Crash course of Java RMI
Here is a simple crash course of RMI api provided by Java.
- RMI is basically implementation of Remote Procedural Call
- It differs from CORBA which is used for communication between two different languages.
- These days we have a new implementation of RMI which is RMI/IIOP (RMI over Internet Inter-ORB protocol). This implementation internally uses CORBA.
| Server | Client |
| Create and interface that extends java.rmi.Remote interface provided by Java api. | Create a client which has information about the Remote interface that server has created. But it does not have any clue about the implementation of that interface. |
| Create a class which implements that interface to act as a remote object and also extends java.rmi.server.UnicastRemoteObject. By extending this makes sure that all the things that need to be done behind the scene to work as a remote object are performed by UnicastRemoteObject. This class will override a constructor which throws java.rmi.RemoteException because super class does the same. | Using JNDI it will lookup for the registered objects and will access the required method. |
| Create a server that binds this remote object with the RMI registry using JNDI. | |
| Create stub and skeleton of remote object using rmic command. | |
| Start the registry using rmi registry command | |
| Run the server object. |
Important things that needs to be considered:
(1) Security is one thing that can stop you from accessing Remote object in that case you can play with the Security manager or can create a simple policy file with contents as shown below:
grant codeBase “file:C:/RMI Tutorial/-” {
permission java.security.AllPermission;};
Then run the Java class using following command
java -Djava.security.manager -Djava.security.policy=policy-file RemoteServer
(2) If any method of the Remote Interface has input/output parameters as an Object. That object must be Serializable. That’s where serialization comes in to picture!
FaceMash algorithm from Social network movie.
Hi All,
I saw social network movie and next day when I was discussing this movie with few of my colleagues we were thinking about the algorithm which Mark had taken from one of his friend who use it for Chess game. If you have seen that movie you will surely remember that scene. No body among us was sure what was that algorithm and how does it help in matching girls!
I thought I must search that and found some good points about that. Facemash is an algorithm which is actually based on another algorithm called Elo Rating System.
Wiki says:
The Elo rating system is a method for calculating the relative skill levels of players in two-player games such as chess. It is named after its creator Arpad Elo, a Hungarian-born American physics professor
Algorithm:
Now according to this algorithm following repersents the expected score of Player A
Similarly the expected score for Player B is
Where RA is current rating of Player A and RB is current rating of player B.
When player A will play the match what ever score he will have will be compared with the expected score and that will give a new rating for player A. This will be done using the below formula;
Where SA is the actual score of player A. K is a constant that has two values for Master players K=16 and for weaker players K will be 32.
Now let us take an ecample of rating girls as done in the Social Network movie. For example we have the following table:
| Girl | Current Rating | Result | Score |
| A | 80 | ||
| B | 90 | Lost | 0 |
| C | 30 | Won | 1 |
| D | 50 | Draw | 0.5 |
| E | 40 | Won | 1 |
Now as you can clearly see that A was comapred with 4 girls and she has scored 2.5 points. If you calculate the Expected score using the above formula it will be
2.157(0.485+0.572+0.543+0.557).
Now using the second formula given above we will calculate the new Rating for player A using K = 16 will be
R’A = 80 + 16(2.5-2.157) = 85.2
So this was all about that algorithm. This is widely used in games like football etc.
I hope you guys will like it I will be waiting for your comments 
Building simple WYSIWYG editor(Coming Soon!!)
Coming Soon!!
Java Exceptions…expecting the unexpected(Coming soon!)
Coming Soon!!
Object A….present sir!!(Proxy object ;)) (Coming soon!!)
Coming soon!!
Have a Bite of Bytecode!!
This topic is very different from other Java topics. I am sure a lot of developer think that why should I learn about byte code at all. Well I can give only two reasons of that
(1) Either you are a Byte code engineer for example using a library like BCEL which is used to change/enhance byte code generated by Java compiler.
(2) You are passionate to learn thing under the hood (Under the hood is used mostly by Bill Venners a great writer of a great book Inside the Java Virtual machine worth reading it.)
What is byte code after all??
Just like C and C++ generates assembler code. Java compiler generates byte codes. This byte code is used by JVM to perform instructions written by Programmer in Java language.
What byte code consists of?
Byte code is basically combination of instructions to be executed at run time by JVM. The main things byte code has:
- Opcodes: Opcode is an instruction which will get executed by JVM at run time.
- Information of reference to the objects
- Information about the local variables
There is one command line tool given by sun which comes with Java installer. That command line tool is used to read byte codes generated by Java compiler. For Example we have following java code:
public class BiteOfByteCode{
public void sayHello(String name){
System.out.println(name);
}
}
After compilation following command will give you result as shown below in screen shot.
javap -c BiteOfByteCode
Now as you can see above things doesn’t look familiar at all. Everything under Code: is byte codes generated by javac command i.e. Java compiler. You can see that some weird start of commands. For example any one can understand that load_0 is trying to load something but what the hell is a at the beginning of that command??
This is the character which JVM used to understand the type of variable.
Here is one table that explains few of them:
| Character | Description |
| a | This means the loaded variable is an object |
| i | Integer |
| f | Float |
| c | Char |
| b | Byte |
Before understanding the byte code let’s understand the memory management of JVM. JVM is stack based. For each method it has a separate area called Frame. Below we have a sample display of a frame:
I like to draw such pictures on the board and use them for articles.
Each frame has main 3 components:
- Local Variable array: This array consists of all the local variables/ reference to the local variables and also it consists of a reference to this (Object on which method is invoked) stored at index 0. In case of static method first parameter value will be stored at index 0.
- Oprand stack: This is used by all the instruction to perform specific tasks. For example when an instruction will have to add two variables the data will be popped from the local array of variables and pushed in to the operand stack and result will be pushed back.
- Reference to the Constant pool: We all know that in java we have a concept called constant pool for example for java.lang.String. This reference refers to the constant pool required for the method.
Let’s go through the byte codes now
In the first part it executes constructor of class. As you can see that BiteOfByteCode class does not extend any class so it will extend java.lang.Object class implicitly.
aload_0: This command fetches reference of this from the local array and pushes it to the operand stack. As mentioned earlier reference to this is stored at index 0. That is why it says load_0. Where 0 represents the index of local array and “a” at the beginning represents that instruction is applicable to the object reference.
invokespecial #1: will invoke constructor of super class in our case java.lang.Object. #1 is used to build an index into the runtime constant pool of the class where the reference to name is stored.
From JVM specification:
Runtime Constant Pool
A runtime constant pool is a per-class or per-interface runtime representation of the
constant_pooltable in aclassfile (§4.4). It contains several kinds of constants, ranging from numeric literals known at compile time to method and field references that must be resolved at run time. The runtime constant pool serves a function similar to that of a symbol table for a conventional programming language, although it contains a wider range of data than a typical symbol table.Each runtime constant pool is allocated from the Java virtual machine’s method area (§3.5.4). The runtime constant pool for a class or interface is constructed when the class or interface is created (§5.3) by the Java virtual machine.
return: This will simply return from the constructor.
Let’s look into the method’s byte code now:
getstatic: This will get static object of out because in our example we have used System.out to print value passed to the method.
aload_1: This is similar to the very first instruction in constructor. This will pop the value from local array at index 1 and will load in the operand stack In this case value at index 1 is String object reference which is passed as parameter to the method. Let us say we have int variable instead of String then this instruction will be iload_0 where I represents that Integer needs to be loaded.
Then last one will simply make a call to println method.
This is all about the above byte code structure.
One more thing which is worth knowing is the values coming on the left hand for example in case of constructor first opcode is at 0 then next one at 1 then 4. One may ask how come 4 after 1. This is fairly simple to understand this is basically index as you can see first one takes only 1 memory location second one takes 3 (one for invokespecial, second for #1 and third of object reference).
All this is really very basic idea and there is much more to learn about byte code. I hope you must have enjoyed this tutorial and I am looking forward for your comments/feedback. Keep visiting my blog I will do my best to make it worth reading it.
