Error message when accessing non-public type

[Alex Buckley]

Hi Gunnar,

Thanks. Yes, I suspected you were using ServiceLoader::load to get a 
dummy instance of each custom constraint validator class (in general, 
there might be multiple modules providing a validator for @Min-on-int), 
and then obtaining each dummy's Class object and doing your own 
instantiations of validator classes thereafter.

It wouldn't help if ServiceLoader simply returned the Class objects for 
service implementations, because you still wouldn't be able to 
newInstance() them without the export. You would need ServiceLoader to 
take one of those Class objects back (call it cImpl), along with the 
Class object for a service interface (call it cIntf), and then 
instantiate cImpl on your behalf if a) cIntf.isAssignableFrom(cImpl) and 
b) <<some caller-sensitive conditions>>. No more export needed.

Alex

On 6/21/2016 11:55 PM, Gunnar Morling wrote:
> Hi Alex,
>
> Good question, I should have mentioned some more details on this.
>
> Hibernate Validator is using the service loader here to discover
> constraint validator *types*, but then it itself is going to create (and
> configure) instances of these types based on the annotation properties
> set for specific usages of a constraint. I.e. there will be one instance
> of a validator per constraint usage (the instance obtained from the
> service loader will be dropped once its type has been examined).
>
> E.g. two instances of the constraint validator type for @Min on int
> would be created here:
>
>      class Foo {
>          @Min(1)
>          int bar;
>
>          @Min(2)
>          int baz;
>      }
>
> One might argue that this is a bit mis-using the service loader
> mechanism and we should rather detect some sort of constraint validator
> factory in charge of creating actual constraint validator instances (in
> which case we could take the instance returned from the service loader
> as is).
>
> But then it's working good enough in its current form and is simpler on
> implementors that way (who just need to provide/expose constraint
> validators and nothing more). But it explains why that export is needed
> under Jigsaw.
>
> Alternatively, an API for obtaining just the types of services (not
> instances) might help with this case.
>
> --Gunnar
>