bernielewis-rmi

8/8/2019 BernieLewis-RMI

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Remote Method Invocation in Java

Bennie Lewis

EEL 6897


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AGENDA

Introduction

What is RMI

The Goals of RMI

The RMI system architecture

How RMI works

Distributed garbage collection

Security

Conclusion

Questions


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IntroductionRMI

Javas RMI is an alternative to low level sockets. A

remote method invocation is a form of the RPC that isavailable on other systems. Instead of creating objects

on local machines you create some of the objects onother machines an you communicate with those

objects as you would normally would with local objects


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WHAT IS RMI

RMI is a core package of the JDK 1.1 and above that canbe used to develop distributed application.

It enables software developers to write distributed

applications in which the methods of remote objects canbe invoked from other JVMs


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The Goals of RMI

Support seamless remote invocations on objects in

different java virtual machines.

Support callbacks from servers to clients.

Integrate the distributed object model into the Java

language in a natural way while retaining most of the Java

languages object semantics. Make differences between the distributed object model

and the local java object model apparent.

Make writing reliable distributed applications as simple aspossible.

Preserve the safety provided by the java sun real timeenvironment.


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The RMI system architecture

The RMI system is built in three layers

The stub/skeleton layer

The remote reference layer

The transport layer


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The stub/skeleton layer

The stub/skeleton layer is the interface between theapplication layer and he rest of the system. When a serverapplication is developed. Stubs and skeletons are

generated using the RMI rmic compiler, which generatesproxy classes from the servers bytecodes The stub/skeleton

layer transmits data to the remote reference layer via theabstraction of marshal streams which use objects

serialization. Therefore. This layer does not deal with thespecifics of any transport.


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The remote reference layer

The remote reference layer is a middle layer between thestub/skeleton layer and the transport layer. This layer isresponsible for providing the ability to support varying

remote reference or invocation protocols independent ofthe client stubs and server skeletons.


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The Transport layer

The transport layer is the low-layer that ships marshalstreams between different address spaces. The transportlayer is responsible for setting up connections to remote

address spaces, managing connections, listening forincoming calls, maintaining a table of remote objects that

reside in the same address space, setting up aconnections to this dispatcher.


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How RMI works

When a client invokes an object implementation from theserver the three layers of RMI come in the play. The mostimportant layer to the programmer is the stub/skeleton

layer. RMI comes with an rmic compiler that generatesstubs and skeletons from user defined remote interfaces.

Basically, stubs are client proxies and skeletons are server-side entities. Stubs will allow clients to communicate with

the other layers of the system. This is done automatically asyou will have to in inherit from RMI classes.


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Example package RMI.hello;

import java.rmi.Naming;

import java.rmi.RemoteException;

import java.rmi.RMISecurityManager; import java.rmi.server.UnicastRemoteObject;

public class RMIserver extends UnicastRemoteObject implements Hello

{

public RMIserver() throws RemoteException

{

super();

}

public String sayHello()

{

return "Bennie Lewis Hello World!";

}

public static void main(String args[])

{

if(System.getSecurityManager() == null)

{

System.setSecurityManager(new RMISecurityManager());

}

try

{

RMIserver obj = new RMIserver();

Naming.rebind("//localhost/HelloServer",obj);

System.out.println("HelloServer bound in registry");

}

catch(Exception e)

{

System.out.println("RMIserver err: " + e.getMessage());

e.printStackTrace();

}

}

}


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Example cont. package RMI.hello;

import java.applet.Applet;

import java.awt.Graphics;

import java.rmi.Naming;


public class RMIclient extends Applet

{

String message = "blank";

Hello obj = null;

public void init()

{

try {

obj = (Hello)Naming.lookup("//" + getCodeBase().getHost() + "/HelloServer");

message = obj.sayHello();

}

catch(Exception e)

{

System.out.println("HelloApplet exception:" + e.getMessage());

e.printStackTrace();

}

} public void paint(Graphics g)

{

g.drawString(message,25,50);

}

}


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Example cont.

package RMI.hello;

import java.rmi.Remote;


public interface Hello extends Remote

{ String sayHello() throws RemoteException;

}


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Distributed Garbage Collection

When stand-alone applications are developed using java,objects that are no longer referenced by any client areautomatically deleted. This is a desirable feature when

developing distributed applications. The RMI systemprovides a distributed garbage collector that

automatically deletes remote objects that are no longerreferenced by any client.


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Security

There are a number of security issues that you should beaware of when developing mission-critical systems in RMI.

There is no authentication; a client just requests and object

(stub), and the server supplies it. Subsequentcommunication is assumed to b from the same client.

There is no access control to the objects

There are no security checks on the RMI registry itself; anycallerIs allowed to make request.

There is no version control between the stubs and the

skeletons, making it possible for client to use a down-levelstub to access a more recent skeleton breaking release-to-

release binary compatibility


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Conclusion RMI is an alternative to low level sockets

RMI is a core package of the JDK 1.1 and above that canbe used to develop distributed application.

The RMI system is built in three layers The stub/skeleton

layer, The remote reference layer, The transport layer


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References

Qusay H. Mahmoud,(1999). Distributed Programming withJava. Greenwich CT: Manning Publications Co.

http://java.sun.com/javase/technologies/core/basic/rmi/i

ndex.jsp


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Questions

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