Overriding the Default Error Templates

You can use the built-in Twig error renderer to override the default error templates. Both the TwigBundle and TwigBridge need to be installed for this. Run this command to ensure both are installed:

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$ composer require symfony/twig-pack

When the error page loads, TwigErrorRenderer is used to render a Twig template to show the user.

This renderer uses the HTTP status code and the following logic to determine the template filename:

1. Look for a template for the given status code (like error500.html.twig);
2. If the previous template doesn't exist, discard the status code and look for a generic error template (error.html.twig).

To override these templates, rely on the standard Symfony method for overriding templates that live inside a bundle and put them in the templates/bundles/TwigBundle/Exception/ directory.

A typical project that returns HTML pages might look like this:

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templates/
└─ bundles/
   └─ TwigBundle/
      └─ Exception/
         ├─ error404.html.twig
         ├─ error403.html.twig
         └─ error.html.twig      # All other HTML errors (including 500)

Example 404 Error Template

To override the 404 error template for HTML pages, create a new error404.html.twig template located at templates/bundles/TwigBundle/Exception/:

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{# templates/bundles/TwigBundle/Exception/error404.html.twig #}
{% extends 'base.html.twig' %}

{% block body %}
    <h1>Page not found</h1>

    <p>
        The requested page couldn't be located. Checkout for any URL
        misspelling or <a href="{{ path('homepage') }}">return to the homepage</a>.
    </p>
{% endblock %}

In case you need them, the TwigErrorRenderer passes some information to the error template via the status_code and status_text variables that store the HTTP status code and message respectively.

Additionally you have access to the Exception with exception, which for example allows you to output the stack trace using {{ exception.traceAsString }} or access any other method on the object. You should be careful with this though, as this is very likely to expose sensitive data.

Security & 404 Pages

Due to the order of how routing and security are loaded, security information will not be available on your 404 pages. This means that it will appear as if your user is logged out on the 404 page (it will work while testing, but not on production).

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# config/routes/framework.yaml
when@dev:
    _errors:
        resource: '@FrameworkBundle/Resources/config/routing/errors.xml'
        prefix:   /_error

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<!-- config/routes/framework.xml -->
<?xml version="1.0" encoding="UTF-8" ?>
<routes xmlns="http://symfony.com/schema/routing"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://symfony.com/schema/routing
        https://symfony.com/schema/routing/routing-1.0.xsd">

    <when env="dev">
        <import resource="@FrameworkBundle/Resources/config/routing/errors.xml" prefix="/_error"/>
    </when>
</routes>

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// config/routes/framework.php
use Symfony\Component\Routing\Loader\Configurator\RoutingConfigurator;

return function (RoutingConfigurator $routes) {
    if ('dev' === $routes->env()) {
        $routes->import('@FrameworkBundle/Resources/config/routing/errors.xml')
            ->prefix('/_error')
        ;
    }
};

Overriding Error output for non-HTML formats

To override non-HTML error output, the Serializer component needs to be installed.

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$ composer require symfony/serializer-pack

The Serializer component has a built-in FlattenException normalizer (ProblemNormalizer) and JSON/XML/CSV/YAML encoders. When your application throws an exception, Symfony can output it in one of those formats. If you want to change the output contents, create a new Normalizer that supports the FlattenException input:

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# src/Serializer/MyCustomProblemNormalizer.php
namespace App\Serializer;

use Symfony\Component\ErrorHandler\Exception\FlattenException;
use Symfony\Component\Serializer\Normalizer\NormalizerInterface;

class MyCustomProblemNormalizer implements NormalizerInterface
{
    public function normalize($exception, string $format = null, array $context = []): array
    {
        return [
            'content' => 'This is my custom problem normalizer.',
            'exception'=> [
                'message' => $exception->getMessage(),
                'code' => $exception->getStatusCode(),
            ],
        ];
    }

    public function supportsNormalization($data, string $format = null): bool
    {
        return $data instanceof FlattenException;
    }
}

Overriding the Default ErrorController

If you need a little more flexibility beyond just overriding the template, then you can change the controller that renders the error page. For example, you might need to pass some additional variables into your template.

To do this, create a new controller anywhere in your application and set the framework.error_controller configuration option to point to it:

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# config/packages/framework.yaml
framework:
    error_controller: App\Controller\ErrorController::show

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<!-- config/packages/framework.xml -->
<?xml version="1.0" encoding="UTF-8" ?>
<container xmlns="http://symfony.com/schema/dic/services"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://symfony.com/schema/dic/services
        https://symfony.com/schema/dic/services/services-1.0.xsd">

    <framework:config>
        <framework:error-controller>App\Controller\ErrorController::show</framework:error-controller>
    </framework:config>

</container>

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// config/packages/framework.php
use Symfony\Config\FrameworkConfig;

return static function (FrameworkConfig $framework) {
    // ...
    $framework->errorController('App\Controller\ErrorController::show');
};

The ErrorListener class used by the FrameworkBundle as a listener of the kernel.exception event creates the request that will be dispatched to your controller. In addition, your controller will be passed two parameters:

exception

The original Throwable instance being handled.

logger

A DebugLoggerInterface instance which may be null in some circumstances.

Working with the kernel.exception Event

When an exception is thrown, the HttpKernel class catches it and dispatches a kernel.exception event. This gives you the power to convert the exception into a Response in a few different ways.

Working with this event is actually much more powerful than what has been explained before, but also requires a thorough understanding of Symfony internals. Suppose that your code throws specialized exceptions with a particular meaning to your application domain.

Writing your own event listener for the kernel.exception event allows you to have a closer look at the exception and take different actions depending on it. Those actions might include logging the exception, redirecting the user to another page or rendering specialized error pages.

This approach allows you to create centralized and layered error handling: instead of catching (and handling) the same exceptions in various controllers time and again, you can have just one (or several) listeners deal with them.