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Writing the Interface Definition Language (IDL) file

Note: All command and troubleshooting instructions apply to Java 2 SDK v1.3.0 and its version of idlj only.

Before you start working with Java IDL, you need to install version 1.3 of the Java 2 SDK. J2SDK v1.3.0 provides the Application Programming Interface (API) and Object Request Broker (ORB) needed to enable CORBA-based distributed object interaction, as well as the idlj compiler. The idlj compiler uses the IDL-to-Java mapping to convert IDL interface definitions to corresponding Java interfaces, classes, and methods, which you can then use to implement your client and server code.

This section teaches you how to write a simple IDL interface definition and how to translate the IDL interface to Java. It also describes the purpose of each file generated by the idlj compiler.

These topics are included in this section:

1. Writing Hello.idl
2. Understanding the IDL file
3. Mapping Hello.idl to Java
4. Understanding the idlj Compiler Output

1. Create a new directory, named Hello, for this application.

2. Start your favorite text editor and create a file named Hello.idl in this directory.

3. In your file, enter the code for the interface definition:

   module HelloApp
   {
     interface Hello
     {
       string sayHello();
     };
   };
   

4. Save the file.

Understanding the IDL file

OMG IDL is a purely declarative language designed for specifying programming-language-independent operational interfaces for distributed applications. OMG specifies a mapping from IDL to several different programming languages, including C, C++, Smalltalk, COBOL, Ada, and Java. When mapped, each statement in OMG IDL is translated to a corresponding statement in the programming language of choice. You can use the tool idlj to map an IDL interface to Java and implement the client class. When you map the same IDL to C++ and implement the server in that language, the Java client and C++ server interoperate through the ORB as though they were written in the same language.

The IDL for Hello World is extremely simple; its single interface has a single operation. You need perform only three steps:

1. Declare the CORBA IDL module
2. Declare the interface
3. Declare the operations

A CORBA module is a namespace that acts as a container for related interfaces and declarations. It corresponds closely to a Java package. Each module statement in an IDL file is mapped to a Java package statement.

The module statement looks like this:

module HelloApp
{
    // Subsequent lines of code here.
};

When you compile the IDL, the module statement will generate a package statement in the Java code.

Declaring the Interface

Like Java interfaces, CORBA interfaces declare the API contract an object has with other objects. Each interface statement in the IDL maps to a Java interface statement when mapped.

In your Hello.idl file, the interface statement looks like this:

module HelloApp
{
  interface Hello  // These
  {                // are the 
                   // interface
  };               // statement.
};

When you compile the IDL, this statement will generate an interface statement in the Java code. Your client and server classes will implement the Hello interface in different ways.

Declaring the Operations

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

In your Hello.idl file, the operation statement looks like this:

module HelloApp
{
  interface Hello
  {
    string sayHello();  // This line is the operation statement.
  };
};

The tool idlj reads OMG IDL files and creates the required Java files. The idlj compiler defaults to generating only the client-side bindings. If you need both client-side bindings and server-side skeletons (as you do for our Hello World program), you must use the -fall option when running the idlj compiler. For more information on the IDL-to-Java compiler options, follow the link.

1. Go to a command line prompt.

2. Change to the directory containing your Hello.idl file.

3. Enter the compiler command:

      
   	idlj -fall Hello.idl
   

If you list the contents of the directory, you will see that a directory called HelloApp has been created and that it contains six files. Open Hello.java in your text editor. Hello.java is the signature interface and is used as the signature type in method declarations when interfaces of the specified type are used in other interfaces. It looks like this:

package HelloApp;


/**
* HelloApp/Hello.java
* Generated by the IDL-to-Java compiler (portable), version "3.0"
* from Hello.idl
*/

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

With an interface this simple, it is easy to see how the IDL statements map to the generated Java statements.

IDL Statement		Java Statement
module HelloApp		package HelloApp;
interface Hello		public interface Hello

The single surprising item is the extends statement. All CORBA objects are derived from org.omg.CORBA.Object to ensure required CORBA functionality. The required code is generated by idlj; you do not need to do any mapping yourself.

In previous versions of the idlj compiler (known as idltojava), the operations defined on the IDL interface would exist in this file as well. Starting with J2SDK v1.3.0, the IDL-to-Java mapping puts all of the operations defined on the IDL interface in the operations interface, HelloOperations.java. The operations interface is used in the server-side mapping and as a mechanism for providing optimized calls for co-located clients and servers. For Hello.idl, this file looks like this:

package HelloApp;


/**
* HelloApp/HelloOperations.java
* Generated by the IDL-to-Java compiler (portable), version "3.0"
* from Hello.idl
*/

public interface HelloOperations 
{
  String sayHello ();
} // interface HelloOperations

Because there is only one operation defined in this interface, it is easy to see how the IDL statements map to the generated Java statements.

IDL Statement		Java Statement
string sayHello();		String sayHello();

Understanding the idlj Compiler Output

The idlj compiler generates a number of files. The actual number of files generated depends on the options selected when the IDL file is compiled. The generated files provide standard functionality, so you can ignore them until it is time to deploy and run your program. The files generated by the idlj compiler for Hello.idl, with the -fall command line option, are:

_HelloImplBase.java

This abstract class is the server skeleton, providing basic CORBA functionality for the server. It implements the Hello.java interface. The server class HelloServant extends _HelloImplBase.

_HelloStub.java

This class is the client stub, providing CORBA functionality for the client. It implements the Hello.java interface.

Hello.java

This signature interface contains the Java version of our IDL interface. The Hello.java interface extends org.omg.CORBA.Object, providing standard CORBA object functionality. It also extends IDLEntity, and is used as the signature type in method declarations when interfaces of the specified type are used in other interfaces.

HelloHelper.java

This final class provides auxiliary functionality, notably the narrow() method required to cast CORBA object references to their proper types.

HelloHolder.java

This final class holds a public instance member of type Hello. It provides operations for out and inout arguments, which CORBA allows, but which do not map easily to Java's semantics.

HelloOperations.java

This operations interface contains the single method sayHello(). The IDL-to-Java mapping puts all of the operations defined on the IDL interface into this file. The operations interface is used in the server-side mapping and as a mechanism for providing optimized calls for co-located clients and servers.

When you write the IDL interface, you do all the programming required to generate all these files for your distributed application. The next steps are to implement the client and server classes. In the steps that follow, you will create HelloClient.java and HelloApplet.java client classes and the HelloServer.java class.

Troubleshooting

• Error Message: "idlj" not found

  If you try to run idlj on the file Hello.idl and the system cannot find idlj, it is most likely not in your path. Make certain that the location of idlj (the J2SDK v.1.3 .bin directory) is in your path, and try again.

• Mapping IDL to Java: Overview

  Provides the basics on mapping IDL constructs to the corresponding Java statements.

• Chapter 3 of the OMG CORBA/IIOP specification

  Provides the complete specification for OMG Interface Definition Language. At this writing, the specification can be downloaded from http://www.omg.org/technology/documents/new_formal/corba.htm.