Bukkit/Spigot - How to wait until BukkitRunnable is finished

Question

In my onDisbale() method in my Main class I have a loop which creates and starts new BukkitRunnables. I'm getting a error in console: org.bukkit.plugin.IllegalPluginAccessException: Plugin attempted to register task while disabled
I need to somehow wait in my onDisable() method until all the BukkitRunnables I create in the loop are finished. How to do that?
My code looks like this:

@Override
public void onDisable() {
    for (Player p : Bukkit.getOnlinePlayers()) {
        new PlayerDataSaverRunnable().runTaskAsynchronously(this);
    }
}

Answer 1

The onDisable method is the very last thing that gets called before your plugin is disabled and the server shuts down. So, as the error message says, you unfortunately can't schedule any new tasks in the onDisable function.

You mentioned in a comment that you were trying to write to a file in the plugins folder, and under normal circumstances you'd want to do that asynchronously. But, because onDisable only ever gets called when the server is shut down (or when the entire server is reloaded with /reload), it's perfectly fine to run code here that blocks the main thread and could potentially take a few seconds to run — in the case of a shutdown, by the time this method gets called, all the players will have already been kicked off the server, and so there's no "lag" to complain about. If your plugin is super advanced and has to save a bunch of stuff, I don't think any server owners would complain even if it took 10 or so seconds to disable.

Of course, you would have to be saving something crazy for it to take a whole 10 seconds to save. More than likely, most files will save in just a few milliseconds.

If you're really dead-set on disabling the plugin as fast as possible, you might consider having some async task that runs every 5 minutes or so and auto-saves the files. Then, in onDisable, you could only save files that changed since the auto-saver was last run. That's a good practice anyways, just incase the server crashes or the power goes out and the onDisable method doesn't get a chance to run. But, then again, I would still recommend that you save everything in the onDisable method (that's what I do for all of my plugins, as well), even if it will take a few seconds and block the main thread, just so you can be 100% sure that everything gets saved correctly.