[hibernate-issues] [Hibernate-JIRA] Commented: (HV-421) Reconsider behavior of parameter validation for inheritance hierarchies

[Gunnar Morling (JIRA)]

    [ http://opensource.atlassian.com/projects/hibernate/browse/HV-421?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=39864#action_39864 ] 

Gunnar Morling commented on HV-421:
-----------------------------------

The problem with conjoining all the preconditions of a method in a hierarchy is that a caller could get a constraint violation although he completely fulfilled the contract he has seen. So if a caller invokes foo() on a variable which's static type is A, a constraint violation might be the result if the variable's runtime is B and the precondition at B#foo() is stronger without any chance for the caller to know in advance.

So most PbC implementations choose to disjoin the preconditions in a hierarchy, causing the weakest precondition in the hierarchy to apply. Other implementations choose to prohibit multiple preconditions in a hierarchy at all (e.g. Microsoft's Code Contracts). Given the complexity of the matter at hand and the chance we might get something wrong here, I feel we might well start with such a conservative approach in 4.2 and add support for the inheritence scenario later on. WDYT?

> Reconsider behavior of parameter validation for inheritance hierarchies
> -----------------------------------------------------------------------
>
>                 Key: HV-421
>                 URL: http://opensource.atlassian.com/projects/hibernate/browse/HV-421
>             Project: Hibernate Validator
>          Issue Type: Bug
>          Components: engine
>            Reporter: Gunnar Morling
>            Assignee: Gunnar Morling
>             Fix For: 4.2.0.Beta2
>
>
> Let A extend B and A#foo() override B#foo(). When validating an invocation of A#foo() the current implementation will evaluate all parameter constraints defined at A#foo() *and* B#foo(). That way foo()'s preconditions defined in B are strengthened by A. 
> According to the ["Programming by contract"|http://en.wikipedia.org/wiki/Programming_by_contract] article on WP this is not allowed, subtypes may only weaken preconditions defined by supertypes. The common implementation pattern for this is to combine the preconditions within a hierarchy by a logical OR, meaning the weakest precondition in the hierarchy applies.
> Note that postconditions (return value constraints) may be strengthened (but not weakened) by subtypes. Therefore the current implementation (AND combination) should be correct here.

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