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Java RMI
A. Thirugnanasambandha Murthy
Asst. Prof (GL),
Thiruvalluvar University

Unit -2
Java RMI
What Is Java RMI?
• A distributed interoperability technology for Java
– Packages java.rmi, java.rmi.server and java.rmi.registry
– Tools rmic and rmiregistry
• Java’s answer to CORBA
– Java is the interface definition language
– Key limitation: both client and server must be written in Java
• Part of Enterprise JavaBeans
– Along with JDBC (Java DataBase Connectivity), Java Security, Java
Messaging Services, Java Transactions Services, Java IDL, other APIs
– like CORBA’s common object services
• Detailed documentation is available at
http://java.sun.com/
Java RMI = Java Remote Method Invocation

Unit -2
Java RMI
Distributed Architecture

Unit -2
Java RMI
Client/Server Programming
with Java RMI (I)
 Define an extension of interface
java.rmi.Remote
– Declare the methods the remote object exports
– Declare each such method to throw java.rmi.RemoteException
 Define a subclass of
java.rmi.server.UnicastRemoteObject that also
implements the extension of java.rmi.Remote
defined in step 1
 Write a server program that creates an instance
of the class defined in step 2
– Use synchronized wherever necessary to make server thread-safe
– The server program must register the instance with a registry service
– Default registry server is started by running rmiregistry

Unit -2
Java RMI
Client/Server Programming
with Java RMI (II)
 Compile server program with javac, and then use
rmic to generate stub and skeleton classes for
the remote object
– Stub and skeleton take care of all networking
– Limitation of rmic: the remote object class cannot be nested
 Write a client program that uses the remote
object
– The client uses class Naming to obtain a reference to the remote
object, using the interface name from step 1 (not the class name)
– The client invokes methods on the remote object through this
reference
 Run the server and then run the client

Unit -2
Java RMI
Client/Server Execution
with Java RMI (I)
Client Server
Client’s machine Server’s machine
Registry
Server

Unit -2
Java RMI
with Java RMI (II)
Client Server
Remote
Object
Registry
Server

Unit -2
Java RMI
with Java RMI (III)
Client Server
Remote
Object
Naming.rebind(…)
skeleton
Registry
Server

Unit -2
Java RMI
with Java RMI (IV)
Client Server
Remote
Object
Naming.lookup(…)
skeleton
stub
Registry
Server

0
Unit -2
Java RMI
with Java RMI (V)
Client Server
Remote
Object
skeleton
stub
Remote Method
Invocation
Registry
Server

1
Unit -2
Java RMI
Default RMI Registry
• Server on machine.com registers object with local
RMI registry using some string name
Naming.rebind(“remoteObj”, referenceToRemoteObj)
• Client refers to objects with URL
– Local client: rmi:///remoteObj
– Remote client: rmi://machine.com/remoteObj
– If client code contains
Naming.lookup(“rmi://” + objName)
» objName would be “/remoteObj” for local object
» objName would be “machine.com/remoteObj” for remote
object
• Registry is bound to port 1099 by default
– Accepts optional port number as argument
– Non-default port number must then be added to URL

2
Unit -2
Java RMI
Example:
Air Traffic Control
• Air traffic control is divided into several sectors
• Each sector is managed by a different controller
• Airplanes check in with a controller and then
periodically send the controller position reports
• The controller analyzes position reports and tells
an airplane when its position is too close to other
planes

3
Unit -2
Java RMI
Air Traffic Control:
ATC Exception
package ATC;
public class ATCException extends Exception {
public ATCException(String msg) { super(msg); }
}
• For server-specific exceptions
• Different from RemoteException, which is thrown as a
result of RMI failure

4
Unit -2
Java RMI
Position Report Data
package ATC;
public class PositionReport implements java.io.Serializable {
public int Latitude;
public int Longitude;
public int Heading;
public int Airspeed;
public PositionReport(int la, int lo, int h, int a) {
Latitude = la;
Longitude = lo;
Heading = h;
Airspeed = a;
}
}
• Java objects can be passed in RMIs, as long as they are
serializable
– Usually accomplished with an implementation of (empty) interface
Serializable
– Stub and skeleton take care of serialization/deserialization

5
Unit -2
Java RMI
Remote Controller’s
Interface
package ATC;
import java.rmi.*;
public interface ControllerInterface extends Remote {
public void CheckIn(int flight) throws RemoteException;
public void CurrentPosition(int flight, PositionReport pos)
throws RemoteException, ATCException;
}

6
Unit -2
Java RMI
Remote Controller Class (I)
package ATC;
import java.net.*; // for InetAddress
import java.rmi.*;
import java.rmi.server.*;
public class Controller extends UnicastRemoteObject
implements ControllerInterface {
protected String myName;
public Controller(String port, String name) throws RemoteException {
super();
try {
myName = "//" + InetAddress.getLocalHost().getHostName() +
":" + port + "/" + name;
Naming.rebind(myName, this);
}
catch (Exception ex) { System.err.println("Exception " + ex); }
}
…
}

7
Unit -2
Java RMI
Remote Controller Class (II)
public class Controller extends UnicastRemoteObject
implements ControllerInterface {
…
public void CheckIn(int flight) throws RemoteException
{
record existence of new flight
}
public void CurrentPosition(int flight, PositionReport pos)
throws RemoteException, ATCException
{
if (pos.latitude < -90 || pos.latitude > 90
|| pos.longitude < -180 || pos.longitude > 180
|| pos.heading < 1 || pos.heading > 360
|| pos.airspeed < 50 || pos.airspeed > 700)
throw new ATCException("flight " + String.valueOf(flight) +
": invalid position");
else if ( flight is too close to other airplanes )
throw new ATCException("flight " + String.valueOf(flight) +
": dangerous position");
}
}

8
Unit -2
Java RMI
ATC Server
package ATC;
import java.rmi.*;
public class ATCServer {
public static void main(String[] args) {
try {
Controller c = new Controller(args[0], args[1]);
}
catch (Exception ex) {
System.err.println("Exception " + ex);
System.err.println("usage: java ATCServer port# controllerName");
}
}
}

9
Unit -2
Java RMI
Airplane Client (I)
package ATC;
import java.rmi.*;
public class Airplane {
protected int myFlightNumber;
protected String myController;
protected PositionReport myPosition
= new PositionReport(34, -118, 180, 350);
try {
myFlightNumber = Integer.parseInt(args[0]);
myController = args[1];
ControllerInterface c = // The interface, not the class!
(ControllerInterface)Naming.lookup("rmi://" + myController);
c.CheckIn(myFlightNumber);
…
}
…
}
}

0
Unit -2
Java RMI
Airplane Client (II)
public class Airplane {
…
try {
…
for (;;) {
c.CurrentPosition(myFlightNumber, myPosition);
java.lang.Thread.sleep(200);
update position
}
}
catch (RemoteException ex)
{ System.err.println("RemoteException " + ex); }
catch (ATCException ex)
{ System.err.println("ATCException " + ex); }
catch (Exception ex) {
System.err.println("Exception " + ex);
System.err.println("usage: java Airplane flight# controllerName");
}
}
}

1
Unit -2
Java RMI
GNUmakefile
CLASSFILES=ATC/ATCException.class
ATC/ATCServer.class
ATC/Airplane.class
ATC/Controller.class
ATC/ControllerInterface.class
ATC/PositionReport.class
REMOTECLASS=ATC/Controller.class
RMIFILES=ATC/Controller_Skel.class
ATC/Controller_Stub.class
.SUFFIXES: .java .class
all: $(CLASSFILES) $(RMIFILES)
$(RMIFILES) : $(REMOTECLASS)
rmic ATC.Controller
mv -f *.class ATC
.java.class :
javac $<

2
Unit -2
Java RMI
Compiling the Example
elysees 1332> ls
ATC/ GNUmakefile
elysees 1333> ls ATC
ATCException.java Controller.java
ATCServer.java ControllerInterface.java
Airplane.java PositionReport.java
elysees 1334> gmake all
+elysees+ javac ATC/ATCException.java
+elysees+ javac ATC/ATCServer.java
+elysees+ javac ATC/Airplane.java
+elysees+ rmic ATC.Controller
+elysees+ mv -f Controller_Skel.class Controller_Stub.class ATC
elysees 1335> ls ATC
ATCException.class Controller.java
ATCException.java ControllerInterface.class
ATCServer.class ControllerInterface.java
ATCServer.java Controller_Skel.class
Airplane.class Controller_Stub.class
Airplane.java PositionReport.class
Controller.class PositionReport.java
elysees 1336>

3
Unit -2
Java RMI
Running the Example:
Local Client and Server
Server on elysees (using the default registry port number):
Client on elysees:
elysees 1355> java ATC.Airplane 100 /LosAngeles
ATCException ATC.ATCException: flight 100: invalid position
elysees 1356> java ATC.Airplane 100 elysees.ics.uci.edu/LosAngeles
elysees 1357>
elysees 1352> rmiregistry &
[1] 5398
elysees 1353> java ATC.ATCServer 1099 LosAngeles &
[2] 5407
elysees 1354>

4
Unit -2
Java RMI
Running the Example:
Remote Client and Server
Server on elysees (using a non-default registry port number):
Client on octavian:
octavian 1356> java ATC.Airplane 100 /LosAngeles
RemoteException java.rmi.ConnectException: Connection refused
to host: [octavian.ics.uci.edu:1099]; nested exception is:
java.net.ConnectException: Connection refused
octavian 1357> java ATC.Airplane 100 elysees.ics.uci.edu/LosAngeles
RemoteException java.rmi.ConnectException: Connection refused
to host: [elysees.ics.uci.edu:1099]; nested exception is:
java.net.ConnectException: Connection refused
octavian 1358> java ATC.Airplane 100 elysees.ics.uci.edu:1033/LosAngeles
octavian 1359>
elysees 1352> rmiregistry 1033 &
[1] 5398
elysees 1353> java ATC.ATCServer 1033 LosAngeles &
[2] 5407
elysees 1354>

5
Unit -2
Java RMI
Other Possible
Architectures
• A program or remote object can act as both a
client and a server
• Example: 3-Tiered Client/Server
Client
Program
Legacy
Program
Remote
Data
Object
Remote
Data
Object
Server
Program
Remote
Business
Object
Remote
Business
Object

6
Unit -2
Java RMI
Principal Similarities
Between CORBA and RMI
• Suitable for distributed, object-oriented,
client/server systems
• Synchronous interaction via remote procedure
call (RPC)
• RPC implemented via client stubs and server
skeletons, which hide networking and data
representation
• Objects can be both client and server

7
Unit -2
Java RMI
Principal Differences
Between CORBA and RMI
• CORBA
– Independent of implementation language
– Operation invocations can be formed statically or dynamically
– Object implementation binding can be static or dynamic
– The CORBA object adapters define various object execution
semantics
– Many integrated object services are available
• RMI
– Requires objects to be programmed in Java
– Objects are platform independent
– Operation invocations are formed statically
– Mapping of names to objects is static
– Many object services are being defined within Enterprise JavaBeans
– Mobile code support

8
Unit -2
Java RMI
Sidebar Discussion:
Dynamic Proxies in Java
• Introduced in J2SEv1.3 (JDK1.3)
• Created using the java.lang.reflect.Proxy class
• Allows dynamic creation of objects that
implement an arbitrary interface class without
writing code
• All methods are called through a single invoke()
method, which you implement:
– public Object invoke(Object proxy, Method method, Object[] args)
throws Throwable
• You can get a proxy with a call like this:
– ActionListener listener =
(ActionListener)Proxy.createProxy(myInvocationHandler,
ActionListener.class);
» Some detail omitted for clarity

9
Unit -2
Java RMI
Why are Dynamic Proxies
useful?
• A generic “listener” for Java events
– Consider a generic debugging framework for Swing events that can
print every event to the screen
» Don’t need to create a new implementation for each type of
listener (ActionListener, WindowListener, MouseListener, etc.)
• Dynamically create stubs for an RMI-like
middleware
– This is implemented in a middleware called LJM (Lightweight Java
Middleware), which ships with ArchStudio 3
– LJM allows you to do basically what RMI does except without
having to use rmic
– LJM does not currently support mobile code aspects of RMI

0
Unit -2
Java RMI
Discussion
• How would we extend the example to allow a
Controller to give an Airplane instructions for
avoiding a dangerous position?
• How would we extend the example to allow the
Controller to indicate an ATC exception condition
outside of a call to CurrentReport()?
• How would we extend the example to allow the
Controller to tell the Airplane to check in with a
different Controller?

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