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RMI-IIOP Programmer's Guide   Copyright © 1999 Sun Microsystems, Inc.  Copyright © 1999 International Business Machines Corporation. All Rights Reserved.

Documentation Contents

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

Introduction

Background Reading

• 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 documentation.
• The Java IDL web page will familiarize you with Sun's CORBA/IIOP implementation.
• The Java IDL trail in the Java Tutorial.
• The Java Language to IDL Mapping document is a detailed technical specification of RMI-IIOP.

What is RMI-IIOP?

RMI

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

The JDK's CORBA/IIOP implementation is known as Java IDL. Along with the idltojava 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

The New rmic Compiler

Here are the new rmic flags:

-iiop -- Generates IIOP stubs/ties.
-idl -- Generates IDL.
-noValueMethods -- Stops generation of IDL for methods and constructors within IDL valuetypes.
-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.
-factory -- Uses factory keyword.
-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

The -iiop Flag

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 -idl Flag

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

The -noValueMethods Flag

See the RMIC tool page (Solaris Version/Windows version) for a complete rmic description.

The New idlj Compiler

How to Make RMI Programs Use IIOP

1. If you are using the RMI registry for naming services, you need to switch to JNDI with the CosNaming plugin. 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 initialNamingContext = new InitialContext();

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

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

use:

         import javax.naming.*;
         ...
         initialNamingContext.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.

On the server side, use the PortableRemoteObject.toStub() call to obtain a stub, then use writeObject() to serialize this stub to an ObjectOutputStream. 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, 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);

3. Either change your remote implementation classes to inherit from javax.rmi.PortableRemoteObject, or explicitly export implementation objects after creation by calling PortableRemoteObject.exportObject().

 
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 or use any of the RMI DGC facilities. Use PortableRemoteObject.unexportObject() to unexport objects that are no longer in use. This has no effect for objects exported to JRMP on 1.1.6.

 
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:

     _<implementionName>_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 900. If you want to use a different port number (for example, port 1050), use the ORBInitialPort modifier:

     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>:900
           <appl_class>

This example uses the default name service port number of 900. If you specified a different port in step 7, 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>:900

This example uses the default name service port number of 900. If you specified a different port in step 7, 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.

Restrictions When Running RMI Programs Over IIOP

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 DGC interfaces

Converting the RMI Hello World Program to RMI-IIOP

Here's the RMI Hello World players:

• HelloImpl.java is the RMI server.
• Hello.java is the remote interface implemented by HelloImpl.
• HelloApplet.java is the RMI client.
• Hello.html loads HelloApplet.

If you haven't already, go through the RMI Hello World example. Once you've completed this example, take the following steps.

Adapt the Implementation Class (Server) to RMI-IIOP:

1. Import javax.rmi.server.PortableRemoteObject rather than javax.rmi.server.UnicastRemoteObject:

     //Goodbye
     //import java.rmi.server.UnicastRemoteObject;

     //Hello
     import javax.rmi.PortableRemoteObject;

2. Import the JNDI naming package:

     import javax.naming.*;

3. Make HelloImpl extend PortableRemoteObject rather than UnicastRemoteObject:

     public class HelloImpl
                  extends PortableRemoteObject
                  ...

4. Use the JNDI registry, rather than the RMI registry, by adding the following code:

     Context initialNamingContext = new InitialContext();

This step provides an initial JNDI naming context (and will also need to be done in the client).
 
5. Use JNDI rebind(), rather than the RMI version:

Old code:

     HelloImpl obj = new HelloImpl("HelloServer");
     Naming.rebind("HelloServer", obj);

New code:

     HelloImpl obj = new HelloImpl("HelloServer"); //unchanged
     initialNamingContext.rebind("HelloServer",obj);

1. Import the PortableRemoteObject package:

     import javax.rmi.PortableRemoteObject;

2. Create a JNDI initial naming context, and pass this to the CosNaming plugin:

     import java.util.*;
     import javax.naming.*;
     ...
     Hashtable env = new Hashtable();
     env.put("java.naming.corba.applet", this);
     //The next two values will be specifiable
     //By applet tag params in future releases
     env.put("java.naming.factory.initial",
             "com.sun.jndi.cosnaming.CNCtxFactory");
     env.put("java.naming.provider.url", "iiop://<hostname>:900");
     Context ic = new InitialContext(env);

3. Use JNDI lookup(), rather than the RMI version, AND replace the Java remote interface cast with a call to javax.rmi.PortableRemoteObject.narrow():

Old code:

     import java.rmi.*;
     ...
     Hello obj = (Hello)Naming.lookup("//" +
                     getCodeBase().getHost() + "/HelloServer");

New code:

     import javax.naming.*;
     ...
     Hello obj =
      (Hello)PortableRemoteObject.narrow(
                         initialNamingContext.lookup("HelloServer"),
                         Hello.class);

Add the following properties to Hello.html applet tag:

<param name="java.naming.factory.initial" value="com.sun.jndi.cosnaming.CNCtxFactory">
<param name="java.naming.provider.url" value="iiop://<hostname>:900">

Compile the Java Source Files

javac -d $HOME/public_html/codebase Hello.java HelloImpl.java HelloApplet.java

Make sure that your search path finds the rmic command in the $RMI_IIOP_HOME/bin directory.

rmic -iiop -d $HOME/public_html/codebase examples.hello.HelloImpl

Start the JNDI Name Server

tnameserv

tnameserv -ORBInitialPort 1050

Start the Hello Server

java -Djava.naming.factory.initial=com.sun.jndi.cosnaming.CNCtxFactory
     -Djava.naming.provider.url=iiop://<hostname>:900
     examples.hello.HelloImpl

Start the Hello Client

Use appletviewer to load Hello.html.

appletviewer Hello.html

<html>
<title>Hello World</title>
<center> <h1>Hello World</h1> </center>

The message from the HelloServer is:
<p>
<applet
        code="examples.hello.HelloApplet"
        width=500 height=120>

</applet>
</HTML>

Converting the Client Applet to an Application

Adapt the Client Application to RMI-IIOP:

1. Convert HelloApplet to an application:
• Copy HelloApplet.java (the original RMI version) to HelloApp.java.
• Change the class name (for example, to HelloApp).
• Remove the extends Applet clause.
• Change init() to main().
• Move String message ="";.
• Eliminate the paint() method.
  
  
2. Use the JNDI registry, rather than the RMI registry:

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

3. Use JNDI lookup(), rather than the RMI version, AND replace the Java remote interface cast with a call to javax.rmi.PortableRemoteObject.narrow():

Old code:

     import java.rmi.*;
     ...
     Hello obj = (Hello)Naming.lookup("//" +
                     getCodeBase().getHost() + "/HelloServer");

New code:

     import javax.naming.*;
     ...
     Hello obj = (Hello)PortableRemoteObject.narrow(
                         initialNamingContext.lookup("HelloServer"),
                         Hello.class);

The host and port will be designated when starting the server.

javac -d $HOME/public_html/codebase HelloApp.java

Start the Name Server and Hello Server

Start these the same as in the applet example.

Start the Hello Application Client

Here's how:

java -Djava.naming.factory.initial=com.sun.jndi.cosnaming.CNCtxFactory
     -Djava.naming.provider.url=iiop://<hostname>:900
      examples.hello.HelloApp

Other Things You Should Know

Servers Need to be Thread Safe

Hashtables with Identical Vector Keys

1. A 1.1 RMI application creates a hashtable.
2. The application puts a value into the hashtable using a vector key A.
3. The application puts another value into the hashtable using a vector key B. Vector B is structurally identical to vector A, but is a different object.
4. The hashtable now has two entries with keys A and B.
5. The 1.1 RMI application sends the hashtable, by value, to a 1.2 RMI application.
6. The hashtable on the 1.2 side has one entry with key B. This is because the deserialization code for the hashtable uses 1.2 rules to populate it, and these rules compare vector keys by value instead of by object identity.
7. If the hashtable is ever sent back across RMI from 1.2 to 1.1, it will still have one entry with key B.

Interoperating with Other ORBs

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.
• Port 900 is reserved on Solaris.  You must use a port number greater than 1024 when running on Solaris.
• On Solaris, you may experience an "out of file descriptors" problem when running RMI-IIOP applications.  This may occur if you are using too many file descriptors, which can happen because each JAR file in the classpath consumes one file descriptor and the JDK 1.1.6 setup adds numerous JAR files to the classpath.  Before running applications, set the file descriptor limit to a number greater than its default value of 64.  For example, set the ulimit number to 90 or higher, as follows:

ulimit -n 90