Handling Invalid JSON¶

In our controller, we’re already checking to see if the JSON is invalid, but right now, we’re throwing a normal PHP Exception message, which results in the 500 error:

// src/KnpU/CodeBattle/Controller/Api/ProgrammerController.php
// ...

private function handleRequest(Request $request, Programmer $programmer)
{
    // ...

    if ($data === null) {
        throw new \Exception(sprintf('Invalid JSON: '.$request->getContent());
    }

    // ...
}

To make this a 400 error, we could do 2 things. First, we could create a new Response object and set its status code to 400. That’s what we’re already doing with the validation error.

Second, in Silex, we could throw a special HttpException:

// src/KnpU/CodeBattle/Controller/Api/ProgrammerController.php
// ...

private function handleRequest(Request $request, Programmer $programmer)
{
    // ...

    if ($data === null) {
        throw new HttpException(
            400,
            sprintf('Invalid JSON: '.$request->getContent())
        );
    }

    // ...
}

In most frameworks, if you throw an exception, it results in a 500 status code. That’s true in Silex too, unless you throw this very special type of exception where the status code is the first argument.

First, make sure it’s working by running the test:

Awesome! So why am I throwing an exception instead of just returning a normal 400 response? The problem is that we’re inside handleRequest, so if I return a Response object here, it won’t actually be sent back to the user unless we also return that value from newAction and updateAction. That just gets confusing and a bit ugly.

Instead, if we throw an exception, the normal execution will stop immediately and the user will definitely get the 400 response. So being able to throw an exception like this makes my code easier to write and understand. Double threat!

The disadvantage is complexity. When I throw an exception, I need to have some other magic layer that is able to convert it into a proper response. In Silex, that magic layer is smart enough to see my HttpException and create a response with a 400 status code instead of 500.

If this doesn’t make sense yet, keep following along.

ApiProblem for Invalid JSON¶

Since invalid JSON is a “problem”, we should really send back an application/problem+json response. Let’s first update the test to look for this Content-Type header and then look for a type field that’s equal to invalid_body_format:

# features/api/programmer.feature
# ...

Scenario: Error response on invalid JSON
  # the rest of the scenario
  # ...
  And the "Content-Type" header should be "application/problem+json"
  And the "type" property should equal "invalid_body_format"

To make this work, we’ll create a new ApiProblem object. But first, let’s add the new invalid_body_format type as a constant to the class and give it a title:

// src/KnpU/CodeBattle/Api/ApiProblem.php
// ...

class ApiProblem
{
    // ...
    const TYPE_INVALID_REQUEST_BODY_FORMAT = 'invalid_body_format';

    static private $titles = array(
        // ...
        self::TYPE_INVALID_REQUEST_BODY_FORMAT => 'Invalid JSON format sent',
    );

    // ...
}

Next, instantiate the new ApiProblem in the controller:

// src/KnpU/CodeBattle/Controller/Api/ProgrammerController.php
// ...

private function handleRequest(Request $request, Programmer $programmer)
{
    // ...

    if ($data === null) {
        $problem = new ApiProblem(
            400,
            ApiProblem::TYPE_INVALID_REQUEST_BODY_FORMAT
        );

        // ...
    }

    // ...
}

But now what? When we had validation errors, we just created a new JsonResponse, passed $problem->toArray() as data, and returned it. But here, we want to throw an exception instead so that the normal flow stops.

We’re going to fix this in two steps. First, we will throw an Exception, but we’ll put the ApiProblem inside of it. Second, we’ll hook into the magic layer that handles exceptions and extend it so that it transforms the exception into a Response with a 400 status code. Again, this is a little more complicated, so if it doesn’t make sense yet, watch our implementation.