Once you have compiled your plugin using the 4.2 API you will need to reload it to test it. Follow these steps to reload your plugin without restarting jEdit:
From the Plugins menu open the Plugin Manager.
On the Manage tab uncheck Hide libraries. This will allow you to see plugins that are not loaded.
Find the plugin on the Manage tab and uncheck it. This will unload the plugin. You will get a warning if this plugin does not support dynamic reloading. If you get that warning you will need to restart jEdit to reload the plugin until the plugin is converted over to the 4.2 API.
Recheck the plugin to reload it.
The Activator plugin provides a very convenient way to test the reloading behavior of your plugin, as well as restart it for debugging purposes.
If you have reached this point in the text, you are probably serious about writing a plugin for jEdit. Good luck with your efforts, and thank you for contributing to the jEdit project.