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This method is called by
ErrorController and its job is to return a
FlattenException that contains the status code, headers and body that should be set on the final error
FlattenException::createFromThrowable intelligently sets the status code and headers. But we still need to, somehow, figure out what content to send back, like a JSON error, or an HTML page that says: "Please send help!".
To do that,
SerializerErrorRenderer first tries to figure out what format the user wants - like HTML or JSON. The
$this->format property is actually a callback that points down here to this
getPreferredFormat() method. This is a fancy way of getting the request and calling
$request->getPreferredFormat(). And... hey! We know that method! I'll hit Shift + Shift and open
HttpFoundation. Search for
prepare(). This method is called by a listener to the
kernel.response event. It normalizes a few things... including setting the
Content-Type header if it hasn't already been set. To help with that, it calls
$request->getPreferredFormat() to try to figure out if the user wants HTML, JSON or something else. One of the ways it figures this out is by looking at the
Accept header on the request.
SerializerErrorRenderer, we're once again using
$request->getPreferredFormat(), which will return a simple string like
render(), this is pretty cool: it says:
Hey serializer! Can you try to serialize the
FlattenExceptionobject into this format?
html, this will fail with a
NotEncodableValueException: the serializer doesn't handle HTML. We'll talk about that case in a minute. But if the format is
xml or some other format that the serializer does support, this will convert the exception to that format!
We can see this. If we refresh the page... we see the big HTML exception - and we'll see the code that makes this soon. Copy the URL, find your terminal and use
curl to fetch that URL. But also pass a
-H flag to add a header:
curl https://localhost:8000/news/foo -H "Accept: application/json"
This will change the "preferred format" on the request to
json. And... check it out! It's a 404 status code but in a JSON format! We can even use
text/xml to see this in XML.
curl https://localhost:8000/news/foo -H "Accept: text/xml"
How does this work? One of the normalizers in the serializer is called
ProblemNormalizer. I'll hit Shift + Shift to open it:
If you don't know much about the serializer component, the important thing to know is that a normalizer is responsible for taking an object and converting it into an array of data. Thanks to the
supportsNormalization() method, this class is used when you try to normalize a
This normalizer creates a response format that follows an HTTP specification: it helps us return an official, standardized error response. It's pretty simple: it sets keys for
$debug mode, it also adds
trace. Also, the
detail key in debug mode will be the exception message... but in production, it will be the "status text", which is a generic "Not Found" message... or something similar, based on the status code. That's done so that your exception messages don't "leak" to the public.
normalize() method is passed the
$exception, which is the
FlattenException. But if you look back at
SerializerErrorRenderer, it also passes the original exception as an
exception key on the
$context - that's the 3rd argument to
So this gives us a really nice error response body, without any work. If you wanted to change this data, you could do that by adding your own custom normalizer. We actually talk about this in our API Platform Security Tutorial. You could decorate the
ProblemNormalizer... and maybe just add or tweak some data or you could create an entirely new normalizer. Heck, you could use the
supports - you need to implement
ContextAwareNormalizerInterface to make that work - and make that new normalizer responsible for only normalizing
FlattenException classes for a specific, original exception. If you want to try that and have problems, let us know.
Ok, close that class up. Next, let's find out what happens if the format is not something that the serializer can handle. Like, HTML.