Mapping Hello.idl to Java

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:


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:


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:


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:


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


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


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.


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


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.


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.

For More Information

• 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

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