Getting Started with Java IDL:
Writing the Interface Definition

Note: All command and troubleshooting instructions apply to the Java 2 Platform, Standard Edition, version 1.4 and its version of idlj only.

Before you start working with Java IDL, you need to install version 1.4 of J2SE. J2SE v.1.4 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 language 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:

Writing Hello.idl
Understanding the IDL file
Mapping Hello.idl to Java
Understanding the idlj Compiler Output

Writing `Hello.idl`

To create the Hello.idl file,

Create a new directory, named Hello, for this application.
Start your favorite text editor and create a file named Hello.idl in this directory.

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

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

Save the file.

Understanding the IDL file

OMG IDL is the language used to describe the interfaces that client objects call and object implementations provide. An interface definition written in OMG IDL completely defines the interface and fully specifies each operation's parameters. An OMG IDL interface provides the information needed to develop clients that use the interface's operations.

Clients are written in languages for which mappings from OMG IDL concepts have been defined. The mapping of an OMG IDL concept to a client language construct will depend on the facilities available in the client language. OMG specifies a mapping from IDL to several different programming languages, including C, C++, Smalltalk, COBOL, Ada, Lisp, Python, and Java. When mapped, each statement in OMG IDL is translated to a corresponding statement in the programming language of choice.

For example, you could use the tool idlj to map an IDL interface to Java and implement the client class. When you mapped the same IDL to C++ and implemented the server in that language, the Java client (through the Java ORB) and C++ server (through the C++ ORB) interoperate as though they were written in the same language.

The IDL for "Hello World" is extremely simple; its single interface has but two operations. You need perform only three steps:

Declare the CORBA IDL module
Declare the interface
Declare the operations

Declaring the CORBA IDL Module

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 lines
  {                // declare the 
                   // interface
  };               // statement.
};

When you compile the IDL, this statement will generate an interface statement in the Java code.

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 an operation statement.
    oneway void shutdown();   // This line is another
  };
};

The interface definition for our little "Hello World" application is now complete.

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.

New in J2SE v.1.4: The default server-side mapping generated when either the -fall or -fserver arguments are used conform to Chapter 11, Portable Object Adapter (POA) of the CORBA 2.3.1 Specification (formal/99-10-07). For more information on the POA, link to Portable Object Adapter.

The advantages of using the Portable Object Adaptor (POA) are:

Allow programmers to construct object implementations that are portable between different ORB products.
Provide support for objects with persistent identities.
Provide support for transparent activation of objects.
Allow a single servant to support multiple object identities simultaneously.

Make sure that the jdk/bin directory (or the directory containing idlj, java, javac, and orbd) are in your path.
Go to a command line prompt.
Change to the directory containing your Hello.idl file.
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:

//Hello.java
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, in conformance with the CORBA 2.3.1 Specification (formal/99-10-07), 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:

//HelloOperations.java
package HelloApp;


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

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

Because there are only two operations 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();`
`oneway void shutdown();`		`void Shutdown ();`

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. Under J2SE v.1.4, the files generated by the idlj compiler for Hello.idl, with the -fall command line option, are:

HelloPOA.java
This abstract class is the stream-based server skeleton, providing basic CORBA functionality for the server. It extends org.omg.PortableServer.Servant, and implements the InvokeHandler interface and the HelloOperations interface. The server class, HelloServant, extends HelloPOA.
_HelloStub.java
This class is the client stub, providing CORBA functionality for the client. It extends org.omg.CORBA.portable.ObjectImpl and implements the Hello.java interface.
Hello.java
This 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 the HelloOperations interface and org.omg.CORBA.portable.IDLEntity.
HelloHelper.java
This class provides auxiliary functionality, notably the narrow() method required to cast CORBA object references to their proper types. The Helper class is responsible for reading and writing the data type to CORBA streams, and inserting and extracting the data type from Anys. The Holder class delegates to the methods in the Helper class for reading and writing.
HelloHolder.java
This final class holds a public instance member of type Hello. Whenever the IDL type is an out or an inout parameter, the Holder class is used. It provides operations for org.omg.CORBA.portable.OutputStream and org.omg.CORBA.portable.InputStream arguments, which CORBA allows, but which do not map easily to Java's semantics. The Holder class delegates to the methods in the Helper class for reading and writing. It implements org.omg.CORBA.portable.Streamable.
HelloOperations.java
This interface contains the methods sayHello() and shutdown(). The IDL-to-Java mapping puts all of the operations defined on the IDL interface into this file, which is shared by both the stubs and skeletons.

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 the HelloClient.java client class and the HelloServer.java server 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.1.4 bin directory) is in your path, and try again.

For More Information

IDL to Java Language Mapping Overview
Provides the basics for mapping IDL constructs to the corresponding Java statements.
Chapter 3 of the OMG CORBA 2.3.1 specification, formal/99-10-07
Provides the complete specification for OMG Interface Definition Language. At this writing, the specification can be downloaded from http://cgi.omg.org/c gi-bin/doc?formal/99-10-07.

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Getting Started with Java IDL: Writing the Interface Definition

Understanding the IDL file

Getting Started with Java IDL:
Writing the Interface Definition