On 2018-03-07T15:14:11 +0000 Stephen Colebourne <scolebou...@joda.org> wrote: > > Effectively what is needed is another way for a library to be informed > of the presence of the calling application. One possible solution to > this would be to allow users to write module initialization code in > module-info.java. Then an application coder would have a solid > reliable place to put code that registers the additional configuration > files with the low-level library.
There's a pattern I've seen used in OSGi that I've considered adapting for my own use (exposing resource-only modules that don't contain any code and yet need to be able to cause some other piece of code to instantiate services on behalf of the module): They call it the "extender pattern". https://dzone.com/articles/osgi-42-extender-pattern-and Briefly, what happens is that you subscribe to an interface that tells you when bundles (OSGi terminology for artifacts containing modules, more or less) are added to or removed from the system. In Jigsaw, this would probably equate to publishing some sort of event that can be observed whenever someone creates a new module layer. When you get notified that a bundle has been added, you can scan the manifest of the bundle (via the standard jar manifest APIs if you like) and can then, for example, look for manifest fields that tell you where in the jar file to find application-specific config files. X-My-Extra-Config-File: /com/example/config.xml The listening party can then read the config file, instantiate services as necessary, etc. It'd need to be handled in a way that ensured that you don't essentially race the module resolution code; Just because your code didn't get a chance to subscribe to "module became available" events until after they'd appeared doesn't mean you should miss the events. OSGi does handle this (it's written into the spec), but I don't know quite how the implementations handle it. -- Mark Raynsford | http://www.io7m.com
