It should not return null after it detects that the plugin has been loaded

[JiaRG]

pf4j/pf4j/src/main/java/org/pf4j/AbstractPluginManager.java

Line 811 in f80bc95

return null;

If null is returned, a null pointer exception will occur here

pf4j/pf4j/src/main/java/org/pf4j/AbstractPluginManager.java

Line 193 in f80bc95

return pluginWrapper.getDescriptor().getPluginId();

Should the return loaded PluginWrapper using getPlugin(String pluginId) method?

[decebals]

I understand what you say, but if we do the modifications according to your recommendation (return PluginWrapper in loadPluginFromPath when someone try to load a plugin multiple time), we don't have a "real" feedback if we loaded or not the plugin from path. The result is the same if you load a new plugin from a disk path or you tried to load a plugin multiple times.
What about if we throw a PluginException with the message "Plugin '{}' already loaded with id '{}'", pluginPath, pluginId?
Also, I am curios to know other opinions from other people.

[JiaRG]

Well, I agree with you and throw a PluginException, such as RepeatLoadPluginException.

[decebals]

I added PluginAlreadyLoadedException.
We can evolve this PluginException hierarchy and add other subclasses like: PluginNotFoundException, InvalidPlugin(Descriptor)Extension, ...
I don't know, it can be a good idea.