Chapter 4 CORBA

8/12/2019 Chapter 4 CORBA

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The Common Object Request

Broker Architecture (CORBA)


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CORBA

Defines a framework for object-orienteddistributed applications.

Allows distributed programs in differentlanguages and different platforms to interact as

though they were in a single programminglanguage on one computer.

Brings advantages of OO to distributed systems.

Allows you design a distributed application as aset of cooperating objects and to reuse existingobjects.


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The Basic Architecture

object clientnaming service

naming

lookup

stub

ORB

network

operating

system

object

implementation

skeleton

ORB

network

operating

system

logical data flow

physical data flow


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Object Request Broker (ORB)

A software component that mediates transfer ofmessages from a program to an object located ona remote host.

Hides underlying network communications from a

programmer. ORB allows you to create software objects whose

member functions can be invoked by clientprograms located anywhere.

A server program contains instances of CORBAobjects.


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Object Request Broker (ORB)

1) When a client invokes a member function on a

CORBA object, the ORB intercepts the function

call.

2) ORB directs the function call across the network

to the target object.

3) The ORB then collects the results from the

function call returns these to the function call.


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Implementation Detail


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Cross-language CORBA application

object client written in Java

stub in Java generated by compil ing

the CO RBA object interface

ORB written in Java

object implementation written

in C++

skeleton in C++ generated bycompiling the CORBA object

interface

ORB written in C++


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CORBA: A Software Bus


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CORBA Object Interface

A distributed object is defined using a software file similar to

the remote interface file in Java RMI.

Since CORBA is language independent, the interface isdefined using a universal language with a distinct syntax,known as the CORBA I nter face Defini tion Language(I DL ).

The syntax of CORBA IDL is similar to Java and C++.However, object defined in a CORBA IDL file can beimplemented in a large number of diverse programminglanguages, including C, C++, Java, COBOL, Smalltalk, Ada,Lisp, Python, and IDLScript.

For each of these languages, OMG has a standardized mappingfrom CORBA IDL to the programming language, so that acompiler can be used to process a CORBA interface togenerate the proxy files needed to interface with an objectimplementation or an object client written in any of the

CORBA-compatible languages.


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CORBA Application Diagram

An ORB compatible with the IOR protocol will allow an

object reference to be registered with and retrieved fromany IOR-compliant directory service.

CORBA object references represented in this protocol arecalled I nteroperable Object References (IORs).


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Interoperable Object Reference(IOR)

An IOR is a string that contains encoding forthe following information:

The type of the object.

The host where the object can be found.

The port number of the server for that object.

An object key, a string of bytes identifying the

object.

The object key is used by an object server tolocate the object.


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Interoperable Object Reference(IOR)

The following is an example of the stringrepresentation of an IOR [5]:IOR:000000000000000d49444c3a677269643a312e3000000

00000000001000000000000004c0001000000000015756c74

72612e6475626c696e2e696f6e612e6965000009630000002

83a5c756c7472612e6475626c696e2e696f6e612e69653a677269643a303a3a49523a67726964003a

The representation consists of thecharacter prefix IOR: followed by a

series of hexadecimal numeric characters,each character representing 4 bits ofbinary data in the IOR.


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Object Servers and Object Clients

As in Java RMI, a CORBA distributed object is

exported by an object server, similar to the object

server in RMI.

An object client retrieves a reference to a

distributed object from a naming or directory

service, to be described, and invokes the methods

of the distributed object.


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CORBA Object References

As in Java RMI, a CORBA distributed object is

located using an object reference. Since CORBA

is language-independent, a CORBA object

reference is an abstract entity mapped to a

language-specific object reference by an ORB, in arepresentation chosen by the developer of the

ORB.


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CORBA Naming Service

To export a distributed object, a CORBA object

server contacts a Naming Service to binda symbolicname to the object. The Naming Service maintains adatabase of names and the objects associated withthem.

To obtain a reference to the object, an object clientrequests the Naming Service to look up the objectassociated with the name .

The API for the Naming Service is specified in

interfaces defined in IDL, and includes methods thatallow servers to bind names to objects and clients toresolve those names.


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CORBA Naming Service

naming context1

naming context1 naming context2

naming context1 naming context1

object

name1

object

namen

...

...

...

...


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A Naming Context

A naming context correspond to a folder or directory

in a file hierarchy, while object names corresponds toa file.

The full name of an object, including all theassociated naming contexts, is known as a compound

name. The first component of a compound namegives the name of a naming context, in which thesecond component is accessed. This processcontinues until the last component of the compound

name has been reached. Naming contexts and name bindings are created using

methods provided in the Naming Serviceinterface.


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A CORBA object name

The syntax for an object name is as follows:

where the sequence of naming contexts

leads to the object name.


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Example of a naming hierarchy

As shown, an object representing the mens

clothing department is named

store.clothing.men, where store and clothing

are naming contexts, and men is an object

name.store

clothing Appliances

women men

...television

...


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Object Adapters

In the basic architecture of CORBA, the

implementation of a distributed object interfaces with

the skeleton to interact with the stub on the object

client side. As the architecture evolved, a software

component in addition to the skeleton was needed on

the server side: an object adapter.

distributed object

implementation

object adapter

ORB


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Object Adapter

An object adapter simplifies the

responsibilities of an ORB by assisting anORB in delivering a client request to an objectimplementation.

When an ORB receives a clients request, itlocates the object adapter associated with theobject and forwards the request to the adapter.

The adapter interacts with the objectimplementations skeleton, which performsdata marshalling and invoke the appropriatemethod in the object.


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The Portable Object Adapter

There are different types of CORBA object

adapters.

The Portable Object Adapter, or POA, is a

particular type of object adapter that is defined

by the CORBA specification. An object

adapter that is a POA allows an object

implementation to function with different

ORBs, hence the word portable.


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Copyright 1997 Alex Chaffee

CORBA Flow

Client Virtual Machine

Client

Server Virtual Machine

Stub

Remote

Object

Skeleton

Name Server Virtual Machine

Fred

Server


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CORBA Flow


Client


Stub

Remote

Object

Skeleton


Fred

Server

1

2

1. Server Creates Remote Object2. Server Registers Remote Object


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CORBA Flow


Client


Stub

Remote

Object

Skeleton

Name ServerVirtual Machine

Fred

Server

4

3. Client requests object from Name Server

4. Name Server returns remote reference

(and stub gets created)

3


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CORBA Flow


Client


Stub

Remote

Object

Skeleton


Fred

Server

6

5. Client invokes stub method6. Stub talks to skeleton

7. Skeleton invokes remote object

method

5 7


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A Java IDL application example


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IDL to Java Type Mapping

IDL Java

module package

interface interface

string/wstring Stringlong int

long long long

float float

double double

exception class

operation Method


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V D

+ CORBA IDL:module Math{

interface MathLibrary {

long add( in long x, in long y );

string About( in string version );};

};

+ Java :package Math;

public interface MathLibrary {

int add (int x, int y);

String About(String version);

}


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The CORBA Interface file Hello.idl

01. module HelloApp

02. {

03. interface Hello

04. {

05. string sayHello();

07. };

08. };


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Compiling the IDL file (using Java 1.4)

The IDL file should be placed in a directory dedicated to the

application. The file is compiled using the compiler idljusing acommand as follows:

idlj -fallHello.idl

The fall command option is necessary for the compiler togenerate all the files needed.

In general, the files can be found in a subdirectory named App when an interface file named .idl iscompiled.

If the compilation is successful, the following files can be found in

a HelloAppsubdirectory:HelloOperations.java Hello.java

HelloHelper.java HelloHolder.java

_HelloStub.java HelloPOA.java

These files require no modifications.


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idlj Generated Files


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The *Operations.java file

There is a file HelloOperations.java

found in HelloApp/ after you compiled using

idlj

It is known as a Java operationsinterface in

general

It is a Java interface file that is equivalent to

the CORBA IDL interface file (Hello.idl)


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HelloApp/HelloOperations.java

package HelloApp;

public interface HelloOperations

{String sayHello ();

} // interface HelloOperations


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HelloApp/Hello.java

This interface contains the Java version of our IDL interface. The

Hello.java interface extends org.omg.CORBA.Object, providingstandard CORBA object functionality. It also extends theHelloOperations interface andorg.omg.CORBA.portable.IDLEntity.

package HelloApp;

public interface Hello extends HelloOperations,org.omg.CORBA.Object,org.omg.CORBA.portable.IDLEntity

{

} // interface Hello


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HelloHelper.java, the Helper class

The Java class HelloHelper provides auxiliary

functionality needed to support a CORBA object in thecontext of the Java language.

In particular, a method, narrow,allows a CORBA

object reference to be cast to its corresponding type inJava, so that a CORBA object may be operated on using

syntax for Java object.

The Helper class is responsible for reading and writing

the data type to CORBA streams, and inserting andextracting the data type from anythings.


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HelloHolder.java, the Holder class

This final class holds a public instance

member of type Hello.

Whenever the IDL type is an out or an inout

parameter, the Holder class is used (In IDL, a

parameter may be declared to be outif it is an

output argument, and inout if the parameter

contains an input value as well as carries an

output value).


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_HelloStub.java

The Java class HelloStub is the stub file, the

client-side proxy, providing CORBA

functionality for the client .

It extends

org.omg.CORBA.portable.ObjectImpl

and implements the Hello.javainterface.


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HelloPOA.java, the server skeleton

The Java class HelloPOAis the skeleton, the

server-side proxy, combined with the portable

object adapter.

It extends org.omg.PortableServer.Servant, and

implements the InvokeHandler interface and the

HelloOperationsinterface.

The server class HelloImplextends HelloPOA
http://java.sun.com/j2se/1.4/api/org/omg/PortableServer/Servant.htmlhttp://java.sun.com/j2se/1.4/api/org/omg/PortableServer/Servant.htmlhttp://java.sun.com/j2se/1.4/api/org/omg/PortableServer/Servant.htmlhttp://java.sun.com/j2se/1.4/api/org/omg/PortableServer/Servant.htmlhttp://java.sun.com/j2se/1.4/api/org/omg/PortableServer/Servant.htmlhttp://java.sun.com/j2se/1.4/api/org/omg/PortableServer/Servant.htmlhttp://java.sun.com/j2se/1.4/api/org/omg/PortableServer/Servant.htmlhttp://java.sun.com/j2se/1.4/api/org/omg/PortableServer/Servant.html


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The application

Server-side Classes

On the server side, two classes need to be

provided: the servant and the server.

The servant, HelloImpl, is the implementation of

the HelloIDL interface; each Helloobject is an

instantiation of this class.


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The Servant - HelloApp/HelloImpl.java

// The servant -- object implementation -- for the Hello

// example. Note that this is a subclass of HelloPOA,

// whose source file is generated from the

// compilation of Hello.idl using j2idl.

01.import HelloApp.*;02. import org.omg.CosNaming.*;

03. import java.util.Properties; 04. class HelloImpl extends HelloPOA{

05. private ORB orb;

06. public void setORB(ORB orb_val) {

07. orb = orb_val;

08. }

09. // implement sayHello() method

10. public String sayHello() {

11. return "\nHello world !!\n";

12. }

13. } //end class

The server HelloApp/HelloServer java


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The server - HelloApp/HelloServer.java

The HelloServer class has the server's main() method, which:1. Creates and initializes an ORB instance

2. Gets a reference to the root POA and activates the POAManager

3. Creates a servant instance (the implementation of one CORBAHello object) and tells the ORB about it

4. Gets a CORBA object reference for a naming context in which toregister the new CORBA object

5. Gets the root naming context

6. Registers the new object in the naming context under the name

"Hello"

7. Waits for invocations of the new object from the client


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Understanding HelloServer.java

1. Creating and Initializing an ORB Object

A CORBA server needs a local ORB object, as does the CORBA

client.

The ORB variable is declared and initializedinside the try-catch block.

ORB orb = ORB.init(args, null);


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2. Get a Reference to the Root POA and Activate the

POAManager

The ORB obtains the initial object references to services

such as the Name Service using the method

resolve_initial_references The reference to the root POA is retrieved and the POAManager is

activated from within the try-catch block.

POA rootpoa =

POAHelper.narrow(orb.resolve_initial_references("RootPOA")); rootpoa.the_POAManager().activate();


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Managing the Servant Object

3. Creates a servant instance: The servant inherits from

the interface generated by idlj and actually performs thework of the operations on that interface. Our HelloServer

needs a HelloImpl.

We instantiate the servant object inside the try-catchblock, just after activating the POA manager, as shown:

HelloImpl helloImpl = new HelloImpl();

In the next line of code, setORB(orb) is defined on the

servant so that ORB.sayHello() can be called as part ofthe sayHello operation. This step is required because of

the sayHello() method defined in Hello.idl.

helloImpl.setORB(orb);
http://download.oracle.com/javase/1.4.2/docs/guide/idl/jidlGlossary.htmlhttp://download.oracle.com/javase/1.4.2/docs/guide/idl/jidlGlossary.html


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Managing the Servant Object

4. get the object reference associated with the

servant.

The narrow() method is required to cast CORBA

object referencesto their proper types.

org.omg.CORBA.Object ref =

rootpoa.servant_to_reference(helloImpl);

Hello href = HelloHelper.narrow(ref);
http://download.oracle.com/javase/1.4.2/docs/guide/idl/jidlGlossary.htmlhttp://download.oracle.com/javase/1.4.2/docs/guide/idl/jidlGlossary.htmlhttp://download.oracle.com/javase/1.4.2/docs/guide/idl/jidlGlossary.htmlhttp://download.oracle.com/javase/1.4.2/docs/guide/idl/jidlGlossary.html


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Obtaining the Initial Naming Context

5. Get the root naming service:

we call orb.resolve_initial_references()to get an object

reference to the name server:

org.omg.CORBA.Object objRef =

orb.resolve_initial_references("NameService");The string "NameService" is defined for all

CORBA ORBs. When you pass in that string,

the ORB returns a naming context object thatis an object reference for the name service


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Narrowing the Object Reference

As with all CORBA object references, objRef is a

generic CORBA object.

To use it as a NamingContextExt object, you must

narrow it to its proper type. The call to narrow() is

just below the previous statement:NamingContextExt ncRef =

NamingContextExtHelper.narrow(objRef);

The ncRef object is now an

org.omg.CosNaming.NamingContextExt and you canuse it to access the naming service and register the

server.


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Registering the Servant with the Name Server

6. Registers the new object in the naming context under

the name "Hello": we create a new NameComponent array

String name = "Hello";

NameComponent path[] = ncRef.to_name( name );

Finally, we pass path and the servant object to thenaming service, binding the servant object to the"Hello" id:

ncRef.rebind(path, href); Now, when the client calls resolve("Hello") on the

initial naming context, the naming service returns anobject reference to the Hello servant.

The server - HelloApp/HelloServer java


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The server HelloApp/HelloServer.java

public class HelloServer {

public static void main(String args[]) {

try{// create and initialize the ORB


// get reference to rootpoa & activate the POAManager

POA rootpoa = POAHelper.narrow

(orb.resolve_initial_references("RootPOA"));rootpoa.the_POAManager().activate();

// create servant and register it with the ORB

HelloImplhelloImpl = new HelloImpl();

helloImpl.setORB(orb);

// get object reference from the servant

org.omg.CORBA.Object ref =

rootpoa.servant_to_reference(helloImpl);

// and cast the reference to a CORBA reference

Hello href = HelloHelper.narrow(ref);


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HelloApp/HelloServer.java - continued

// get the root naming context

// NameService invokes the transient name serviceorg.omg.CORBA.Object objRef =

orb.resolve_initial_references("NameService");

// Use NamingContextExt, which is part of the

// Interoperable Naming Service (INS) specification.

//To use it as a NamingContextExt object, you must

// narrow it to its proper type

NamingContextExt ncRef=


// bind the Object Reference in Naming



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HelloApp/HelloServer.java - continued

//This operation converts a Stringified Name into an

// equivalent array of Name Components.NameComponentpath[] = ncRef.to_name( name );ncRef.rebind(path, href);

System.out.println("HelloServerready and waiting ...");

// wait for invocations from clients

orb.run();}

catch (Exception e) {

System.err.println("ERROR: " + e);

e.printStackTrace(System.out);

}

System.out.println("HelloServer Exiting ...");

}

}


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Importing Required Packages

// The package containing our stubs

import HelloApp.*;

// HelloServer will use the naming service

import org.omg.CosNaming.*;

// The package containing special exceptions thrown by the name service

import org.omg.CosNaming.NamingContextPackage.*;// All CORBA applications need these classes

import org.omg.CORBA.*;

// Classes needed for the Portable Server Inheritance Model

import org.omg.PortableServer.*;

import org.omg.PortableServer.POA;// Properties to initiate the ORB

import java.util.Properties;

Th bj li li i


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The object client application

A client program can be a Java application, an applet,

or a servlet. The client code is responsible for:

creating and initializing the ORB,

looking up the object using the Interoperable Naming

Service, invoking the narrow method of the Helper object to cast

the object reference to a reference to a Hello objectimplementation,

and invoking remote methods using the reference. The objects sayHello method is invoked to receive a

string.

U d di H ll Cli j


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Understanding HelloClient.java

Importing Required Packages

import HelloApp.*; // the package containing our stubs// HelloClient will use the Naming Service

import org.omg.CosNaming.*;

import org.omg.CosNaming.NamingContextPackage.*;// All CORBA applications need these classes

import org.omg.CORBA.*;

C ti ORB Obj t


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Creating an ORB Object

1. The ORB variable is declared and initialized inside

the try-catch block.


Obt i i th I iti l N i C t t


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Obtaining the Initial Naming Context

2. Call orb.resolve_initial_references() to get an object

reference to the name server:



N i th Obj t R f


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Narrowing the Object Reference

As with all CORBA object references, objRef is a generic

CORBA object. To use it as a NamingContextExt object,you must narrow it to its proper type.

NamingContextExt ncRef =


R l th Obj t R f i N i


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Resolve the Object Reference in Naming

To publish a reference in the Naming Service to the Hello object

implementing the Hello interface, you first need an identifyingstring for the Hello object.


Finally, we pass name to the naming service's resolve_str() method

to get an object reference to the Hello server and narrow it to aHello object:

helloImpl = HelloHelper.narrow(ncRef.resolve-str(name));

System.out.println("Obtained a handle on server object: " +

helloImpl);

I ki th H ll () O ti


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Invoking the sayHello() Operation

System.out.println(helloImpl.sayHello());


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// A sample object client application.

import HelloApp.*;

import org.omg.CosNaming.*; public class HelloClient{

static Hello helloImpl;

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




NamingContextExt ncRef =NamingContextExtHelper.narrow(objRef);

helloImpl =

HelloHelper.narrow(ncRef.resolve_str(Hello));

System.out.println(helloImpl.sayHello());

C ili d R i J IDL li ti


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Compiling and Running a Java IDL application

1. Create and compile the Hello.idl file on the server

machine:idlj -fall Hello.idl

2. Copy the directory containing Hello.idl (including

the subdirectory generated by id l j)

to the clientmachine.

3. In theHel loApp directory on the client machine:create HelloClient.java. Compile the *.java files,

including the stubs and skeletons (which are in thedirectoryHelloApp):

javac *.java HelloApp/*.java

C ili d R i J IDL li ti


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4. In the HelloApp directory on the server machine:

Create HelloServer.java. Compile the .javafiles:javac *.java HelloApp/*.java

On the server machine: Start the Java Object RequestBroker Daemon, orbd, which includes a Naming Service.

To do this on Unix:orbd -ORBInitialPort 1050 -ORBInitialHost

servermachinename&

To do this on Windows:start orbd -ORBInitialPort 1050 -ORBInitialHost

servermachinename



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5. On the server machine, start the Hello server, as

follows:java HelloServer ORBInitialHost -ORBInitialPort 1050

6. On the client machine, run the Hello application

client. From a DOS prompt or shell, type:java HelloClient -ORBInitialHost nameserverhost

-ORBInitialPort 1050

all on one line.

Note that nameserverhostis the host on which theIDL name server is running. In this case, it is theserver machine.



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7. Kill or stop orbdwhen finished. The name

server will continue to wait for invocations

until it is explicitly stopped.

8. Stop the object server.


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http://download.oracle.com/javase/1.4.2/docs/gui

de/idl/jidlExample.html

Summary 1


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Summary-1

You have been introduced to

the Common Object Request Broker

Architecture(CORBA), and

a specific CORBA facility based on the

architecture: Java IDL

Summary 2


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Summary-2

The key topics introduced with CORBA are:

The basic CORBA architecture and its emphasis on objectinteroperability and platform independence

Object Request Broker (ORB) and its functionalities

The Inter-ORB Protocol (IIOP) and its significance

CORBA object reference and the Interoperable ObjectReference (IOR) protocol

CORBA Naming Serviceand the Interoperable NamingService(INS)

Standard CORBA object services and how they areprovided.

Object adapters, portable object Adapters (POA) andtheir significance.

Summary 3


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Summary-3

The key topics introduced with Java IDLare:

It is part of the Java TM2 Platform, Standard Edition (J2SE) Java packages are provided which contain interfaces and

classes for CORBA support

Tools provided for developing a CORBA application

include idlj(the IDL compiler) and orbd(the ORB andname server)

An example application Hello

Steps for compiling and running an application.

Client callback is achievable.

CORBA tookits and Java RMI are comparable and alternativetechnologies that provide distributed objects. An applicatonmay be implemented using either technology. However, thereare tradeoffs between the two.

Documents

Chapter 4 CORBA