How can I use "/server " start/stop/restart any server on my waterfall server?

I have this code in my ServerStartStop.java

 package eu.pandacrew.pandacrewnetwork.Commands; import net.md_5.bungee.api.ChatColor; import net.md_5.bungee.api.CommandSender; import net.md_5.bungee.api.ProxyServer; import net.md_5.bungee.api.plugin.Command; import java.io.IOException; public class ServerStartStop extends Command {   public ServerStartStop() {       super("server", "pandacrew.command.server");   }   @Override   public void execute(CommandSender sender, String[] args) {       if (args.length >= 2) {           String action = args[0].toLowerCase();           String serverName = args[1];           switch (action) {               case "start":                   startServer(sender, serverName);                   break;               case "stop":                   stopServer(sender, serverName);                   break;               case "restart":                   restartServer(sender, serverName);                   break;               default:                   sender.sendMessage(ChatColor.RED + "Invalid command. Use /server <start|restart|stop> <server>");                   break;           }       } else {           sender.sendMessage(ChatColor.RED + "Invalid number of arguments. Use /server <start|restart|stop> <server>");       }   }   private void startServer(CommandSender sender, String serverName) {       // Implement code here to start the server       try {           Runtime.getRuntime().exec("java -jar /opt/PandaCrew/" + serverName + "/spigot.jar");           sender.sendMessage(ChatColor.GREEN + "Server '" + serverName + "' started!");       } catch (IOException e) {           sender.sendMessage(ChatColor.RED + "Error starting the server. Check the server configuration.");       }   }   private void stopServer(CommandSender sender, String serverName) {       // Was für einen code hier?       sender.sendMessage(ChatColor.GREEN + "Server '" + serverName + "' stopped!");   }   private void restartServer(CommandSender sender, String serverName) {       // Was für einen code hier?       sender.sendMessage(ChatColor.GREEN + "Server '" + serverName + "' restarted!");   } }

I want to have a plugin on my root server (network in /opt/PandaCrew), where I have a waterfall server and a few spigot servers, that can start and stop servers. My code doesn't work when I try to start the server. I also have no idea how to stop or restart it. Can someone help me?
Thanks in advance!