Java rmi

JAVA RMI – INTRODUCTION
RMI stands for Remote Method Invocation. It is a mechanism that allows an object residing in one
systemto access/invoke anobject running on another JVM.
RMI is used to build distributedapplications; it providesremote communication between Java
programs. It is providedin the package java.rmi.
Architecture of an RMI Application
In an RMI application, we write two programs, a server programand a client program.
Inside the server program, a remote object is createdand reference of that object is made available
for the client .
The client program requeststhe remote objectson the server and tries to invoke its methods.
The following diagram shows the architecture of an RMI application.
Let us now discuss the componentsof this architecture.
Transport Layer − Thislayer connectsthe client and the server. It manages the existing connection
and
also sets up new connections.
Stub − A stub is a representationof the remote object at client. It resides in the client system; it acts
as a gateway for the client program.
JVM
residesontheserver
residesontheclient
usingtheregistry
Skeleton− This is the object which resides on the server side. stub communicateswith this skeleton
to pass request to the remote object.
RRL − It is the layer which manages the referencesmade by the client to the remote object.

Working of an RMI Application
The following points summarize how an RMI application works−
When the client makes a callto the remote object, it is receivedby the stub which eventually passes
this request to the RRL.
When the client-side RRL receivesthe request, it invokesa method called invoke of the object
remoteRef. It passes the request to the RRL on the server side.
The RRL on the server side passes the request to the Skeletonwhich finally invokesthe
requiredobject on the server.
The result is passed all the way back to the client.
Marshalling and Unmarshalling
Whenever a client invokesa method that acceptsparameters on a remote object, the parameters are
bundled intoa message before being sent over the network. These parameters may be of primitive
type or objects. Incase of primitive type, the parametersare put together and a header is attached to
it. In case the parameters are objects, then they are serialized. Thisprocessis known as marshalling.
At the server side, the packed parametersare unbundled and then the required method is invoked.
Thisprocessis known as unmarshalling.
RMI Registry
RMI registry is a namespace on which all server objectsare placed. Each time the server createsan
object, it registersthis object with the RMIregistry (using bind or reBind methods). These are
registeredusing a unique name known as bind name.
To invoke a remote object, the client needs a reference of that object. At that time, the client fetches
the object from the registry using its bind name (using lookupmethod).
The following illustration explains the entire process−
RemoteReferenceLayer proxyontheserver
Goals of RMI
Following are the goals of RMI −
To minimize the complexity of the application.
To preserve type safety.
Distributed garbage collection.
Minimize the difference between working with localand remote objects.
To developan RMI Application we require the followingclasses and interfaces.
1.interface
2.Implementation class
3.Server Application
4.Client Application.
Defining Interface.
Add.java
import java.rmi.*;
public interface Addextends Remote
{
public int sum(int a,int b) throws RemoteException;
}

DefiningImplementationClass.
AddImpl.java
importjava.rmi.*;
importjava.rmi.server.*;
publicclassAddImpl extendsUnicastRemoteObjectimplementsAdd
{
publicAddImpl()throwsRemoteException
{
super();
}
publicintsum(inta,intb) throwsRemoteException
{
intc=a+b;
returnc;
}
}
ServerProgram
AddServer.java
importjava.rmi.*;
importjava.rmi.server.*;
publicclassAddServer
{
publicstaticvoidmain(Stringargs[])
{
tTry
{
Addob=newAddImpl();
Naming.rebind(“khaja”,ob);
System.out.println(“Serverready”);
}
catch(Exceptione)
{
System.out.println(e);
}
}
}

ClientProgram
AddClient.java
importjava.rmi.*;
importjava.util.*;
publicclassAddClient
{
publicstaticvoidmain(Stringargs[])
{
try
{
Addob=(Add)Naming.lookup(“khaja”);
Scannersc=newScanner(System.in);
System.out.println(“Enterthe FirstNumber“);
inta=sc.nextInt();
System.out.println(“Enterthe Second Number“);
intb=sc.nextInt();
intc=ob.sum(a,b);
System.out.println(“sum=”+c);
}
catch(Exceptione)
{
System.out.println(e);
}
}
}
CompilationandExecutionSteps
Step1:compile 4 .javafiles
Step2:create stubsandskeleton(optional fromjdk1.5)
>rmic AddImpl
Step3:startrmiregistry
>start rmiregistry
Step4:startserver
>start javaAddServer
Step5:Run Client
>javaAddClient

Java rmi

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