Java mapping Clients and Servers

Understand the IDL (Interface Definition Language) Basics

• IDL is used in CORBA to define the interfaces that objects present to the outside world.
• Learn the IDL syntax: module, interface, struct, enum, typedef, attribute, operation.

Example IDL:

module HelloApp {
  interface Hello {
    string sayHello();
    oneway void shutdown();
  };
};

Learn the IDL-to-Java Mapping Rules

Each IDL construct maps to a Java construct as per the CORBA specifications.

Use the idlj Compiler

Compile your .idl file using the idlj compiler:

idlj -fall Hello.idl

This generates:

• _Hello.java — The client stub interface.
• HelloHelper.java — Helper class for marshalling/unmarshalling.
• HelloHolder.java — Used for out/inout parameters.
• _HelloStub.java — Client-side stub that communicates with the ORB.
• HelloPOA.java — Abstract server skeleton to be extended for implementation.

Dissect the Generated Files

• _Hello.java

  An interface for client applications.

• _HelloStub.java

  Implements _Hello.java, marshals requests to the remote object.

• HelloPOA.java

  Abstract base class for the servant. You override it to implement business logic.

• HelloHelper.java

  Provides helper methods like:

  Hello narrow(org.omg.CORBA.Object obj);
  void insert(Any a, Hello that);
  Hello extract(Any a);
  

• HelloHolder.java

  Used for out and inout parameters.

Implement the Server

You write a class that extends HelloPOA and implements the sayHello() method.

public class HelloImpl extends HelloPOA {
  public String sayHello() {
    return "Hello, world!";
  }
  public void shutdown() {
    orb.shutdown(false);
  }
}

Compile and Run Using ORB

Use ORB.init(args, null) and bind the servant in the CORBA Naming Service. Then, use the stub on the client side.

Review IDL-Java Type Mappings

Use a Visual Diagram

Create a visual mapping flow:

IDL Hello.idl
    ↓
idlj compiler
    ↓
Java Interfaces + Stubs + Skeletons
    ↓
Implement HelloPOA.java → Run ORB server
    ↓
Use _HelloStub.java in the client via Naming Service

Deepen by Debugging and Tracing

Use logging or IDE breakpoints to trace:

• how the client calls the stub,
• how the stub uses the ORB,
• how the ORB routes to the skeleton (servant),
• how results are marshalled back.

1. Define a simple IDL for a calculator.
2. Generate Java bindings using `idlj`.
3. Implement the server logic.
4. Write a CORBA client to invoke the operations.
5. Extend the IDL with new operations or structs and regenerate.