Java-RMI Lab

Outline

• Let first builds a simple home-made framework

• This is useful to understand the main issues

• We see later how java-rmi works and how it solves the same issues

Generic architecture

object A object BskeletonRequestproxy for B

Reply

CommunicationRemote Remote referenceCommunication

modulemodulereference module module

for B’s class& dispatcher

remoteclient server

servant

Local proxy: Remote methods appear as local Remote Reference Module: local-remote object mapDispatcher: Dispatch the request to a SkeletonSkeleton: Invoke the method on B; send back the reply

Implementation

object Aproxy for Counter

Remote Communication

modulereference module

client

We’d like something like this…

Client Object

Receive input

Call method on the proxy

Implementation

public int inc(int value){//marshal value

//prepare a request

//send the request to the Server//The address is in the//Remote Reference Module

//… on in a remote reference data structure

}

public int dec(int value){

}

object Aproxy for Counter

Remote Communication

modulereference module

client

Proxy…

Implementation Communication module…

object Bskeleton

Communication Remote reference

modulemodule

for B’s class

& dispatcher

remote

server

servant

CommunicationModuleRegister an object

(uses Object Table)

Initialize

Receive request

Dispatch to the Skeleton

Implementation Skeleton

object Bskeleton

Communication Remote reference

modulemodule

for B’s class

& dispatcher

remote

server

servant

Skeleton &Dispatcher

Initialize(bound to a Counter)

Receive request

Dispatch to the Object

Send the reply

Implementation details

• See RMI-LAB on the web site

• Exercise: add a new object…

Lesson learned

• Key (and borrowing) aspects• Managing communication • Managing remote reference • Implements support modules (proxy,

skeleton)

Java-rmi solution

• Managing communication– Embedded into the JVM

• Managing remote reference – Rmiregistry, remote reference layer

• Implements support modules (proxy, skeleton) – Proxy: automatically generated from the code – Skeleton: no longer needed thanks to

‘reflection’ (see later)

Lab1: Remote counterimport java.rmi.*; //Import rmi API

public interface Counter extends Remote //makes Counter a remote interface{int inc(int i) throws RemoteException; //manage or throws this exception….int dec(int i) throws RemoteException;

}

Step2: interface implementationimport java.rmi.*;

public class Counter_impl implements Counter{

private int counter;

public Counter_impl() throws RemoteException {counter = 0;}public int inc(int i) throws RemoteException {counter++;return counter;}public int dec(int i) throws RemoteException {counter--;return counter;}

}

import java.rmi.RemoteException;import java.rmi.server.UnicastRemoteObject;import java.rmi.registry.LocateRegistry;import java.rmi.registry.Registry;public class Server{

public static void main(String args[]) throws RemoteException{

//Create a remote object..Counter c = new Counter_impl() ;

Counter stub = (Counter)UnicastRemoteObject.exportObject(c, 0);

//bind "counter" to the stubRegistry registry = LocateRegistry.getRegistry();registry.rebind("counter", stub);System.out.println("Counter bound");

}}

Export object = It can receive requests

Client

import java.rmi.RemoteException;import java.rmi.registry.LocateRegistry;import java.rmi.registry.Registry;public class Client{

public static void main(String args[]){try{

Registry registry = LocateRegistry.getRegistry(args[0]);Counter c = (Counter) registry.lookup("counter");int i = c.inc(10);System.out.println(i);

}catch (Exception e){e.printStackTrace();}}

}

Running the example

• On the same machine via classpath– Run rmiregisrty: rmiregistry (make sure CLASSPATH is

unset)– Run the server: java -classpath . Server– Run the client: java -classpath . Client localhost

Class downloading

• JAVA-RMI allows to download the definition of an object's class if the class is not defined in the receiver's Java virtual machine.

• Classes definitions are typically made network accessible through a web server

Class downloading • Class downloading are

controlled by a Security manager

• Without a security manager installed, RMI will not download classes, other than from the local class path.

• This restriction ensures that the operations performed by downloaded code are subject to a security policy.

Installing Security Manager…if (System.getSecurityManager() == null) { System.setSecurityManager(new SecurityManager());

}…

java -Djava.rmi.server.codebase="http://xxxx. " -Djava.security.policy=“java.policy”

codebase = Where classes are network accessible

policy file

grant { permission java.security.AllPermission; };

LAB2

• Account example

LAB3

• Shared whiteboard