COMPE 515 ADVANCED JAVA PROGRAMMING FOR THE WEB

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COMPE 515ADVANCED JAVA PROGRAMMING FOR THE WEB

• F. Cemile Serçe

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Corba (Common object REQUEST broker archICTECTURE)
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Background

• RMI is a powerful mechanism for distributing and
  processing objects in a platform-independent
  manner, however...
• it only works with objects that have been created
  using Java

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Background

• CORBA, a more generic approach to the development
  of distributed systems
• allows to work with objects written in a variety
  of programing languages
• by a client which themselves may be written in a
  variety of programming languages

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Background

• CORBA, product of Object Management Group (OMG)
• not a specific implementation, but a
  specification for creating and using distributed
  objects
• an individual implementation constitutes an ORB,
  such as
• VisiBroker
• Orbix
• Java IDL

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ORB

• The ORB is an abstract entity that acts as the
  middleman in all remote method invocations.
• The ORB finds a server that can handle a method
  invocation, passes the request to the server,
  receives the response and forwards it to the
  client.
• The functions handled by an ORB are actually
  implemented in both client and server

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Background

• RMI uses Java to define the interfaces for its
  objects, CORBA uses a special language called
  Interface Definition Language (IDL) to define
  those interfaces
• In order for any ORB to provide access to
  software objects in a particular programming
  language, the ORB has to provide a mapping from
  the IDL to the target language

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Remote Method Invocation using the same ORB
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Remote Method Invocation using the different ORBs
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IDL Language Mapping

• OMG defines mappings to different languages
• C, C, Java, Smalltalk, COBOL, Ada, Lisp, PL/1,
  Python, and IDLscript
• Every ORB has an IDL compiler, creating
• A STUB and
• A SKELETON

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IDL
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Java IDL

• Java TM IDL
• distributed objects.
• interaction among objects regardless of whether
  they're written in the Java programming language
  or another language such as C, C, COBOL, or
  others

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ORB

• To support interaction between objects in
  separate programs, Java IDL provides an Object
  Request Broker, or ORB.
• The ORB is a class library
• enables low-level communication between Java IDL
  applications and other CORBA-compliant
  applications

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Example

• A simple CORBA distributed application using Java
  IDL
• "Hello World" program as a distributed
  application
• The Hello World program has a single operation
  that returns a string to be printed

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Distributed Objects

• Any relationship between distributed objects has
  two sides
• the client (calls a remote interface)
• the server (provides a remote interface),
• common to most distributed object standards,
  including RMI, CORBA, etc.
• both define object-level rather than application
  level interaction

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IIOP Internet Inter-ORB ProtocolA one-method
distributed object shared between a CORBA client
and server
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On Client

• The application includes a reference for the
  remote object
• The object reference has a stub method,
• stub is actually wired into the ORB,
• Stub invokes the ORB's connection, which forwards
  the invocation to the server.

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On Server

• the ORB uses skeleton code to translate the
  remote invocation into a method call on the local
  object
• skeleton
• translates the call and calls the method being
  invoked
• transforms results or errors, and sends them back
  to the client via the ORBs.

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Between ORBs

• a shared protocol, IIOP (Internet Inter-ORB
  Protocol)
• IIOP, which is based on the standard TCP/IP
  internet protocol, defines how CORBA-compliant
  ORBs pass information back and forth
• Like CORBA and IDL, the IIOP standard is defined
  by OMG

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Hello World Application

• Define the remote interface
• Compile the remote interface
• Implement the server
• Implement the client
• Start the applications

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1. Define the remote interface

• define the interface using IDL
• idlj automatically maps from IDL, generating all
  java language stub and skeleton source files
• WRITING IDL file

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WRITING IDL

• Writing Hello.idl
• create a new directory, named Hello
• start a text editor and create a file named
  Hello.idl
• enter the following code
• module HelloApp
• interface Hello
• string sayHello()
• oneway void shutdown()
• save the file

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Understanding IDL

• Declaring the CORBA IDL Module
• a namespace that acts as a container for related
  interfaces and declarations.
• mapped to Java package statement

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Understanding IDL

• Declaring the Interface
• declare the API contract an object has with other
  objects
• mapped to java interface statement

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Understanding IDL

• Declaring the Operations
• CORBA operations are the behavior that servers
  promise to perform on behalf of clients that
  invoke them
• Each operation in the IDL generates a
  corresponding method statement in the generated
  Java interface.

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2. Compile the remote interface

• idlj
• reads OMG IDL files and creates the required Java
  files
• defaults to generating only the client-side
  bindings
• if you need both, use the -fall option when
  running the idlj compiler

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2. Compile the remote interface

• Make sure that the j2sdk/bin directory is in your
  path
• Go to a command line prompt.
• Change to the directory containing your Hello.idl
  file.
• Enter the compiler command
• idlj -fall Hello.idl
• .....Generates a number of files......

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HelloPOA.java

• HelloPOA.java
• This abstract class is the stream-based server
  skeleton
• _HelloStub.java
• the client stub
• Hello.java
• Java version of our IDL interface
• HelloHelper.java
• the narrow() method required to cast CORBA object
  references to their proper types
• HelloHolder.java
• holds a public instance member of type Hello
• HelloOperations.java
• This interface contains the methods sayHello()
  and shutdown()

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3. Implement the Server

• Consists of two classes
• Servant (HelloImpl)
• implementation of Hello IDL interface
• subclass of HelloPOA
• servant provides methods sayHello(),
  shutdown().
• servant methods are like ordinary java methods
• Server
• ??

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3. Implement the Server

• Consists of two classes
• Servant
• Server
• Creates and initializes an ORB instance
• Gets a reference to the root POA and activates
  the POAManager
• Creates a servant instance (the implementation of
  one CORBA Hello object) and tells the ORB about
  it
• Gets a CORBA object reference for a naming
  context in which to register the new CORBA object
• Gets the root naming context
• Registers the new object in the naming context
  under the name "Hello
• Waits for invocations of the new object from the
  client

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Creating HelloImpl

• public class HelloImpl extends HelloPOA
• private ORB orb
• public void setORB(ORB orb_val)
• orb orb_val
• // implement sayHello() method
• public String sayHello()
• return "\nHello world !!\n"
• // implement shutdown() method
• public void shutdown()
• orb.shutdown(false)

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Creating HelloServer
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4. Implement the client
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5. Start the applications

• start orbd
• start orbd -ORBInitialPort 1050 ORBInitialHost
  localhost
• start the Hello server
• start java HelloServer -ORBInitialPort 1050
  -ORBInitialHost localhost
• Run the client application
• java HelloClient -ORBInitialPort 1050
  -ORBInitialHost localhost
• The client prints the string from server to the
  command line