Transcript Overview of Distributed Object Computing

CORBA Overview;
Developing Basic CORBA
Applications
DS 520 - Distributed Systems Frameworks
Review
 Anatomy and Requirements of Distributed Computing
 Advantages and Features of DOC Systems
 General Architecture of DOC Systems
 Overview of Object Management Architecture and CORBA
Distributed Systems Frameworks
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Distributed Computing
 Can think of DC as:
 breaking down an application into individual computing agents
 distributed over a network
 work together on a cooperative task
 Motivation for DC:
 parallel processing: can solve larger problems without larger computers
 applications and data may be difficult to relocate
 redundant processing agents for fault tolerant systems
 Flavors of Distributed Programming
 Messaging
 Asynchronous, Open-loop: programmer must maintain context
 Examples; socket programming, cgi (based on http), etc.
 Remote Invocation
 Synchronous (usually), Closed-loop, Location transparency (looks like a local call)
 Examples: Remote Procedure Call (RPC), Distributed Objects
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General Pattern for Remote Invocation
Client Code
Server Code
Stub
Skeleton
Infrastructure
 Call:
 marshal arguments
 convert to network format
 locate server
 transmit data
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 Serve:
 receive data
 convert & unmarshal
 invoke method
 marshall return value
 transmit data
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Anatomy of a Distributed Application
 3 layers of a distributed application:
 network level (TCP/IP, network protocols)
 higher-level services (directory services, security protocols, etc.)
 application level
 At the application level:
 processes
 threads
 objects: processes can be made up of one or more objects which can be accessed
by one or more threads within the process
 agents: an “independent” functional element (e.g., in a banking application we
may have a customer agent, a transaction agent, an information brokerage agent)
 Example (the customer agent):
 object1: running on client machine (2 threads: listening for data; updating display)
 object2: running on the bank server (issuing queries; sending data back to client)
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Requirements for a Distributed Application
 Partitioning and Distributing Data and Functions
 data-driven distribution
 functional distribution
 object-based distribution
 Flexible, Extendible Communication Protocols
 allocation of tasks to agents has a direct influence on the complexity of the
communication protocol (type of data, amount of data, persistence of connections)
 may also be dictated by legacy systems that need to be incorporated
 Multi-threading Requirements
 server object may need to service multiple remote agents at once
 effective way to optimize resources
 Security Requirements
 authentication of agent identities
 define resource access levels
 data encryption
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Features of DOC Systems
 Object Interface Specification
 to allow clients to access objects regardless of implementation details
 Object Manager
 The core of a distributed object system (e.g., ORB, or Registry in RMI)
 Manages object skeletons and object references on the server
 When a client requests a new object, the object manager
 locates the skeleton for the class of the requested object
 creates new instance based on skeleton; stores new object in the object storage
 sends a reference to the new object back to the client
 Registration / Naming Service
 Acts as an intermediary between the object client and the object manager
 Once the interface to an object is defined, an implementation of the interface
must be registered with the service so that it can be addressed by clients
 Object Communication Protocol to handle remote object requests
 Must support a means of transmitting and receiving object and method
references, and data in the form of objects or basic data types
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General Architecture for a DOC System
Registration
Service
Object
Skeleton
Object
Storage
Server
Implementation
Object
Interface
Specification
IDL Compilers
Object Manager
Naming Service
Client Stub
Interface
Client Application
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Remote Object Transactions at Runtime
Server Object
Implementation
Object
Skeleton
4. Object
Interactions
2. Resolve
Object
Object Manager Naming Service
1. Request
Object
Client
Application
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3. Object
Handle
Object
Stubs
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OMG’s Mission
 Develop a single architecture, using object technology, for
distributed application integration, guaranteeing:
 reusability of components;
 interoperability & portability;
 basis in commercially available software
 Consensus-based approach
 Focus on swiftly-developed, easily usable (“off the shelf”)
component standards:
 Single terminology for object-orientation.
 Common abstract framework.
 Common reference model.
 Common interfaces & protocols
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Long Term Vision
WEB Server
WEB Browser
Java Enabled
IIOP
-- HTML
-- GIF, JPEG
-- AV, WAV
-- Java
Hot Java
Java Beans
ActiveX
Java Interpreter
OMG IDL
Programs
IIOP (CORBA)
P
R
O
G
R
A
M
S
IIOP
Sea of
Objects
(CORBA)
Active
X
APP
COM/CORBA
Legacy
DCE APP
DCE
DCE
Object Management Architecture
Non-standardized
application-specific
interfaces
Application Interfaces
Vertical
domain-specific
interfaces
Domain Interfaces
Horizontal
facility interfaces
CORBAfacilities
Object Request Broker
CORBAservices
General service interfaces
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Overview of CORBA Objects
 CORBA objects differ from typical programming language objects:
 CORBA objects can be located anywhere on a network.
 CORBA objects (like Java objects) can run on any platform.
 CORBA objects can be written in any of several languages.
 CORBA object developers need know nothing of where their clients
will be, what hardware or OS they will run on, or what language they
will be written in.
 CORBA objects approach universal accessibility.
 A client of an object has access to an object reference for the object,
and invokes operations on the object.
 A client knows only the logical structure of the object according to its interface and
experiences the behavior of the object through invocations.
 Client code has no knowledge of the implementation of the object or which ORB is used
to access the implementation.
 An object implementation provides the semantics of the object, usually
by defining data for the object instance and code for the object's
methods.
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A Request
 A request consists of:
 Target object (target object identified by a unique object reference)
 Operation
 Parameters (the input, output and in-out parameters defined for the operation;
may be specified individually or as a list)
 Optional request context
 Results (the result values returned by the operation)
Client
Object Implementation
Client Proxy
(stub code)
Skeleton
code
Request
ORB
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CORBA Framework Elements
 Object Request Broker (ORB)
 This is the object manager in CORBA
 Mechanisms for specifying interfaces
 Interface Definition Language (IDL) - for static interface definitions
 Dynamic Invocation Interface (DII) - lets clients access interfaces as first-class
objects at run-time from an Interface Repository.
 Internet Inter-Orb Protocol (IIOP)
 A binary protocol for communication between ORBs.
 Was added in CORBA 2.0
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Today
 CORBA Framework and Components
 CORBA IDL
 Steps in Developing CORBA Applications
 Examples
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Object Request Broker (ORB)
 The Object Manager in CORBA
 On client side the ORB is responsible for
 accepting requests for a remote object
 finding implementation of the object
 accepting client-side reference to the remote object(converted to a language
specific form, e.g., a Java stub object)
 routing client method calls through the object reference to the object
implementation
 On server side the ORB
 lets object servers register new objects
 receives requests from the client ORB
 uses object’s skeleton interface to invoke object’s activation method
 creates reference for new object and sends it back to client
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OMG IDL
 OMG Interface Definition Language (IDL):
 mappings for many languages/compilers;
 independent of any particular language/compiler;
 multiple-inheritance, public interface-structured specification language;
 not for implementation.
 primary support for interoperability between static and dynamic requests
mechanisms.
 IDL Structure
 Module
Module auction {
exception NotAllowed {};
struct Sale {
int price;
string item;
}
 a namespace
 Interface
 abstract type
 multiple inheritance
interface Auction {
void bid (in long price)
raises NotAllowed;
}
 Struct
 structured data
}
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Internet Inter-Orb Protocol (IIOP)
 CORBA specification is neutral with respect to network protocols
 the CORBA standard specifies what is known as the General Inter-ORB Protocol (GIOP)
 GIOP is a high-level standard protocol for communication between ORBs
 not used directly; instead, it is specialized by a particular protocol that would then be
used directly
 Internet Inter-ORB Protocol (IIOP)
 IIOP is the GIOP-based protocol for TCP/IP networks
 As of the 2.0 version of the CORBA specification, vendors are required to implement the
IIOP protocol
 CORBA Networking Model
 CORBA applications are built on top of GIOP-derived protocols such as IIOP
 these protocols, in turn, rest on top of TCP/IP, DCE, or other underlying transport
protocol the network uses
 an application architecture can be designed to use a bridge that would interconnect, for
instance, DCE-based application components with IIOP-based ones.
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Passing Objects by Reference
 In a distributed application, there are two possible methods for
an application component to obtain access to an object in
another process:
 When an object is passed by reference, the object itself remains "in place"
while an object reference for that object is passed. Operations on the object
through the object reference are actually processed by the object itself.
 When an object is passed by value, the object's state is copied and passed to its
destination (via object serialization), where a new copy of the object is
instantiated. Operations on that object's copy are processed by the copy, not by
the original object.
 Note: in CORBA, objects are generally passed by reference (however, CORBA
3.0 specification has now added pass-by-value semantics).
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Object References
 An Object Reference is the information needed to specify an object
within an ORB.
 The representation of an object reference handed to a client is only valid for the lifetime
of that client.
 The language mapping also provides additional ways to access object references in a
typed way for the convenience of the programmer.
 There is a distinguished object reference, the null reference, guaranteed to be different
from all object references, that denotes no object. In Java, this is a Java null.
 To invoke a CORBA object, you need a reference for the object. There
are two ways to get a reference for a CORBA object:
 from another object, such as a factory or a name service
 from a string that was specially created from an object reference
 Interoperable Object References
 CORBA uses IOR as a “pointer” to a specific instance of a class in a distributed
environment
 encodes host, port, object identity
 may be externalized (using object_to_string)
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CORBA Components
 Client stub
 Each stub represents (it is a proxy) an object operation (a possible request) which a client
invokes in a language-dependent manner (e.g., by calling a subroutine which represents
the operation).
 The stubs make calls on the rest of the ORB using interfaces that are private to JavaIDL.
 Alternatively, a client may dynamically construct and invoke request objects which can
represent any object operation.
 Implementation Skeleton
 Each skeleton provides the interface through which a method receives a request
(dynamic and static skeletons)
 Object Adapter
 Purpose is to interface an object's implementation with its ORB
 Each object adapter provides access to those services of an ORB (such as activation,
deactivation, object creation, object reference management) used by a particular type of
object implementation.
 ORB Interface
 The interface to the small set of ORB operations common to all objects, e.g., the
operation which returns an object's interface type.
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CORBA Components
Object Implementation
Client
Dynamic
Invocation
Client
Stubs
ORB
Interface
Implementation
Skeletons
Object
Adapter
ORB Core
standard interface
Proprietary ORB interface
One interface per object adaptor
Normal call interface
Up call interface
One interface per object operation
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Client Side
Clients perform requests using object references.
Clients may issue requests through
object interface stubs (static) or
dynamic invocation interface.
Client
Dynamic
Invocation
Client
Stubs
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ORB
Interface
Clients may access general ORB
services:
• Interface Repository.
• Context Management.
• List Management.
• Request Management.
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Implementation Side
Implementations receive requests through skeletons (without
knowledge of invocation approach).
Object Implementation
ORB
Interface
Implementation
Skeletons
Object
Adapter
The Object Adapter provides for:
• management of references;
• method invocation;
• authentication;
• implementation registration;
• activation/deactivation.
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Static v. Dynamic Invocation
 Static Invocation
 Static interfaces are generated in form of client stubs by the IDL (pre-)
compiler.
 This means that the structure of the object has to be known before hand (at
compile time).
 Allows for better type checking; less runtime overhead; self-documentation.
 Dynamic Invocation
 Dynamic Invocation Interface (DII) allows clients to invoke operations on
remote objects without having access to object stubs (another way to do this
without dynamic invocation is to download static client stubs via a Java
applet).
 Clients must discover interface-related information at runtime (e.g., using the
interface repository)
 Servers can offer new services anytime without the need for recompilation on
the client side.
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Dynamic Requests
 The Dynamic Invocation Interface (DII) allows clients to
dynamically:





discover objects;
discover objects’ interfaces;
create requests;
invoke requests;
receive responses.
 Major features of Dynamic Invocation Interface:
 requests appear as objects themselves;
 requests are reusable;
 invocation may be synchronous or asynchronous;
 requests may be generated dynamically, statically or in combination approach.
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CORBA Interface Repository
 The Interface Repository is a service that provides persistent
objects that represent the IDL information in a form available at
runtime.
 Note: The JavaIDL runtime does not include an implementation of an Interface
Repository and one is not generally required by clients at runtime.
 Using the IR, it is possible for a program to encounter an object whose
interface was not known at compile time, yet be able to determine what
operations are valid on the object and make invocation on it.
 Interface Repository provides:
 Dynamic client access to interface definitions to construct a request.
 Dynamic type-checking of request signatures.
 Traversal of inheritance graphs.
 ORB-to-ORB interoperability.
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CORBA Implementation Repository
 The Implementation Repository contains information that allows the
ORB to locate and activate implementations of objects.
 Ordinarily, installation of implementations and control of policies
related to the activation and execution of object implementations is
done through operations on the Implementation Repository.
 In addition to its role in the functioning of the ORB, the
Implementation Repository is a common place to store additional
information associated with implementations of ORB objects. (e.g.,
debugging information, administrative control, resource allocation,
security, etc)
 The Implementation Repository supports the implementation of object
servers. It is not needed by clients in order to access servers.
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Summary of CORBA Interfaces
Implementation
Installation
IDL Interface
Definitions
Interface
Repository
Accesses
Client
Stubs
Includes
Client
Implementation
Skeletons
Implementation
Repository
Includes
Describes
Object Implementation
 All objects are defined in IDL by specifying their interfaces.
 Object definitions (interfaces) are manifested as objects in the Interface
Repository, as client stubs, and as implementation skeletons.
 Descriptions of object implementations are maintained as objects in the
Implementation Repository.
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Summary: CORBA Remote Method Invocation
 Clients use “object interfaces” through language mapping
 Java clients should work on any ORB that supports the Java language bindings.
 Clients can call any object instance remotely, so long as the object instance
implements the interface.
 Clients can call remote objects statically or dynamically
 The server cannot tell whether the client is using static or dynamic invocation.
 Objects are identified using a unique id: Interoperable Object
Reference (IOR)
 CORBA passes objects by reference
 IOR was Introduced in CORBA 2.0
 Object references can be converted to strings and back to “live” objects via
ORB interface functions.
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Steps in Developing CORBA
Applications
 Write specification for each object using IDL
 Use IDL Compiler (e.g., idl2java) to generate:
 Client Stub code
 Server Skeleton code
 Write the client (in Java, can be applications or applets)
 Write the server object implementation code (the “servant”)
 Compile the client and server code
 Start the server
 Run the client application
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The Structure of CORBA IDL
(See Section 2.3.4 of Brose, Vogel, Duddy)
 Modules
 provide namespaces for a set of related class descriptions
 map to Java packages with the same name
 Interfaces
 specification of the operations a client can invoke on a remote object
 in addition to operations, interfaces include constructs such as:
 constant declarations
 attributes (can be read/write or readonly; implementation automatically creates get
and set operations for these attributes)
 exceptions (raised if the operations do not perform successfully)
 Operations
 CORBA equivalent of methods in Java
 IDL defines the operations’ signature: parameters and return values/types
 parameters can be in, out, or inout
 IDL also defines what exceptions can be raised by the operation
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Data Types in CORBA IDL
 Basic Types
 short, long, unsigned long, unsigned short, float, double, long double, char,
wchar, boolean, string, octet, etc.
 Constructed Types
 struct and union (similar to C++; can be used in conjunction with a typedef)
 sequence (variable sized arrays of objects)
 any (generic type which represents any possible IDL types; similar to the Java
Object type)
 enum (enumerated type with named integer values)
 arrays
 valuetypes (similar to interfaces; preceded with keyword valuetype to provide
pass-by-value semantics)
 Each CORBA IDL data type gets mapped to a native data type via
the appropriate language binding (e.g, IDL-to-Java mapping).
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IDL Syntax
module <identifier> {
<type declarations>;
<constant declarations>;
<exception declarations>;
interface <identifier> [:<inheritance>] {
<type declarations>;
<constant declarations>;
<attribute declarations>;
<exception declarations>;
[<op-type>]<identifier>(<parameters>)
[raises <exceptions>]
[<op-type>]<identifier>(<parameters>)
[raises <exceptions>]
. . .
};
interface <identifier> [:<inheritance>] {. . .};
. . .
};
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An IDL Example
module MyAnimals {
interface Dog: Pet, Animal {
attribute short age;
exception NotInterested{string explanation};
void Bark(in short how_long)
raises (NotInterested);
void Sit(in string where)
raises (NotInterested);
. . .
};
interface Cat: Animal {
void Eat();
. . .
};
};
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Another IDL Example
module RoomBooking {
interface Meeting {
readonly attribute string purpose;
readonly attribute string participants;
oneway void destroy();
};
interface MeetingFactory {
Meeting CreateMeeting(in string purpose,in string participants);
};
interface Room {
// A Room provides methods to view, make,cancel bookings.
enum Slot { am9, am10, am11, pm12, pm1, pm2, pm3, pm4 };
const short MaxSlots = 8;
typedef Meeting Meetings[ MaxSlots ];
exception NoMeetingInThisSlot {};
exception SlotAlreadyTaken {};
readonly attribute string name;
Meetings View();
void Book( in Slot a_slot, in Meeting
raises(SlotAlreadyTaken);
void Cancel( in Slot a_slot )
raises(NoMeetingInThisSlot);
a_meeting )
};
};
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The CORBA Count Example
The Count Program
 the client will invoke a method increment() on the remote object Count;
 increment() adds 1 to the value of the attribute sum and returns the value
to the client program;
 the initial value of sum is set by the client (sum is a read/write attribute, so the
client can use CORBA’s built-in accessor functions to manipulate it.
// Count.idl
First we must start
with an IDL
definition for the
Count object:
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module Counter
{
interface Count
{
attribute long sum;
long increment();
};
};
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The IDL Compiler
 Before using the IDL compiler or running the program make sure that
the PATH and CLASSPATH environment variables are properly set
up:
 Your PATH must point to the Java runtime environment directory as well as the
ORB runtime directory ( \inprise\vbroker\bin);
 Your CLASSPATH variable must include the current directory (“.”), the usual
Java classes and libraries (including the directory where your program classes
will be stored) as well as the following jar files required by Visibroker:
In Visibroker 3.x
\inprise\vbroker\lib\vbjcosnm.jar
\inprise\vbroker\lib\vbjorb.jar
\inprise\vbroker\lib\vbjapp.jar
\inprise\vbroker\lib\vbjtools.jar
\inprise\vbroker\lib\vbjgk.jar
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In Visibroker 4.x
\inprise\vbroker\lib\vbdev.jar
\inprise\vbroker\lib\vbjdev.jar
\inprise\vbroker\lib\vbjorb.jar
\inprise\vbroker\lib\vbjmigration.jar
\jdk1.2.2\lib\servlet.jar
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The IDL Compiler
 Now you can compile the IDL definition:
> idl2java -no_tie Count.idl
 Important Notes:
 if you are using Visibroker 3.3, you need JDK 1.1.8; as VBJ 3.3 is not
compatible with Java 2 platform
 in VBJ 4.x, idl2java compiler generated code is based on the POA (portable
object adapter) semantics; in order to use BOA semantics, must use:
> idl2java -boa Count.idl
also some additional casting needs to be done in BOA initialization (more on
this later, but see Chapters 30 and 31 of the VBJ 4.x Programmer’s Guide)
 in VBJ 3.x, the idl2java compiler generated code is based on BOA semantics
(also the API for the stub and skeleton code is different than those in VBJ 4.x)
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What Does the IDL Compiler Generate?
Count.idl
VBJ 3.x
Implementation Skeleton
Client Stub
idl2java
Client-Side
Server-Side
_st_Count.java
CountHelper.java
CountHolder.java
_CountImplBase.java
Count.java
_example_Count.java
Note: these source files will be a part of the Java package Counter and will be placed in
a directory with the same name.
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What Does the IDL Compiler Generate?
Count.idl
VBJ 4.x using POA
Implementation Skeleton
Client Stub
idl2java
Client-Side
Server-Side
_CountStub.java
CountHelper.java
CountHolder.java
CountPOA.java
Count.java
Note: if -boa option is used with idl2java compiler, _CountImplBase.java is also
generated to be used in the object implementation.
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Generated Classes & Interfaces
Counter._st_Count: implements the client-side stub for the Count object; it
provides the marshaling functions for the client. [_CountStub in VBJ 4.x].
Counter._CountImplBase: implements the server-side skeleton for Count; it
unmarshals the arguments passes by the client. [CountPOA in VBJ 4.x using POA].
Counter.CountHelper: provides some additional utility functions for the client,
including the narrow operation for downcasting CORBA object references, and (in
this case) the Visibroker specific bind operation for finding the remote object.
Counter.CountHolder: it is used as a wrapper for passing out and inout
parameters of type Count (Java only natively supports in parameters).
Counter.Count: contains the Java interface corresponding to the IDL interface for
Count; the object implementation for Count must implement this interface.
Counter._example_Count: a sample class for the Count object implementation
which could be filled-in by the programmer. [not generated in VBJ 4.x].
package Counter;
public interface Count extends org.omg.CORBA.Object {
public void sum(int sum);
public int sum();
public int increment();
}
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Class Hierarchy for BOA Count Interface
class
org.omg.CORBA.portable.Skeleton
(provided by the ORB)
class
org.omg.CORBA.portable.ObjectImpl
(provided by the ORB)
Extends
interface
Counter.Count
(generated by id2java)
Implements
abstract class
Counter._st_Count
(generated by id2java)
Extends
abstract class
Counter._CountImplBase
(generated by id2java)
Extends
Note: in case of POA, the object skeleton “CountPOA”
extends org.omg.CORBA.PortableServer.Servant
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class CountImpl
(written by programmer
and used by the server)
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Steps in Developing the Count Application
 Write a client program in Java:
 this can be an applet or an application;
 in this case we write a Java application CountClient.java.
 Write the server-side code for the Count application:
 write a server application which creates and serves instance(s) of the Count object
(instances of the class CountImpl);
 write the object implementation (CountImpl).
 Object Implementation:
 CORBA inheritance model: the implementation class is always derived from the
corresponding _XXXImplBase class generated by IDL compiler (in this case from
_CountImplBase; [Note: with POA this is XXXPOA, e.g., CountPOA]
 this inheritance allows the servant class to obtain the properties of both CORBA and Java
object models;
 the alternative to inheritance model is to use delegation (in CORBA terminology the Tie
method); if -no_tie option is not used with idl2java relevant classes will be
generated;
 in this case we create our CountImpl by modifying the file _example_Count.java.
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_example_Count.java
package Counter;
public class _example_Count extends Counter._CountImplBase {
public _example_Count(java.lang.String name) {
super(name);
}
public _example_Count() {
super();
}
public int increment() {
// implement operation...
return 0;
}
public void sum(int sum) {
// implement attribute writer...
}
public int sum() {
// implement attribute reader...
return 0;
} }
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The BOA Count Object Implementation
// CountImpl.java: The Count Implementation
class CountImpl extends Counter._CountImplBase
{
private int sum;
// Constructors
CountImpl(String name)
{
super(name);
System.out.println("Count Object Created");
sum = 0;
}
// get sum
public int sum() { return sum; }
// set sum
public void sum(int val) { sum = val; }
// increment method
public int increment() {
sum++;
return sum;
}
}
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Tasks Performed by the Main Server Class
 Initialize the Object Request Broker
 done by creating an instance of the ORB pseudo-object;
 the static Java method init() on the class org.omg.CORBA.ORB returns
an instance of an ORB.
 Initialize the Basic Object Adaptor
 a reference to an instance of the BOA is obtained via the method BOA_init().
 Create the object (an instance of the class implementation)
 in this case we create an instance of CountImpl class
 Activate the newly created object
 in BOA this is done by calling the obj_is_ready() method of the ORB
instance on the object;
 this action exports the object to the ORB.
 Wait for incoming requests
 in BOA this is done by calling the impl_is_ready() method of the ORB
instance on the object;
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// CountServer.java: The Count Server main program
class CountServer {
static public void main(String[] args) {
try {
// Initialize the ORB
org.omg.CORBA.ORB orb = org.omg.CORBA.ORB.init(args, null);
// Initialize the BOA
org.omg.CORBA.BOA boa = orb.BOA_init();
The
BOA
Count
Server
// Note: if using VBJ 4.x with -boa option the BOA
// initialization must be done with following cast:
// com.inprise.vbroker.CORBA.BOA boa =
//
((com.inprise.vbroker.CORBA.ORB)orb).BOA_init();
// Create the Count object
CountImpl count = new CountImpl("My Count");
// Export to the ORB the newly created object
boa.obj_is_ready(count);
// Ready to service requests
boa.impl_is_ready();
}
catch(org.omg.CORBA.SystemException e) {
System.err.println(e);
}
}
}
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Tasks Performed by the Client
 Initialize the ORB
 done the same way as for the server.
 Locate the Remote Object
 in this case client binds to the object via the Visibroker bind() method.
 Perform Operations via Remote Methods
 set the remote sum attribute to zero;
 calculate the start time;
 invoke the increment method 1000 times;
 calculate the elapsed time;
 print the results.
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The BOA Count Client
// CountClient.java
Static Client, VisiBroker for Java
class CountClient{
public static void main(String args[]) {
try {
// Initialize the ORB
org.omg.CORBA.ORB orb = org.omg.CORBA.ORB.init(args, null);
// Bind to the “persistent” Count Object reference
Counter.Count counter = Counter.CountHelper.bind(orb, "My Count");
// Set sum to initial value of 0
counter.sum((int)0);
// Calculate Start time
long startTime = System.currentTimeMillis();
// Increment 1000 times
for (int i = 0 ; i < 1000 ; i++ ) { counter.increment();}
// Calculate stop time; print out statistics
long stopTime = System.currentTimeMillis();
System.out.println("Avg Ping = " + ((stopTime-startTime)/1000f)+" msecs");
System.out.println("Sum = " + counter.sum());
}
catch(org.omg.CORBA.SystemException e) {
System.err.println("System Exception");
System.err.println(e); }
}
}
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Building and Running the Count Example
 Compile the client, server, and the implementation classes
vbjc -d \CorbaJavaBook.2e\classes CountClient.java
vbjc -d \CorbaJavaBook.2e\classes CountServer.java
vbjc -d \CorbaJavaBook.2e\classes CountImpl.java
Note: \CorbaJavaBook.2e\classes is where the Java
bytecode classes will be stored (without -d option classes are
generated in the current directory).
 Start the Visibroker Smart Agent (OSAgent)
 osagent -c
OR
start osagent -c
 OSAgent may be installed as an NT Service or started using the icon in the
program menu
 Start the Count Server: vbj CountServer
 Execute the Client Application:
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vbj CountClient
52
The Count
Program in
Action
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Next time
 More Examples of CORBA Applications
 CORBA Object References
 The POA version of Count Application
 More on IDL elements
 read/readonly attributes
 oneway methods
 parameter passing mechanisms
 exceptions and exception handling
 Discussion of Project Phase I
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