Example of how to use plugin channel for bukkit-bungee communication

BukkitPlug.java

import java.io.ByteArrayOutputStream;
import java.io.DataOutputStream;

import org.bukkit.Bukkit;
import org.bukkit.plugin.java.JavaPlugin;
import org.bukkit.scheduler.BukkitRunnable;
import org.bukkit.scheduler.BukkitTask;

public class BukkitPlug extends JavaPlugin {
    BukkitTask task;
    @Override
    public void onEnable() {
        task = new RunStuff().runTaskTimer(this, 0l, 120L);
    }
    
    @Override
    public void onDisable() {
        task.cancel();
    }
    
    public class RunStuff extends BukkitRunnable {
        Plugin plug;
        public RunStuff(Plugin sender) {
            plug = sender;
        }
        
        public void run() {
            ByteArrayOutputStream outbytes = new ByteArrayOutputStream();
            DataOutputStream out = new DataOutputStream(outbytes);
            out.writeUTF("Stuff");
            if (Bukkit.getOnlinePlayers().length != 0)
                Bukkit.getOnlinePlayers()[0].sendPluginMessage(plug, "BungeePlug", outbytes.getBytes());
        }
    }
}

BungeePlug.java

import com.google.common.eventbus.Subscribe;

import net.md_5.bungee.api.plugin.Plugin;
import net.md_5.bungee.api.plugin.Listener;
import net.md_5.bungee.api.ProxyServer;
import net.md_5.bungee.api.event.PluginMessageEvent;

public class BungeePlug extends Plugin implements Listener {
    public void onEnable() {
        ProxyServer.getInstance().getPluginManager().registerListener(this);
    }
  
    public void onDisable() {
        // Do nothing
    }
    
    @Subscribe
    public void onPluginMessage(PluginMessageEvent e) {
        
    }
}