New York University

Computer Science Department

Courant Institute of Mathematical Sciences

 

RMI-IIOP Example

(JavaSoft Whitepaper Adapted for the Course)

 

 

Course Title: Application Servers                                                                                                     Course Number: g22.3033-011

Instructor: Jean-Claude Franchitti                                                                                                      Session: 4

 

Introduction

This document will teach you 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. This document is for RMI programmers who want to write RMI-IIOP programs, or convert existing RMI programs to RMI-IIOP. <!-- BACKGROUND READING--> <!-- WHAT IS RMI-IIOP? -->

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.

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 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.

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 objects can use the IIOP protocol, and communicate with CORBA objects. This solution is known as RMI-IIOP. RMI-IIOP combines RMI-style ease of use with CORBA cross-language interoperability.

The New rmic Compiler

The RMI-IIOP software comes with a new rmic compiler that can generate IIOP stubs and ties, and emit IDL.

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

<!--

<p>

The <code>-iiop</code> flag <i>cannot</i> be combined with any of

the JRMP flags (<code>-v1.1</code>, <code>-vcompat</code>, <code>-v1.2</code>)

because JRMP and IIOP use the same name for stub files (for example,

<code>Foo</code> becomes <code>Foo_Stub</code>). So the last generator would

overwrite the previous one's output.

<p>

When none of the "generator" flags is specified (<code>-iiop</code>,

<code>-idl</code>, <code>-v1.1</code>, <code>-vcompat</code>,

<code>-v1.2</code>), <code>rmic</code> defaults to one of two

JRMP choices depending on the JDK version:

If JDK 1.2 is being used, the default is <code>-vcompat</code>, else the default

is <code>-v1.1</code>.

<p>

Note that choosing <code>-vcompat</code> or <code>-v1.2</code> when

running on JDK 1.1.6 will result in an error. Without this behavior,

the generated code would fail to compile, resulting in an error that's

not very useful.

-->

The new rmic behaves differently than previous versions when no output directory (-d option) is specified. In the JDK, the stub and tie files are always written into the current working directory when no -d option is specifed, regardless of package. Here is a description of the new rmic behavior:

·        For each input class which has source that must be compiled, .class files are created in a directory chosen as follows:

1.     If the -d option is present, use specified directory as the root, creating subdirectories as needed; else...

2.     If the source file is not zipped, use the directory containing the source file; else...

3.     Exit with "can't write" error.

·        For each input class, all generated files (.idl and/or _Stub/_Tie/_Skel and their .class files) are created in a directory chosen as follows:

1.     If the -d option is present, use specified directory as the root, creating subdirectories as needed; else...

2.     Search the classpath for the input class file. If found and not zipped, use the directory containing the class file; else...

3.     Search the classpath for the input class source file. If found and not zipped, use the directory containing the source file; else...

4.     Search the classpath for an existing subdirectory whose relative path matches the package of the input class. If found and not zipped, use it; else...

5.     If the input class has no package, use the current working directory (from the System property user.dir); else...

6.     If the current working directory is in the classpath, use it as the root, creating subdirectories as needed; else...

7.     Exit with an error message which says that the -d option is required.

<!--

There is one other flag that we might want to document.  It is only valid when

-iiop and/or -idl flags are present:

<blockquote>

  -always Forces re-generation even when existing stubs/ties/idl is newer than

the input class.

</blockquote>

This one may never be needed by most users. I use it when I change rmic itself

and need to ensure that stubs are re-generated. I did not include this one in

the usage message that rmic dumps.

<p>

-->

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.

The -idl Flag

Using rmic with the -idl option generates OMG IDL for the classes specified and any classes referenced.
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.

The -noValueMethods Flag

The -noValueMethods option, when used with -idl, ensures that methods and initializers are notincluded in valuetypes emitted during IDL Generation. These are optional for valuetypes and are otherwise omitted.

The New idlj Compiler

The RMI-IIOP software includes a new IDL-to-Java compiler. This compiler supports the new 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. <!-- The RMI/IIOP COOKBOOK -->

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 JNDI with the CosNaming plugin. You need to do the following:

a.      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();

b.     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);

c.     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.Util.write_RemoteObject(myORB.create_output_stream(), myWombat);

// 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(PortableRemoteObject.toStub(myWombat));

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);

You will also need to specify the -nolocalstubs option with the rmic -iiop command.
 

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, use the following command:

     tnameserv -ORBInitialPort 1050

The CLASSPATH must have previously been modified as necessary. Alternatively, the settings described in the installation instructions can be passed on the command line using the -classpath option. If the -classpath approach is used on JDK 1.1, the classes.zip file from the JDK must also be specified as the last item in the classpath.
 

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 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 8.

The CLASSPATH must have previously been modified as necessary. Alternatively, the settings described in the installation instructions can be passed on the command line using the -classpath option. If the -classpath approach is used on JDK 1.1, the classes.zip file from the JDK must also be specified as the last item in the classpath.
 

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 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 8.

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 26.4.2 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

In the following example you'll convert the RMI Hello World example to RMI-IIOP. You will also convert RMI Hello World applet client to an application.

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

The RMI Hello World example uses a developement directory of $HOME/jdk1.1/mysrc/example/hello and a deployment directory of $HOME/public_html/codebase, where $HOME is your home directory. Though you don't have to, the following example assumes that you use these directories.

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);

Here are the changes you need to make to HelloApplet.java:

1.     Import the PortableRemoteObject package:

     import javax.rmi.PortableRemoteObject;

<!-- ORIGINAL STEP TWO

 <li><b>(Current Release)</b> Create a JNDI (<code>javax.naming</code>)

     initial naming context, and start the ORB explicitly:

     <pre>

     import java.util.*;

     import javax.naming.*;

     import org.omg.CORBA.ORB;

     ...

     Hashtable env = new Hashtable();

     ORB orb = ORB.init(this, null);

     env.put("java.naming.corba.orb", orb);

     //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://&lt;hostname&gt;:900");

     Context initialNamingContext = new InitialContext(env);

     </pre>

     <b>(Future Releases)</b> Create a JNDI initial naming context,

     and pass <code>this</code> to the <code>CosNaming</code>

     plugin:

     <pre>

     import java.util.*;

     import javax.naming.*;

     ...

     Hashtable env = new Hashtable();

     env.put("java.naming.corba.applet", this);

     Context ic = new InitialContext(env);

     </pre>

--><!-- NEW STEP TWO -->

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);

Specify Naming Properties in the Applet Tag

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"

Note: Setting these parameters in the applet tag doesn't currently work. See the readme.

Compile the Java Source Files

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

Generate the Stub and Tie Classes

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

This will generate the files Hello_Stub.class (the client-side proxy) and HelloImpl_Tie.class (the server-side proxy) in the $HOME/public_html/codebase/examples/hello directory.

Start the JNDI Name Server

tnameserv

This starts the JNDI name server with the default port of 900. If you want to use a different port number (Solaris users must use a port above 1024), use a command line like:

tnameserv -ORBInitialPort 1050

Make sure your CLASSPATH is set, as necessary.

Start the Hello Server

java -Djava.naming.factory.initial=com.sun.jndi.cosnaming.CNCtxFactory

     -Djava.naming.provider.url=iiop://<hostname:900

     examples.hello.HelloImpl

Make sure your CLASSPATH is set, as necessary.

Start the Hello Client

Use appletviewer to load Hello.html.

appletviewer Hello.html

As a reminder, here's what Hello.html looks like:

<html

<titleHello World</title

<center <h1Hello World</h1 </center

 

The message from the HelloServer is:

<p

<applet

        code="examples.hello.HelloApplet"

        width=500 height=120

 

</applet

</HTML

If all goes according to plan, appletviewer will echo the HelloServer's message.

Converting the Client Applet to an Application

Here's how to change the applet client to an application client:

Adapt the Client Application to RMI-IIOP:

1.     Convert HelloApplet to an application:

a.    Copy HelloApplet.java (the original RMI version) to HelloApp.java

b.     Change the class name (for example, to HelloApp)

c.     Remove the extends Applet clause

d.    Change init() to main()

e.    Move String message ="";

f.       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.

Compile the HelloApp Source

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

You don't need to regenerate your stub and tie.

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

You'll see the server's message printed to the client console.