[bv-dev] Node#getElementDescriptor() for user-created nodes

[Gunnar Morling]

Hi experts,

One of the remaining issues on our way to 1.1 is BVAL-336 [1], which is
about what to return from Node#getElementDescriptor() for nodes added by
the user via the ConstraintViolationBuilder API.

Note that as per the spec only sub paths can be created by the user, i.e.
user added nodes always represent properties but e.g. no methods or
parameters. Based on that, it seems right to return the correct
PropertyDescriptor in case a user adds a node for an existing property.

Things get tricky though when nodes are added for non-existent properties.
Emmanuel, Hardy and I identified the following options to address this
situation:

#1 Disallow adding non-existent nodes

In case a user adds a node but no property with that names exist, an
exception is thrown. The problem is elegantly avoided that way, but we
might break some 1.0 user code (currently the spec is not really clear
whether added nodes must exist or not).

#2 return null from getElementDescriptor()

When invoking getElementDescriptor() on a Node representing a non-existent
property, null could be returned. This seems consistent (there is no
element), but causes additional null checking when traversing a path.

#3 return a PropertyDescriptor from getElementDescriptor()

Since all user-added nodes represent properties, a PropertyDescriptor could
be returned. hasConstraints() would return false,
getConstraintDescriptors() the empty set etc. This would allow to handle
all nodes and their descriptors uniformly when traversing a path. Question
is what to return from PropertyDescriptor#getElementClass(). Options are to
return null (signaling that the property is non-existent) or Object.class.

#4 return a VirtualDescriptor from getElementDescriptor()

We could create a new Kind, VIRTUAL, and an accompanying type
VirtualDescriptor and return an instance of this. Behavior of the methods
on the descriptor would be basically the same as in #3; getKind() returning
Kind.VIRTUAL would allow for a very explicit checking whether the node
exists or not.

Any feedback on the options (ideally with arguments pro/con) or other
alternatives are highly welcome. As life goes, Emmanuel's, Hardy's and my
preferences are equally distributed between these options :)

Thanks,

--Gunnar

[1] https://hibernate.onjira.com/browse/BVAL-336
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