public class Instances {
    
    /**
     * Async result
     */
    public static interface Reply {
        /**
         * @param result
         */
        void set(Serializable result);
    }

    /**
     * Interface to receive object from another instance
     */
    public static interface ReceiverInterface {
        /**
         * Callback function when message arrived.
         *
         * You could not block the callback thread, or the communication will be interrupted.
         *
         * @param frominstance pid of the sender instance
         * @param object java object received
         */
        public void onReceive(int frominstance, Serializable object);
    }
    
    /**
     * Get the first (pid) of previous instances
     * @return pid of first instance
     */
    public native static int getFirstInstance();
    
    /**
     * Get current instance (pid)
     * @return pid of current instance
     */
    public native static int getCurrentInstance();
    
    /**
     * Get all instances (pid) of current program
     * @return array of pid, the first one is the first instance, the second is the second, and so on
     */
    public native static int[] getInstances();
    
    /**
     * Send object to the first instance
     * @param object object to send
     */
    public static void sendObject(Serializable object) {
        sendObject(getFirstInstance(), object);
    }
    
    /**
     * Send object to specific instance (pid)
     * @param toinstance pid of target instance
     * @param object object to send
     */
    public static void sendObject(int toinstance, Serializable object) {
        sendObject(toinstance, object, null);
    }
    
    /**
     * Send object to specific instance (pid)
     * @param toinstance pid of target instance
     * @param object object to send
     * @param reply async callback
     */
    public native static void sendObject(int toinstance, Serializable object, Reply reply);
    
    /**
     * The reply should be 3rd parameter of onReceive().
     * But for compatibility, get the reply during running of onReceive(),
     * you can do the reply later asynchronously but you need to get the instance before return of onReceive()
     * @param object
     */
    public native static Reply getReply();
    
    /**
     * Wait the object to be sent by sendObject().
     * Usually it's not required to flush() unless it's going to exit.
     */
    public native static void flush();
    
    /**
     * Set the receiver instance
     * @param thereceiver the receiver instance
     */
    public native static void setReceiver(ReceiverInterface thereceiver);
}

package hello;

import java.io.Serializable;
import java.util.Arrays;

import com.regexlab.j2e.Instances;

/**
 * To control the single instance. The second instance will send its arguments to
 * the first instance and then exits.
 */
public class HelloSingleInstance implements Instances.ReceiverInterface {

	public static void main(String[] args) throws InterruptedException {
		
		// set the Receiver
		Instances.setReceiver(new HelloSingleInstance());
		
		// check whether this is the first instance
		int first   = Instances.getFirstInstance();
		int current = Instances.getCurrentInstance();
		
		if(first == current) {
			System.out.println("This is the first instance");
		}
		
		// else this is NOT the first instance
		else {
			System.out.println("This is NOT the first instance.");
			
			// send the 'args' to the first instance 
			Instances.sendObject(first, args);
			
			// flush() before it's going to exit, usually flush() is not required
			Instances.flush();
			
			// then exit
			System.exit(1);
		}
		
		// your main task of your program
		System.out.print("Sleeping 200 seconds for TESTing ");
		for(int i=0; i<200; i++) {
			System.out.print(".");
			Thread.sleep(1000);
		}
	}

	public void onReceive(int frominstance, Serializable object) {
		String [] args = (String[])object;
		
		// you can do something with the args
		System.out.println("\nAnother instance ("+frominstance+") launched and his args = " + Arrays.asList(args));
	}

}