Contents

• CORBA
• CORBA to Java Mapping
• Jini Proxies
• Simple Example
  • Corba Server in Java
  • Corba Client in Java
  • Jini Service
  • Jini Server and Client
  • Building the Example
  • Running the Example
• Room Booking Example
  • CORBA Objects
  • Multiple Objects
  • Exceptions
  • Interfaces for Single Thin Proxy
  • RoomBookingBridge Implementation
  • Other Classes
  • Building the Example
  • Running the Example
• Migrating a CORBA Client to Jini
• Jini Service as a CORBA Object
• Summary

CORBA is also an infrastructure for distributed systems. It is targeted at a slightly different ``space'' to Jini, and has some minor and major differences.

1. It allows specification of objects which can be distributed. The concentration is on distributed objects rather than on distributed services
2. It is language-independent, using an Interface Definition Language (IDL) for specification of interfaces
3. CORBA objects can be implemented in a number of languages, including C, C++, SmallTalk and Java
4. Current versions of CORBA pass remote object references, rather than complete object instances. Each CORBA object lives within a server, and the object can only act within this server. This is more restricted than Jini, where an object can have instance data and class files sent to a remote location and execute there. This limitation in CORBA may change in future with ``pass by value'' parameters to methods

IDL is a language that allows the programmer to specify the interfaces of a distributed object system. The syntax is similar to C++, but does not include any implementation-level constructs. So it allows definitions of data types (such as structures and unions), constants, enumerated types, exceptions and interfaces. Within interfaces it allows declaration of attributes and operations (methods). The complete IDL specification can be found on the Object Management Group (OMG) site http://www.omg.org.

The book ``Java Programming with CORBA'' by Andreas Vogel and Keith Duddy (http://www.wiley.com/compbooks/vogel) contains an example of a room booking service specified in CORBA IDL and implemented in Java. This defines interfaces for Meeting, a factory to produce them, MeetingFactory and a Room. A room may have a number of meetings in slots (hourly throughout the day), and there are support constants, enumerations and typedefs to support this. In addition, exceptions may be thrown under various error conditions. The IDL that follows differs slightly from that given in the book, in that definitions of some data types that occur within interfaces have been ``lifted'' to a more global level, since the mapping from IDL to Java has changed slightly for elements nested within interfaces since that book was written.

module corba {
module RoomBooking {

    interface Meeting {
    
        // A meeting has two read-only attributes which describes
        // the purpose and the participants of that meeting.
    
        readonly attribute string purpose;
        readonly attribute string participants;

        oneway void destroy();
    };
    
    interface MeetingFactory {
    
        // A meeting factory creates meeting objects.
    
        Meeting CreateMeeting( in string purpose, in string participants);
    };

    // Meetings can be held between the usual business hours.
    // For the sake of simplity there are 8 slots at which meetings
    // can take place.
    
    enum Slot { am9, am10, am11, pm12, pm1, pm2, pm3, pm4 };
    
    // since IDL does not provide means to determine the cardinality
    // of an enum, a corresponding constant MaxSlots is defined.
    
    const short MaxSlots = 8;
    
    exception NoMeetingInThisSlot {};
    exception SlotAlreadyTaken {};
        
    interface Room {
    
        // A Room provides operations to view, make and cancel bookings.
        // Making a booking means associating a meeting with a time-slot
        // (for this particular room).
    
        // Meetings associates all meetings (of a day) with time slots
        // for a room.
    
        typedef Meeting Meetings[ MaxSlots ];
    
        // The attribute name names a room.
    
        readonly attribute string name;
    
        // View returns the bookings of a room.
        // For simplicity, the implementation handles only bookings
        // for one day.
    
        Meetings View();
    
        void Book( in Slot a_slot, in Meeting  a_meeting )
            raises(SlotAlreadyTaken);
    
        void Cancel( in Slot  a_slot )
            raises(NoMeetingInThisSlot);
    };
}; 
};

CORBA has bindings to a number of languages. The most recent addition is Java, and this binding is still under active development (that is, the core is basically settled, but some parts are still changing). This binding must cover all elements of IDL. A horribly brief summary is:

Module

A module is translated to a java package. All elements within the module becomes classes or interfaces within the package

Basic types

Most of the basic types map in a straightforward manner: a CORBA int becomes a Java int, a CORBA string becomes a Java java.lang.String, and so on. Some are a little tricky, such as the unsigned types which have no Java equivalent.

Constant

Constants within a CORBA IDL interface are mapped to constants within the corresponding Java interface. Constants which are ``global'' have no direct equivalent in Java, and so are mapped to Java interfaces with a single field which is the value.

Enum

Enumerated types have no direct Java equivalent, and so are mapped into a Java interface with the enumeration as a set of integer constants.

Struct

A CORBA IDL structure is implemented as a Java class with instance variables for all fields.

Interface

A CORBA IDL interface translates into a Java interface.

Exception

A CORBA IDL exception maps to a final Java class.

A Jini service exports a proxy object that acts within the client, on behalf of the service. The proxy may be ``fat'' or ``thin'', depending on circumstances. In the chapter on MindStorms, the proxy has to be thin, since all it does is pass on requests to the real service, which is linked to the hardware device. The service cannot move since it has to talk to a particular serial port, so the proxy is thin. (The proxy may have an extensive user interface, but the Jini community seems to feel that any user interface should be in Entry objects rather than in the proxy itself.) Proxy objects created as RMI proxies are similarly thin, just passing on method calls to the service.

CORBA services can be delivered to any accessible client. Each service is limited to the server on which it is running, so they are essentially immobile. They can be found by a variety of methods, such as a CORBA naming or trading service. These search methods can be run by any client, anywhere. A search will return a reference to a remote object, which is essentially a thin proxy to the CORBA service. Similarly, if a CORBA method call creates and returns an object, then it will return a remote reference to that object which will continue to exist on the server where it was created. (The new CORBA standards will allow objects to be returned by value. This is not yet commonplace, and will probably be restricted to a few languages such as C++ and Java.)

The simplest way to make a CORBA object available to a Jini federation is to build a Jini service that is at the same time a CORBA client. The service acts as a bridge between the two protocols. Really, this is just the same as MindStorms - anything that requires talking a different protocol (hardware or software) will require a bridge between the two.

Most CORBA implementations use IIOP - that is, they are TCP based. The current Jini implementation is also TCP based. So there is a confluence of transport methods, which normally would not occur. A bridge would usually be fixed to a particular piece of hardware, but here it is not necessary due to this confluence. A Jini proxy need not report back to its server since it can negotiate directly with the CORBA service over the TCP network. In many instances it can be a ``fat'' proxy. This is similar in concept to the fat proxy in the chapter on Service Choices: there the fat proxy used HTTP to talk to a service within another protocol, but this can also be done with the CORBA protocols. A proxy may need to be thin in some cases such as when it is used by an applet: security restrictions may mean that it cannot talk to any host but only to the one the applet came from. In such a case it would have to perform all CORBA client activities back on its `home' server.

The CORBA IDL tutorial at http://java.sun.com/products/jdk/1.2/docs/guide/idl/ gives a ``hello world'' example. This is defined by the IDL

module corba {
    module HelloApp {
        interface Hello {
	    string sayHello();
        };
    };
};

/*
 * File: ./corba/HelloApp/Hello.java
 * From: Hello.idl
 * Date: Tue Aug 24 11:30:21 1999
 *   By: idltojava Java IDL 1.2 Aug 11 1998 02:00:18
 */

package corba.HelloApp;
public interface Hello
    extends org.omg.CORBA.Object, org.omg.CORBA.portable.IDLEntity {
    String sayHello()
;
}

A server for the hello IDL can be written in any language with a CORBA binding, such as C++. Rather than get diverted into other languages, though, we shall stick to a Java implementation. However, this language choice is not forced on us by CORBA.

The server must create an object that implements the Hello interface. This is done by creating a servant that inherits from the _HelloImplBase, and then registering it with the CORBA ORB (Object Request Broker - the CORBA ``backplane''). The server must also find a name server and register the name and the implementation. The servant implements the Hello interface. The server can just sleep to continue existence after registering the servant.

/**
 * CorbaHelloServer.java
 */
package corba;

import corba.HelloApp.*;
import org.omg.CosNaming.*;
import org.omg.CosNaming.NamingContextPackage.*;
import org.omg.CORBA.*;

public class CorbaHelloServer {
    
    public CorbaHelloServer() {
	
    }
    
    public static void main(String[] args) {
	try {
	    // create a Hello implementation object
	    ORB orb = ORB.init(args, null);
	    HelloServant helloRef = new HelloServant();
	    orb.connect(helloRef);

	    // find the name server
	    org.omg.CORBA.Object objRef = 
		orb.resolve_initial_references("NameService");
	    NamingContext ncRef = NamingContextHelper.narrow(objRef);

	    // bind the Hello service to the name server
	    NameComponent nc = new NameComponent("Hello", "");
	    NameComponent path[] = {nc};
	    ncRef.rebind(path, helloRef);

	    // sleep
	    java.lang.Object sleep = new java.lang.Object();
	    synchronized(sleep) {
		sleep.wait();
	    }
	} catch(Exception e) {
	    e.printStackTrace();
	}
    }
    
} // CorbaHelloServer

class HelloServant extends _HelloImplBase {
    public String sayHello() {
	return("hello world");
    }
}

A standalone client finds a proxy implementing the Hello interface by methods such as looking up a CORBA name server. The name server returns a org.omg.CORBA.Object which is cast to the interface type by the HelloHelper method narrow() (the Java cast method is not used). This proxy object can then be used to call methods back in the CORBA server

/**
 * CorbaHelloClient.java
 */
package corba;

import corba.HelloApp.*;
import org.omg.CosNaming.*;
import org.omg.CORBA.*;

public class CorbaHelloClient {
    
    public CorbaHelloClient() {
	
    }
    
    public static void main(String[] args) {
	try {
	    ORB orb = ORB.init(args, null);

	    // find the name server
	    org.omg.CORBA.Object objRef = 
		orb.resolve_initial_references("NameService");
	    NamingContext ncRef = NamingContextHelper.narrow(objRef);

 	    // find the Hello proxy
	    NameComponent nc = new NameComponent("Hello", "");
	    NameComponent path[] = {nc};
	    org.omg.CORBA.Object obj = ncRef.resolve(path);
	    Hello helloRef = HelloHelper.narrow(obj);

	    // now invoke methods on the CORBA proxy
	    String hello = helloRef.sayHello();
	    System.out.println(hello);
	} catch(Exception e) {
	    e.printStackTrace();
	}
    }
    
} // CorbaHelloClient

In order to make the CORBA object accessible to the Jini world, it must be turned into a Jini service. At the same time it must remain in a CORBA server, so that it can be used by ordinary CORBA clients. So we do nothing to the CORBA server. Instead, we need to build a Jini service that will act as a CORBA client. This service will then be able to deliver the CORBA service to Jini clients.

The Jini service can be implemented as a fat proxy delivered to a Jini client and performing most if its activity within this Jini client. The processes that run in this, with their associated Jini and CORBA objects, look like:

Figure 18.1: CORBA and Jini services

The Java interface for this service is quite simple, and basically just copies the interface for the CORBA service, as

/**
 * JavaHello.java
 */
package corba;

import java.io.Serializable;

public interface JavaHello extends Serializable {
    
    public String sayHello();
} // JavaHello

The implementation of this Jini interface will basically act as a CORBA client. Its sayHello() method will contact the CORBA naming service, find a reference to the CORBA Hello object and call its sayHello() method. The Jini service can just return the string it gets from the CORBA service.

/**
 * JavaHelloImpl.java
 */
package corba;

import org.omg.CosNaming.*;
import org.omg.CORBA.*;
import corba.HelloApp.*;

public class JavaHelloImpl implements JavaHello {
    
    protected Hello helloRef = null;
    protected String[] argv;

    public JavaHelloImpl(String[] argv) {
	this.argv = argv;
    }

    public String sayHello() {
	// Hello helloRef = null;

	if (helloRef == null) {
	    helloRef = getHelloRef();
	}
	// now invoke methods on the CORBA proxy
	String hello = helloRef.sayHello();
	return hello;
    }

    protected Hello getHelloRef() {
	ORB orb = null;
	// Act like a CORBA client
	try {
	    orb = ORB.init(argv, null);

	    // find the CORBA name server
	    org.omg.CORBA.Object objRef = 
		orb.resolve_initial_references("NameService");
	    NamingContext ncRef = NamingContextHelper.narrow(objRef);

 	    // find the CORBA Hello proxy
	    NameComponent nc = new NameComponent("Hello", "");
	    NameComponent path[] = {nc};
	    org.omg.CORBA.Object obj = ncRef.resolve(path);
	    Hello helloRef = HelloHelper.narrow(obj);
	    return helloRef;
	} catch(Exception e) {
	    e.printStackTrace();
	    return null;
	}
    }
} // JavaHelloImpl

The Jini server which exports the service doesn't contain anything new

package corba;


// import com.sun.jini.lookup.JoinManager;
import net.jini.lookup.JoinManager;
import net.jini.core.lookup.ServiceID;
// import com.sun.jini.lookup.ServiceIDListener;
// import com.sun.jini.lease.LeaseRenewalManager;
import net.jini.lookup.ServiceIDListener;
import net.jini.lease.LeaseRenewalManager;
import net.jini.discovery.LookupDiscovery;
import net.jini.discovery.LookupDiscoveryManager;

/**
 * JavaHelloServer.java
 */

public class JavaHelloServer implements ServiceIDListener {
    
    public static void main(String argv[]) {
	new JavaHelloServer(argv);

        // keep server running forever to
        // - allow time for locator discovery and
        // - keep re-registering the lease
        Object keepAlive = new Object();
        synchronized(keepAlive) {
            try {
                keepAlive.wait();
            } catch(java.lang.InterruptedException e) {
                // do nothing
            }
        }
    }

    public JavaHelloServer(String[] argv) {

	JoinManager joinMgr = null;
	try {
	    LookupDiscoveryManager mgr = 
		new LookupDiscoveryManager(LookupDiscovery.ALL_GROUPS,
					   null /* unicast locators */,
					   null /* DiscoveryListener */);
	    joinMgr = new JoinManager(new JavaHelloImpl(argv),
				      null,
				      this,
				      mgr,
				      new LeaseRenewalManager());
	} catch(Exception e) {
	    e.printStackTrace();
	    System.exit(1);
	}
    }

    public void serviceIDNotify(ServiceID serviceID) {
	System.out.println("got service ID " + serviceID.toString());
    }
    
} // JavaHelloServer

Similarly, the Jini client doesn't contain anything new

package client;

import corba.JavaHello;

import java.rmi.RMISecurityManager;
import net.jini.discovery.LookupDiscovery;
import net.jini.discovery.DiscoveryListener;
import net.jini.discovery.DiscoveryEvent;
import net.jini.core.lookup.ServiceRegistrar;
import net.jini.core.lookup.ServiceTemplate;


/**
 * TestCorbaHello.java
 */

public class TestCorbaHello implements DiscoveryListener {

    public static void main(String argv[]) {
	new TestCorbaHello();

        // stay around long enough to receive replies
        try {
            Thread.currentThread().sleep(10000L);
        } catch(java.lang.InterruptedException e) {
            // do nothing
        }
    }

    public TestCorbaHello() {
	System.setSecurityManager(new RMISecurityManager());

	LookupDiscovery discover = null;
        try {
            discover = new LookupDiscovery(LookupDiscovery.ALL_GROUPS);
        } catch(Exception e) {
            System.err.println(e.toString());
            System.exit(1);
        }

        discover.addDiscoveryListener(this);

    }
    
    public void discovered(DiscoveryEvent evt) {

        ServiceRegistrar[] registrars = evt.getRegistrars();
 
        for (int n = 0; n < registrars.length; n++) {
	    System.out.println("Service found");
            ServiceRegistrar registrar = registrars[n];

	    new LookupThread(registrar).start();
	}

	    // System.exit(0);
    }

    public void discarded(DiscoveryEvent evt) {
	// empty
    }

    class LookupThread extends Thread {

	ServiceRegistrar registrar;

	LookupThread(ServiceRegistrar registrar) {
	    this.registrar = registrar;
	}


	public void run() {

	    Class[] classes = new Class[] {JavaHello.class};
	    JavaHello hello = null;
	    ServiceTemplate template = new ServiceTemplate(null, classes, 
							   null);
	    
	    try {
		hello = (JavaHello) registrar.lookup(template);
	    } catch(java.rmi.RemoteException e) {
		e.printStackTrace();
		System.exit(2);
	    }
	    if (hello == null) {
		System.out.println("hello null");
		return;
	    }
	    String msg;
	    try {
		msg = hello.sayHello();
		System.out.println(msg);
	    } catch(Exception e) {
		// we may get a CORBA runtime error
		System.err.println(e.toString());
	    }
	}
    }

} // TestCorbaHello

The major additional step is to build the Java classes from the IDL specification. There are a number of CORBA IDL to Java compilers. One of these is the Sun compiler idltojava which is available from java.sun.com. This (or another compiler) needs to be run on the IDL file to produce the Java files in the package corba.HelloApp. These are standard Java files, and can be compiled using your normal Java compiler. They may need some CORBA files in the classpath if required by your vendor's implementation of CORBA. Files produced by idltojava do not need any extra classes.

The Jini server, service and client are also normal Java files, and can be compiled like earlier Jini files, with the classpath set to include the Jini libraries.

There are a large number of elements and processes that must be set running to get this running satisfactorily

1. A CORBA name server must be set running. In the JDK 1.2 distribution is a server, tnameserv. By default this runs on TCP port 900. Under Unix access to this port is restricted to system supervisors. It can be set running on this port by a supervisor, or started during boot time. An ordinary user will need to use the option -ORBInitialPort port to run it on a port above 1024. For example,

   
   tnameserv -ORBInitialPort 1055
   

   All CORBA services and clients should also use this port number
2. The Java version of the CORBA service can then be started by

   
   java corba.CorbaHelloServer -ORBInitialPort 1055
     

3. Typical Jini support services will need to be running, such as a Jini lookup service, the RMI daemon rmid, and HTTP servers to move class definitions around
4. The Jini service can be started by

   
   java corba.JavaHelloServer -ORBInitialPort 1055
     

5. Finally, the Jini client can be run by

   
   java client.TestCorbaHello -ORBInitialPort 1055
     

The IDL for a room booking problem was briefly discussed in an earlier section. This has a few more complexities over the previous example. The problem is to have a set of rooms, and for each room a set of bookings that can be made on that room. The bookings may be made on the hour, from 9am till 4pm (this only covers the bookings for one day). Bookings once made may be cancelled. A room can be queried for the set of bookings it has: it returns an array of meetings, which are null if no booking has been made, or a non-null meeting which has details of the participants and the purpose of the meeting.

The additional features of this example are

1. Each room is implemented as a separate CORBA object. There is also a ``meeting factory'' which produces more objects. This is a system with multiple CORBA objects residing on many CORBA servers. There are several possibilities for dealing with multiple objects
2. Some of the methods return CORBA objects. These may need to be exposed to clients. This is not a problem if the client is a CORBA client, but here we will have Jini clients
3. Some of the methods throw user-defined exceptions, in addition to CORBA-defined exceptions. Both of these need to be handled appropriately

CORBA defines a set of ``primitive'' types such as integers of various sizes, chars, etc. The language bindings specify the primitive types they turn into for each language. For example, the CORBA wide character wchar becomes a Java Unicode char. Things are different for non-primitive objects, though.

The IDL for the room booking defines CORBA interfaces for Meeting, MeetingFactory and Room. The implementation of these can be in any suitable language, and need not be in Java. The Java binding will convert these into Java interfaces. A CORBA client written in Java will get objects that implement these interfaces, but these will essentially be just references to remote CORBA objects. Two things are certain about these references:

1. CORBA interfaces generate Java interfaces such as Hello. These inherit from org.omg.CORBA.portable.IDLEntity, which implements Serializable. So the references can be moved around like Jini objects. But they lose their link to the CORBA ORB that created them, and may end up in a different ``namespace'' where the reference makes no sense. So CORBA references cannot be usefully moved around. The closest one can come at present is to convert them to ``string-ified'' form and move that around. This may change when CORBA pass-by-value objects become common. Note that the serialization method that gives a string representation of a CORBA object is not the same as the Java one: the CORBA method serializes the remote reference, the Java method serializes the object's instance data
2. They do not subclass from UnicastRemoteObject or Activatable. The Java runtime will not use an RMI stub for them

If a Jini client gets local references to these objects, and keeps them local, then it can use them via their Java interfaces. If they need to be moved around the network, then appropriate ``mobile'' classes will need to be defined and the information copied across to them from the local objects. For example, the CORBA Meeting interface generates the Java interface

/*
 * File: ./corba/RoomBooking/Meeting.java
 * From: RoomBooking.idl
 * Date: Wed Aug 25 11:30:25 1999
 *   By: idltojava Java IDL 1.2 Aug 11 1998 02:00:18
 */

package corba.RoomBooking;
public interface Meeting
    extends org.omg.CORBA.Object, org.omg.CORBA.portable.IDLEntity {
    String purpose();
    String participants();
    void destroy()
;
}

/**
 * JavaMeeting.java
 */
package corba.common;

import java.io.Serializable;
import org.omg.CORBA.*;
import corba.RoomBooking.*;
import java.rmi.RemoteException;

public interface JavaMeeting extends Serializable {
    String getPurpose();
    String getParticipants();
    Meeting getMeeting(ORB orb);
} // JavaMeeting

/**
 * JavaMeetingImpl.java
 */
package corba.RoomBookingImpl;

import corba.RoomBooking.*;
import org.omg.CORBA.*;
import corba.common.*;

/**
 * A portable Java object representing a CORBA object.
 */
public class JavaMeetingImpl implements JavaMeeting {
    protected String purpose;
    protected String participants;
    protected String corbaObj;

    /**
     * get the purpose of a meeting for a Java client
     * unaware of CORBA
     */ 
    public String getPurpose() {
	return purpose;
    }

    /**
     * get the participants of a meeting for a Java client
     * unaware of CORBA
     */ 
    public String getParticipants() {
	return participants;
    }

    /**
     * reconstruct a meeting using a CORBA orb in the target JVM
     */
    public Meeting getMeeting(ORB orb) {
	org.omg.CORBA.Object obj = orb.string_to_object(corbaObj);
	Meeting m = MeetingHelper.narrow(obj);
	return m;
    }

    /**
     * construct a portable Java representation of the CORBA
     * Meeting, using the CORBA orb on the source JVM
     */
    public JavaMeetingImpl(Meeting m, ORB orb) {
	purpose = m.purpose();
	participants = m.participants();
	corbaObj = orb.object_to_string(m);
    }
    
} // JavaMeetingImpl

The implementation of the room booking problem in the Vogel and Duddy book runs each room as a separate CORBA object, each with its own server. A meeting factory creates meeting objects which are kept within the factory server, and passed around by reference. So for a distributed application with ten rooms, there will be eleven CORBA servers running.

There are several possible ways of bringing this set of objects into the Jini world so that they are accessible to a Jini client, including

1. A Jini server may exist for each CORBA server
   1. Each Jini server may export fat proxies, which build CORBA references in the same Jini client
   2. Each Jini server may export a thin proxy, with a CORBA reference held in each of these servers
2. A single Jini server may be built for the federation of CORBA objects
   1. The single Jini server exports a fat proxy, which builds CORBA references in the Jini client
   2. The single Jini server exports a thin proxy, with all CORBA references within this single server

We can have one Jini server for each of the CORBA servers. The Jini servers can be running on the same machines as the CORBA ones, but there is no necessity to do so. Applet security restrictions, for example, may mean that all the Jini servers are running on a single machine, the same one as an applet's HTTP server.

The Jini proxy objects exported by each Jini server may be fat ones, which connect directly to the CORBA server. Thus each proxy becomes a CORBA client, as was dealt with in the ``hello world'' example. Within the Jini client, we do not just have one proxy, but many proxies. Because they are all running within the same address space, they can share CORBA references - there is no need to package a CORBA reference as a portable Jini object. In addition, the Jini client can just use all of these CORBA references directly, as instance objects of interfaces. Diagramatically, this appears as

Figure 18.2: CORBA and Jini services for fat proxies

The CORBA servers are all accessed from within the Jini client. This may rule out this arrangement if the client is an applet, and the servers are on different machines.

The proxies exported can be thin, such as RMI stubs. In this case each Jini server is acting as a CORBA client.

Figure 18.3: CORBA and Jini services for thin proxies

If all the Jini servers are collocated on the same machine, then this becomes a possible architecture suitable for applets. The downside of this approach is that all the CORBA references are within different JVMs. In order to move the reference for a meeting from the Jini meeting factory to one of the Jini rooms, it may be necessary to wrap it in a portable Jini object, as discussed above. The Jini client will also need to get information about the CORBA objects, which can be gained from these portable Jini objects.

An alternative to Jini servers for each CORBA server is to have a single Jini server into the CORBA federation. This can be a feasible alternative when the set of CORBA objects form a complete system or module, and it makes sense to treat them as a unit. There are then the choices again of where to locate the CORBA references, either in the Jini server or in a proxy. Placing them in a fat proxy looks like

Figure 18.4: CORBA and Jini services for single fat proxy

Placing all the CORBA references on the server side of a Jini service means that a Jini client only needs to make one network connection to the service. This is probably the best option from a security viewpoint of a Jini client.

Figure 18.5: CORBA and Jini services for single thin proxy

CORBA methods can throw exceptions of two types: system exceptions and user exceptions. System exceptions subclass from RuntimeException, and so are unchecked. These do not need to have explicit try...catch clauses around them. If an exception is thrown, it will be caught by the Java runtime, and generally halt the process with an error message. This would result in a CORBA client dying, which would generally be undesirable. Many of these system exceptions will be caused by the distributed nature of CORBA objects, and probably should be caught explicitly. If they cannot be handled directly, then to bring them into line with the Jini world, they can be wrapped as ``nested exceptions'' within a Remote exception and thrown again.

User exceptions are declared in the IDL for the CORBA interfaces and methods. These exceptions are checked, and need to be explicitly caught (or re-thrown) by Java methods. If a user exception is thrown, this will be because of some semantic error within one of the objects, and will be unrelated to any networking or remote issues. They should be treated as they are, without wrapping them in Remote exceptions.

This and the following sections consider issues in building a single thin proxy for a federation of CORBA objects. The Vogel and Duddy book gives a CORBA client to interact with the CORBA federation, and this is used as the basis for the Jini services and clients.

Using a thin proxy means that all CORBA-related calls will be placed in the service object, and made available to Jini clients only by means of portable Jini versions of the CORBA objects. These portable objects are defined by the interfaces

/**
 * JavaRoom.java
 */
package corba.common;

import corba.RoomBooking.*;
import java.io.Serializable;
import org.omg.CORBA.*;
import java.rmi.RemoteException;

public interface JavaRoom extends Serializable {
    String getName();
    Room getRoom(ORB orb);
} // JavaRoom

/**
 * JavaMeeting.java
 */
package corba.common;

import java.io.Serializable;
import org.omg.CORBA.*;
import corba.RoomBooking.*;
import java.rmi.RemoteException;

public interface JavaMeeting extends Serializable {
    String getPurpose();
    String getParticipants();
    Meeting getMeeting(ORB orb);
} // JavaMeeting

The bridge interface between the CORBA federation and the Jini clients has to provide methods for making changes to objects within the CORBA federation, and to obtain information from them. For the room booking system this requires the ability to book and cancel meetings within rooms, and also the ability to view the current state of the system. Viewing is accomplished by three methods: updating the current state, and getting the list of rooms and of bookings for a room.

/**
 * RoomBookingBridge.java
 */

package corba.common;

import java.rmi.RemoteException;
import corba.RoomBooking.*;
import org.omg.CORBA.*;

public interface RoomBookingBridge extends java.io.Serializable {
    
    public void cancel(int selected_room, int selected_slot)
	throws RemoteException, NoMeetingInThisSlot;
    public void book(String purpose, String participants,
		     int selected_room, int selected_slot)
	throws RemoteException, SlotAlreadyTaken;
    public void update()
	throws RemoteException, UserException;
    public JavaRoom[] getRooms()
	throws RemoteException;
    public JavaMeeting[] getMeetings(int room_index)
	throws RemoteException;
} // RoomBookingBridge

The room booking Jini bridge has to perform all CORBA activities, and to wrap these up as portable Jini objects. A major part of this is locating the CORBA services, which here are the meeting factory and all of the rooms. We do not want to get too involved in these issues here. The meeting factory can be found in essentially the same way as the hello server was earlier, by looking up its name. Finding the rooms is harder as these are not known in advance. Essentially, the equivalent of a ``directory'' has to be set up on the name server, which is known as a ``naming context''. Rooms are registered within this naming context by their servers, and the client gets this context and then does a search for its contents. The Jini component of this object is that it subclasses from UnicastRemoteObject, and implements a RemoteRoomBookingBridge which is a remote version of RoomBookingBridge. It is also worthwhile noting how CORBA exceptions are caught and wrapped in Remote exceptions.

/**
 * RoomBookingBridgeImpl.java
 */
package corba.RoomBookingImpl;

import org.omg.CORBA.*;
import org.omg.CosNaming.*;
import corba.RoomBooking.*;
import corba.common.*;
import java.rmi.RemoteException;
import java.rmi.server.UnicastRemoteObject;
import java.util.Vector;

public class RoomBookingBridgeImpl extends UnicastRemoteObject implements RemoteRoomBookingBridge {

    private MeetingFactory meeting_factory;
    private Room[] rooms;
    private Meeting[] meetings;
    private ORB orb;
    private NamingContext room_context;
   
    public RoomBookingBridgeImpl(String[] args)
	throws RemoteException, UserException {
	try {
            // initialise the ORB
            orb = ORB.init(args, null);
        }
        catch(SystemException system_exception ) { 
            throw new RemoteException("constructor RoomBookingBridge: ",
				      system_exception); 
        }
	init_from_ns();
	update();
    }
    
    public void init_from_ns()
	throws RemoteException, UserException {

        // initialise from Naming Service
        try {
            // get room context
	    String str_name = "/BuildingApplications/Rooms/";
	    org.omg.CORBA.Object objRef = orb.resolve_initial_references("NameService");
	    NamingContext ncRef = NamingContextHelper.narrow(objRef);
	    NameComponent nc = new NameComponent(str_name,  " ");
	    NameComponent path[] = {nc};
    
	    org.omg.CORBA.Object roomRef = ncRef.resolve(path);
            room_context = NamingContextHelper.narrow(roomRef);
            if( room_context == null ) {
                System.err.println( "Room context is null," );
                System.err.println( "exiting ..." );
                System.exit( 1 );
            }
	    
            // get MeetingFactory from Naming Service
	    str_name = "/BuildingApplications/MeetingFactories/MeetingFactory";
	    nc = new NameComponent(str_name, " ");
	    path[0] = nc;
            meeting_factory = MeetingFactoryHelper.narrow(ncRef.resolve(path));
            if( meeting_factory == null ) {
                System.err.println(
                    "No Meeting Factory registred at Naming Service" );
                System.err.println( "exiting ..." );
                System.exit( 1 );
            }
        }
        catch(SystemException system_exception ) { 
	    throw new RemoteException("Initialise ORB", system_exception);
        }
    }

    public void update()
	throws RemoteException, UserException {

        try {
            // list rooms
            // initialise binding list and binding iterator
            // Holder objects for out parameter
            BindingListHolder blHolder = new BindingListHolder();
            BindingIteratorHolder biHolder = new BindingIteratorHolder();
            BindingHolder bHolder = new BindingHolder();
            Vector roomVector = new Vector();
            Room aRoom;
    
            // we are 2 rooms via the room list
            // more rooms are available from the binding iterator
            room_context.list( 2, blHolder, biHolder );

            // get rooms from Room context of the Naming Service
            // and put them into the roomVector
            for(int i = 0; i < blHolder.value.length; i++ ) {
                aRoom = RoomHelper.narrow(
                    room_context.resolve( blHolder.value[i].binding_name ));
                roomVector.addElement( aRoom );
            }

            // get remaining rooms from the iterator
            if( biHolder.value != null ) {
                while( biHolder.value.next_one( bHolder ) ) {
                    aRoom = RoomHelper.narrow(
                        room_context.resolve( bHolder.value.binding_name ) );
                    if( aRoom != null ) {
                        roomVector.addElement( aRoom );
                    }
                }
            }

            // convert the roomVector into a room array
            rooms = new Room[ roomVector.size() ];
            roomVector.copyInto( rooms );

            // be fiendly with system resources
            if(  biHolder.value != null )
                biHolder.value.destroy();
        }
    
        catch(SystemException system_exception) {
	    throw new RemoteException("View", system_exception);
            // System.err.println("View: " + system_exception); 
        }
    }

    public void cancel(int selected_room, int selected_slot)
	throws RemoteException, NoMeetingInThisSlot {
        try {
            rooms[selected_room].Cancel(
                Slot.from_int(selected_slot) );
            System.out.println("Cancel called" ); 
        }
        catch(SystemException system_exception) { 
	    throw new RemoteException("Cancel", system_exception);
        }
    }

    public void book(String purpose, String participants,
			int selected_room, int selected_slot) 
	throws RemoteException, SlotAlreadyTaken {
        try {
            Meeting meeting =
                meeting_factory.CreateMeeting(purpose, participants);
            System.out.println( "meeting created" );
            String p = meeting.purpose();
            System.out.println("Purpose: "+p);
            rooms[selected_room].Book(
                Slot.from_int(selected_slot), meeting );
            System.out.println( "room is booked" ); 
        }
        catch(SystemException system_exception ) { 
	    throw new RemoteException("Booking system exception", system_exception);
        }
    }

    /**
     * return a list of the rooms as portable JavaRooms
     */
    public JavaRoom[] getRooms() {
	int len = rooms.length;
	JavaRoom[] jrooms = new JavaRoom[len];
	for (int n = 0; n < len; n++) {
	    jrooms[n] = new JavaRoomImpl(rooms[n]);
	}
	return jrooms;
    }

    public JavaMeeting[] getMeetings(int room_index) {
	Meeting[] meetings = rooms[room_index].View();
	int len = meetings.length;
	JavaMeeting[] jmeetings = new JavaMeeting[len];
	for (int n = 0; n < len; n++) {
	    if (meetings[n] == null) {
		jmeetings[n] = null;
	    } else {
		jmeetings[n] = new JavaMeetingImpl(meetings[n], orb);
	    }
	}
	return jmeetings;
    }
} // RoomBookingBridgeImpl

The Java classes and servers implementing the CORBA objects are mainly unchanged from the implementations given in the Vogel and Duddy book. They can continue to act as CORBA servers to the original clients. I replaced the ``easy naming'' naming service by a later one with the slightly more complex standard mechanism for creating contexts and placing names within this context. This can use the tnameserv CORBA naming server, for example.

The Vogel and Duddy room booking client was messed around a little bit, but its essential structure remained unchanged. The GUI elements, for example, were not altered. All CORBA-related code was hived off into the bridge classes.

The Vogel and Duddy code samples can all be downloaded from a public Web site (http://www.wiley.com/compbooks/vogel), and come with no author attribution or copyright claim. The client is also quite lengthy since it has plenty of GUI inside. So I won't complete the code listing here. The code for all my classes, and the modified code of the Vogel and Duddy classes, is given in the subdirectory corba of the programs.zip file.

The IDL interface RoomBooking.idl needs to have an IDL to Java run on it. This produces classes in the package corba.RoomBooking. These can all be compiled using the standard Java classes and any CORBA classes needed.

The Jini server, service and client are also normal Java files, and can be compiled like earlier Jini files, with the classpath set to include the Jini libraries.

There are a large number of elements and processes that must be set running to get this running satisfactorily

1. A CORBA name server must be set running, as in the earlier problem. For example,

   
   tnameserv -ORBInitialPort 1055
   

   All CORBA services and clients should also use this port number
2. A CORBA server should be started for each room, with the first parameter being the ``name'' of the room, as in

   
   java corba.RoomBookingImpl.RoomServer "freds room" -ORBInitialPort 1055
     

3. A CORBA server should be started for the meeting factory

   
   java corba.RoomBookingImpl.MeetingFactoryServer -ORBInitialPort 1055
     

4. Typical Jini support services will need to be running, such as a lookup service, the RMI daemon rmid, and HTTP servers to move class definitions around
5. The Jini service can be started by

   
   java corba.RoomBookingImpl.RoomBookingBridgeServer -ORBInitialPort 1055
     

6. Finally, the Jini client can be run by

   
   java corba.RoomBookingImpl.RoomBookingClientApplication -ORBInitialPort 1055
     

Both of the examples in this chapter started life as pure CORBA systems written by other authors, with CORBA objects delivered by servers to a CORBA client. The clients were both migrated to Jini clients of a Jini service acting as a front-end to CORBA objects, in a series of steps. For those in a similar situation, it may be worthwhile to spell out the steps I went through in doing this for the room booking problem.

1. The original client was a single ``monoblock'' client, mixing GUI elements, CORBA calls and glue to hold it all together. The first step was to decide on the architectural constraint: one Jini service, or many.
2. A single Jini service was chosen (for no other reason than it looked to offer more complexities). This implied that all CORBA related calls had to be collected into a single object, the RoomBookingBridgeImpl. At this stage, the RoomBookingBridge interface was not defined: that came after the implementation was completed (ok, I hang my head in shame, but I was trying to adapt existing code rather than starting from scratch). At this time, the client was still running as a pure CORBA client - no Jini mechanisms had been introduced.
3. Once all the CORBA related code was isolated into one class, another architectural decision had to be made, as to whether this was to function as a fat or thin proxy. The decision to make it thin in this case was again based on interest rather than functional reasons.
4. The GUI elements left behind in the client needed to access information from the CORBA objects. In the thin proxy model, this meant that portable Jini objects had to be built to carry information out of the CORBA world. This led to the interfaces such as JavaRoom and the implementations as JavaRoomImpl. The GUI code in the client had no need to directly modify fields in these objects, so they ended up as ``readonly'' versions of their CORBA sources. (If a fat proxy had been used, this step of creating portable Jini objects would not have been necessary.)
5. The client was modified to use these portable Jini objects, and the RoomBookingBridgeImpl was changed to return these from its methods. Again, this was all still done within the CORBA world, and no Jini services were yet involved. This looked like a good time to define the RoomBookingBridge interface, when everything had settled down.
6. Finally, the RoomBookingBridgeImpl was turned into a UnicastRemoteObject and placed into a Jini server. The client was changed to lookup a RoomBookingBridge service rather than create a RoomBookingBridgeImpl object.

At the end of this, I had an implementation of a Jini service, with a thin RMI proxy. The CORBA objects and servers had not been changed at all. The original CORBA client had been split into two, with the Jini service implementing all of the CORBA lookups. These were exposed to the client through a set of ``facades'' that gave it the information it needed.

The client was still responsible for all of the GUI aspects, and so was acting as a ``knowledgeable'' client. If needed, these GUI elements could be placed into Entry objects, and also exported as part of the service.

We have looked at making CORBA objects into Jini services. Is it possible to go the other way, and make a Jini service appear as a CORBA object in a CORBA federation? Well, it should be. Just as there is a mapping from CORBA IDL to Java, there is also a mapping of a suitable subset of Java into CORBA IDL. So a Jini service interface can be written as a CORBA interface. A Jini client could then be written as the implementation of a CORBA serve to this IDL.

At present, with a paucity of Jini services, it does not seem worthwhile yet to explore this in detail. This may change in the future, though.

CORBA is a separate distributed system to Jini. However, it is quite straightforward to build bridges between the two systems, and there are a number of different possible architectures. This makes it possible for CORBA services to be used by Jini clients.