[External] : Re: The definition of `requires`

[Alex Buckley]

On 12/23/2024 7:08 AM, David Lloyd wrote:
> It would also open the door to replacing C with E: a module with 
> different bits, dependencies, *and* name and exports, as a part of 
> upgrading B to a new version, without breaking A.
> 
> Not only that, but if M decides it needs C - or it needs something that 
> needs C - you are not necessarily locked into the problem that A and B 
> must agree on a version of C, if there is an isolation mechanism. 

Layers let you have multiple versions of C and B. One layer can have 
class loaders that load an A->B->C configuration, while another layer 
has class loaders that load an upgraded A->B'->E configuration.

This fulfills one of the requirements for the module system: 
https://openjdk.org/projects/jigsaw/spec/reqs/#isolated-dynamic-configurations

A layer is an abstraction that combines class loaders and module 
declarations in order for the JVM to enforce accessibility and thereby 
provide encapsulation. See 
https://www.youtube.com/watch?v=QnMDsI2GbOc&t=1848s.

But if C and E don't want encapsulation, because they export all their 
public classes, then having C and E in different layers doesn't buy you 
anything beyond what you get with different class loaders. I understand 
the kind of isolation you're looking for, but we never claimed that 
layers would provide it.

Alex