# JavaTM IDL FAQ

This is a document that attempts to answer many of the questions you may have about using JavaTM IDL technology. We will continue to update this document with your questions as we receive them. If you do not find your question answered here, please check the user-supported forum for Java IDL technology, which is available at http://forum.java.sun.com. For comments please send us email at javaidl@sun.com.

Errors

- Why do I get this message when I try to run HelloServer: Exception in thread "main" java.lang.NoClassDefFoundError: HelloServer/java

- My existing programs aren't running using J2SE 1.4. Are there any compatibility issues that could affect this?

- Why can't I connect using IOR's from a Linux server when the IOR contains 127.0.0.0 as the host address?

Features

- What is the thread model supported by the JDK 1.4 CORBA implementation?

- Does Java IDL contain notification/event services?

- Does Java IDL contain an Interface Repository?

- How can I run the Hello World example on two machines?

- Is Java IDL compliant with OMG CORBA specifications?

Compatibility/Interoperability

- How can I test whether my JDK ORB Client can communicate with an ORBIX Server?

- Do you have any examples that use a CORBA client and a Java server application?

- How do I connect to a 3rd-party Naming Service from Sun's ORB?

- Can I use the Java IDL ORB with a C++ CORBA server? (interoperability)

General

- Where can I download the idltojava compiler?

- What are my options for developing CORBA applications using Java technology?

- What is the difference between Java IDL and RMI-IIOP?

## Errors

### Why do I get this message when I try to run HelloServer: "Exception in thread "main" java.lang.NoClassDefFoundError: HelloServer/java"

> The most common reason for this error when running HelloServer (and also HelloClient) from the Java IDL tutorial is that `HelloServer` is not in the classpath. For more information on setting the classpath variable, see Setting the classpath.

> The second most common reason occurs when a space is added between quotation marks in the following line of code:

>     NameComponent nc = new NameComponent("Hello", "");

> There should be no spaces between the quotation marks. Without spaces, this value is passed as a null. With a space between the quotations, a space character is passed.

### My existing programs aren't running using J2SE 1.4. Are there any compatibility issues that could affect this?

> The `idlj` compiler now generates server-side mappings based on the POA Inheritance Model. For compatibility with existing applications, a new flag has been added to the `idlj` compiler to allow it to generate server-side mappings based on the ImplBase Inheritance Model, `-oldImplBase`. Existing applications that need to talk to servers created in J2SE 1.3 or prior may need to update their MAKEFILE so that this flag is used, however, new applications that have no such requirements should not generate these deprecated mappings.

### Why can't I connect using IOR's from a Linux server when the IOR contains 127.0.0.0 as the host address?

> On Red Hat Linux installations `InetAddress.getLocalHost()` may return an `InetAddress` corresponding to the loopback address (127.0.0.1). This arises because the default installation creates an association in `/etc/hosts` between the hostname of the machine and the loopback address. To ensure that `InetAddress.getLocalHost()` returns the actual host address, update the `/etc/hosts` file or the name service configuration file (`/etc/nsswitch.conf`) to query dns or nis before searching hosts.

## Features

### What is the thread model supported by the JDK 1.4 CORBA implementation?

> The Java CORBA ORB shipped in J2SE is multi-threaded. On the server side, we have a thread pool such that each incoming request is handled by a separate thread. If all the pool's threads are in use when a new request comes in, a new thread is created and added to the pool. The thread is returned to the pool after the request is finished.

> The Java CORBA ORB is threaded to allow scalability and concurrent request processing. The SINGLE_THREAD option for the POA threading policy is not supported.

> The threading model for the Java CORBA ORB is implicit: the user does not set the thread policy for the ORB. There are no external, user-level APIs exposed for controlling the threading model or the number of threads.

### Does Java IDL contain notification/event services?

> No, it does not. If you need an events service, you can implement one, purchase one off the shelf, or search for one freely available to start things off with.

### Does Java IDL contain an Interface Repository?

> No, it does not. If you need an Interface Repository, you can implement one, purchase one off the shelf, or search for one freely available to start things off with.

### How can I run the Hello World example on two machines?

> You need to use the `-ORBInitialHost <Host Name where Name Service is Launched>` option when starting the client and/or server (whichever is not running on the same machine as the name server). This way the client and server know where to find the Name Service. As example can be found on the Web at http://java.sun.com/j2se/1.4/docs/guide/idl/tutorial/jidl2machines.html.

### Is Java IDL technology compliant with CORBA specifications?

> With the `idlj` compiler in J2SE 1.3, we are CORBA 2.3 compliant in some areas but not in others. In J2SE 1.4, we are more compliant with CORBA 2.3. See the CORBA compliance statement at http://java.sun.com/j2se/1 .4/docs/api/org/omg/CORBA/doc-files/compliance.html.

> RMI-IIOP also mostly conforms to CORBA/IIOP 2.3.1, with GIOP 1.2 support along the lines of compliance with CORBA 2.3.

### How can I specify a different ORB?

> To use a different ORB than the Java CORBA ORB in your application, you can set the `org.omg.CORBA.ORBClass` property to the ORB of your choice. For example, to explicitly set the ORB to the POA ORB, you would set the property as follows in your code:

>     public class MyApp {

>         public static void main( String args[] ) {
>             Properties properties = System.getProperties();
>             properties.put( "org.omg.CORBA.ORBClass",
>                 "com.sun.corba.se.internal.POA.POAORB" );

>             try {
>                  ORB orb = ORB.init( args, properties);

>                  ...

> Replace the path to the POA ORB with the ORB of your choice.

> We haven't tested the interoperability of the Java ORB with other vendor's ORBs.

## Compatibility/Interoperability

### How can I test whether my JDK ORB Client can communicate with an ORBIX Server?

> First, we have not done compatibility testing with any other ORBs from any other vendors.

> Second, try this. Convert the Interoperable Object References (see IOR discussion in INS tutorial) on the ORBIX server to a string using the `ORB.object_to_string()` method. Write that string to a file.

> On the client side, read that string and convert it to an object reference using the `ORB.string_to_object()` method, then try communicating with it.

> To communicate with the Java IDL Naming Service or ORBIX Name Server, you must use Interoperable Naming Service (INS) or by stringifying `RootNamingContext`'s reference as explained above.

### Do you have any examples that use a CORBA client and a Java server application?

> A white paper that includes an example application with a C++ client and an Enterprise JavaBeans component can be found on the Web site at: http://java.sun.com/j2se/1.4/docs/guide/rmi-iiop/interop.pdf.

### How do I connect to a 3rd-party Naming Service from Sun's ORB?

> The recommended way is to use the Inter-Operable Naming Service (INS), if this option is supported by the 3rd-party Naming Service.

> To use the Sun ORB with another vendor's Naming Service,

> 1. Start the 3rd-party name server on Host and Port.

> 2. Pass the following argument to `ORB.init()`:

>                -ORBInitRef NameService=corbaloc:iiop:1.2@:/NameService

> When you do `orb.resolve_initial_references( "NameService" )`, you should be able to connect to 3rd-party Name Service. If you are still not able to connect, try these troubleshooting hints:

> - Verify that the 3rd-party Name Service supports INS.
> - Verify that the host and port information is accurate.
> - Verify that the 3rd-party Name Service has been started successfully.
> - Verify that the 3rd-party Name Service supports GIOP 1.2. If not, refer to the Name Server's documentation for the correct the GIOP version, and modify the `corbaloc:` URL accordingly.
> - Determine if the 3rd-party Name Service has a different object key it uses to contact `NameService`. If so, refer to the Name Server's documentation.

### Can I use the Java IDL ORB with a C++ CORBA server? (interoperability)

> The Java IDL ORB is an ORB completely written using Java technology. The `idlj` compiler generates code that follows the conventions defined in the IDL to Java Language Mapping Specification. The Java ORB does not provide a compiler that generates code in any languages other than the Java platform. If you want to test interoperability between the Java ORB and an ORB written in another language (such as C++), you will need to find an ORB written in that language and a compiler that conforms to the specific language mapping. Language mapping specifications are available from the Object Management Group Web site at http://www.omg.org/technology/documents/formal/corba_language_mapping_specs.htm. A user who wants to use the Java platform on one side and C++ on the other will only share the IDL. You will have to use your C++ ORB's tools to generate the stubs and skeletons for use with the C++ ORB, but you don't have to change the IDL at all. To find a vendor that provides a CORBA ORB and a language mapping compiler in the language you are working with, search the Web for "C++ CORBA ORB", or equivalent.

> A white paper that includes an example application with a C++ client and an Enterprise JavaBeans component can be found on the Web site at: http://java.sun.com/j2se/1.4/docs/guide/rmi-iiop/interop.html.

> Although it is true that ORBs written in different languages should be able to talk to each other, we haven't tested the interoperability of the Java ORB with other vendor's ORBs.

## General

### Where can I download the idltojava compiler?

> We highly recommend that you use the latest version of the IDL-to-Java compiler, `idlj` for all new code. If you still need to use `idltojava` for legacy applications, you can download the `idltojava` compiler for JDK 1.2 from http://developer.java.sun.com/developer/earlyAccess/jdk12/idltojava.html.

> To get the latest version of the IDL-to-Java compiler, download the latest version of JavaTM 2 Platform, Standard Edition (J2SETM). When J2SE is installed, `idlj` will be located in the `bin` directory.

> Other troubleshooting information related to `idltojava` is located in the Java IDL FAQ for J2SE v.1.3.

### What are my options for developing CORBA applications using Java technology?

> CORBA technology as part of the Java 2 platform consists of an Object Request Broker (ORB) written in the Java language (with a small bit of native code), the RMI programming model, and the IDL programming model.

> The "magic" that gives us the cross-language, cross-vendor interoperability is the Internet InterORB Protocol, or IIOP. IIOP can be a transport protocol for distributed applications written in either IDL or Java RMI. IIOP allows distributed objects to conform to OMG's CORBA specification.

> When using the IDL programming model, the interface is everything! It defines the points of entry that can be called from a remote process, such as the types of arguments the called procedure will accept, or the value/output parameter of information returned. Using IDL, the programmer can make the entry points and datatypes that pass between communicating processes act like a standard language.

> CORBA is a language-neutral system in which the argument values or return values are limited to what can be represented in the involved implementation languages. In CORBA, object orientation is limited only to objects that can be passed by reference (the object code itself cannot be passed from machine-to-machine) or are predefined in the overall framework. Passed and returned types must be those declared in the interface.

> With RMI, the IDL and the implementation language are the same thing, so you don't have to worry about mapping from one to the other. Language-level objects (the code itself) can be passed from one process to the next. Values can be returned by their actual type, not the declared type. Or, you can compile the interfaces to generate IIOP-compliant stubs and skeletons to talk to objects written on other machines in other CORBA-compliant languages.

### What is the difference between Java IDL and RMI-IIOP?

> This is a fundamental question and it's important to understand the distinction between these two ways of integrating the Java programming language with CORBA.

> Java IDL is for CORBA programmers who want to program in the Java programming language based on interfaces defined in CORBA Interface Definition Language (IDL). This is "business as usual" CORBA programming, supporting the Java language in exactly the same way as other languages like C++ or COBOL.

> RMI-IIOP (Remote Method Invocation over Internet Inter-ORB Protocol) is for developers who want to use the Java programming language to program to the Java RMI interfaces, but use IIOP as the underlying transport. RMI-IIOP provides interoperability with other CORBA objects implemented in various languages - but only if all the remote interfaces are originally defined as Java RMI interfaces. It is of particular interest to programmers using Enterprise JavaBeans (EJB), since the remote object model for EJB's is RMI-based.

> There are several scenarios that will define how you will want to create distributed CORBA applications. Here are some of them:

- > Java IDL! If you have been developing CORBA applications using IDL for some time, you will probably want to stay in this environment. Create the interfaces using IDL, and define the client and server applications using the Java programming language to take advantage of its "Write Once, Run AnywhereTM" portability, its highly productive implementation environment, and its very robust platform.

- > RMI-JRMP! If all of your applications are written in the Java programming language, you will probably want to use Java RMI technology to enable communication between Java objects on different virtual machines and different physical machines. Using Java RMI without its IIOP option leverages its strengths of code portability, security, and garbage collection.

- > RMI-IIOP! If you are writing most of your new applications using the Java programming language, but need to maintain legacy applications written in other programming languages as well, you will probably want to use Java RMI with its IIOP compiler option.

###

## Limitations

For more information on product limitations, read Limitations.

### Can I use multiple profiles in Interoperable Object References (IOR)?

> If an IOR contains multiple profiles, the J2SE v.1.4 ORB always uses the first one.

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