Distinguish between conversion failure and validation failure in o:viewParamValidationFailed - validation

Question

I am using the OmniFaces <o:viewParam> taghandler in my current project. I like it, it's great. And even greater is the <o:viewParamValidationFailed>. Now, we are able to send an error if validation or conversion fails. But I wonder, whether it is possible to distinguish between conversion failure and validation failure.
Let's say we want to send a Bad Request if the given view param in malformed and can not be converted; for that matter send a Not Found if conversion succeeded, but the object could not be found in the database; and send a Forbidden if the successfully fetched object should not be accessed by the user.
Does anybody know a way to achieve this?

Answer 1

It's unfortunately not possible to distinguish between a ConverterException and ValidatorException when you've only UIInput#isValid() at hands. Theoretically, you could check and test the faces message to see if it represents a conversion or validation error, but this is not a robust approach, certainly not when it's localized.
On the other hand, it's possible to declare multiple view parameters on the same parameter name. You do not necessarily need to specify a value to set it as model value.
Here's an example based on your description, note that the model value is only set on the last one:
<o:viewParam name="foo">
<f:converter converterId="yourFooConverter" />
<o:viewParamValidationFailed sendError="400" />
</o:viewParam>
<o:viewParam name="foo">
<f:converter converterId="yourFooConverter" />
<f:validateRequired />
<o:viewParamValidationFailed sendError="404" />
</o:viewParam>
<o:viewParam name="foo" value="#{bean.foo}">
<f:converter converterId="yourFooConverter" />
<f:validateRequired />
<f:validator validatorId="yourRestrictedAccessValidator" />
<o:viewParamValidationFailed sendError="403" />
</o:viewParam>
To avoid the expensive job of calling the DB on every conversion, let the YourFooConverter implementation store the converted value as a custom attribute of the FacesContext and then check it on every pass.