Демон?
Не вопрос.

Вот пример из проекта, который я пишу

---------------------------
nata.cpp

...

JNIEXPORT jboolean JNICALL Java_nata__1fork
  (JNIEnv *, jobject){

jboolean result;
pid_t parpid;
          if((parpid=fork())<0)
            {            
             printf("\ncan't fork");
             exit(1);              
            }
          else if (parpid!=0)
          exit(0);      
          setsid();    
        printf ("%d",getpid());
         result=true;

return result;

}

...
----------------------------------
nata.java

    public native boolean _fork();

public nata()
{
    try
    {
    System.loadLibrary("nata");
    }catch (Exception e)
    {
    System.err.println ("ERROR:\nNo such library \"libnata.so\".\nCheck your library path.");
    }

}
...
public boolean fork ()
    {
    boolean res = false;
        try
        {
        res = _fork ();
        }catch (Exception e)
        {
        System.err.println ("ERROR:\nin library libnata.so: no such instruction \"_fork ()\"\ntry open nata.cpp and edit this void");
        res=false;
        }
    return res;
    }
----------------------------------------------
Пример собственно демона:
Server.java

import java.rmi.Naming;
import java.rmi.registry.Registry;
import java.rmi.registry.LocateRegistry;
import java.rmi.RemoteException;
import java.rmi.NotBoundException;
public class Server
{
    public final String SERVER_NAME = "NATA SERVER v0.1";

    public static void main (String[] args) throws Exception  
    {
new nata().fork(); //Тут и создается демон
    
            CS server = new CS(args[0]);
            server.runServer();
        
        

    }
}
-------------------