Transcript corba-and-java_edited

Introduction
• CORBA loves Java!
• CORBA provides a platform-independent,
language-independent way to write
applications that can invoke objects that
live across the room or across the planet.
• Java is an object-oriented language that's
ideal for writing the clients and servers
living on the Object Bus.
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What is CORBA?
• Common Object Request Broker Architecture
• Communication infrastructure for distributed objects
• Allows a heterogeneous, distributed collection of objects to
collaborate transparently
What is the OMG?
• Designers of CORBA
• Consortium of 700+ companies
– Not including Microsoft
• Members:
• platform vendors
• database vendors
• software tool developers
• corporate developers
• software application vendors
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Basic CORBA Architecture
Server
Client
response
request
ORB
ORB
“Object Bus”
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CORBA Objects
• Examples
– Service
– Client
– Component
– Business object
• CORBA objects approach universal accessibility
– Any Language
– Any Host on network
– Any Platform
CORBA Elements
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ORB
CORBA Services
CORBA Facilities
Application Objects
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ORB
• Object Request Broker
– “Object Bus”
• Handles all communication among objects
• Each host (machine) has its own ORB
• ORBs know how to talk to each other
• ORB also provides basic services to client
ORB Responsibilities
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Find the object implementation for the request
Prepare the object implementation to receive the request
Communicate the data making up the request
Retrieve results of request
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IIOP
• Internet Inter-Orb Protocol
• Network or “wire” protocol
• Works across TCP/IP (the Internet protocol)
What is an ORB really?
• Not a separate process
• Library code that executes in-process
• Listens to TCP ports for connections
– One port per local object
• Opens TCP sockets to other objects
– N ports per remote machine
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ORB Features
• Method invocations
– Static and Dynamic
– Remote objects or CORBA services
• High-level language bindings
– Use your favorite language; ORB translates
• Self-describing
– Provides metadata for all objects and services
• Local or remote
– Same API wherever target object lives
• Preserves context
– Distributed security and transactions
• Coexistence with legacy code
– Just provide a wrapper object
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IDL
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Interface Definition Language
Defines protocol to access objects
Like a contract
Well-specified
Language-independent
IDL Example
module Calc {
interface Adder {
long add(in long x, in long y);
}
}
• Defines an object called Adder with a method called add
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Stubs and Skeletons
• Stub
– lives on client
– pretends to be remote object
• Skeleton
– lives on server
– receives requests from stub
– talks to true remote object
– delivers response to stub
Client vs. Server
• in CORBA, a client is a client relative to a particular object
• i.e. an object with a reference to a “server” object
• A client may also act as a server
– If it has an IDL and stubs and skeletons
• Technically, a CORBA server contains one or more CORBA objects
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Stubs and Skeletons
Client Host Machine
Server Host Machine
Client
Object
Remote
Object
Stub
Skeleton
ORB
IIOP
ORB
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CORBA Services
• APIs for low-level, common tasks
• Life Cycle Service
– creating, copying, moving, removing objects
• Naming Service
– Register objects with a name
– Look up objects by name
• Concurrency Control Service
– Obtain and release exclusive locks
• Transaction Service
– Two-phase commit coordination
– Supports nested transactions
• Persistence Service
– Storing objects in a variety of databases
– RDBMS, OODBMS, file systems
• Security Service
– Authentication, ACLs, encryption, etc.
• Event Service
– Uncoupled notifications
•Relationship
•Externalization
•Query
•Licensing
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CORBA Facilities
• Frameworks for specialized applications
• Distributed Document Component Facility
– OpenDoc
• In progress:
– Agents
– Business Objects
– Internationalization
Three Tiers
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User Interface Tier
Business Logic Tier
Data Storage Tier
Can use CORBA objects in each tier
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N-Tier Design with CORBA
Storage “Tier”
ORB
DB
ORB
ORB
ORB
DB
Data Object
ORB
ORB
ORB
ORB
Client Tier
TP Monitor
ORB
Business Object
Tier
Service “Tier”
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Java IDL - Using CORBA from Java
Java CORBA Products
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The Java 2 ORB
VisiBroker for Java
OrbixWeb
Netscape Communicator
Various free or shareware ORBs
Java IDL
• Should be named “Java CORBA”
– More than just IDL
– Full implementation of CORBA in 100% Java
• Three Parts
– ORB
– Naming Service
– idltojava compiler
• Ships with JDK 1.2
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The Java ORB
• 100% Java
• Generic
• Allows Java IDL applications to run either as stand-alone Java
applications, or as applets within Java-enabled browsers
• Uses IIOP
Other Java ORBs
• Visigenic (Inprise) VisiBroker
– Netscape Communicator
– Oracle Web Server 3.0
– Free download
• Iona OrbixWeb
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IDL to Java Type Mapping
IDL Type
boolean
char / wchar
octet
short / unsigned short
long / unsigned long
long long / unsigned long long
float
double
string / wstring
Java Type
boolean
char
byte
short
int
long
float
double
String
IDL vs. Java vs. C++ concepts
IDL
module
Java
package
C++
namespace
interface
interface
abstract
class
operation
method
member
function
attribute
pair of
methods
pair of
functions
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IDL Modules
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Map to Java packages
It has the root level name of the module
Clutters up the package hierarchy
e.g. module Calc ->
– package Calc
– interface Calc.Adder
– not package ORG.omg.CORBA.modules.Calc
IDL Attributes
• Map to pair of functions
• IDL
– string name;
• Java
– public void name(String val);
– public String name();
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idltojava
• Development tool
• Automatically generates java stubs, skeletons, helpers, holders, ...
• Generates stubs for specific remote interfaces
Skeletons
• Java objects call stub methods
• Stubs communicate with CORBA objects
– and vice versa
• Transparent integration
Stubs
• ORB passes request to skeleton (like a stub)
• Skeleton calls local implementation
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Remote Interfaces and Stubs
IDL Interface
implements
implements
extends
Client
Stub
Skeleton
Remote Object
(Server)
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idltojava input
• Calc.idl
module Calc {
interface Adder {
long add(in long x, in long y);
}
}
idltojava output
• idltojava Calc.idl
 Adder.java (a Java interface that maps the
IDL interface)
 _AdderStub.java (a client stub)
 _AdderImplBase.java (a server skeleton)
 AdderHelper.java (a helper class)
 AdderHolder.java (a holder class)
• Must compile these files and put them in the CLASSPATH
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CORBA Flow
Client Virtual Machine
Server Virtual Machine
Client
Remote
Object
Skeleton
Stub
Server
“Fred”
Name Server Virtual Machine
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CORBA Flow
Client Virtual Machine
Server Virtual Machine
1. Server Creates Remote Object
Client2. Server Registers Remote Object
Remote
Object
1
Skeleton
Stub
Server
2
“Fred”
Name Server Virtual Machine
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CORBA Flow
Client Virtual Machine
Server Virtual Machine
3. Client requests object from
Client Name Server
4. Name Server returns remote
reference
(and stub gets created)
Skeleton
Stub
3
Remote
Object
Server
4
“Fred”
Name Server Virtual Machine
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RMI Flow
Client Virtual Machine
Server Virtual Machine
5. Client invokes stub method
Client 6. Stub talks to skeleton
7. Skeleton invokes remote object
method
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Remote
Object
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Stub
Skeleton
Server
“Fred”
Name Server Virtual Machine
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