Java extensibility and JPMS (ServiceLoader)

[Johannes Spangenberg]

> *Ad 2 - Provider module cannot be optional dependency*
> -------------------------------------
> Problem: We cannot declare `requires static` on a module that has the 
> ServiceLoader provider interface (or abstract class)

Note that there was a relative long discussion about this exact topic in 
April 2023:

https://mail.openjdk.org/pipermail/jigsaw-dev/2023-April/014846.html

I personally think that the current implementation is too restrictive as 
I see very little gain in this runtime error, but there was also a 
noticeable opposition.

> *Ad 4 - Duality of definition between classpath and module path*
> -------------------------------------
> Problem: To support services, we MUST declare them twice - once in 
> `provides` in module-info.java, once through META-INF/services
> Possible solution: Java could read module descriptors even when 
> running in classpath mode to add service implementations and merge it 
> with META-INF/services information

While I did also run into this issue, I would favor a solution during 
the build phase. Maybe some project still uses Java 8, where the JDK 
does not yet know about the module-info. The behavior should not change 
between different versions of the runtime.

PS: I am not affiliated with the project, besides being subscribed to 
this mailing list.
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