How to open a package from a module in the boot layer to a module in another layer?

[Alan Bateman]

On 12/01/2025 09:12, Code Ranger wrote:
> 1. I see that JDK-8347283 was closed  with your comment. I don't agree 
> with you decision, but I can be wrong. Let me explain, why I think 
> that is wrong using this concrete example.
>
> Boot layer has jdk modules and main application modules. The CORE 
> module of the application creates plugins with child layers. No one in 
> the world can know what plugins will be created and used when the main 
> application starts. Now, somewhere in the future, the module A from 
> plugin X will require module B from boot layer to open its package to 
> module A. I believe, that the only solution to do it is to provide the 
> controller of the boot layer.
>
> 2. Now, I tried to do apiModule.addOpens in CORE module. I got:
>
> java.lang.IllegalCallerException: com.foo.api.packagename is not open 
> to module com.foo.core
>     at java.base/java.lang.Module.addOpens(Module.java:918) ~[?:?]
>
>
> Therefore, as far as I understand, for the solution you are suggesting 
> to work, the packagename package of the API module must be opened to 
> the Core module. But the main problem, as I've said above - no one 
> knows in advance which packages of which modules from the boot layer 
> will need to be opened/exported, etc., to the plugins that will be 
> created by users and deployed dynamically.

Your second mail reveals a bit more but it's not clear if your issue is 
one of these scenarios:

1. A serialization library that is incompatible with strong 
encapsulation. If so, that's a different discussion.

2. The API and CORE modules have some interesting internals that the 
authors of these modules have chosen not to export. The author of a 
plugin doesn't agree. As you've found, this can only be facilitated with 
cooperation from code in CORE. It may additionally require CLI option if 
API won't open its packages to CORE. This means running with --add-opens 
API/com.foo.api=CORE so that CORE can open API's package to the plugin 
module in the child layer.

3. A badly behaved plugin that depends on the internals of standard or 
JDK modules. This would be whack-a-mole and require a combination of CLI 
options and code in CORE to open java.* and jdk.* packages to the 
offending code in the plugin.

-Alan