Make PluginIndex reusable

[Squareys]

Hii @dietzc !

I found out that PluginIndex re-adds all plugins of the PluginFinder everytime a new Context is created. This results in duplicates which are explicitly allowed by PluginIndex. Also, plugins are re-discovered every Context creation, which eliminates the performance gain we hoped for.

I believe this PR will solve that problem.

Greetings,
Jonathan

[dietzc]

is this potentially also a bug on SciJava side @ctrueden?

Thanks @Squareys

[ctrueden]

Yeah, I suppose you could call it an unintuitive quirk of when plugin discovery happens. Looking at it now, I think it would probably make more sense to discover all the plugins immediately upon creation of the PluginIndex, and get rid of the public discover() method. We could change this for SJC3; see scijava/scijava-common#196.

In the meantime, there is another workaround which shouldn't require you to create a new subclass:

PluginIndex pluginIndex = new PluginIndex(null);
ArrayList<PluginInfo<?>> plugins = new ArrayList<>();
pluginFinder.findPlugins(plugins);
pluginIndex.addAll(plugins);

That is: passing null to the PluginIndex ensures no PluginFinder is linked to it, so every time you create a new Context with it, its discover() method will do nothing.

I didn't test this, but in theory it should work.