Inconsistency with service loading by layer or by class loader

[Alan Bateman]

On 14/12/2024 17:10, David Lloyd wrote:
> :
>
> The reason to lazily load and link modules is basically analogous to 
> the reason that we lazily load and link classes. In fact there is very 
> little difference. While requiring all classes to be preloaded may 
> have some benefits in certain circumstances (take GraalVM for 
> example), it also significantly restricts flexibility in a few 
> important ways, and of course has a heavy performance cost, and so is 
> not generally considered to be a good strategy for application 
> runtimes or even the Java launcher itself.
>
> For modules, we face the same issue. I can, for example, create a 
> single application layer with a thousand modules in it. Before the 
> application can start, every JAR has to be opened and every module has 
> to be loaded, which entails parsing or dynamically creating 
> descriptors (generally a combination of these things), each with 
> dozens of support objects for things like dependencies, exports, and 
> services, and then their internal graphs have to be resolved and wired 
> and checked for consistency. So there is a significant performance 
> cost there.

Startup with a large number of modules on the application module path is 
an interesting topic. Reliable configuration means the modules required 
by the initial module must be recursively enumerated to create the 
module graph, and this does mean the module path must be scanned (once) 
to find the modules.

Efforts to date have been focused on shifting the work to link-time with 
jlink. When the module system was introduced we included a jlink plugin 
to generate the module graph (the Configuration) at link time, then a 
fast reconstitute of the Configutation at run-time. In JDK 12 it went a 
step further with CDS support for object archiving and enabling CDS by 
default. It improved again in JDK 16 with the archiving of the boot 
layer. The overall effect of all these efforts is all code to initialize 
the module system and the boot layer disappears from startup.

I think it would be interesting to explore shifting the generation of 
the Configuration for application / child layers to either build time or 
link time. There is a lots of interesting directions to explore. We had 
many ideas in this area during JDK 9 but had to focus on the core 
system. I suspect this could be a lot more fruitful direction that 
wouldn't forsake reliable configuration.

-Alan
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