1. Java Remote Method Invocation (RMI)
In Java we implement object systems:
JVM 1
JVM 2
distribution
scheme
thread 1
thread 2
execution
scheme O1 O2 O3

2. Programs/objects running on different virtual machines:
separately compiled/linked
allocated in different address spaces
 object by value message passing
Approach: (very natural in object technology)
definition of behavior  interface
implementation of behavior  class

3. The concept client object client object server object server object
STUB
SKELETON
server
object
server
object

4. RMI architecture layers
client
server
stubs & skeletons
stubs & skeletons
RMI
system
remote
reference layer
remote
reference layer
transport layer

5. Stubs:
plays the role of PROXY
Skeletons:
helper class: generated automatically
(Java 2: new protocol + reflection classes
 no skeleton class)

6. Remote Reference Layer (RRL)
RRL defines the invocation semantics
Connections: unicast, point-to-point (currently)
Stages:
server instantiates service
exporting to RMI
optional: naming and registering
(Java 2: activable remote object (i.e. “on-demand instantiation”)

7. On TCP/IP: Java Remote Method Protocol (JRMP)
network cable
Transport layer
JRE
JRE
JRE
JRE
Host OS
Host OS
Network layer
Network layer
On TCP/IP: Java Remote Method Protocol (JRMP)
Other proprietary protocols are also in use!!
(Java 2 v1.3: RMI-IIOP (straight link to CORBA))

8. The big question: How can a client find a particular service?
Naming
The big question: How can a client find a particular service?
RMI registry
lookup
register
client
server
stubs & skeletons
stubs & skeletons
RMI

9. RMI can use various directory services.
RMI includes: rmiregistry service:
- the registry service runs on each machine, which
hosts remote objects and accepts requests for
services
- default TCP/IP port: 1099
Client: the RMI registry service provides lookup method
specification of required remote object service: URL
rmi://<host_name>
[:<name_service_port>]
/<service_name>

10. Using RMI
Components of an RMI based application:
interface definitions for remote services
implementations of remote services
stub and skeleton files
server(s) to host the services
RMI naming service
client program
optional: class file provider
Example: remote calculator service and client program

11. Step 1: write and compile service interfaces
import java.rmi.Remote;
import java.rmi.RemoteException;
public interface Calculator extends Remote {
public long add(long a, long b) throws RemoteException;
public long sub(long a, long b) throws RemoteException;
public long mul(long a, long b) throws RemoteException;
public long div(long a, long b) throws RemoteException; }

12. Step 2: write and compile service implementation
import java.rmi.server.UnicastRemoteObject;
import java.rmi.RemoteException;
public class CalculatorImpl extends UnicastRemoteObject
implements Calculator {
// Implementations must have an explicit constructor
// in order to declare the RemoteException exception
public CalculatorImpl() throws RemoteException {
super(); }
public long add(long a, long b) throws RemoteException {
return a + b;
public long sub(long a, long b) throws RemoteException {
return a - b;
public long mul(long a, long b) throws RemoteException {
return a * b;
public long div(long a, long b) throws RemoteException {
return a / b;

13. Step 3: generate stub and skeleton classes
RMI compiler: part of JDK
rmic CalculatorImpl
Result:
CalculatorImpl_Stub.class
CalculatorImpl_Skel.class

14. Step 4: write remote service class
import java.rmi.Naming;
public class CalculatorServer {
public CalculatorServer() {
try {
Calculator c = new CalculatorImpl();
Naming.rebind("rmi://localhost:1099/CalculatorService", c);
} catch (Exception e) {
System.out.println("Trouble: " + e); }
public static void main(String args[]) {
new CalculatorServer();

15. Step 5: write client program
import java.rmi.Naming;
import java.rmi.RemoteException;
import java.net.MalformedURLException;
import java.rmi.NotBoundException;
public class CalculatorClient {
public static void main(String[] args) {
try {
Calculator c = (Calculator)
Naming.lookup("rmi://localhost/CalculatorService");
System.out.println( c.sub(4, 3) );
System.out.println( c.add(4, 5) );
System.out.println( c.mul(3, 6) );
System.out.println( c.div(9, 3) ); }
catch (MalformedURLException murle) { <.. some code here ..>
catch (RemoteException re) { <.. some code here ..>
catch (NotBoundException nbe) { <.. some code here ..>
catch (java.lang.ArithmeticException ae){
<.. some code here ..>

16. Remarks on parameter passing in RMI
Parameters in a single JVM:
primitive types: by value
object types: “object reference by value”
Parameters in RMI:
object types: (of course) by value
 object serialization
local reference to a remote object:
reference to the proxy