1. Remote Method Invocation with Java-RMI
Automatically generated
Skeleton
- Demarshalling
- Dispatch
Stub
- Marshalling
Network
ServerI
Server
Client
Contains only functionality

2. What does the client see?
A remote object (proxy, stub) can
be casted to all interface the server implements,
be accessed only via interface methods.
On a remote invocation
non-remote arguments are copied (using serialization),
remote arguments are given as reference,  different to local invocation
java.rmi.RemoteException can always be thrown.
Remote Objects always support the interface Remote. (Tag interface)

3. How to build the server? Class diagram: RemoteObject Remote
Provided by the JDK
RemoteServer
UnicastRemoteObject
Activatable
Provided by the programer
ServerI
ServerImpl
- every method which is remotely invocable must be declared to throw RemoteException.
- Skeletons/Stubs are created with the JDK tool rmic.

4. Coordination with a naming service
The naming service (rmiregistry)
is a remote Java object,
manages relationships between Strings (i.e. names) and remote Java objects (on the same host).
remote Java objects register themselves with java.rmi.Naming.bind(String name,Remote obj); java.rmi.Naming.rebind(String name,Remote obj);
the name has the form: “//host:port/identifier“ (default: host=localhost, port=1099)
clients use the name to receive a stub: Remote Naming.lookup(String name);

5. Examplary Server Server interface: interface ServerI extends Remote
{ public void doSomeService()
throws RemoteException;}
Server implementation:
class Server extends UnicastRemoteObject
implements ServerI
{ public Server() throws RemoteException
{ super();}
public void doSomeService()
throws RemoteException
{ System.out.println("Working hard..");}
....

6. Initialisation+Registration
...
public static void main(String args[])
{try { // create instance
Server myServer = new Server();
// bind it to the naming service
Naming.rebind(“//elfe:4711/HardServer", myServer);
// done
} catch(Exception ex)
{ ex.printStackTrace();
System.exit(5); }

7. Remote Invocation of the Server
public class Client
{ static public void main(String args[])
{ try { // locate Server
Remote rs = Naming.lookup(“//elfe:4711/HardServer");
ServerI server = (ServerI) rs;
// use service
server.doSomeService();
// done
} catch(Exception ex)
{ ex.printStackTrace();
System.exit(5); }
} }

8. Compilation, Configuration, Startup
Compile classes
jedi> javac Server.java ServerI.java Client.java
Create Skeletons and Stubs
jedi> rmic Server
Start naming service on elfe
elfe> rmiregistry 4711 &
Start server on elfe
elfe> java Server &
Start client somewhere
jedi> java Client
Working hard on elfe

9. Installation of JacORB
Source code with examples and documentation:
local installation in the Sun pool: setenv JACO /import/public/opt/JacORB1_1 setenv PATH $JACO/bin:$PATH setenv CLASSPATH $JACO/lib/idl.jar:$JACO/lib/jacorb.jar:$CLASSPATH cp $JACO/jacorb_properties.template ~/.jacorb_properties edit ~/.jacorb_properties (an install script is on the WWW)
adapt the URL settings in the property file; at least this entry: jacorb.NameServerURL=file:/....

10. Overview about CORBA Client Server Servant Servant Impl. Servant IDL
Interface
Skeleton
Stub
POA
POA
ORB

11. Class Hierarchie for CORBA Servers for the Interface Foo
Servant
<<interface>>
FooOperations
FooPOA
FooPOATie
FooDelegateImpl
Delegate allows use of inheritence:
new FooPOATie(new FooDelegateImpl())
Delegates to
<<interface>>
Foo
FooImpl
Servants are no CORBA objects

12. The CORBA COS Naming Service
A naming context
stores relationships between names and Objects,
is itself a CORBA object (allows hierarchical, potentially cyclic name spaces).
name = sequence<name_components>, name_component=(string id, string kind)
ORB provides bootstrapping with the root context NameServer (ORB uses jacorb.NameServerURL; it points to an IOR)
bind an object in a subcontext NamingContext.bind(Name n, Object o);
resolve an object Object Object NamingContext.resolve(Name n);
create a new sub context NamingContext NamingContext.new_context(); NamingContext.bind_new_context(Name n);
bug in JDK  use NamingContextExt !

13. Examplary Server now in CORBA
Server IDL interface:
interface Server { void doSomeService();};
Server implementation:
class ServerImpl extends ServerPOA { public void doSomeService()
{ System.out.println("Working hard..");}
// main method follows

14. Initialisation+Registration
static public void main(String args[]) { try { // access CORBA, activate POA
ORB orb = org.omg.CORBA.ORB.init(args, null);
POA poa = POAHelper.narrow(orb.resolve_initial_references("RootPOA"));
poa.the_POAManager().activate(); // from now on requests are handled !!! // get name server
NamingContextExt nc =
NamingContextExtHelper.narrow(orb.resolve_initial_references("NameService"));
// generate a server and register it at the POA
org.omg.CORBA.Object cob = poa.servant_to_reference(new ServerImpl());
// generate a name
NameComponent [] name = new NameComponent[1];
name[0] = new NameComponent( "HardServer", "some kind");
// bind server to name
nc.bind(name, cob);
// done
System.out.println("Server is ready."); orb.run();
} catch(Exception ex){ex.printStackTrace();}
} } // main, class

15. Remote Invocation of the Server
class Client
{ static void main(String args[])
{ try{
// get ORB and naming service
ORB orb = org.omg.CORBA.ORB.init(args, null);
NamingContextExt nc =
NamingContextExtHelper.narrow(orb.resolve_initial_references("NameService"));
// generate the name
NameComponent [] name = new NameComponent[1];
name[0] = new NameComponent( "HardServer", "some kind");
// query naming service, cast the stub
org.omg.CORBA.Object cob = nc.resolve(name);
Server server = ServerHelper.narrow(cob); // use the generated Helper class
// use service
server.doSomeService();
} catch(Exception ex){ ex.printStackTrace(); }
} }

16. Compile the Example
Generate skeleton,stubs, helper classes from the IDL > idl server.idl
Skeletons: ServerPOA.java,ServerPOATie.java
Stub: _ServerStub.java
Helpers: ServerHelper.java, ServerHolder.java
Interfaces: Server.java, ServerOperations.java
compile the classes > javac *.java (take care of your CLASSPATH!)

17. Startup your Scenario
Start the naming service > ns NSIOR NSIOR should be: file(jacorb.NameServerURL) E.g.: file:/home/tnfink/ns.ior  /home/tnfink/ns.ior
start the server jedi> jaco ServerImpl Server is ready.
Start the client troll> jaco Client Working hard (auf jedi)

18. Miscellaneous Methods
Convert CORBA object references to Java Strings: String ORB.object_to_string(org.omg.CORBA.Object obj); org.omg.CORBA.Object ORB.string_to_object(String ior);
test for existance of an CORBA object (i.e. stub) boolean Object._non_existent();
test if two CORBA objects are equal boolean Object._is_equivalent(Object other); a return of false does not guarantee inequality!
Duplicate a CORBA object (reference) Object Object.duplicate();

19. Miscellaneous Tools, Parameters
naming service GUI >nmg
analysis of IORs > dior -f ns.ior IOR components----- TypeId : IDL:omg.org/CosNaming/NamingContextExt:1.0 Profile Id : TAG_INTERNET_IOP IIOP Version : 1.1 Host : Port :
Parameter for debug output (0=none, 1=important, ) jacorb.verbosity = 0

20. Java RMI  CORBA Java RMI is easy to learn and use,
does not support heterogeneity, scalability, reliability
CORBA
needs effort for learning
is a sophisticated industrial tool for the real world.