IBM SDK for Java

RMI-IIOP Programmer's Guide

Preface

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 (see Restrictions when running RMI programs over IIOP), 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 help you with this technology:

• The Java RMI home page contains links to RMI documentation, examples, specification, and more: http://java.sun.com/products/jdk/rmi/index.html
• The RMI trail in the Java Tutorial: http://java.sun.com/docs/books/tutorial/rmi
• The RMI API Javadoc HTML contains the most up-to-date RMI API documentation: http://java.sun.com/javase/6/docs/api/java/rmi/package-summary.html
• The Java IDL Web page will familiarize you with Sun's CORBA/IIOP implementation: http://java.sun.com/javase/6/docs/technotes/guides/idl/index.html
• The Java IDL Trail in the Java Tutorial: http://java.sun.com/j2se/1.5.0/docs/guide/idl/GShome.html
• The Java IDL Trail in the Java Tutorial: http://java.sun.com/javase/6/docs/technotes/guides/idl/GShome.html

What are RMI, IIOP, and RMI-IIOP?

RMI

With RMI, you can write distributed programs in the Java programming language. RMI is easy to use, you do not 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. For a quick introduction to writing RMI programs, see the RMI tutorial Web page: http://java.sun.com/docs/books/tutorial/rmi, which 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 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 Standard Edition v6 CORBA/IIOP implementation is known as Java IDL. Along with the IDL to Java (idlj) compiler, Java IDL can be used to define, implement, and access CORBA objects from the Java programming language.

The Java IDL Web page: http://java.sun.com/j2se/1.5.0/docs/guide/idl/index.html, 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: http://java.sun.com/j2se/1.5.0/docs/guide/idl/GShome.html.

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 (see Restrictions when running RMI programs over IIOP), 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 ease of use with CORBA cross-language interoperability.

Using RMI-IIOP

The rmic compiler

Purpose

The rmic compiler generates IIOP stubs and ties, and emits IDL, in accordance with the Java Language to OMG IDL Language Mapping Specification: http://www.omg.org/cgi-bin/doc?formal/01-06-07.

Parameters

-iiop

Generates stub and tie classes. A stub class is a local proxy for a remote object. Clients use stub classes to send calls to a server. Each remote interface requires a stub class, which implements that remote interface. The remote object reference used by a client is a reference to a stub. Tie classes are used on the server side to process incoming calls, and dispatch the calls to the correct 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 has methods that 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.

-poa

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 Mapping Specification: http://www.omg.org/cgi-bin/doc?formal/01-06-07.

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. The class serves as the base class for all POA servant implementations. It provides a number of methods that can be called by the application programmer, as well as methods that are called by the POA itself and might be overridden by the user to control aspects of servant behavior.

Valid only when the -iiop option is present.

-idl

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

Tip: 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. Java clients or servers can use the original RMI-IIOP types.

IDL provides a purely declarative means of specifying the API for an object. IDL is independent of the programming language used. The IDL is used as a specification for methods and data that can be written in and called from any language that provides CORBA bindings. Java and C++ are such languages. For a complete description, see the Java Language to OMG IDL Mapping Specification: http://www.omg.org/cgi-bin/doc?formal/01-06-07.

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

-always

Forces regeneration even when existing stubs, ties, or IDL are newer than the input class. Valid only when -iiop or -idl options are present.

-noValueMethods

Ensures that methods and initializers are not included in valuetypes emitted during IDL Generation. Methods and initializers are optional for valuetypes and are otherwise omitted.

Only valid when used with -idl option.

-idlModule <fromJavaPackage[.class]> <toIDLModule>

Specifies IDLEntity package mapping. For example: -idlModule sample.bar my::real::idlmod.

Only valid when used with -idl option.

-idlFile <fromJavaPackage[.class]> <toIDLModule>

Specifies IDLEntity file mapping. For example: -idlFile test.pkg.X TEST16.idl.

Only valid when used with -idl option.

More Information

For more detailed information about the rmic compiler, see the RMIC tool page:

• Solaris, Linux®, AIX®, and z/OS® version: http://java.sun.com/javase/6/docs/technotes/tools/solaris/rmic.html
• Windows® version: http://java.sun.com/javase/6/docs/technotes/tools/windows/rmic.html

The idlj compiler

Purpose

The idlj compiler generates Java bindings from an IDL file. 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.

More Information

For usage information on the idlj compiler, see IDL-to-Java Compiler User's Guide.

Making RMI programs use IIOP

Before you begin

To use these instructions, your application must already use RMI.

Procedure

1. If you are using the RMI registry for naming services, you must switch to CosNaming:
   1. In both your client and server code, create an InitialContext for JNDI. For a Java application use the following code:

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

      For an applet, use this alternative code:

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

   2. Modify all uses of RMI registry lookup(), bind(), and rebind() to use JNDI lookup(), bind(), and rebind() instead. Instead of:

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

      use:

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

2. If you are not using the RMI registry for naming services, you must have some other way of bootstrapping your initial remote object reference. For example, your server code might 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, you must also ensure that object references are connected to an ORB before serialization and after deserialization.
   1. 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. For example:

      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);

   2. 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. For example:

      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);

   The JNDI approach is much simpler, so it is preferable to use it whenever possible.
3. Either change your remote implementation classes to inherit from javax.rmi.PortableRemoteObject, or explicitly to 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. Do not depend on distributed garbage collection (DGC) or use any of the RMI DGC facilities. Use PortableRemoteObject.unexportObject() to make the ORB release its references to an exported object that is 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 tnameserv command The CosNaming server uses the default port number of 2809. If you want to use a different port number, use the -ORBInitialPort parameter.
8. When starting client and server applications, you must specify some system properties. When running an application, you can specify properties on the command line:

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

9. If the client is an applet, you must specify some properties in the applet tag. For example:

   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 specify a different port in the previous step, 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.

Results

Your application can now communicate with CORBA objects using RMI-IIOP.

Connecting IIOP stubs to the ORB

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. The servant instance is created when a PortableRemoteObject constructor is called as a base class. This is known as implicit invocation.

Later, when the application calls PortableRemoteObject.toStub(), the ORB creates the corresponding Stub object and associates it with the cached Tie object. But because 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 OMG IDL Mapping Specification (http://www.omg.org/cgi-bin/doc?formal/01-06-07) says this 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 (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, observe the following restrictions.

• Make sure all constant definitions in remote interfaces are of primitive types or String and evaluated at compile time.
• Do not 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 OMG IDL Mapping Specification for more information: http://www.omg.org/cgi-bin/doc?formal/01-06-07
• Do not inherit the same method name into a remote interface more than once from different base remote interfaces.
• Be careful when using names that are identical other than their case. The use of a type name and a variable of that type with a name that differs from the type name in case only is supported. Most other combinations of names that are identical other than their case are not supported.
• Do not depend on runtime sharing of object references to be preserved exactly when transmitting object references to IIOP. Runtime sharing of other objects is preserved correctly.
• Do not use the following features of RMI, which do not work in RMI-IIOP:
  • RMISocketFactory
  • UnicastRemoteObject
  • Unreferenced
  • The Distributed Garbage Collector (DGC) interfaces

Additional information

Servers must be thread safe

Because remote method invocations on the same remote object might 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 older ORBs cannot handle the IIOP encodings for Objects By Value. This support is needed to send RMI value classes (including strings) over IIOP.

When do I use UnicastRemoteObject vs PortableRemoteObject?

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

Known limitations

• 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 this should not be a problem.

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