BackEnd Service Provider.

[Alan Bateman]

On 21/04/2020 14:01, Doug Simon wrote:
> Hi Gary,
>
> I cannot understand why --add-reads does not work and have reached out 
> to Alan Bateman for more insight.
>
I read through the thread [1] and I don't see an obvious solution to this.

The main issue here is that org.grfstuff.compiler wants to provide an 
implementation of org.graalvm.compiler.hotspot.HotSpotBackendFactory but 
that service type is not in org.grfstuff.compiler and 
org.graalvm.compiler.hotspot is not exported to org.grfstuff.compiler by 
any of the modules that it reads. The post resolution consistency check 
that it trips up on is specified in the Configuration API docs if you 
are interested.

I assume `--add-exports 
jdk.internal.vm.compiler/org.graalvm.compiler.hotspot=org.grfstuff.compiler` 
was added in an attempt to workaround this but that CLI option is the 
equivalent of jdk.internal.vm.compiler invoking Module::addExports after 
the module graph has been reified. It doesn't impact resolution or any 
of the post resolution consistency checks at run-time. Yes, there is an 
unfortunate difference between between compile-time and run-time in this 
regard but that is something for another discussion.

I don't think there are any obvious workarounds. The Module API defines 
addXXX methods to export or open packages at run-time, extend 
readability or add a service dependences but it doesn't define 
addProvides. I think it came up once during JDK 9 but more in the 
context of symmetry rather than a compelling use-case. It's not clear 
that the issue under discussion here is compelling enough, needs more 
thought as the scenario is a bit weird. If org.grfstuff.compiler was co 
developed with jdk.internal.vm.compiler then it could use a qualified 
export of course.

Initially I thought you could workaround this by moving 
org.grfstuff.compiler  to the class path but this it's not going to work 
as ServiceLodaer usage in Graal seems to use module layers as congtext, 
so it will not find implementations on the class path.

-Alan

[1] https://mail.openjdk.java.net/pipermail/graal-dev/2020-April/005949.html