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

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)

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.

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);

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..");} ....

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); }

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); } } }

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

Installation of JacORB Source code with examples and documentation: http://www.inf.fu-berlin.de/~brose/jacorb 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:/....

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

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

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 !

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

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

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(); } } }

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!)

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)

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();

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 : 160.45.110.127 Port : 1281 ... Parameter for debug output (0=none, 1=important, ... 10) jacorb.verbosity = 0

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.