

For now, this is just a super fancy way of calling the error_controller. Instead of executing it directly, it creates a Request with an _controller attribute and tells HttpKernel to handle it. Neato!

Next, let's jump into error_controller itself and find out exactly how Symfony renders an error page. Because, it's a smart process: it renders the exception page in dev, the error page in prod and even changes format - like rendering JSON - when requested.