Need suggestion for plugin development using Eclipse

Hey Guys,

We have a custom plugin in our Jira Data center, Now we got request to add some functionality to it and after upgrading our jira to 9.4.19 it stopped working. I am new to this area. Could you please explain how to write the code in Eclipse IDE and any other trouble shooting techniques, how to test the plugin in IDE itself and any references you know.

Thanks in advance!