# RMI-IIOP Programmer's Guide

## Copyright Information

Note: Before using this information and the product it supports, be sure to read the general information under Notices.

This edition of the User Guide applies to RMI-IIOP, as described below, and to all subsequent releases and modifications until otherwise indicated in new editions.

(c) Copyright Sun Microsystems, Inc. 1995, 2002, 901 San Antonio Rd., Palo Alto, CA 94303 USA. All rights reserved.

(c) Copyright International Business Machines Corporation, 1998, 2004. All rights reserved.

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## Table of Contents

- Introduction

- Background Reading

- What is RMI-IIOP?
- The New rmic Compiler
- The New idlj Compiler
- How to Make RMI Programs Use IIOP
- Restrictions When Running RMI Programs Over IIOP
- Converting the RMI Hello World Program to RMI-IIOP
- Other Things You Should Know
- Notices

- Trademarks

## Introduction

This document discusses how to write Java Remote Method Invocation (RMI) programs that can access remote objects by using the Internet Inter-ORB Protocol (IIOP). By making your RMI programs conform to a small set of restrictions, your RMI programs can access CORBA objects. RMI-IIOP gives you RMI ease-of-use coupled with CORBA/IIOP language interoperability.

### Background Reading

Here are some sites to get you up to speed with this technology:

- The RMI-IIOP home page contains links to RMI-IIOP documentation, sample code, specifications, news, other related web sites, and more. Check it out!
- The RMI-IIOP FAQ page answers many basic RMI-IIOP questions.
- The Java RMI home page contains links to RMI documentation, examples, specification, and more. Make sure you read this.
- The RMI trail in the Java Tutorial.
- The RMI API javadoc contains the most up-to-date RMI API documentation.
- The Java IDL web page will familiarize you with Sun's CORBA/IIOP implementation.
- The Java IDL Trail in the Java Tutorial.

## What is RMI-IIOP?

### RMI

With RMI you can write distributed programs in the Java programming language. RMI is easy to use, you don't need to learn a separate interface definition language (IDL), and you get Java's inherent "write once, run anywhere" benefit. Clients, remote interfaces, and servers are written entirely in Java. RMI uses the Java Remote Method Protocol (JRMP) for remote Java object comunication. To get a quick introduction to writing RMI programs, see the RMI Tutorial web page. That document describes writing a simple "Hello World" RMI program.

RMI lacks interoperability with other languages, and, because it uses a non-standard communication protocol, cannot communicate with CORBA objects.

### IIOP, CORBA, and Java IDL

IIOP is CORBA's communication protocol. It defines the way the bits are sent over a wire between CORBA clients and servers. CORBA is a standard distributed object architecture developed by the Object Management Group (OMG). Interfaces to remote objects are described in a platform-neutral interface definition language (IDL). Mappings from IDL to specific programming languages are implemented, binding the language to CORBA/IIOP.

The Java(TM) 2 Platform, Standard Edition (J2SE) v1.4 CORBA/IIOP implementation is known as Java IDL. Along with the `idlj` compiler, Java IDL can be used to define, implement, and access CORBA objects from the Java programming language.

The Java IDL web page gives you a good, Java-centric view of CORBA/IIOP programming. To get a quick introduction to writing Java IDL programs, see the Getting Started: Hello World web page.

### RMI-IIOP

Previously Java programmers had to choose between RMI and CORBA/IIOP (Java IDL) for distributed programming solutions. Now, by adhering to a few restrictions, RMI server objects can use the IIOP protocol, and communicate with CORBA client objects written in any language. This solution is known as RMI-IIOP. RMI-IIOP combines RMI-style ease of use with CORBA cross-language interoperability.

## The rmic Compiler

The RMI-IIOP software comes with an `rmic` compiler that can generate IIOP stubs and ties, and emit IDL, in accordance with the Java Language to OMG IDL Language Mapping Specification.

Here are the `rmic` flags that provide support for RMI-IIOP:

> -iiop
>   Generates IIOP stubs/ties
> -iiop -poa
>   Generates IIOP stubs/ties that work with a Portable Object Adapter (POA).
> -idl
>   Generates IDL
> -always
>   Forces re-generation even when existing stubs/ties/idl are newer than the input class. Only valid when -iiop and/or -idl flags are present.
> In addition the following options may be used along with the `-idl` option:
> -noValueMethods
>   Stops generation of IDL for methods and constructors within IDL valuetypes
> -idlModule <fromJavaPackage<.class>> <toIDLModule>
>   Specifies IDLEntity package mapping. For example:
> -idlModule foo.bar my::real::idlmod
> -idlFile <fromJavaPackage<.class>> <toIDLFile>
>   Specifies IDLEntity file mapping. For example:
> -idlFile test.pkg.X TEST16.idl

For more detailed information on the `rmic` compiler refer to the RMIC tool page (Solaris Version/Windows version).

### The -iiop Flag

Using `rmic` with the `-iiop` option generates stub and tie classes. A stub class is a local proxy for a remote object. Stub classes are used by clients to send calls to a server. Each remote interface requires a stub class, which implements that remote interface. The client's reference to a remote object is actually a reference to a stub. Tie classes are used on the server side to process incoming calls, and dispatch the calls to the proper implementation class. Each implementation class requires a tie class.

Stub classes are also generated for abstract interfaces. An abstract interface is an interface that does not extend `java.rmi.Remote`, but whose methods all throw either `java.rmi.RemoteException` or a superclass of `java.rmi.RemoteException`. Interfaces that do not extend `java.rmi.Remote` and have no methods are also abstract interfaces.

### The -iiop -poa Flag

New to this release of J2SE is the `-iiop -poa` option. Using the `-iiop` flag with the `-poa` option changes the inheritance from `org.omg.CORBA_2_3.portable.ObjectImpl` to `org.omg.PortableServer.Servant`. This type of mapping is nonstandard and is not specified by the Java Language to IDL Mapping Specification.

The `PortableServer` module for the Portable Object Adapter (POA) defines the native `Servant` type. In the Java programming language, the `Servant` type is mapped to the Java `org.omg.PortableServer.Servant` class. It serves as the base class for all POA servant implementations and provides a number of methods that may be invoked by the application programmer, as well as methods which are invoked by the POA itself and may be overridden by the user to control aspects of servant behavior.

### The -idl Flag

Using `rmic` with the `-idl` option generates OMG IDL for the classes specified and any classes referenced. This option is only required if you have a CORBA client written in another language that needs to talk to a Java RMI-IIOP server.

Note: After the OMG IDL is generated using `rmic -idl`, use the generated IDL with an IDL-to-C++ or other language compiler, but not with the IDL-to-Java language compiler. "Round tripping" is not recommended and should not be necessary. The IDL generation facility is intended to be used with other languages such as C++. Java clients or servers can use the original RMI-IIOP types.

IDL provides a purely declarative, programming language independent means for specifying the API for an object. The IDL is used as a specification for methods and data that can be written in and invoked from any language that provides CORBA bindings. This includes Java and C++ among others. See the Java Language to IDL Mapping (OMG) document for a complete description.

Note: The generated IDL can only be compiled using an IDL compiler that supports the CORBA 2.3 extensions to IDL.

### The -noValueMethods Flag

The `-noValueMethods` option, when used with `-idl`, ensures that methods and initializers are not included in `valuetype`s emitted during IDL Generation. These are optional for `valuetype`s and are otherwise omitted.

## The idlj Compiler

The RMI-IIOP software includes an IDL-to-Java compiler. This compiler supports the CORBA Objects By Value feature, which is required for interoperation with RMI-IIOP. It is written in Java, and so can run on any platform. See the IDL-to-Java Compiler User's Guide for details of how to use this compiler.

## How to Make RMI Programs Use IIOP

The following steps are a general guide to converting an RMI application to RMI-IIOP.

1. If you are using the RMI registry for naming services, you need to switch to `CosNaming`. You need to do the following:

1. In both your client and server code, you need to create an `InitialContext` for JNDI using the following code:

             import javax.naming.*;
             ...
             Context ic = new InitialContext();

2. Modify all uses of RMI registry `lookup()`, `bind()`, and `rebind()` to use JNDI `lookup()`, `bind()`, and `rebind()` instead. For example, instead of your RMI server using:

             import java.rmi.*;
             ...
             Naming.rebind("MyObject", myObj);

use:

             import javax.naming.*;
             ...
             ic.rebind("MyObject", myObj);

3. If the client is an applet, the client applet needs to pass `this` to the JNDI `CosNaming` plugin. Replace the above code with the following:

            import java.util.*;
            import javax.naming.*;
            ...
            Hashtable env = new Hashtable();
            env.put("java.naming.applet", this);
            Context ic = new InitialContext(env);

2. If you are not using the RMI registry for naming services, you have some other way of bootstrapping your initial remote object reference. For example, your server code may be using Java serialization to write an RMI object reference to an `ObjectOutputStream` and passing this to your client code for deserializing into an RMI stub. When doing this in RMI-IIOP, it is also necessary to ensure that object references are connected to an ORB before serialization and after deserialization. The following code example shows how to do this.
   On the server side, use the `PortableRemoteObject.toStub()` call to obtain a stub, then use `writeObject()` to serialize this stub to an `ObjectOutputStream`. If necessary, use `Stub.connect()` to connect the stub to an ORB before serializing it. The code to do this looks something like:

>     org.omg.CORBA.ORB myORB = org.omg.CORBA.ORB.init(new String[0], null);
>     Wombat myWombat = new WombatImpl();
>     javax.rmi.CORBA.Stub myStub = (javax.rmi.CORBA.Stub)PortableRemoteObject.toStub(myWombat);
>     myStub.connect(myORB);
>     // myWombat is now connected to myORB.  To connect other objects to the
>     // same ORB, use PortableRemoteObject.connect(nextWombat, myWombat);
>     FileOutputStream myFile = new FileOutputStream("t.tmp");
>     ObjectOutputStream myStream = new ObjectOutputStream(myFile);
>     myStream.writeObject(myStub);

On the client side, use `readObject()` to deserialize a remote reference to the object from an `ObjectInputStream`. Before using the deserialized stub to call remote methods, it must be connected to an ORB. You could do this with code like:

>     FileInputStream myFile = new FileInputStream("t.tmp");
>     ObjectInputStream myStream = new ObjectInputStream(myFile);
>     Wombat myWombat = (Wombat)myStream.readObject();
>     org.omg.CORBA.ORB myORB = org.omg.CORBA.ORB.init(new String[0], null);
>     ((javax.rmi.CORBA.Stub)myWombat).connect(myORB);
>     // myWombat is now connected to myORB.  To connect other objects to the
>     // same ORB, use PortableRemoteObject.connect(nextWombat, myWombat);

As you can see, the JNDI approach is much simpler, so use it whenever possible.
3. Either change your remote implementation classes to inherit from `javax.rmi.PortableRemoteObject`, or explicitly export implementation objects after creation by calling `PortableRemoteObject.exportObject()`. For more discussion on this topic read Connecting IIOP stubs to the ORB.
4. Change all the places in your code where there is a Java cast of a remote interface to use `javax.rmi.PortableRemoteObject.narrow()`.
5. Don't depend on distributed garbage collection (DGC) or use any of the RMI DGC facilities. Use `PortableRemoteObject.unexportObject()` to unexport objects that are no longer in use.
6. Regenerate the RMI stubs and ties using the `rmic` command with the `-iiop` option. This will produce stub and tie files with the following names:

         _<implementationName>_Tie.class
         _<interfaceName>_Stub.class

7. Before starting the server, start the `CosNaming` server (in its own process) using the following command:

         tnameserv

This uses the default port number of 2809. If you want to use a different port number, use the following command:

         tnameserv -ORBInitialPort 1050

8. When starting client and server applications, specify the following system properties:

         java -Djava.naming.factory.initial=com.sun.jndi.cosnaming.CNCtxFactory
              -Djava.naming.provider.url=iiop://<hostname>:2809
               <appl_class>

This example uses the default name service port number of 2809. If you specified a different port in step 8, you need to use the same port number in the provider URL here. The <hostname> in the provider URL is the host name that was used to start the `CosNaming` server in step 7.
9. If the client is an applet, specify the following properties in the applet tag:

         java.naming.factory.initial=com.sun.jndi.cosnaming.CNCtxFactory
        java.naming.provider.url=iiop://<hostname>:2809

This example uses the default name service port number of 2809. If you specified a different port in step 8, you need to use the same port number in the provider URL here. The <hostname> in the provider URL is the host name that was used to start the `CosNaming` server in step 7.

### Connecting IIOP stubs to the ORB

When your application uses IIOP stubs, as opposed to JRMP stubs, you need to properly connect the IIOP stubs with the ORB before invoking operations on the IIOP stubs (this is not necessary with JRMP stubs). This section discusses the extra 'connect' step required for the IIOP stub case.

The `PortableRemoteObject.exportObject()` call only creates a tie object and caches it for future usage. The created tie does not have a delegate or an ORB associated. This is known as explicit invocation.

The `PortableRemoteObject.exportObject()` happens automatically when the servant instance is created. This happens when a `PortableRemoteObject` constructor is called as a base class. This is known as implicit invocation.

Later, when `PortableRemoteObject.toStub()` is called by the application, it creates the corresponding Stub object and associates it with the cached Tie object. But since the Tie is not connected and does not have a delegate, the newly created stub also does not have a delegate or ORB.

The delegate is set for the stub only when the application calls `Stub.connect(orb)`. Thus, any operations on the stub made before the ORB connection is made will fail.

The Java Language to IDL Mapping Specification says the following about the `Stub.connect()` method:

> The `connect` method makes the stub ready for remote communication using the specified ORB object `orb`. Connection normally happens implicitly when the stub is received or sent as an argument on a remote method call, but it is sometimes useful to do this by making an explicit call (e.g., following deserialization). If the stub is already connected to `orb` (i.e., has a delegate set for `orb`), then connect takes no action. If the stub is connected to some other ORB, then a `RemoteException` is thrown. Otherwise, a delegate is created for this stub and the ORB object `orb`.

For servants that are not POA-activated, `Stub.connect(orb)` is necessary as a required setup.

## Restrictions When Running RMI Programs Over IIOP

To make existing RMI programs run over IIOP, you need to observe the following restrictions.

1. Make sure all constant definitions in remote interfaces are of primitive types or String and evaluated at compile time.
2. Don't use Java names that conflict with IDL mangled names generated by the Java to IDL mapping rules. See section 28.3.2 of the Java Language to IDL Mapping specification for the Java to IDL name mapping rules.
3. Don't inherit the same method name into a remote interface more than once from different base remote interfaces.
4. Be careful when using names that differ only in case. The use of a type name and a variable of that type whose name differs from the type name only in case is supported. Most other combinations of names that differ only in case are not supported.
5. Don't depend on runtime sharing of object references to be preserved exactly when transmitting object references across IIOP. Runtime sharing of other objects is preserved correctly.
6. Don't use the following features of RMI:

- `RMISocketFactory`
- `UnicastRemoteObject`
- `Unreferenced`
- The Distributed Garbage Collector (DGC) interfaces

## Other Things You Should Know

### Servers Need to be Thread Safe

Since remote method invocations on the same remote object may execute concurrently, a remote object implementation must be thread-safe.

### Interoperating with Other ORBs

RMI-IIOP should interoperate with other ORBS that support the CORBA 2.3 specification. It will not interoperate with older ORBs, because these are unable to handle the IIOP encodings for Objects By Value. This support is needed to send RMI value classes (including strings) over IIOP.

NOTE: 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.

### When do I use UnicastRemoteObject vs PortableRemoteObject?

`UnicastRemoteObject` should be used as the superclass for the object implementation in RMI programming. `PortableRemoteObject` should be used in RMI-IIOP programming. If `PortableRemoteObject` is used, you can switch the transport protocol to either JRMP or IIOP during runtime.

### Known Problems

- JNDI 1.1 does not support `java.naming.factory.initial=com.sun.jndi.cosnaming.CNCtxFactory` as an Applet parameter. Instead, it must be explicitly passed as a property to the `InitialContext` constructor. This capability is supported in JNDI 1.2.
- When running the Naming Service on Unix based platforms, you must use a port number greater than 1024. The default port is 2809, so should cause no problems.

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