//main
public class main extends JavaPlugin implements PluginMessageListener{

    public static HashMap<String, Integer> playerCounts = new HashMap<String, Integer>();
    public static main plugin;
    public static int playercount = 0;   [...]

//main (in onEnable)
plugin = this;

Bukkit.getMessenger().registerOutgoingPluginChannel(this, "BungeeCord");
Bukkit.getMessenger().registerIncomingPluginChannel(this, "BungeeCord", this);

//main
     @Override
        public void onPluginMessageReceived(String channel, Player player, byte[] message) {
                if (!channel.equals("BungeeCord")) return;
                try {
                        DataInputStream in = new DataInputStream(new ByteArrayInputStream(message));
                        String command = in.readUTF();
                        if (command.equals("PlayerCount")) {
                                String server = in.readUTF();
                                int playerCount = in.readInt();
                                playerCounts.put(server, playerCount);
                        }
                } catch (Exception e) {
                        e.printStackTrace();
                }
        }

     public static void getPlayerCount(String Server){
            try {
                final ByteArrayOutputStream b = new ByteArrayOutputStream();
                final DataOutputStream out = new DataOutputStream(b);
                out.writeUTF("PlayerCount");
                out.writeUTF(Server);
                Bukkit.getServer().sendPluginMessage(plugin, "BungeeCord", b.toByteArray());
            } catch (IOException e2) {
                e2.printStackTrace();
            }
        }


//joinsign.java
package de.florian.thebadexp.cloud;

import java.io.ByteArrayOutputStream;
import java.io.DataOutputStream;
import java.io.IOException;
import java.util.Random;

import org.bukkit.Bukkit;
import org.bukkit.block.Sign;
import org.bukkit.entity.Player;
import org.bukkit.event.EventHandler;
import org.bukkit.event.Listener;
import org.bukkit.event.block.Action;
import org.bukkit.event.block.SignChangeEvent;
import org.bukkit.event.player.PlayerInteractEvent;


import de.florian.mineraster.system.main;

public class joinsign implements Listener{

    private main plugin;
    int online = 0;
    int max = 16;
    int sched3;
    public static Sign JoinSign;


    public joinsign(main main) {
        this.plugin = main;
    }




     @EventHandler
     public void onSignChange(SignChangeEvent e){
            if(e.getLine(0).equalsIgnoreCase("[joink]")){
                Sign sign = (Sign) e.getBlock().getState();
                main.getPlayerCount("knockout");
                if (main.playerCounts.containsKey("knockout")){
                online = main.playerCounts.get("knockout");
                } else {
                    main.getPlayerCount("knockout");
                }
                e.setLine(0, "§a§l§m-------------");
                e.setLine(1, "§lKnockout-1");
                e.setLine(2, online +"/"+max);
                e.setLine(3, "§a§l§m-------------");
                if (!Bukkit.getScheduler().isCurrentlyRunning(sched3)){
                    @SuppressWarnings("unused")
                    int sched3 = Bukkit.getScheduler().scheduleSyncRepeatingTask(plugin, new Runnable() {

                        @Override
                        public void run() {
                            if (main.playerCounts.containsKey("knockout")){
                                online = main.playerCounts.get("knockout");
                                } else {
                                    main.getPlayerCount("knockout");
                                    online = main.playerCounts.get("knockout");
                                }
                            System.out.println("Sign Change to "+ online);
                                sign.setLine(0, "§a§l§m-------------");
                                sign.setLine(1, "§lKnockout-1");
                                sign.setLine(2, ""+online +"/"+max);
                                sign.setLine(3, "§a§l§m-------------");
                                sign.update();
                            }
                        }, 10, 20*4);
                }
         }
     }

     @EventHandler
        public void onSignInteract(PlayerInteractEvent e){
            if(e.getAction() == Action.RIGHT_CLICK_BLOCK){
                if(e.getClickedBlock().getState() instanceof Sign){
                    Player p = e.getPlayer();
                    Sign s = (Sign) e.getClickedBlock().getState();

                    if(s.getLine(1).contains("Knockout-1")){
                        ByteArrayOutputStream b = new ByteArrayOutputStream();
                        DataOutputStream out3 = new DataOutputStream(b);

                        try {
                            out3.writeUTF("Connect");
                            out3.writeUTF("knockout");
                        } catch (IOException ex){
                            System.err.println("Es gab einen Fehler:");
                            ex.printStackTrace();

                        }

                        p.sendPluginMessage(plugin, "BungeeCord", b.toByteArray());

                    }

                }
            }
        }



}