JAVA MDU DISTRIBUTED

1. UNIT 4 DISTRIBUTED OBJECTS
1) Distributed Object Model
Remote Object - an object whose methods can be invoked from another
Virtual Machine.
Remote Interface - a Java interface that declares the methods of a
remote object.
Remote Method Invocation (RMI) - the action of invoking a method of
a remote interface on a remote object.
2) How Client Server Communicates in Distributed
Systems OR Roles of Client & Server in
Distributed Systems
In the traditional client/server model, the client
translates the request to an intermediary
transmission format & sends the request data to the server.The server
parses the request format,computes the response & formats the
response for transmission to the client.The client then parses the
response & displays it to the user.Now you have to design a
transmission format & you have to write code for conversion between
your data & the transmission format.Now what we want is a
mechanism by which the client programmer makes a regular method
call,the problem is that the object that carries out the work is not
located in the same virtual machine.The solution is to install a proxy
for the server object on the client.The client calls the proxy,making a
regular method call.The client proxy contacts the server.
similarly the programmer of the server object doesn’t want to fuss
with the client communication.The solution is to install a second proxy
object on the server.The server proxy communicates with the client
proxy & it makes regular method calls to the server objects.

2. How do proxies communicate with each other ?
a) RMI (Remote Method Invocation) – It supports method calls b/w
distributed java objects.
b) CORBA (Common Object Request Broker Architecture) – It supports
method calls b/w objects of any programming language.It uses the Internet
Inter-ORB protocol, or IIOP, to communicate between objects.
c) SOAP (Simple Object Access Protocol) – It is programming language
neutral.It supports method calls through web services.
( draw diagram of this on page no.279 of ur book)
3) Remote Method Invocation (RMI)
If you have an access to an object on a different machine,
you can call methods of the remote object.of course, the method
parameters must somehow be shipped to the other machine,the server
must be informed to execute the method & the return value must be
shipped back.
In RMI, the object whose methods makes the remote call is called the
client object. The remote object is called the server object.The
computer running the java code that calls the remote method is the
client for that call.
The computer hosting the object that processes the call is the server for
that call.
Registration (binding)
A server can register its remote objects with a naming
service – the rmiregistry.Once registered, each remote
object has a unique URL.
Obtaining a remote object reference.

3. A client can use the naming service to lookup a URL to obtain a
remote object reference.
 The application can pass and return remote object references
as part of its operation.
(draw diagram of this on page no.281 of ur book)
Stubs & Parameter Marshalling :
when client code invokes a remote method on a remote object, it
actually calls an ordinary method on a proxy object called a stub.The stub
resides on the client machine.The stub packages the parameters used in
the remote method into a block of bytes. This packaging uses a device
independent encoding for each parameter.
The process of encoding the parameters is called parameter
marshalling.The Purpose of parameter marshalling is to convert the
parameters into a format suitable for transport from one m/c to another.
Stub functions include:
a) Initiating a call to the remote object.
b) A description of the method to be called.
c) The marshalled parameters.
d) Unmarshals the return value or exception from the
server.
e) Return value to the caller.
Skeleton
A proxy object on server side is called Skeleton.
Skeleton function includes:
a) It unmarshals the parameters.
b) It locates the object to be called.
c) It calls the desired method.

4. d) It captures & marshals the return value or
exception of the call.
e) Returns marshalled data back to the stub.
( draw diagram of this on page no.282)
This process is complex, but it is completely automatic & to a large extent
transparent to the programmer.
Dynamic Class Loading
When u pass a remote object to another program, either as
a parameter or return value of a remote method, then
that program must have the class file for that object.
For ex: consider a method with return type Product.The client program
needs the class file product.class to compile,but now suppose server
constructs & return Book object & Book is a subtype of Product.Client
have no idea where to find the class file Book.class. Therefore a class
loader that will load required classes from the server is required.RMI uses
a special RMIClassLoader that loads at runtime the classes required for
remote invocations.whenever a program loads new code from another
network location,there is a security issue.For that reason,you need to use
a security manager in RMI client applications.This is a safety mechanism
that protects the program from viruses in stub code.For specialized
applications,programmers can substitute their own class loaders &
security managers.

5. RMI Architecture
Client Virtual Machine Server Virtual Machine
Client Remote
Object
Skeleton
Stub Server
RmiRegistry
Registry Virtual Machine

6. RMI Flow
1. ServerVirtual Machine Object
Client Creates Remote Server Virtual Machine
2. Server Registers Remote Object
Client Remote
Object
1
Skeleton
Stub Server
2
RmiRegistry
Registry Virtual Machine

7. RMI Flow
Client Virtual Machine Server Virtual Machine
Client Remote
3. Client requests object from Registry
Object
4. Registry returns remote reference
(and stub gets created)
Skeleton
Stub Server
3 4
RmiRegistry
Registry Virtual Machine

8. RMI Flow
Client Virtual Machine Server Virtual Machine
Client Remote
Object
5 7
6
Skeleton
Stub Server
5. Client invokes stub method
6. Stub talks to skeleton
7. Skeleton invokes remote object
method “Fred”
Registry Virtual Machine
4) Setup for Remote Method Invocation (RMI)
1) Defining a remote interface.
2) Implementing the remote interface.
3) Write the server program to create server objects.
4) Write the client program that uses the remote object.
5) Generating stubs & skeletons.
6) Starting the registry & registering the objects
7) Running the server & client.
1. Define the Remote Interface –
Your client program needs to manipulate server objects,but it doesnot
actually have copies of all of them.

9. The objects themselves reside on the server.The client code must still
know what it can do with those objects.Their capabilities are expressed
in an interface that is shared between the client & server & so resides
simulateneously on both machines.
To define the remote service, we write a remote interface.All interfaces
for remote objects must extend Remote interface defined in java.rmi
package.
All methods in those interfaces may throw a RemoteException.Such
exceptions are thrown during a remote method call if there is a network
problem
import java.rmi.*;
public interface Hello extends Remote
{
public String sayHello() throws RemoteException;
}
2. Implementing the Remote Interface
On the server side,you must implement the class that
actually carries out the methods advertised in the remote
interface.Declare the remote interface being implemented.
You can tell that the class is a server for remote methods because it
extends UnicastRemoteObject,which is a concrete java class that makes
objects remotely accessible. A UnicastRemoteObject object resides on a
server.This is the class that we extend for all the server classes.Define a
constructor for the remote object.
Give an implementation for each method on the remote interface.By
convention, the name of a class implementing an interface refers to the
name of the interface and is ended by Impl.
import java.rmi.*;
import java.rmi.server.*;
public class HelloImpl extends UnicastRemoteObject
implements Hello

10. {
public HelloImpl() throws RemoteException
{
super();
}
public String sayHello() throws RemoteException
{
return "Hello! How r u? ";
}
}
3. Server Program
import java.rmi.*;
import java.rmi.server.*;
public class HelloServer
{
public static void main(String args[])
{
try
{
Hello h=new HelloImpl();
Naming.rebind("server",h);
System.out.println("object is registered");
System.out.println("now server is waiting");
}
catch(Exception e)
{
System.out.println("error:"+e);
}
}
}
Locating Server Objects

11. To access a remote object that exists on the server,the client needs a
local stub object.Then how can a client request such a stub? The most
common method is to call a remote method of another server object &
get a stub object as a return value.
A server programs registers objects with the bootstrap registry service &
the client retrieves stubs to those objects.You register a server object by
giving the bootstrap registry service a reference to the object & a name.
RMI URLs start with rmi:// & are followed by a server, an optional port
number, another slash, & the name of the remote object. Eg:
rmi://localhost:99:port/central_warehouse
For security reasons,an application can bind,unbind or rebind registry
object refrences only if it runs on the same host as the registry.
Registry used for client bootstraping to get the initial reference.
java.rmi.Naming
a) registry access
b) URL format : prot://host:port/name
c) default protocol : rmi
d) default host : localhost
e) default port : 1099
f) all static methods:
void bind(String name, Remote obj);
void rebind(String name, Remote obj);
void unbind(String name); // unbinds the name
Remote lookup(String url); // returns a stub
4. Client program
import java.rmi.*;
public class HelloClient
{
public static void main(String args[])
{

12. try
{
Hello h=(Hello)Naming.lookup("rmi://localhost/server");
System.out.println("client: Hello!");
System.out.println("server:" +h.sayHello());
}
catch(Exception e)
{
System.out.println("Error:"+e);
}
}
}
5. Registering the Object
import java.rmi.*;
import java.rmi.server.* ;
import javax.naming.*;
public class ShowBindings {
public static void main (String args[]) {
try {
Context namingContext=new InitialContext();
NamingEnumeration e = namingContext.list(“rmi:”);
while(e.hasMore())
System.out.println(e.next.getName());
}
catch(Exception e) {
e.printStackTrace();
}
}
}
Then follow these steps:
1) Compile the source files for the
interface, implementation, client & server classes.

13. 2) run rmic on the implementation class
3) Start the rmi registry as:
Start rmiregistry
4) start the server.
5) Run the client.
(draw diagram of this on page no.293)
5) Parameter Passing in Remote Methods
There are two ways of passing parameters in remote methods:
1) Passing Non remote Methods -
When a remote object is passed from the server to the client, the client
receives a stub. Using the stub, it can manipulate the server object by
invoking remote methods. The object, however stays on the server. It is
also possible to pass & return any objects with a remote method call,
not just those that implements the Remote interface. The client gets a
copy of the string. Then, after the call, the client has its own string
object to work with.
This means that there is no need for any further connection to any
object on the server to deal with that string.
Whenever an object that is not a remote object needs to be transported
from one Java virtual machine to another, the Java virtual machine
makes a copy & sends the copy across the network connection. This
technique is very different from parameter passing in a local method.
When u pass objects into a local method or return them as method
results, only object references are passed. However, object references
are memory addresses of objects in the local java virtual machine. This
information is meaningless to a different java virtual machine. The RMI
mechanism can also make copies of more complex objects, provided

14. they are serializable.RMI uses the serialization mechanism to send
objects across a network. This means that only the information in any
classes that implement the serializable.
Eg: An object of type customer is then sent to the server.
Because customer is not a remote object, a copy of the object is made
on the server. The server program sends back an array list of products.
The array list contains those products that match the customer profile &
it always contains that one item namely, a copy of the book core java.
Again ArrayList is not a remote class, so the array list is copied from the
server back to its client.
2) Passing Remote Objects –
In remote passing objects, the client receives a stub object, then saves it
in an object variable with the same type as the remote interface. The
client can now access the actual object on the server through the
variable. The client can copy this variable in its own local machine – all
those copies are simply references to the same stub. Only remote
interfaces can be accessed through the stub. A remote interface is any
interface extending Remote. All local methods are inaccessible through
the stub. A local method is any method that is not defined in a remote
interface. Local methods can run only on the virtual machine containing
the actual object.
Stubs are generated only from classes that implement a remote
interface & only the methods specified in the interfaces are provided in
the stub classes. If a subclass doesn’t implement a remote interface but
a superclass does, & an object of the subclass is passed to a remote
method, only the superclass methods are accessible. A remote class can
implement multiple interfaces. For Eg:
interface StockUnit extends Remote
{
public String getStockCode() throws RemoteException;
}
Class BookImpl extends ProductImpl implements StockUnit

15. {
public BookImpl(String title, String theISBN, int age1, int age2, String
hobby) throws RemoteException
{
super(title + “Book” , age1, age2, hobby) ;
ISBN =theISBN;
}
public String getStockCode() throws RemoteException
{
Return ISBN;
}
private String ISBN ;
}
Now when a book object is passed to a remote method, the recipient
obtains a stub that has access to the remote methods in both the Product &
the StockUnit class.
Cloning Remote Objects –
Stubs do not have a clone method, so you cannot clone a remote object
by invoking clone on the stub. The reason is:
If clone were to make a remote call to tell the server to clone the
implementation object, then the clone method would need to throw a
RemoteException.
6) Server Object Activation
We used a server program to instantiate & register objects so that the
clients could make remote calls on them. However in some cases, it may
be wasteful to instantiate lots of server objects & have them wait for
connections, whether or not client objects use them. The activation
mechanism lets you delay the object construction so that a server object
is only constructed when atleast one client invokes a remote method on
it.

16. To take advantage of activation, the client code is completely
unchanged. The client simply requests a remote reference & make calls
through it.
However the server program is replaced by an activation program that
constructs activation descriptors of the objects that are to be
constructed at a later time, 7 binds receivers for remote method calls
with the naming service. When a call is made for the first time, the
information in the activation descriptor is used to construct the object.
A server object that is used in this way should extend the Activable class
& implement one or more remote interfaces. For ex:
Class ProductImpl extends Activable implements Product
{
……………
}
You must provide a constructor that takes two parameters:
a) An activation ID
b) A single object containing all construction information wrapped in a
MarshallObject.
When you place a serialized (or marshalled) copy of the construction
information inside the activation descriptor. Your server object constructor
should use the get method of the MarshalledObject class to deserialize the
construction information.
To construct the activation descriptors, you need the following:
a) The activation group ID for the virtual machine
which the object should be constructed.
b) The name of the class
c) The URL string from which to load the class files.
d) The marshalled construction information.
Pass the descriptor to the static Activable.register method.

17. It returns an object of some class that implements the
remote interfaces of the implementation class.You can
bind that object with the naming service:
Product p = (Product) Activable.register(desc);
namingContext.bind(“toaster”, p) ;
The activation program exits after registerin & binding the activation
receivers. The server objects are constructed only when the first remote
method call occurs.
7) CORBA
CORBA lets u make calls between objects written in any language.
CORBA is an industry standard for distributed object computing,
developed by Object Management Group (OMG).
CORBA depends on having an ORB(Object Request Broker) available on
both client & server. ORB act as a kind of universal translator for
interobject CORBA communication.
It Provides a set of common services eg: create, copy, move or destroy
objects, to a “naming service” that allows you to search for objects if
you know their name.
How CORBA differ from RMI:
RMI is Java-only.
RMI is not an industry standard (it is a product of Sun).
RMI does not provide such a rich set of services.
RMI is simpler to use and integrates smoothly with Java.
Steps for Implementing CORBA objects
1) Write the Interface that’ specifies how the object works,
using IDL, the interface definition language for defining
CORBA interfaces. IDL is a special language to specify
interfaces in a language neutral form.

18. 2) Using the IDL complier for the target language(s),
generate the needed stub & helper classes.
3) Add the implementation code for the server objects,
using the language of your choice. Compile the
implementation code.
4) Write a server program that creates & registers the
server objects.The most convenient method for
registration is to use the CORBA naming service, a service that is
similar to rmiregistry.
5) Write a client program that locates the server objects
invokes services on them.
6) Start server program on server & client program on
client.
A CORBA Example –
1) first you initialize the ORB.The ORB is a code library that knows
how to talk to other ORB’s & how to marshal & unmarshal
parameters.
ORB orb = ORB.init(args , null) ;
2) Next , you locate the naming service that helps you
locate other objects.However in CORBA, the
naming service is just another CORBA object.To
call the naming service, you first need to locate
it.The call
String [] services = orb.list_initial_services();
lists the name of the standard services to which the
ORB can connect.
3) To obtain an object reference to the service, you use
the resolve_initial_references method.It returns a
generic CORBA object, an instance of the class org.

19. corba.Object.
org.omg.CORBA.Object
object=orb.resolve_initial_references(“NameService”);
4) Convert this reference to a NamingContext reference so that you
can invoke the methods of the NamingContext interface.
NamingContext
namingcontext=NamingContextHelper.narrow(object);
5) Now you have the naming context, you can use it to
locate the object that the server placed into it.The naming context
associates names with server objects.
6) A name component consists of an ID & a kind.The
ID is a name for the component & Kind is some indication of the
type of the component.
IDL (Interface Definition Language)
IDL specifies interfaces b/w CORBA objects.
CORBA is language independent, bcoz interfaces described in IDL can be
mapped to any programming language.
A client written in c++ can communicate with server written in java.
IDL syntax is :
interface Product {
string getDescription();
};
Differences b/w IDL & java :
a) The interface definition ends with a semicolon.
b) string is written in lower case.
c) In CORBA, the string contain only 8 bit characters,

20. whereas in java programming language, string
contains 16 bit Unicode characters.
IDL compiler generates a number of other source files – the stub class for
communicating with the ORB & three helper classes.
IDL compiler generates a class with suffix Holder for every interface. for
ex: when a Product interface is compiled, it automatically generates a
ProductHolder class.
In IDL you can use sequence construct to define an arrays of variable
size.u must define a type before u can declare sequence parameters. Ex:
interface Warehouse {
ProductSeq find(Customer c);
};
IDL can contain constants also. Ex:
interface Warehouse {
const int SOLD_OUT =404;
};
In IDL, u can group definitions of interfaces,types,exceptions & constants
into modules. Ex:
module corejava {
interface Product {
……
};
interface Warehouse {
…………
};
};
8) Remote Method Calls with SOAP (Simple Object
Access Protocol)

21. SOAP is an XML protocol, like CORBA’s IIOP ,that provides a protocol for
invoking remote methods.
In recent years, web services have emerged as a popular technology for
remote method calls.
A web service has two components:
a) A server can be accessed with SOAP transport
protocol.
b) A description of the service in the Web
Description Language (WSDL) format.
A primary attraction of web services is that they are language neutral.
WSDL:
WSDL is analogous to IDL.It describes the interface of the web service,
the methods that can be called & their parameter & return types.
The WSDL descriptor describes the services in a language independent
manner.For Ex: the WSDL for the Amazon web services (located at
http://soap.amazon.com/schemas3/AmazonWebServices.wsdl)
describes an AuthorSearchRequest operation.
To make web services easy to understand, we look at an example :
The Amazon web service allows a programmer to interact with the Amazon
system for a wide variety of purposes.For ex: you can get listings of all
books with a given author or title, or you can fill shopping carts & place
orders.Amazon makes this service available for use by companies that want
to sell items to their customers, using the Amazon systems as a fulfillment
backend.
(draw diagram on page no 337)