RMI-IIOP Programmer's Guide

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 CORBA clients in any language can access RMI-IIOP servers. RMI-IIOP gives you RMI ease-of-use coupled with CORBA/IIOP language interoperability.

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 communication. 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 because it does not use CORBA-IIOP as the communication protocol.

IIOP, CORBA, and Java IDL

IIOP is CORBA's communication protocol using TCP/IP as the transport. It specifies a standard for client and server communication. 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) 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.

Table of Contents

• The rmic Compiler
• The idlj Compiler
• How to Make RMI Programs Use IIOP
• Restrictions When Running RMI Programs Over IIOP
• Other Things You Should Know

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, in accordance with the compliance statement.

Here are the primary rmic flags that support the CORBA/IIOP functionality:

-iiop -- Generates IIOP stubs/ties.
-iiop -poa -- Generates IIOP stubs/ties that work with a Portable Object Adapter (POA).
-idl -- Generates IDL.

The following options are used in conjunction with the -idl option.

-noValueMethods -- Stops generation of IDL for methods and constructors within IDL valuetypes.
-always -- Forces regeneration even when existing stubs/ties/idl are newer than the input class.  Only valid when -iiop and/or -idl flags are present.
-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, read the rmic documentation.

The -iiop Flag

Using rmic with the -iiop option generates IIOP stub and tie classes instead of Java Remote Method Protocol (JRMP) stub and skeleton 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.

QUESTION: I have a RMI-IIOP application which was running on J2SE 1.3. When I generate new stubs and skeletons using rmic -iiop -poa <RMI Interface>, it does not work. Why ?

ANSWER: There are 2 different ways to compile and run RMI-IIOP programs, which are listed below. We do not recommend that you combine these.

1. The default RMI-IIOP behavior. An example of an application that uses the default RMI-IIOP behavior can be found at Tutorial: Getting Started Using RMI-IIOP.
2. POA-based RMI-IIOP. An example of an application that uses the non-standard POA-based RMI-IIOP behavior can be found at Getting Started Using RMI-IIOP: Example Using POA-based server-side model.

The -iiop -poa Flag

New to this release of Java SE 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 OMG IDL Language 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. You would want to use this option when you have a CORBA client in another language that wants to talk to an 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 valuetypes emitted during IDL Generation. These are optional for valuetypes and are otherwise omitted.

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

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. In the createORB() method, do not pass the org.omg.ORBClass property. Instead, use ORB.init( args, null ). 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() 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.*;
                       ...
                       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.

   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(). 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:

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

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

        orbd -ORBInitialPort port#
       

   You must specify the port number when starting ORBD.
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>:1050
                  <appl_class>
       

   This example uses the name service port number of 1050. 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>:1050
       

   This example uses the name service port number of 1050. 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.

For tutorials that describe creating a new RMI-IIOP application, link to Getting Started Using RMI-IIOP: Example Using POA-based server-side model or Tutorial: Getting Started Using RMI-IIOP.

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 Collection (DGC) interfaces

Other Things You Should Know

Servers Need to be Thread Safe

Since remote method invocation on the same remote object may execute concurrently, a remote object implementation needs to make sure its implementation is 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 Solaris, you must use a port number greater than 1024.
• 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
      

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 Tutorial gives a step-by-step example for creating and running a simple RMI-IIOP application.
• 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 Tutorial is the newest version of the IDL trail from the Java Tutorial.
• The Java Language to IDL Mapping, ptc/00-01-06 document is a detailed technical specification of RMI-IIOP. For further information on how the specification is implemented in this version of Java SE, see the compliance document.