You can use IntelliJ to make mods in. Im sitting in it now and I know other modders are also using it.
There is an eclipse folder but i dont think about that.
The problem with onCreated is that its run twice. Both on server and client.
I have some code where I random choose an enchantment and I can see that it runs it twice.
It gives me a problem because I want to take levels from the player when the item is crafted.