Documentation Contents

# Java IDL FAQ

This is a document that attempts to answer many of the questions you may have about using Java IDL technology.

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 this release. 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?
- What does this minor code mean?
- Can I add multiple classpaths when starting a server using `servertool`?
- What do I do when the naming service (`tnameserv`, `orbd`) fails to run on the Linux platform??

Features

- What is the thread model supported by the CORBA implementation in this release?
- Does Java IDL contain notification/event services? an Interface Repository?
- How can I run the Hello World example on two machines?
- Is Java IDL compliant with OMG CORBA specifications?

Compatibility/Interoperability

- With which CORBA specifications does J2SE 5.0 comply?
- How can I specify a different ORB?
- How can I test whether my JDK ORB Client can communicate with another vendor's ORB implementation?
- 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

- Should I use Java IDL or Java RMI-IIOP technology?
- 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 Java RMI-IIOP?
- What are the limitations of Java IDL and Java RMI-IIOP in this release?
- What are the standard tags for ...?

## 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 class path.

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 this release. Are there any compatibility issues that could affect this?

Information on compatibility between J2SE 1.4.x and J2SE 5.0 can be found in CORBA Compatibility Information - J2SE 5.0.

### 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.

### What does this minor code mean?

The most recent versions of the standard (OMG) minor code exceptions can be found on the OMG Web site at http://www.omg.org/.

Some of the most common Sun minor code exceptions are the following:

- COMM_FAILURE/201. vmcid: SUN minor code: 201 literally means "CONNECT_FAILURE". This may be caused by a `java.net.SocketException`, usually one of `BindException`, `ConnectException`, or `NoRouteToHostException`.

  Some things to verify are:

  - Is a naming service running? If not, start the ORBD naming service as described in the `orbd` man page (Solaris, Linux, or Mac OS X or Windows).

  - Are the `-ORBInitialHost` and `-ORBInitialPort` values being set correctly for the naming service? If you are uncertain about what the settings should be, read the `orbd` man page (Solaris, Linux, or Mac OS X or Windows).

  - Are the client and server applications aware of the port number (and machine name, if applicable) where the Naming Service is running? Read the `orbd` man page (Solaris, Linux, or Mac OS X or Windows) for more information on how to do this.

- COMM_FAILURE/208. vmcid: SUN minor code: 208 literally means "CONNECTION ABORT", which generally means the connection has been dropped.

- COMM_FAILURE/209. vmcid: SUN minor code: 209 literally means "CREATE_LISTENER_FAILED": `Unable to create the listener thread on the specific port. Either the post is taken or there was an error creating the daemon thread`. This generally indicates that the port on which you are trying to run the naming service is in use by another process. If you are running on Solaris, you could discover whether or not something is running on this port using the following terminal prompt command:

      netstat | grep port_number

- OBJECT_NOT_EXIST/204. vmcid: SUN minor code: 204 literally means "SERVANT_NOT_FOUND". It is only thrown in one place: `corba.INSSubcontract.getINSReference`.

- MARSHAL/217. vmcid: SUN minor code: 217 means that your client tried to send either a `wchar` or `wstring` in GIOP 1.0, which is not legal in the spec.

- MARSHAL/202. vmcid: SUN minor code: 202 means that the code is attempting to marshal an object that derives from `org.omg.CORBA.Object`, but that particular instance has never been connected to an ORB. When using the POA, you need to register the object with the POA first. If you need more information on how to register an object with the POA, refer to Portable Object Adapter (POA) or the Getting Started with Java™ IDL tutorial.

- BAD_PARAM/201. vmcid: SUN minor code: 201 literally means "NULL_PARAM". This exception often occurs because a Java `null` was given to a `write` method such as `write_string`, `write_octet_array`, etc. You cannot return a Java `null` as the result of a Java method.

- org.omg.CORBA.INTERNAL. vmcid: SUN minor code: 208 means `Unable to determine local hostname using InetAddress.getLocalHost().getHostName()`.

  The ORB uses `InetAddress.getLocalHost().getHostName()` to create a reference to the name service for looking for and/or binding references. It also uses `InetAddress.getLocalHost().getHostName()` on the server side to create remote object references (i.e., IORs) that contain the name/port of the server (rather than a dotted-decimal/port pair).

  To avoid the call to `getHostName`, you can set the following properties (refer to the `orbd` man page [Solaris, Linux, or Mac OS X or Windows] if you are not sure how to do this):

  - Set `com.sun.CORBA.ORBServerHost` to the DNS name or dotted-decimal address of the server if the ORB is acting as a server.

  - Set `com.sun.CORBA.ORBInitialHost` to the DNS name or dotted-decimal address of the name server.

  NOTE: These properties are proprietary and are subject to deletion or change.

### Can I add multiple classpaths when starting a server using `servertool`?

The longer version of this question was as follows: My server code needs multiple JAR files in the classpath. When I register my server using the `servertool`, and try to specify the classpath, I get the syntax of the `register` command. I assume my command line is exceeding the limit.

How do I specify a really long classpath while registering the server in the `servertool`?

The `servertool` does accept multiple JAR files, however, there is currently a bug with the length of the command line buffer in `servertool`. Read more about the `servertool` wrapper bug, 4482166.

The workaround is to call the `ServerTool` class to start the server, rather than starting the server from within `servertool`, such as:

    ${JAVA_HOME}/bin/java com.sun.corba.se.internal.Activation.ServerTool
    -ORBInitialPort ${ORB_INITIAL_PORT} -cmd register -server
    sample.MyServer -classpath jar1:jar2:jar3 -applicationName sample

Please note that the name of the `servertool` class may change in future releases.

### What do I do when the naming service (`tnameserv`, `orbd`) fails to run on the Linux platform?

If the naming service fails to startup on Linux, include the following lines in the `/etc/hosts` file:

    <Local Host IP Address>   localhost
    <Local Host IP Address>   <HostName>

where <Local Host IP Address> is the IP address where the naming service (`tnameserv`, `orbd`) is started.

## Features

### What is the thread model supported by the CORBA implementation in this release?

The Java CORBA ORB shipped in Java SE 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? an Interface Repository?

No, it does not. If you need one of these services, you can implement one, you can purchase one off the shelf, or you can search for one freely available to start things off with. You can plug-in 3rd party services such as these to the ORB using INS technology.

### 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. An example can be found at Java IDL: The "Hello World" Example on Two Machines.

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

For compliance information for CORBA technology shipped with J2SE 5.0, read the CORBA compliance statement at Official Specifications for CORBA support in J2SE 5.0.

## Compatibility/Interoperability

### With which CORBA specifications does J2SE 5.0 comply?

See Official Specifications for CORBA support in J2SE 5.0.

### How can I specify a different ORB?

To use an ORB other 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, use code as shown in the example to explicitly set the ORB to the ORB-implementation you would prefer to use:

    public class MyApp {

        public static void main( String args[] ) {
            Properties properties = System.getProperties();
            properties.put( "org.omg.CORBA.ORBClass",
                "<com.other_company.package.ORB>" );

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

                 ...

Check the vendor's documentation for information on setting properties specific to their ORB implementation.

### How can I test whether my JDK ORB Client can communicate with another company's ORB implementation?

This ORB has been tested as part of the J2EE certification process, however, it has not been tested with all vendor's standalone CORBA ORBs.

If INS is supported in the other vendor's service, then, try this. Convert the Interoperable Object References (see Java ID: Interoperable Naming Service Example) on the other vendor's ORB server to a string using the `ORB.object_to_string()` method using INS. Write that string to a file.

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

A developer guide that includes an example application with a C++ client and an Enterprise JavaBeans component can be found on the Web site at: Enterprise JavaBeans™ Components and CORBA Clients: A Developer's Guide.

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

The recommended way is to use the Java IDL: Interoperable Naming Service (INS) Example, 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 <H> and Port. <P>
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/. 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 developer guide that includes an example application with a C++ client and an Enterprise JavaBeans component can be found on the Web site at: Enterprise JavaBeans™ Components and CORBA Clients: A Developer's Guide.

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

### Should I use Java IDL or Java RMI-IIOP technology?

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 technology 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 Java in exactly the same way as other languages like C++ or COBOL.

Java Remote Method Invocation over Internet Inter-ORB Protocol ("RMI-IIOP") technology is for Java programmers who want to program to the Java Remote Method Invocation ("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™) technology, since the EJB remote object model is based on Java RMI technology.

### Where can I download the idltojava compiler?

The idltojava compiler is no longer available for download. We highly recommend that you use the latest version of the IDL-to-Java compiler, idlj.

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

### 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 Java 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. Java 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. Java 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 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.
- Java 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.

### What are the limitations when using Java IDL and/or Java RMI-IIOP?

For more information on product limitations, read Limitations.

### What are the standard tags for ...?

The most recent versions of standard tag allocation files can be found at the OMG Web site: http://www.omg.org/.

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