Getting Started Using RMI-IIOP:
Example Using POA-based server-side model

This tutorial shows you the steps to follow to create a distributed version of the classic "Hello World" program using Java™ Remote Method Invocation (RMI) over Internet Inter-ORB Protocol (IIOP). RMI-IIOP adds CORBA (Common Object Request Broker Architecture) capability to Java connectivity to many other programming languages and platforms. RMI-IIOP enables distributed Web-enabled Java remote Management Group. Runtime components include a Java ORB for distributed computing using IIOP communication.

RMI-IIOP is for Java programmers who want to program to the RMI interfaces, but use IIOP as the underlying transport. RMI-IIOP provides interoperability with other CORBA objects implemented in various languages - but only if all the remote interfaces are originally defined as Java RMI interfaces. It is of particular interest to programmers using Enterprise JavaBeans (EJB), since the remote object model for EJBs is RMI-based.

Another option for creating distributed applications is Java™ IDL. Java IDL is for CORBA programmers who want to program in the Java programming language based on interfaces defined in CORBA Interface Definition Language (IDL). This is "business as usual" CORBA programming, supporting Java in exactly the same way as other languages like C++ or COBOL.

Tutorial: The Hello World Application

The distributed Hello World example uses a client application to make a remote method call via IIOP to a server, running on the host from which the client was downloaded. When the client runs, "Hello World!" is displayed.

This tutorial is organized as follows:

The steps to write the source files
The steps to compile the class files
The steps to start the Naming Service, server, and client

Write or Download the Source Files

There are three tasks to complete in this section:

Define the functions of the remote class as an interface written in the Java programming language
Write the implementation class
Write the server class
Write a client program that uses the remote service

The source files created in this tutorial (and which can be downloaded here) are:

HelloInterface.java - a remote interface
HelloImpl.java - a remote object implementation that implements HelloInterface
HelloServer.java - an RMI server that creates an instance of the remote object implementation and binds that instance to a name in the Naming Service
HelloClient.java - a client application that invokes the remote method, sayHello()

Define the functions of the remote class as an interface written in the Java programming language

In the Java programming language, a remote object is an instance of a class that implements a Remote interface. Your remote interface will declare each of the methods that you would like to call from other machines. Remote interfaces have the following characteristics:

The remote interface must be declared public. Otherwise, a client will get an error when attempting to load a remote object that implements the remote interface, unless that client is in the same package as the remote interface.
The remote interface extends the java.rmi.Remote interface.
Each method must declare java.rmi.RemoteException (or a superclass of RemoteException) in its throws clause, in addition to any application-specific exceptions.
The data type of any remote object that is passed as an argument or return value (either directly or embedded within a local object) must be declared as the remote interface type (for example, HelloInterface) not the implementation class (HelloImpl).

For this example, create all of the source files in the same directory, for example, $HOME/mysrc/RMIHelloPOA. Here is the interface definition for the remote interface, HelloInterface. The interface contains just one method, sayHello:

//HelloInterface.java
import java.rmi.Remote;

public interface HelloInterface extends java.rmi.Remote {
  public void sayHello() throws java.rmi.RemoteException;
}

Because remote method invocations can fail in very different ways from local method invocations (due to network-related communication problems and server problems), remote methods will report communication failures by throwing a java.rmi.RemoteException. If you want more information on failure and recovery in distributed systems, you may wish to read A Note on Distributed Computing.

Write The Implementation Class

At a minimum, a remote object implementation class, HelloImpl.java must:

Declare that it implements at least one remote interface
Define the constructor for the remote object
Provide implementations for the methods that can be invoked remotely

An explanation of each of the preceding steps follows the source for HelloImpl.java:

//HelloImpl.java
import javax.rmi.PortableRemoteObject;

public class HelloImpl extends PortableRemoteObject implements HelloInterface {
  public HelloImpl() throws java.rmi.RemoteException {
    super();     // invoke rmi linking and remote object initialization
  }

  public void sayHello() throws java.rmi.RemoteException {
    System.out.println( "It works!  Hello World!!" );
  }
}

Implement a remote interface

In the Java programming language, when a class declares that it implements an interface, a contract is formed between the class and the compiler. By entering into this contract, the class is promising that it will provide method bodies, or definitions, for each of the method signatures declared in that interface. Interface methods are implicitly public and abstract, so if the implementation class doesn't fulfill its contract, it becomes by definition an abstract class, and the compiler will point out this fact if the class was not declared abstract.

The implementation class in this example is HelloImpl. The implementation class declares which remote interface(s) it is implementing. Here is the HelloImpl class declaration:

  public class HelloImpl extends PortableRemoteObject
    implements HelloInterface{

As a convenience, the implementation class can extend a remote class, which in this example is javax.rmi.PortableRemoteObject. By extending PortableRemoteObject, the HelloImpl class can be used to create a remote object that uses IIOP-based transport for communication.

Define the constructor for the remote object

The constructor for a remote class provides the same functionality as the constructor for a non-remote class: it initializes the variables of each newly created instance of the class, and returns an instance of the class to the program which called the constructor.

In addition, the remote object instance will need to be "exported". Exporting a remote object makes it available to accept incoming remote method requests, by listening for incoming calls to the remote object on an anonymous port. When you extend javax.rmi.PortableRemoteObject, your class will be exported automatically upon creation.

Because the object export could potentially throw a java.rmi.RemoteException, you must define a constructor that throws a RemoteException, even if the constructor does nothing else. If you forget the constructor, javac will produce the following error message:

        HelloImpl.java:3: unreported exception java.rmi.RemoteException; must be
        caught or declared to be thrown. 
        
        public class HelloImpl extends PortableRemoteObject implements HelloInterface{
               ^ 
        1 error

To review: The implementation class for a remote object needs to:

Implement a remote interface
Export the object so that it can accept incoming remote method calls
Declare its constructor(s) to throw at least a java.rmi.RemoteException

Here is the constructor for the HelloImpl class:

  public HelloImpl() throws java.rmi.RemoteException { 
    super(); 
  }

Note the following:

The super method call invokes the no-argument constructor of javax.rmi.PortableRemoteObject, which exports the remote object.
The constructor must throw java.rmi.RemoteException, because RMI's attempt to export a remote object during construction might fail if communication resources are not available.

java.rmi.RemoteException is a checked exception, not a runtime exception.

Although the call to the superclass's no-argument constructor, super(), occurs by default (even if omitted), it is included in this example to make clear the fact that the superclass will be constructed before the class.

Provide an implementation for each remote method

The implementation class for a remote object contains the code that implements each of the remote methods specified in the remote interface. For example, here is the implementation for the sayHello() method, which returns the string "It works! Hello World!!" to the caller:

  public void sayHello() throws java.rmi.RemoteException {
    System.out.println( "It works!  Hello World!!" );
  }

Arguments to, or return values from, remote methods can be any data type for the Java platform, including objects, as long as those objects implement the interface java.io.Serializable. Most of the core classes in java.lang and java.util implement the Serializable interface. In RMI:

By default, local objects are passed by copy, which means that all data members (or fields) of an object are copied, except those marked as static or transient.
Remote objects are passed by reference. A reference to a remote object is actually a reference to a stub, which is a client-side proxy for the remote object. We'll create stubs later in this tutorial in the section: Use rmic to generate stubs and skeletons.

Write The Server Class

A server class is the class which has a main method that creates an instance of the remote object implementation, and binds that instance to a name in the Naming Service. The class that contains this main method could be the implementation class itself, or another class entirely.

In this example, the main method is part of HelloServer.java, which does the following:

Creates a Portable Object Adapter (POA) with the appropriate policies
Activates the POA Managers
Creates an instance of the servant and activates the Tie, creates an instance of a remote object
Publishes the object reference
Prepares to accept requests from the client

An explanation of each of the preceding steps follows the source for HelloServer.java:

//HelloServer.java
import javax.naming.InitialContext;
import javax.naming.Context;
import javax.rmi.PortableRemoteObject ;
//Please note that internal Sun APIs
//may change in future releases.
import com.sun.corba.se.internal.POA.POAORB;
import org.omg.PortableServer.*;
import java.util.*;
import org.omg.CORBA.*;
import javax.rmi.CORBA.Stub;
import javax.rmi.CORBA.Util;

public class HelloServer {
  public HelloServer(String[] args) {
    try {
      Properties p = System.getProperties();
      // add runtime properties here
      //Please note that the name of the servertool 
      //class may change in future releases.
      p.put("org.omg.CORBA.ORBClass", 
          "com.sun.corba.se.internal.POA.POAORB");
      p.put("org.omg.CORBA.ORBSingletonClass", 
          "com.sun.corba.se.internal.corba.ORBSingleton");

      ORB orb = ORB.init( args, p );

      POA rootPOA = (POA)orb.resolve_initial_references("RootPOA");

      // STEP 1: Create a POA with the appropriate policies
      Policy[] tpolicy = new Policy[3];
      tpolicy[0] = rootPOA.create_lifespan_policy(
        LifespanPolicyValue.TRANSIENT );
      tpolicy[1] = rootPOA.create_request_processing_policy(
        RequestProcessingPolicyValue.USE_ACTIVE_OBJECT_MAP_ONLY );
      tpolicy[2] = rootPOA.create_servant_retention_policy(
        ServantRetentionPolicyValue.RETAIN);
      POA tPOA = rootPOA.create_POA("MyTransientPOA", null, tpolicy);
          
      // STEP 2: Activate the POA Manager, otherwise all calls to the
      // servant hang because, by default, POAManager will be in the 
      // HOLD state.
      tPOA.the_POAManager().activate();

      // STEP 3: Instantiate the Servant and activate the Tie, If the
      // POA policy is USE_ACTIVE_OBJECT_MAP_ONLY
      HelloImpl helloImpl = new HelloImpl();
      _HelloImpl_Tie tie = (_HelloImpl_Tie)Util.getTie( helloImpl );
      String helloId = "hello";
      byte[] id = helloId.getBytes();
      tPOA.activate_object_with_id( id, tie );


      // STEP 4: Publish the object reference using the same object id
      // used to activate the Tie object.
      Context initialNamingContext = new InitialContext();
      initialNamingContext.rebind("HelloService", 
        tPOA.create_reference_with_id(id, 
          tie._all_interfaces(tPOA,id)[0]) );
      System.out.println("Hello Server: Ready...");
      
      // STEP 5: Get ready to accept requests from the client
      orb.run();
    } 
      catch (Exception e) {
        System.out.println("Problem running HelloServer: " + e);
        e.printStackTrace();
      } 
  }


  public static void main(String args[]) {
    new HelloServer( args );
  }
}

Create a Portable Object Adapter (POA) with the appropriate policies

The main method of the server first needs to create a Portable Object Adapter (POA) with the appropriate policies. For example:

      Policy[] tpolicy = new Policy[3];
      tpolicy[0] = rootPOA.create_lifespan_policy(
        LifespanPolicyValue.TRANSIENT );
      tpolicy[1] = rootPOA.create_request_processing_policy(
        RequestProcessingPolicyValue.USE_ACTIVE_OBJECT_MAP_ONLY );
      tpolicy[2] = rootPOA.create_servant_retention_policy(
        ServantRetentionPolicyValue.RETAIN);
      POA tPOA = rootPOA.create_POA("MyTransientPOA", null, tpolicy);

The Portable Object Adaptor (POA) is designed to provide an object adapter that can be used with multiple ORB implementations with a minimum of rewriting needed to deal with different vendors' implementations. POA support was introduced in J2SE version 1.4.

The POA is also intended to allow persistent objects -- at least, from the client's perspective. That is, as far as the client is concerned, these objects are always alive, and maintain data values stored in them, even though physically, the server may have been restarted many times, or the implementation may be provided by many different object implementations.

The POA allows the object implementor a lot more control. Previously, the implementation of the object was responsible only for the code that is executed in response to method requests. Now, additionally, the implementor has more control over the object's identity, state, storage, and lifecycle.

In this example, the policy values include:

The LifespanPolicyValue can have the following values:
- TRANSIENT - The objects implemented in the POA cannot outlive the POA instance in which they are first created.
- PERSISTENT - The objects implemented in the POA can outlive the process in which they are first created.
The RequestProcessingPolicyValue can have the following values:
- USE_ACTIVE_OBJECT_MAP_ONLY - If the object ID is not found in the Active Object Map, an OBJECT_NOT_EXIST exception is returned to the client. The RETAIN policy is also required.
- USE_DEFAULT_SERVANT - If the object ID is not found in the Active Object Map or the NON_RETAIN policy is present, and a default servant has been registered with the POA using the set_servant operation, the request is dispatched to the default servant.
- USE_SERVANT_MANAGER - If the object ID is not found in the Active Object Map or the NON_RETAIN policy is present, and a servant manager has been registered with the POA using the set_servant_manager operation, the servant manager is given the opportunity to locate a servant or raise an exception.
The ServantRetentionPolicyValue can have the following values.
- RETAIN - to indicate that the POA will retain active servants in its Active Object Map. If no ServantRetentionPolicy is specified at POA creation, the default is RETAIN.
- NON_RETAIN - to indicate Servants are not retained by the POA.

For more information on POA policies, refer to Chapter 11, Portable Object Adapter of the CORBA/IIOP 2.3.1 Specification at http://www.omg.org/cgi-bin/doc?formal/99-10-07

Activate the POA Managers

Each POA object has an associated POAManager object. A POA Manager may be associated with one or more POA objects. A POA Manager encapsulates the processing state of the POAs it is associated with. In this step, the POA Manager is activated. If this step is missing, all calls to the Servant would hang because, by default, the POA Manager will be in the HOLD state.

      tPOA.the_POAManager().activate();

Create an instance of a remote object and activate the Tie

The main method of the server needs to create an instance of the remote object implementation, or Servant. For example:

    HelloImpl helloImpl = new HelloImpl();

The constructor exports the remote object, which means that once created, the remote object is ready to accept incoming calls.

When using RMI-IIOP technology, your implementations use delegation (known as the Tie model) to associate your implementation with the interface. When you create an instance of your implementation, as above, you also need to create a Tie object to associate it with a CORBA interface. The next few lines of code activate the Tie, but only if the POA policy is USE_ACTIVE_OBJECT_MAP_ONLY.

      _HelloImpl_Tie tie = (_HelloImpl_Tie)Util.getTie( helloImpl );
      String helloId = "hello";
      byte[] id = helloId.getBytes();
      tPOA.activate_object_with_id( id, tie );

Publish the object reference using the same object id used to activate the Tie object

For a caller (client, peer, or client application) to be able to invoke a method on a remote object, that caller must first obtain a reference to the remote object.

Once a remote object is registered on the server, callers can look up the object by name (using a naming service), obtain a remote object reference, and then remotely invoke methods on the object. In this example, we use the Object Request Broker Daemon (orbd), which is a daemon process containing a Bootstrap Service, a Transient Naming Service, a Persistent Naming Service, and a Server Manager.

For example, the following code binds the name "HelloService" to a reference for the remote object:

      Context initialNamingContext = new InitialContext();
      initialNamingContext.rebind("HelloService", 
        tPOA.create_reference_with_id(id, 
          tie._all_interfaces(tPOA,id)[0]) );
      System.out.println("Hello Server: Ready...");

Note the following about the arguments to the rebind method call:

The first argument, "HelloService", is a java.lang.String, representing the name of the remote object to bind
The second argument, tPOA.create_reference_with_id(id, tie._all_interfaces(tPOA,id)[0] is the object id of the remote object to bind

Prepare to accept requests from the client

The following line of code, when called by the main thread, enables the ORB to perform work using the main thread.

      orb.run();

Write a client program that uses the remote service

The client application in this example remotely invokes the sayHello method in order to get the string "Hello World!" to display when the client application runs. Here is the code for the client application:

//HelloClient.java
import java.rmi.RemoteException;
import java.net.MalformedURLException;
import java.rmi.NotBoundException;
import javax.rmi.*;
import java.util.Vector;
import javax.naming.NamingException;
import javax.naming.InitialContext;
import javax.naming.Context;

public class HelloClient {

  public static void  main( String args[] ) {
    Context ic;
    Object objref;
    HelloInterface hi;

    try {
      ic = new InitialContext();
    } catch (NamingException e) {
        System.out.println("failed to obtain context" + e);
        e.printStackTrace();
        return;
    }
        
    // STEP 1: Get the Object reference from the Name Service
    // using JNDI call.
    try {
      objref = ic.lookup("HelloService");
      System.out.println("Client: Obtained a ref. to Hello server.");
    } catch (NamingException e) {
        System.out.println("failed to lookup object reference");
        e.printStackTrace();
        return;
    }

    // STEP 2: Narrow the object reference to the concrete type and
    // invoke the method.
    try {
      hi = (HelloInterface) PortableRemoteObject.narrow(
        objref, HelloInterface.class);
      hi.sayHello();
    } catch (ClassCastException e) {
        System.out.println("narrow failed");
        e.printStackTrace();
        return;
      } catch( Exception e ) {
            System.err.println( "Exception " + e + "Caught" );
            e.printStackTrace( );
            return;
        }
  }
}

First, the client application gets a reference to the remote object implementation (advertised as "HelloService") from the Name Service using Java Naming and Directory Interface [TM] (JNDI) calls. Like the Naming.rebind method, the Naming.lookup method takes java.lang.String value representing the name of the object to look up. You supply Naming.lookup() the name of the object you want to look up, and it returns the object bound to that name.

Naming.lookup() takes care of these tasks:
- The Name Service returns the _HelloImpl_Stub instance bound to that name
- The lookup method receives the remote object's (HelloImpl) stub instance and loads the stub class (_HelloImpl_Stub)
- Naming.lookup returns the stub to its caller (HelloClient)
The client application invokes the remote sayHello() method on the server's remote object, causing the string "It works! Hello World!!" to be displayed on the command line.

Compile Class Files

The source code for this example is now complete and the directory contains four files:

HelloInterface.java contains the source code for the remote interface
HelloImpl.java contains the source code for the remote object implementation
HelloServer.java contains the source code for the server
HelloClient.java contains the source code for the client application

In this section, you compile the remote object implementation file, HelloImpl.java, in order to create the .class files needed to run rmic. You then run the rmic compiler to create stubs and skeletons. A stub is a client-side proxy for a remote object which forwards RMI-IIOP calls to the server-side dispatcher, which in turn forwards the call to the actual remote object implementation. The last task is to compile the remaining .java source files to create .class files.

The following tasks will be completed in this section:

Compile the remote object implementation
Use rmic to generate stubs and skeletons
Compile the source files

Compile the remote object implementation

To create stub and skeleton files, the rmic compiler must be run on the fully-qualified package names of compiled class files that contain remote object implementations. In this example, the file that contains the remote object implementations is HelloImpl.java. In order to generate the stubs and skeletons, we must first compile HelloImpl.java, as follows:

    javac -d . -classpath . HelloImpl.java

The "-d ." option indicates that the generated files should be placed in the directory from which you are running the compiler. The "-classpath ." option indicates that files on which HelloImpl.java is dependent can be found in this directory.

Use `rmic` to generate skeletons and stubs

To create CORBA-compatible stub and skeleton files, run the rmic compiler with the -poa -iiop option. The rmic -poa -iiop command takes one or more class names as an argument and produces class files of the form _MyImpl_Tie.class and _MyInterface_Stub.class. The remote implementation file, HelloImpl.class, is the class name to pass in this example.

For an explanation of rmic options, refer to the Solaris Operating Environment rmic manual page or the Microsoft Windows rmic manual page.

To create the stub and skeleton for the HelloImpl remote object implementation, run rmic like this:

    rmic -poa -iiop HelloImpl

The preceding command creates the following files:

_HelloInterface_Stub.class - the client stub
_HelloImpl_Tie.class - the server skeleton

Compile the source files

To compile the source files, run the javac command as follows:

    javac -d . -classpath . HelloInterface.java HelloServer.java HelloClient.java

This command creates the class files HelloInterface.class, HelloServer.class, and HelloClient.class. These are the remote interface, the server, and the client application respectively. For an explanation of javac options, you can refer to the Solaris javac manual page or the Microsoft Windows javac manual page.

Start the Naming Service, server, and client application

The following tasks will be completed in this section:

Start the Naming Service
Start the server
Run the client application

Start the Naming Service

For this example, we will use the Object Request Broker Daemon, orbd, which includes both a Transient and a Persistent Naming Service, and is available with every download of J2SE 1.4 and higher.

For a caller (client, peer, or client application) to be able to invoke a method on a remote object, that caller must first obtain a reference to the remote object.

Once a remote object is registered on the server, callers can look up the object by name, obtain a remote object reference, and then remotely invoke methods on the object.

To start the Naming Service, run orbd from the command line. This command produces no output and is typically run in the background. For more on the orbd tool, you can refer to the orbd manual page.

For this example, on the Solaris operating system:

    orbd -ORBInitialPort 1060&

or, on the Microsoft Windows operating system:

    start orbd -ORBInitialPort 1060

You must specify a port on which to run orbd. For this example the port of 1060 is chosen because in the Solaris operating environment, a user must become root to start a process on a port under 1024.

You must stop and restart the server any time you modify a remote interface or use modified/additional remote interfaces in a remote object implementation. Otherwise, the type of the object reference bound in the Naming Service will not match the modified class.

Start the server

Open another terminal window and change to the directory containing the source files for this example. The command for running the client has been spread out below to make it easier to read, but should be typed without returns between the lines. The following command shows how to start the HelloServer server. Of course, if you used a port other than 1060 or a host other than localhost when starting the orbd tool, replace those values in the command below with the actual values used to start orbd.

    java 
      -classpath . 
      -Djava.naming.factory.initial=com.sun.jndi.cosnaming.CNCtxFactory
      -Djava.naming.provider.url=iiop://localhost:1060 
      HelloServer

For an explanation of java options, you can refer to the Solaris java manual page or the Microsoft Windows java manual page.

The output should look like this:

Hello Server: Ready ...

Run the client application

Once the Naming Service and server are running, the client application can be run. From a new terminal window, go to the source code directory, and run the client application from the command line, as shown below. The command for running the client has been spread out below to make it easier to read, but should be typed without returns between the lines. Of course, if you used a port other than 1060 or a host other than localhost when starting the orbd tool, replace those values in the command below with the actual values used to start orbd.

    java 
      -classpath . 
      -Djava.naming.factory.initial=com.sun.jndi.cosnaming.CNCtxFactory
      -Djava.naming.provider.url=iiop://localhost:1060 
      HelloClient

After running the client application, you will see output similar to the following in your terminal window or command prompt window:

Client: Obtained a ref. to Hello server.

The server window will return the following message:

It works! Hello World!!

This completes the tutorial. If you are ready to move on to more complicated applications, here are some sources that may help:

EJB clients interact with the J2EE EJB tier using the RMI-IIOP protocol. For more information on using RMI-IIOP in this way, see the Java 2 Platform, Enterprise Edition Blueprints or the EJB Tutorials.
The Java IDL tutorial includes an example for running the client and server on different machines. The concepts of Running the Hello World Example on Two Machines apply to this example as well.

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Getting Started Using RMI-IIOP: Example Using POA-based server-side model