[bv-dev] Scopes support for custom constraint validators

[Thang Le]

Hi all,

I've come across a scenario which makes me think supporting different
scopes for custom constraint validators would ease the task for writing
efficient validation logic. Similar to Spring IoC, these scopes are
prototype, session & singleton. Currently, the Bean Validation 1.1 spec
suggests a 'prototype' scope should be used for all custom constraint
validation classes. In my opinion, the 'session' (likely singleton as well)
scope proves to be useful when a custom constraint validator needs to do
the SAME  timing pre-processing logic before doing some actual validation
logic on the current targeted object. In such cases, the developer would
want to store the result of the pre-processing logic into an instance
variable of the custom constraint validation class and reuse this result
later when the constraint is executed again on different objects. Below is
a specific use-case from my current project.

The model I need to validate has a tree-like structure. In this model, each
node can be a/an remote/access-point/cluster. I have a model builder which
builds the model tree given a set of nodes. The builder then hands off this
tree to the validator to validate it. I have some constraints which require
to check the uniqueness of a certain property for a set of nodes (e.g: all
access-points in the tree must have unique serial number). These
constraints are written as custom constraints. I recognize these
constraints follow the same pattern which is I need to traverse the tree to
collect all required node and build a multimap out of the collected nodes
and check for uniqueness using the key of the current targeted object. For
above example, the custom constraint is a class constraint of access-point
class. I need to collect all access-points in the tree, build a multimap of
<serialNumber:access-point> pairs. Then I lookup from the map the
collection of access-points based on the serial number of the access-point
being validated. It would be beneficial if the multimap of
<serialNumber:access-point> pairs can be stored in the custom validator so
that I don't need to re-do this expensive task again when validating other
access-points.

There might be some reasons that you wouldn't want to support different
scopes for custom constraint validator. However, I would think when it
comes to writing a constraint for a bean in relation with other beans, such
scopes become handy. Please let me know your thoughts on this feature.

Thang
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