Document how Extensions are instantiated

[dmitry-timofeev]

It would be nice if org.pf4j.PluginManager#getExtensions had API documentation.

Some things that might be useful to know (or know that they are unspecified, or implementation defined):

1. When extensions are instantiated (When the plugin is loaded/started? When explicitly requested)? (see How/When are extension classes instantiated? #232)
2. In which plugin states is it OK to call this method? (In any? In loaded/started only? What happens in forbidden states?)
3. If I call this method several times and get extensions, is it guaranteed that I will get the same instances?
4. What will happen if I pass unknown pluginId or an absent extension type? (empty list/exception?)

[decebals]

3. If I call this method several times and get extensions, is it guaranteed that I will get the same instances?

By default a new instance is created each time. See ExtensionFactory, DefaultExtensionFactory,
SingletonExtensionFactory and ExtensionFinder#find.

Any time, when someone understand a concept or a workflow from the source code, and he/she considers that the documentation is not good enough/clear, please submit a PR with clarifications, here (for javadoc) or on documentation project. Thanks!

[dmitry-timofeev]

I see, thanks! The thing is

1. Not everything could be easily inferred from the code, hence that many questions.
2. Some things may be intentionally unspecified (implementation defined) — the code does not communicate that.

[decebals]

I have said many times, the weaknesses of this project (and probably most of the open source projects) are documentation and test coverage (number of unit tests). Anybody that can help in these directions is an hero for me. If something is not clear from code, please ask and I (and other people) will answer you.
I will always try to improve the javadoc and documentation.