RMI for FUN … 

This tutorial is not for an experienced individual who needs to know the pros and cons in RMI. This I write for anyone who likes to get a quick brief look up in to RMI. Let me explain in simple English what it does as this I write for a beginner who is just as I am ….

What is actually RMI (Remote Method Invocation)
We use RMI to invoke methods of an object that resides in another memory heap (another machine) through a network …. Just think how cool that would be.
And Oh yes we are just going to write a simple code for a simple scenario and see for ourselves …

What I want to do (scenario) :
Think that we need to monitor the current state (wind speed, wind direction and many more other data) of each wind turbine in a wind mill farm…. In every turbine, we have an application that maintains its own state and we from another location needs to monitor the state of each turbine REMOTELY...

Ok here's the code that how I do this... (will discuss after showing the artifacts)

Class diagram that depicts the server side

At the Wind turbine side (server side) ...

import java.rmi.Remote;
import java.rmi.RemoteException;

public interface RemoteWindMill extends Remote{

    public String getStatus() throws RemoteException;

}

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import java.rmi.registry.Registry;
import java.rmi.RemoteException;
import java.rmi.registry.LocateRegistry;
import java.rmi.server.UnicastRemoteObject;
import java.util.Date;

public class WindMillImpl implements RemoteWindMill {

    public WindMillImpl() throws RemoteException {
    }

    public String getStatus() {

        return " Wind Turbine no 001 : "
                + " \n location = Puttlam Srilanka"
                + "\n Time = " + new Date()
                + "\n Coordinates = 06'08''46'''N 81'06''47'''E"
                + "\n Model = M1500"
                + "\n Hub Height = 46m (151 ft)"
                + "\n Rotor Diameter = 43m (141 ft)"
                + "\n Wind Speed = 40kmph "
                + "\n Wind Dirrection = west to east ";


    }

    public static void main(String[] args) {

        try {
            WindMillImpl mill = new WindMillImpl();
            RemoteWindMill stub = (RemoteWindMill) UnicastRemoteObject.exportObject(mill,0);
            Registry registry = LocateRegistry.getRegistry();
            registry.rebind("RemoteWindMill", stub);

        } catch (Exception e) {
        }

    }
}

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At the monitoring side (client side) ...

import java.rmi.registry.LocateRegistry;

import java.rmi.registry.Registry;

public class WindMillMonitor {

    public static void main(String[] args) {

        new WindMillMonitor().monitor();

    }

    public void monitor() {

        try {

            System.out.println("Trying to call the remote method");

            Registry registry = LocateRegistry.getRegistry();

            RemoteWindMill mill = (RemoteWindMill) registry.lookup("RemoteWindMill");

            String s = mill.getStatus();

            System.out.println(s);

        } catch (Exception e) {

        }

    }

}

------------------------------------------------------------------------------- 

You can download the source code from the below link :
http://www.sendspace.com/file/a9z3sq

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In order to run the system..


step1: start rmiregistry in serverside

 step2: run WindMillImpl in serverside

step3: run WindMillMonitor in client side 

What we are really doing here is we are creating something called a stub which adheres to the interface of the RemoteWindMill and assign it in a registry. This stub is the one that is responsible to commuinicate through wires remotely and we don’t need to care about its content because UnicastRemoteObject.exportObject() will create and assign the stub in a registry for us.  This registry is almost like the white pages of a telephone directory where we find the address of a residence that having the phone number we search for.    Ok here’s a small diagram that depicts what I was talking…

In the diagram,

1. WindMill monitor will check for the stub in the RMI registry which resides in another VM. LocateRegistry.getRegistry(String host, int port) in the WindMillMonitor class is responsible for this. Here we don’t specify the IP address of the WindMillImpl hosting machine or the port which WindMillIImpl will be listening as the arguments of the method because this application was written to run on the same machine just for you to get an idea how this RMI really works.
2. Then by invoking registry.lookup("RemoteWindMill") it will search for the RemoteWindMill stub that has already been assigned to the RMI registry in the other VM. And a copy of  that stub is sent to the client side VM. The client sees a stub in it’s VM as almost like a WindMillImpl object because stub adheres to the RemoteWindMill interface.
3. So that the client will directly be talking to the stub thinking it’s the RemoteWindMillImpl object. But under the covers it’s the stub secretly calling the real RemoteWindMillImpl object residing in another VM via a cable.

Resources: 

• Head First : Design Patterns (Eric Freeman et al.)
• Getting started using RMI (http://download.oracle.com/javase/6/docs/technotes/guides/rmi/hello/hello-world.html)
• Image: Hambantota Wind Farm in SriLanka      (http://en.wikipedia.org/wiki/File:HambantotaWindFarm01.JPG)

How to avoid your desktop being idle, while you are not at the seat

As the title implies the aim of this project was to create a small app to make the desktop mouse move continuously for a specified time while you are away from the PC.

How to use the app

Below figure 1.1 shows a snap shot of the app. As you can see you can specify time duration in minutes (integer value) in the empty text field and press start to make your mouse move !!! …  You can only type digits on that text filed and if you try to add any characters in it, it will discard the characters instantly as you press the character key. You are not permitted to start the app while the text field is empty. If so the app will pop out a warning massage (figure 1.2) and will wait till you specify time duration in minutes.The progress bar will show how much time left for the app to continue. After the specified time is over the app will destroy itself giving the user full freedom to use its mouse again.

Figure 1.1 : Robo_Mouse App

Figure 1.2 : Warning massage 

It should be noted that after you start the app it takes the control of the mouse over from the user, and if you want to stop it, you can’t use the mouse now as it’s currently handled by the Robot Mouse app.

How to stop the running app

If you want to exit from the app before the app exits by itself simply press Alt and f4 keys together (Alt + f4).  If that doesn’t work for some reason go to task manager and kill the app. If something extremely goes wrong  as with most other apps restarting your machine will end up the controversy. (As this is just a simple program that has been written for my own interest I won’t take responsibility for any issues arise regarding any damage done to any PC or any electronic device).

Lessons I learnt from this…

In order to display time in every second three main distinct methods of coding been experimented.  Priority was given to the code that can use less objects and consume lesser memory space. Because its been assumed that this app would be used while there are important apps running on the machine.

As the application should run for a user defined time interval , the app should be using the memory  cleverly, otherwise creating many date objects for getting the current time would end up with lots of garbage in the heap. In order to monitor the memory consumption for the selected three alternative methods for updating time, I ran them separately in a while loop for 100000 times.

code snippets for three methods I experimented with are as follows..

        // method 1 ...

        int i = 0;

        Date d = null;

        Calendar c = Calendar.getInstance();

        while (i < 100000) {

            d = new Date();

            c.setTime(d);

            c.getTime();

            i++;

        }

-----------------------------------------------------------------------------------------------------

       // method 2 ...

       int i = 0;

       Calendar c = Calendar.getInstance();

        while (i < 100000) {

            c.setTimeInMillis(System.currentTimeMillis());

            c.getTime();

            i++;

        }

-----------------------------------------------------------------------------------------------------        

        

        // method 3 ...

        int i = 0;

        Calendar c = Calendar.getInstance();

        SimpleDateFormat formatter;

        java.util.Date utilDate = null;

        while (i < 100000) {

            int year = c.get(Calendar.YEAR);

            int month = c.get(Calendar.MONTH);

            int day = c.get(Calendar.DATE);

            int hour = c.get(Calendar.HOUR);

            int min = c.get(Calendar.MINUTE);

            int sec = c.get(Calendar.SECOND);

            String date = year + "/" + month + "/" + day + "  " + hour + ":" + min + ":" + sec;

            try {

                formatter = new SimpleDateFormat("yyyy/MM/dd hh:mm:ss");

                utilDate = formatter.parse(date);

            } catch (ParseException e) {

                System.out.println(e.toString());

            }

            i++;

        }

    }

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The results were significant…

Method for updating time	Used Heap  (Bytes)
Method 1	4,155,904 (depicted in figure 2.1)
Method 2	4,222,864 (depicted in figure 2.2)
Method 3	4,107,760 (depicted in figure 2.3)

Figure 2.3

Figure 2.1

Figure 2.2

It’s been evident & confirmed that creating Date objects in every loop seems costly in the methods. In method 1 creating Date objects was straight forward. In method 2, Date objects been created implicitly by the c.getTime(). I would rather so thank full to anyone who can give a far more better way of dealing this scenario avoiding creating Date objects. As you can see in the code snippets all those anonymous Date objects will be floating in the heap till GC (garbage Collector) collects them.

Resources :
http://www.odi.ch/prog/design/newbies.php
http://netbeans.org/download/magazine/01/nb01_profiler.pdf


you can download the source codes or the app from here ...
If you need to only run the app just download the below .jar file and double click it to execute...

Source codes : Source Codes of Robot Mouse
Robo_Mouse App (Robot_Mouse.jar) : Download RobotMouse App

Deal with evil through strength but affirm the good in man by trust ...