On Mon, Dec 23, 2024 at 11:50 AM Alex Buckley <alex.buck...@oracle.com> wrote:
> 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. > Sure, but that's not really what I'm asking for (you'd then have two As in one system, which is pretty different from the original scenario). If I have A->B at build time, then it should be possible to use A in a run time system that has A->B->C or A->B->D or A->B->E without A having to know or care what B's dependencies are; furthermore, it should be possible for A to bring its own C or D or E without fearing conflicts with transitive dependencies of B (a big part of classical "JAR hell"). This is where the current system seems deficient. Layers can only take you so far, because of two constraining factors: the hierarchical nature of layers which does not seem to suit all (or most) of the ecosystem, and that a parent layer apparently cannot hide modules from a child layer. 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. > Recognizing that the original design goals did not encompass this scenario, why not consider the enhancement? Are these requirements considered a closed book at this point? -- - DML • he/him
