ModuleLayer for java.base only

[Alan Bateman]

On 08/10/2025 20:11, Jaroslav Tulach wrote:
>  From a rationalistic perspective, I am looking for something very different:
>
> I'd like to start from scratch - e.g. `ModuleLayer.empty()` and built
> something on top of it myself. E.g. create a layer with
> `ModuleFinder.ofSystem()` where I include only some JDK modules - like
> `java.base` (but certainly not `org.graalvm.collections`).
>
> Why that isn't possible?
>
> Is there a technical problem in creating multiple layers that would all
> contain `java.base`?
>
> Or I am just first one who's asking for a feature like this?
There can only be one instance of java.base in the runtime, in the same 
way that there is one java.lang.Object class. You are right that 
Configuration.empty() and ModuleLayer.empty() are not interesting 
outside of creating the boot layer or tests.

It is possible to start with `--limit-modules java.base` so that the 
boot layer only contains java.base but you cannot stack module layers 
that contain the platform modules. There are technical and non-technical 
issues that prevent the java.* name space being used by custom class 
loaders, and it's not possible to give away control of the boot or 
platform class loaders without introducing major security + integrity 
risk. The original requirements for Project Jigsaw did envisage allowing 
some means to augment the set of platform modules at runtime but it 
hasn't been a priority to work on. The mundane cases that have been 
important is the dynamic loading of the java.instrument and 
java.management modules, something that is done in a very limited and 
controlled manner in response to commands that load tool agents.

As regards your desire to hide Enso, and the modules that it uses, from 
applications then it is something that has come once or twice, usually 
in the context of some container/application server that wants to hide 
its dependences from applications that it launches. The module system 
does not provide this type of isolation.

-Alan
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